Signs of Winter 6: The Tree of Life and Pondering the Inevitability of Change

The Tree of Life by Gustav Klimt. Photo by thewholegardenwillbow, Wikimedia Commons

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The Tree of Life is a symbol found throughout a myriad of mythologies and religions. From the ancient Middle East to the shared books of Judaism and Christianity, to Buddhism, Islam, Hinduism, and to a diverse array of beliefs of many indigenous civilizations around the world, some form of the Tree of Life stands at the origin point of life on Earth or at some other critical moment in existence. The Tree, in its many forms, touches both the Earth (and its deep, hidden Underworld recesses) and Heaven. Its leaves, its fruit, or its sap create or preserve life, grant immortality and convey knowledge sometimes for good and sometimes for evil.

This incredibly powerful, archetypal symbol was used by a number of 19th Century naturalists to represent the interconnected structure of Nature.  Augier in 1801 drew a Tree to illustrate the unity of a wide range of living plants, and Lamarck in 1809 used a similar (although inverted) model to illustrate the interconnections of an array of the types of animals. Charles Darwin, in his book “On the Origin of Species” in 1859, incorporated a description and also a drawing of the Tree of Life into his narrative not only to provide a visual representation of evolution and his Theory of Natural Selection but also to give it an emotional foundation. As Darwin described his Tree:

The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.”

Komensky’s Centrum Securitas Tree of Life. Public Domain

The ”green” terminal twigs of this tree, then, represent the living species that have arisen from the now extinct ancestral species(the “dead branches and limbs”) just below them. The green terminal twigs are all that can be actually seen: they are the living organisms of our biotic communities. The lower, “dead branches” may have fossilized relics from which their existence can be inferred or they may have to be logically constructed from the shared features and unique characteristics of the observable living species.

There are many implications inherent to this structure of the Tree of Life: 1. Living species are separated from each other. There are barriers that keep the DNA of each species isolated from the DNA of the other contemporary living species. Each living species is a unique, independent entity traveling along its own time path of evolution! And 2, change (i.e. evolution of new species) occurs only in a “vertical” direction! Genetic variation accumulates within every living population, and that variation interacts with the species’ environment to select for the “most fit” characteristics. These more fit individuals will come to dominate the living spaces of our “green twigs” and eventually, sprout up to form new species.

DNA Strand. Human Genome Research Institute. Public Domain

One of the driving engines for the growth of the Tree of Life is the inevitable change that occurs in an organism’s DNA during its replication when an organism makes new cells. A human cell, for example, contains about 3 billion base pairs whose pattern along the DNA molecule forms the information content of the genome. There are four types of bases in DNA (adenine, cytosine, thymine and guanine) and their precise match-ups with their complimentary bases (adenine and thymine always pair together, and cytosine and guanine always pair together) insures the great precision in DNA replication and also the efficiency and fidelity of RNA synthesis (“transcription”) and protein synthesis (“translation”).

The size of the task to replicate all of the base pairs of a DNA molecule, though, and the speed at which this has to occur inevitably leads to mismatched base pairs in the new DNA strand and potentially altered information in the DNA of the forming daughter cell. It is estimated that when DNA replicates an error in base match-up occurs approximately once in every 10,000 base pairs. This would mean that in a replicated human cell’s DNA 300,000 error points would occur! This would significantly change the genetic information in the newly formed cell and represents a unacceptably high rate of error. To prevent this from happening a robust system of enzymes and RNA molecules swarms over the new DNA strand both during and after replication and repairs most of these errors.

Exactly how many errors persist in the daughter cell DNA varies among different types of organisms and also between different sections of a given organism’s genome. A range of 1 to 6 persisting errors, though, in each and every new strand of replicated DNA is a relatively conservative estimate of the overall change. These replication errors are a type of mutation that inevitably increases the variability of a species’ DNA.

Most of these mutations do not alter the functional information of the new DNA, but, occasionally a significant change will occur! That altered DNA, then, will make a different kind of RNA which may have altered functions in the cell (and maybe even lead to a new type of protein being made in the daughter cell).  These new proteins often are non-functional and may lead to the disability and/or death of the daughter cell. In a few instances, though, these altered proteins may lead to some new structure or function that increases the “fitness” (efficiency and survivability) of the daughter cell (and organism)!

So, built into DNA because of an only slightly less than perfect method of replication, is an inevitable process of genetic change and evolution!

Spectrum of light. By Gringer. Wikimedia Commons

Other things can also cause mutations in a cell’s DNA. Exposure to energy sources in an organism’s environment (like UV or ionizing radiation) can directly and randomly change the base pattern of a DNA strand as can exposure of DNA to some of the powerful oxidizing by-products of cellular metabolism (the molecules called the “free radicals”). Toxins in our environment (like PCB’s or the dozens of poisons in cigarette smoke) can also alter the base patterns in, and, therefore, the information content of, a cell’s DNA. These mutations, like the mutations seen in DNA replication errors, may have no effect on the metabolic functioning of a cell, or they may be harmful and lead to cells with altered, often cripplingly inadequate structures or metabolisms. Very occasionally, though, these mutations can lead to proteins that increase a cell’s or organism’s fitness and possibly serve as the foundation for a newly evolving species. Some mutations can also cause cells to break free of their homeostatic control systems and start to replicate wildly. These types of cells will form cancers. Mutagens that cause cancers are called carcinogens.

It is estimated that 300 to 30,000 base pairs are changed via mutation in each generation of a species. Over a thousand generations, then, a considerable number of genes could be altered! Most of these mutations only affect the cells of an organism that form its own tissues and organs. They would not, then, be passed along to offspring and thus have an influence on the growth of their Tree of Life branch unless germ cells (sperm and ova) were altered. These accumulating genetic alterations, though, make each living organism a patchwork cellular quilt of slightly different genomes that becomes increasingly heterogeneous over time.

Simple diagram of a virus. By domdomegg, Wikimedia Commons

Viruses can also influence an organism’s movement up the Tree of Life. A virus is a small piece of either DNA or RNA that is wrapped in a capsule of protein. They are not considered to be true living organisms (they are called “life forms”) and, so, have no specified place of their own on the Tree of Life.

The purpose of a virus is to make more viruses, but sometimes this is accomplished in a roundabout manner by the virus inserting its genetic information into the genome of the cell it has infected. Residing in this host cell’s DNA, the viral genetic information may or may not synthesize entire viruses, but it will be replicated and preserved each time the host cell and then each subsequent daughter cell divides! The viral genes may also participate in vital functions within the host cell organism! It is estimated that 8% of the human genome is of viral origin. This represents 240 million base pairs some of which have been important in our movement up our Tree of Life twig.

So, it is not easy to move up one’s branch on the Tree of Life, but it is inevitable that every species will do so!

In a couple of weeks I will return to these ideas and talk about a major flaw in the Tree of Life metaphor! I will use some new insights and research on the evolutionary origin of land plants to show you that the branches of our tree sometimes fuse at least partially together! Genes, apparently, are able to flow not only vertically up the Tree, but also horizontally between living organisms!



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Signs of Winter 5: Sounds from the Underground!

Birds in a tree. Photo by A. DeSantis, Wikimedia Commons

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Monitoring sounds in an ecosystem (also called “acoustic monitoring”) is a common technique for determining the presence of organisms that are hard to visually detect. Bird songs, for example, are frequently used to determine the presence of shy or furtive species or to locate birds that inhabit particularly dense vegetative habitats. The detection of these birds by sound is an accepted technique for precise species identification in the field.

Bird songs also encode a great deal of information about a biotic community and can relay important survival clues to a wide range of members of that community. Back in July, 2015 I wrote a blog that explored the information content of bird alarm-songs that were triggered by the presence of predators. Different bird species “announced” the presence of particular predators and other species in the community (both birds and mammals) specifically listened to an often species-specific set of these bird alarms and reacted accordingly! The speed at which these alarm signals raced across a habitat were also calculated and shown to be many times faster than the top speeds at which the predators can move!

Photo by D. Sillman

A recent study in Ohio published in PlosOne (September 4, 2019) explored the effect of “bird chatter” (resting, peaceful bird songs and chirps) on eastern gray squirrels. The squirrels (neighborhood street and park dwellers in a small Ohio town), were exposed to a recording of a red-tail hawk (their principal avian predator). These recordings caused the squirrels to freeze in place with their heads and eyes turned upward searching for the incoming hawk. Immediately after the hawk recording, the researchers played either a recording of ambient noise or a recording of ambient noise plus bird chatter. The squirrels who got the recording of just noise remained motionless and vigilant significantly longer than those who were played the bird chatter tape. The squirrels, apparently, interpreted the relaxed bird sounds as an “all clear” signal for the passing hawk and relaxed their defensive postures and returned to other tasks like looking for food or tormenting neighborhood dogs!

The movement of sound through a vegetative ecosystem can be rapid, but its transmission velocity is affected by a variety of site factors. Very open habitats often are exposed to wind, and wind sounds interfere with sound transmission. Very dense, forest habitats, on the other hand, can have significant masses of vegetative materials that absorb sound or cause directional sound waves to echo about. These factors can both dampen sound energy and also decrease its fidelity and, therefore, information content.

Photo by D. Sillman

A factor we have discussed many times before that affects the density and complexity of forest vegetation here in the eastern United States is browsing by white-tailed deer. White-tailed deer population densities are currently at levels here in the East far greater than their pre-European settlement numbers, and their feeding activities have greatly reduced the ground vegetation cover, forest understory complexity and tree seedling densities! Not surprisingly, in these very open, heavily deer-browsed forests the flow of sound (including bird chatter and bird alarm calls) travels over greater distances and with much less distortion than is observed in a denser, non-deer browsed forest (this research was outlined in a second PlosOne article (February 13, 2019)). The consequences of this facilitated sound transmission on bird territory dimensions or community responsiveness have not yet been determined.

Compared to the cacophony of the above-ground portions of an ecosystem, the below-ground sections, i.e. the soil itself, should be very quiet, indeed. That is until you actually tune into it and listen!

Stag beetle grubs. Photo by L. Enking, Flickr

A soil ecologist at Hochschule Geisenheim University in Germany was interested in studying the amount of greenhouse gas (carbon dioxide, to be exact) beetle grubs generate when they feed on plant roots in an agricultural field. The problem was, in order for her to know where the beetle grubs were (and how many there were and what species they were, etc.) she was forced to dig up the field soil and thus destroy the grubs’ habitat. She wondered if she could locate grubs in the intact soil by sound possibly zeroing in on their chewing noises and then do her carbon dioxide measurements on the intact soil.

So, though contacts and previously published research she located an acoustics researcher at the University of York in England who designed some sturdy microphones that could be driven into the soil. Working in the York scientist’s lab, the two scientists used these microphones on soil containing grubs like the ones living in the fields in Germany and found that the beetle grubs, while burrowing underground, made “chirping” noises! These chirps (called “stridulations”) are made when the grub rubs its middle and hind legs together!

They further determined that the two main species of beetle grubs that infest the study area fields in Germany made recognizably different patterns of chirps! Both the location of the grubs and their species identification, then, could be determined by acoustic analysis alone! The ability to know which beetle grub species was dominant in a particular field could be most beneficial for the crop manager or farmer and lead to the application of the most effective method of pest control!

The German soil ecologist and the English acoustics researcher published a paper describing their findings in  Scientific Reports this past summer (12 July, 2019). They are not sure why the grubs chirp underground. They are hopeful, though, that after they gather sufficient data they should be able to come up with models that relate the stridulation patterns and intensities to the density of the grubs in a given soil volume. They also hope to develop techniques to identify and measure a wide range of crop damaging pests using these remarkably non-invasive, acoustic monitoring technologies!

The soil ecologist is also hopeful that she will be able to connect these auditory data to the metabolic rates of the grub community and then be able to make estimates of their production of carbon dioxide after all!

Now there are a three very important lessons or axioms about Science that are wrapped up in this narrative!

  1. Scientists need to communicate and cooperate with other scientists and share ideas and tools!
  2. A scientist always keeps their eyes open when dealing with natural phenomenon (SOMETHING unexpected (and interesting) is almost always going to happen!).
  3. A scientist should not blindly stick to their research plan! They must never hesitate to change and follow some new, beckoning path!  (and maybe that path, just like the T.S.Elliot poem, will eventually curve back to the place that you began!)



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Signs of Winter 4: Natural History of the White Ash (and more!)

Photo by D. Sillman

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In 1984 a group of students and I built a nature trail on the campus of Penn State New Kensington. The students were inspired by a trail we had explored down on the Delmarva Peninsula while on an ecology field trip to Wallop’s Island, Virginia. That trail illustrated the successional changes in the riparian forests of the peninsula, and on our nine hour van ride home, we agreed that we could make a comparable trail in the dynamic forest on our campus. Succession and ecological change were our themes. We dug in with great energy and scant funds and made a trail that lasted until my retirement from Penn State 34 years later.

There are two parts to this natural history of the white ash: the first is set in 2007 when I stood pondering the dense stand of white ash trees at the entrance to our campus nature trail, and the second is set twelve years later, in 2019, after the emerald ash borer had not only had decimated the campus’ white ash trees but, possibly, was well on its way to drive all of the ash tree species (white, green and “other”(see list at end of essay)) of North America into extinction.

White Ash Trees: 2007

White ashes on Nature Trail (c. 2000) Photo by D. Sillman

I am standing just inside the entrance of the campus nature trail. There is a nearly pure stand of white ash trees (Fraxinus americana) around me. The trees are densely packed together (most are just two or three feet apart from their nearest neighbors), and they are very uniform in size and shape: each is just under a foot in trunk diameter and 50 to 60 feet tall. These trees have grown as volunteers in the untended space between a stand of black pines that were originally planted on the south boundary of the campus and the tended lawns along outer edge of the campus’ athletic fields. Somehow this strip escaped the weekly mowing events of the campus’ energetic landscaping crews and was allowed to grow into a dense copse of white ash trees.

There is a roofed, picnic pavilion on the border between the pines and the white ash stand. When it was built, it would have been situated in clear view of a second pavilion that is just outside of the border of the ashes. This second pavilion overlooks the campus’ soccer field and has been tended to and repaired over its four decades of existence. The pavilion hidden behind the ash stand, though, has been ignored and has a crumbling roof, vine covered posts and moss and lichen encrusted beams and timbers.

Many of the upper branches of the ash trees are draped with wild grape vines, and around the trees there is a thick under-story of barberry, Virginia creeper, spicebush, sassafras, and dogwood. The edge of the ash stand that faces the grassy, open fields is a solid wall of raspberry, myrtle, and poison ivy.

Yellow poplars have begun to edge their way up from the oak-poplar forest that backs the pine plantation and are growing among the failing pines. The once densely planted black pines with their needle carpeted, shady, shrub-free, open inter-tree spaces  are now full of light gaps and invading shrubs and weeds (and poplars). The pines, planted not only here on the south boundary but also in a number of other locations across the campus are all succumbing to the stresses of a non-optimal climate and are dying due to a variety of fungal diseases. Each spring and summer more and more of these pines are being cut down.

A number of the ash trees have been broken in wind storms. Their canopy grape vines made them vulnerable to the hard winds that blow across this ridge. There are mounds of dried, broken grape vines scattered about on the forest floor and in the large spaces in between the still standing trees. Young poplar trees are abundant around these gaps and vine piles.

White ash is a very widely distributed hardwood tree of the eastern United States (Schlesinger 1990). It has remarkable ability to tolerate a wide variety of climatic and site variables but requires a moist, fertile soil rich in nitrogen and calcium (Erdman, Metzger, and Oberg 1979). It grows in combination with a very large number of other trees but is seldom the “dominant” or most abundant species on a site except when it is fulfilling its ecological role of a “pioneer” (Eyre 1980).  The large number of ash trees in the present day second and third growth forests of the Eastern and Midwestern United States probably reflect this “pioneering” nature. White ash is a tree that can quickly colonize and establish itself on disturbed or abandoned land (Schlesinger 1990)..

White ash trees come from other white ash trees, of course. The parental tree (or trees) of these ashes could have been an established ash back in the surrounding oak and poplar forest that was up to 500 feet from this edge habitat (Schlesinger 1990). That parental tree would have been a female, seed producing individual that would have been pollinated by a nearby male, pollen producing white ash sometime in April or May forty-plus years ago.

Ash Seeds on Tree Branch. Public Domain

The white ash seeds develop on the female trees in winged “samara” which are shed into the wind in the early fall. The samara can fly many hundreds of feet, and it is convenient that these long, samara flights occur after the deciduous trees in the forest have dropped their potentially obstructive leaves! The samara come to rest on a random patch of soil or leaf litter. A whole cohort of these samaras must have landed in this grassy strip just outside the pines.  Half of these samaras would have had seeds that could have germinated the following spring (Williams and Hauk 1976).

Moisture is the key to seed germination and also to survival of the seedlings, and the shaded edge of the black pines would have been a very conducive place for white ash seedlings to develop. Field planted white ash like these grow relatively slowly for their first few years, but after they have a well established root system, they are able to very rapidly increase in both height and diameter until they quickly come to over shade the surrounding weeds and competing trees (Logan 1973).

Ash seedlings are readily eaten by deer (Schlesinger 1990). Rabbits are also known to eat the bark of white ash trees. Many insects, fungi, bacteria (including phytoplasmas (very small bacteria) that cause the disease “ash yellows”) also damage white ash trees (Hepting 1971).

The ashes around me, though, survived all of these stresses. They are now producing many thousands of seeds each year that fly out in great clouds of samara and settle on the grassy fields and lawns throughout this part of the campus. Any developing ash seedling, though, is quickly terminated by the weekly spring and summer grass mowing.

The encroaching poplars may eventually rise above these ashes and begin to shade out the intolerant mature trees. And, even more eventually, acorns of the more distantly growing white and red and black oaks will be delivered into the poplar/ash stand by careless squirrels, crows, or blue jays and the slow, steady growth of the oaks will sculpt the forest into its “climax” state. That is, until the next disturbance comes along. Then, the ashes might return and begin the succession sequence all over again.

White Ash Trees: 2019

This was one of my favorite sections of our old, and now abandoned campus nature trail: the first hundred yards or so that wound through the volunteer forest of white ash trees. The straight, graceful trunks of the ashes and their long, deep green, lance-shaped leaves made a wonderful living entrance into the constantly changing forest of the trail. I did one of my first post-doctoral research projects in this little stand of ashes: I sampled the soil mites and collembola around the earthworm burrows and compared them to the mites in the non-worm-worked soil ( Hamilton, W.E. and Sillman, D.Y.  1989.  Influence of Earthworm Middens on the Distribution of Soil Microarthropods.  Biology and Fertility of Soils, 8: 279-284).

Photo by D. Herms, Ohio State University, Wikimedia Commons

About six years ago I began to see small, D-shaped holes in the bark of these ashes, and two years later the trees stopped making leaves. Three winters ago, storms blew down several of the trees, and all of the rest are now standing dead alongside the trail. Their presence makes the trail much too dangerous to walk when there is any sort of wind (which, up on the ridge where the campus is located is almost every day).

I arranged to have these trees cut down so that I could come out with a group of student volunteers to try to put the trail back in shape, but the crew that agreed to do the cutting job brought in excessively large machinery and a very impatient attitude. They charged into the woods and tore the trail surface apart making it impassable. They did all sorts of damage and ended up not even cutting down a single, dead tree. When they were finished, the path looked more like a strip mine than a nature trail. The only tiny piece of poetic justice was that they broke their brand new, mini-bulldozer in their blind pushing, plowing and tearing. The only thing to do after that was to close the trail.

Depending on the relative degree of lumping and splitting of species and subspecies designations, there are probably between 40 and 50 species of ash trees (genus Fraxinus) around the world (Atha and Boom, 2017). Ashes are found primarily in the northern (temperate) regions of Europe, Asia and North America and range from relatively modest sizes (like the 30 foot tall velvet ash) to trees of substantial heights and girths (like the potentially 120 foot tall white ash). Fifteen native ash species are found in North America (USDA, NRCS. 2019) (list at the end of this essay) where they have historically made up a substantial portion of our second and third growth deciduous forests (it estimated that there are (or were) 8 billion, wild ash trees in North America and that they made up 60% of the total tree diameter of our northeast forests (Atha and Boom, 2017). There are also seven exotic ashes that have been planted in North America (USDA, NRCS. 2019) (list at the end of this essay). Ashes, both native and exotic, have been extensively planted in urban and suburban habitats as ornamentals and along urban and suburban street as shade trees. Ash lumber is used to make everything from furniture to baseball bats, and ash logs are a highly preferred and commercially attractive form of firewood (Atha and Boom, 2017).

Five of North America’s ash tree species have recently been classified as critically endangered (IUCN 2019). They are teetering on the brink of potential extinction due to the impact of an exotic, invasive beetle from eastern Asia called the emerald ash borer (Agrilus planipennis). Our nature trail ash trees along with most of the ashes in Western Pennsylvania were some of the victims of these rapidly spreading beetles.

Photo by H. Russell, Wikimedia Commons

The emerald ash borer was first observed in 2001 in Detroit, Michigan. (USDA, Forest Service 2019). Ash trees in Detroit were mysteriously dying, and a small, iridescent green beetle was collected from the logs of the dead trees. The next year across the Detroit River in Windsor, Ontario these beetles were also collected from dead and dying ashes. Within a few years  the emerald ash borer was collected in forests from Minnesota to New York, and then the beetle spread north and east across Canada, east and south to the Atlantic Ocean and the Gulf of Mexico, and as far west as Colorado. Currently thirty-one states and two Canadian provinces report the presence of the emerald ash borer. Researchers speculate that the emerald ash borer had been in North America for more than a decade prior to its 2001 “discovery” in Detroit.

The emerald ash borer probably traveled to North America in ash wood used for shipping crates and packing materials in the cargo holds of ships. From wherever it first landed, it then rapidly began its destructive expansion through our deciduous  forests. The very low genetic variability of ash borers collected throughout the United States suggests that there was just a single invasive introduction event of the beetle (Villari et al. 2016).

Adult female emerald ash borers mate within a week of their summer-season (June to August) emergence and may fly ten kilometers or more in their search for a suitable ash tree on which they can lay their eggs. Eggs are stuck into the cracks and crevices of the outer bark of the ash and hatch into larvae in about three weeks (this life cycle is summarized from Villari et al. 2017). The larvae chew their way into the nutrient rich vascular layer (the “phloem”) just beneath the outer bark and begin to tunnel through and feed on this important tissue. Phloem transport sugars and other nutrients throughout the tree, and as it is destroyed, the tree becomes less and less able to sustain itself. After two or three more years of feeding and growing the mature larva fold themselves up into a pupation chamber just beneath the outer bark where they wait out the winter. In April or May they transform into adults. Adults chew their way out of the pupation chambers (leaving behind their characteristic D-shaped exit holes in the outer bark) in May or June and quickly mate. Each adult female lays on average 55 eggs. Counting the exit holes on a single ash tree suggests that dozens to hundreds of adult ash borers may be emerging each spring from each infected ash tree.

According to entomologists and ecologists working at the U.S. Forest Service and Ohio State University, 282 species of native arthropods (insects and spiders) rely on ash trees for their food and shelter, and 44 of these species feed exclusively on ash trees. Many of these insects are in turn food for a variety of birds and mammals. Ashes (especially the very abundant white ash, green ash and blue ash) are keystone species in their respective forest ecosystems and have wide ranging influences on the other plants and animals around them. Loss of these trees will have far ranging impacts on their ecosystems.

Two dead ash trees, Photo by M. Hunter, Wikimedia Commons

Forests attacked by emerald ash borers have almost no ash seeds in their soil seed beds, and after the death of the standing ashes, there are few ash seedlings to replace them.  The sun gaps that form when the ashes die typically fill up with fast growing, exotic invasive plants like oriental bittersweet, honeysuckle and multiflora rose which shade out and choke out native understory plants and most potential tree seedlings. Dr. Kathleen Knight of the U. S. Forest Service describes the post-ash borer forest as a “dense, impenetrable thicket of shrubs.”

The ash trees of East Asia have evolved mechanisms to control the emerald ash borer (Villari et al. 2016). Asian -native, healthy ashes (like the Manchurian ash) are avoided by the adult female ash borer because of the ability of these healthy trees to make inducible, protective chemicals that can kill the borer’s larvae. Emerald ash borers instead seek out sickened, native trees or non-native species of ash that are not able to synthesize these protective chemicals. Some experiments here in North America have involved spraying specific plant alarm chemicals and hormones (like methyl jasmonate) on infected ash trees. Results of these experiments have suggested that these natural plant compounds may help the trees to fight off the emerald ash borer beetles as effectively as insecticides.

The emerald ash borer has been called the “most destructive and economically costly insect ever to invade North America.” Its ecological and economic impact will far exceed that of the two major tree extermination events of the Twentieth Century: the American chestnut blight and Dutch elm disease. The ash borer is ravaging the ash components of our wild forests and destroying the graceful ash trees of our cities and neighborhoods. Monitoring programs and attempts at quarantine and control have been ineffective in stopping or even slowing down this pestilence. Many forest scientists have given up on trying to save these trees. Their extinction will occur, possibly, in the next few years! I mourn the loss of the white ashes of our nature trail and all of the billions of less seen ashes along our ridges and in our valleys.


Ash (Fraximus) species found in North America (compiled from USDA, NRCS. 2019):

Native species: black ash ( F. nigra), green ash (F. pennsylvanica), white ash (F. americana), blue ash (F. quadrangulata), California ash (F. dipetala), Carolina ash (F. caroliniana), Gregg’s ash (F. greggii), pumpkin ash (F. profunda), velvet ash (F. velutina), Chihuahua ash (F. papillosa), Oregon ash (F. latifolia), Goodings ash (F. gooddingii), Mexican ash (F. berlandieriana), single leaf ash (F. anomala), Texas ash (F. albicans)

Introduced, exotic species: European ash (F. excelsior), Manna ash (=flowering ash) (F. ornus), narrow leaf ash (F. angustifolia), Machurain ash (F. mandschurica), Chinese ash (F. chinensis), Marie’s ash (F. mariesi), Shamel ash (F. uhdei).


Atha, D. and B. Boom. 2017. Field Guide to the Ash Trees of Northeastern United States. Center for Conservation Strategy, The New York Botanical Garden, Bronx, NY. 26 pp.

Baker, W. L. 1976. Eastern forest insects. U. S. Department of Agriculture, Miscellaneous Publication  1175. Washington, D. C. 642 p.

Erdmann, G. G., F. T. Metzger, and R. R. Oberg. 1979. Macronutrient deficiency symptoms in seedlings of four northern hardwoods. USDA Forest Service, General Technical Report NC-53. North Central Forest Experiment Station, St. Paul, MN. 36 p.

Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Society of American Foresters, Washington, D. C. 148 p.

Hepting, G. H. 1971. Diseases of forest and shade tree of the United States. U. S. Department of Agriculture, Agriculture Handbook 386. Washington, D. C. 658 p.

IUCN 2019. The IUCN Red List of Threatened Species. Version 2019-2. Downloaded on 3 December, 2019.

Logan, K. T. 1973. Growth of tree seedlings as affected by light intensity. V. White ash, beech, eastern hemlock, and general conclusions. Canadian Forestry Service, Publication 1323. Ottawa, ON, 12 p.

Pennsylvania Department of Conservation and Natural Resources. 2008. “Emerald ash borer.” (January 21, 2008)

Schlesinger, R. C.. 1990.  Fraxinus Americana, L.: White ash. pp 333-338, In, Burns, R. M. and B. H. Honkala (tech coord) “Silvics of North America: Volume 2, Hardwoods.” U.S. Department of Agriculture, Agriculture Handbook 654. Washington, D. C.877 p.

USDA, Forest Service. 2019. Emerald ash borer: Biological control. ( (3 December, 2019)

USDA, NRCS. 2019. Fraxinus. The PLANTS Database (, 2 December 2019). National Plant Data Team, Greensboro, NC 27401-4901 USA.

Villari, C., D. A. Herms, J. G. A. Whitehill. D. Cipollini, P. Bonello. (2016) Progress and gaps in understanding mechanisms of ash tree resistance to emerald ash borer, a model for wood‐boring insects that kill angiosperms. New Phytologist (209): 63–79 doi: 10.1111/nph.13604

Williams, R. D. and S. H. Hanks. 1976. Hardwood nurserymen’s guide. U.S. Department of Agriculture, Agriculture handbook 473. Washington, D.C. 78 p.

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Signs of Winter (bonus): A Walk at Harrison Hills Park

Photo by D. Sillman

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This blog was first published four years ago as “Signs of Winter 8” (January 21, 2016). I thought that a nice walk in the woods would be a pleasant Christmas present for everyone! Enjoy!!

It was a cold, clear, sunny afternoon at the end of the first week of January. Deborah and I had the day to ourselves and decided to go for a short walk up at Harrison Hills Park. We had heard that a beaver had been spotted in the park’s big pond and wanted to see if we could get some pictures.

We got to the parking area about 1 pm and headed down the pond trail. There was a dusting of snow lingering in the shady places along the trail and under it the ground was frozen solid. In places where the sun (which was brilliantly glaring in a bright blue sky!) hit the trail, though, the snow and soil had melted into a slippery layer of mud. It was about 34 degrees air temperature but the sun felt warm on our faces.

We walked through the black cherry dominated woods and eased past the spot where bluebells will be blooming gloriously in the early spring. We walked past the small meadow where one of our bluebird boxes was set (a box that did not have any nests or fledges last year but seemed so perfectly placed for bluebirds that we left it in place for next spring and summer. Maybe the bluebirds will come to their senses and use it this coming season!). There were woodpeckers (mostly downies) pounding on the trees along the trail and a hint of larger noises (pileated woodpeckers?) further in the distance.

Photo by D. Sillman

As we headed down the west facing slope to the pond, there was a constant rustling in the thick shrubs of the surrounding understory. The glare of the sun made it difficult to see colors, but the sizes of the birds shuttling from branch to branch around us made us think of robins. Finally we heard the familiar robin chortles and calls and got some back-sightings of their red breasts and black bodies. The woods were full of robins!

We soon realized why the flock was here. Most of the understory was made up of berry bearing shrubs and vines. Barberry and spice bush and honeysuckle were interlaced with bittersweet vines and they all were covered with mostly red (but also clusters of black) fruit. Further, this hillside was a great solar collector especially in the afternoon as the weak winter sun dropped into the western sky. We could feel the air temperature go up several degrees as we walked along. Added to the food and warmth there was also a dense stand of spruce trees at the top of the hill that would provide sheltered night roosts for the birds. There must have been hundreds of robins twirling about in the dense shrub layer.

We always think of robins as migratory “signs of spring,” but they will stay in Western Pennsylvania through the winter if there is sufficient food. Holly thickets are great places to find overwintering robins, as are some city neighborhoods and sheltered berry thickets like this. It is interesting that most of the plants making this habitat so ideal for the robins are exotic invasive species. Oriental bittersweet, Japanese barberry and Tartarian honeysuckle are the major berry producers here. Each has merited a “noxious weed” designation and has caused extensive native species declines, but here, collectively, they are making life so much easier for the robins!

Photo by D. Sillman

We got down to the pond and walked as quietly as we could in hopes of seeing the beaver. The small island in the middle of the pond had a stacked pile of freshly cut sticks. The pond water had thin skim of ice and, as the sun began to ease down below the forested rise to the west, a shadow slowly crawled across it. You could feel the air temperature falling as the sun drew down. It was going to be another very cold night!

The beaver did not pop up out of his den. We did not get a picture of him. Clusters of alders next to the pond’s edges, though, had been gnawed through and it was obvious that the sticks piled up on the island were mostly from the freshly cut alders. Most were about two inches in diameter and about four or five feet long. The sticks would not only provide some shelter for the beaver but also food, as the inner bark of alder is a rich carbohydrate source (and a favorite food for beavers). We didn’t see any foot prints in the soft mud around the pond but did see piles of gnaw-chips.

Photo by D. Sillman

Beavers instinctively build dams from the small trees and branches that they cut with their powerful front incisors and carry and float and then mud-cement into place along small streams. These dams create protective ponds within which they can build their lodges. This beaver, though, sidestepped all of that instinctive building behavior by selecting an already constructed pond. Lodge construction, though, which will be the eventual outcome of all of those piled up sticks (if the county doesn’t come and trap out the beaver!), seems to be a learned behavior. Lodges set in the middle of a protective pond may have a variety of geometries and styles, but all have a number of underwater entrances and exits and at least two inner chambers (one for drying off after returning from a swim, and the other for sleeping and rearing their young). It will be interesting to see (again, if the county doesn’t intercede) what school of architecture this beaver ascribes to!

Beavers are obligatorily vegetarians. They preferentially eat water plants when they are available but survive on the inner bark of a variety of trees especially through the winter. They cache large quantities of sticks and branches under their lodges for winter consumption when conditions do not allow them to forage freely about.

We stayed around the pond for about an hour. The beaver never showed himself, but the evidence of his activity was everywhere. There was very little alder still standing around the pond. He would have to shift over to some other type of wood if he wanted to continue to add to his lodge or needed to accumulate more food. Poplars and aspens are other highly preferred food trees, but these are not abundant around the pond. Beavers also eat maples (especially red maple), birches, cottonwoods, willows and even pines. I am sure that he would find sufficient forage within a short waddle of the protective pond.



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Signs of Winter 3: Updates on Pollinators and Pesticides

Wild bee. Photo by Pixabay

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There were two interesting published reports about pollinating insects in Great Britain this last spring and summer:

The first, in the March 26, 2019 issue of Nature Communication summarized the findings of a thirty year study on the distribution of British, native pollinators. Looking at the ranges of 353 species of wild bees and hoverflies, researchers discovered that one-third of the species underwent a significant distribution decline over the course of the study. Only 11% of the studied species actually extended their ranges, and these species were predominantly ones that could live in newly expanding agricultural fields (like fields of oilseed rape). Less common, more specialized pollinating species were particularly hard hit while more generalized species maintained their overall distributions. Habitat loss (especially the loss of diverse, natural habitats) and the impacts of agricultural pesticides are highlighted as the most likely reasons for the observed declines, although the considerable loss of pollinating species in northern Britain may also reflect the impacts of rising temperatures due to ongoing climate change.

The second publication (in the June 13, 2019 issue of Current Biology) was an important bit of good news about British pollinators. Although, as we just stated above, many native pollinator species in Britain are in decline, populations of migratory, pollinator species seem to be maintaining their numbers. Each year, clouds of hoverflies migrate from Europe to southern Britain. Researchers utilizing “vertically looking radar” (VLR) have been able to monitor and quantify these migrating hoverflies for the past decade. Further, the numbers observed via these elegant radar systems have been confirmed on the ground via actual counts by participants in the “Hoverfly Recording Scheme,” a British citizen science group of gardening and nature enthusiasts.

Hoverfly. Photo by Alvagaspar. Wikimedia Commons

The total yearly numbers of the two migrating species of hoverflies have fluctuated between one and four billion individuals over the past ten years, but they are showing no sign of any decline. Hoverflies are important pollinators of a wide range of wild and domesticated plants. Their larvae are also significant predators of a range of plant pest species including aphids. Estimates are that the larvae of the migrating hoverflies consume 3 to 10 trillion aphids each year! These migrating hoverflies also transport pollen between plants in Britain and in Europe and are thought to help maintain the species continuity of these two distant vegetative communities.

The non-target or collateral impacts of pesticides on pollinating insects like bees and hoverflies are a major factor in the decline of their populations not only in Great Britain but throughout the world. Neonicotinoid pesticides in particular have been implicated as having serious, deleterious impacts on honey bees, bumblebees and other wild bee species.  A number of specific neonicotinoids have been banned from use on agricultural fields by the European Union after they were shown to inhibit ovarian development in bumblebees, interfere with the ability of honeybees to fly (and, thus, gather nectar and pollen), reduce the winter survival rates in honeybees, decrease the reproductive rates in bumblebees, and make honeybee sperm less active (see Signs of Summer 4, June 26, 2018, Signs of Summer 11, July 7, 2017, and  Signs of Fall 9, November 3, 2016).

Bumblebee. Photo by Alvagaspar, Wikimedia Commons

Further, in the search for non-neonicotinoid pesticides, chemicals like sulfoxaflor (a member of the Sulfoxamine class of pesticides) have been developed. These pesticides, too, have been shown to harm bees.  Bumblebees exposed to sulfoxaflor do not produce queens! These treated bumblebees also had a reduced number of worker bees compared to controls. The researchers stressed that although the bumblebee colonies “survived” the sulfoxaflor treatment, they were significantly stressed and reduced because of it (see Signs of Fall 8, October 25, 2018)

There have been a number of criticisms about the way pesticides are evaluated for their impacts on non-targeted insects. The standard testing methods involve the exposure of a cohort of pollinators to one pesticide or other biocide at a time. In the actual field uses, though, almost all of these pesticides are being applied along with one or two (or more) other pesticides or fungicides. There is fear that these multiple chemicals interact and have synergistic impacts on pollinating species.

An April 10, 2019 paper in the Proceedings of the Royal Society B looked at the impacts of a “safe for bees” pesticide (flupyraifurone (FPF)) (commericially called “Silvanto”) on honey bee populations when it was combined with the commonly used fungicide propionazole (PRO).  Honey bees exposed to both FPF and PRO exhibited a 50% mortality level at pesticide concentrations that were 80% lower than honey bees only exposed to FPF. FPF is one of the proposed “new era” pesticides designed to replace the neonicotinoids. This study, however, suggests that although it is different from the neonicotinoids, it is still unacceptably toxic to honey bees.

Honeybee. Photo by C.J.Sharp, Wikimedia Commons

The United States and especially the current EPA have been very slow in responding to the pesticide induced destruction of pollinating insects. This past spring (May 20, 2019), however, the EPA announced the de-certification (a functional “banning”) of twelve neonicotinoid pesticides. This decision was not initiated by the Federal Government, though, but instead was a consequence of a court settlement between the companies that manufacture these pesticides (Sungenta, Valeat and Bayer) and a consortium of environmental groups. The law suit was based on the Endangered Species Act and stipulated that the continued use of these pesticides was a real and credible threat to a number of endangered organisms.

And, finally, a number of states are stepping in to fill the gaps in the EPA’s oversight and management of pesticide use in the United States. The EPA in spite of overwhelming evidence of the toxic impacts of an organophosphate pesticide called “chloropyrifos” on farm workers and children living near fields in which the pesticide is used decided not to ban its use. Several states, though (including California, Hawaii, New York, Oregon, Connecticut and New Jersey) have initiated their own bans on the use of chloropyrifos. The National Resource Defense Council states that the toxic impacts of this pesticide are well documented and its use should be stopped. Dow Dupont, the company that manufactures chloropyrifos, cites a large data base of reports and studies (many of which were sponsored by the company itself) that indicates that the pesticide is safe.



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Signs of Winter 2: Plastics, Plastics Everywhere

Photo from Grendz,com

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Almost four years ago the World Economic Forum published a report entitled “The New Plastics Economy” (January, 2016). The report listed four extremely disturbing points about plastics in our social and economic systems:

  1. 95% of plastic packaging (valued at $80 to $120 billion per year) is “lost” after its first use.
  2. Worldwide, 14% of plastic packaging is collected for recycling but only 5% is subsequently reprocessed and then reused.
  3. 8 million tons of plastics enter the world’s oceans each year (and this tonnage is increasing each year).
  4. The world’s oceans contain 150 million tons of plastic, and it is predicted that by 2050 there will be more plastic, by weight, in the oceans than fish.

Worldwide use of plastics has increased 2000% in the past 50 years and is expected to increase by additional 200%  in the next 20 years. Plastic production currently utilizes 6% of the world’s annual production of oil and accounts for 1% of the world’s carbon-pollution production.

Photo by jar-O, Flickr

I have written about plastics before. They were invented just over a century ago (Signs of Fall 4, September 28, 2017. )They are one of the physical hallmarks of the Anthropocene (Signs of Summer 10 August 9, 2018). They exist as macro-pollutants in our soil, freshwater, and marine ecosystems and as micro-pollutants that have worked their way into our tap water, table salt and even our beer (Signs of Fall 12, November 22, 2018).

Plastics are extremely complex, organic polymers that have a wide range of functional properties that are generated by their intricate molecular structures. The names of the specific types of plastics reflect these fundamental structures, but since many of these names are difficult to say or remember (or spell!), a simplified classification system describing the seven “grades” of frequently encountered plastics has been devised. Most items made of plastic have their grade (numbers 1 through 7) stamped right on them.

Photo by Pexels. Southpack LLC

Grade 1 is “polyethylene terephthalate” (“PETE” or even shorter “PET”). These are tough plastics that are excellent barriers against liquids and gases. These are the plastics in most drink bottles and food containers. They are immensely recyclable and their long polymers can be used to make textiles, carpets, auto parts and more.

Grade 2 is high density polyethylene (HDPE). These plastics are made up of long, unbranched polymer chains that can be formed into extremely dense and strong objects. Milk and juice jugs, shampoo bottles, etc. are typically made from #2 plastics. These are also very recyclable and can be used to make plastic crates, building materials and fencing.

Grade 3 is polyvinyl chloride (PVC). This is a very strong plastic that can be used to make detergent bottles and plastic toys. It is frequently referred to, though, as the “poison plastic” because it can contain many toxins. It is, because of its hazardous nature, seldom recycled.

Grade 4 is low density polyethylene (LDPE). It is fundamentally similar to Grade 2 plastic except for a higher number of side branches in its molecular structure which reduce its overall density and cause it to form thinner sheets. Grade 4 is used to make plastic grocery bags, garbage bags and many types of squeeze bottles. Handling #4 plastics is sometimes difficult especially multiple material recycling systems: the light, filmy bags tend to blow about in the sorting systems clogging machinery and interfering with the sorting of the other materials. These #4 plastics, though, are very recyclable but are most efficiently handled separately.

Grade 5, 6 and 7 plastics are, respectively, polypropylene (PP), polystyrene (PS (“i.e. Styrofoam”)), and the catchall category of “other.” Small food containers (like yogurt containers) and plastic straws are #5 plastics. The recycling of these materials is possible, but is seldom done. Most recycling systems do not take 5 or 6 or 7 plastics.

Photo by hhach, Pixabay

A logical response to many of the problems caused by plastics was to increase the rates of recovery and recycling of the plastic materials. Many communities vigorously collected plastics from their citizens and sent these plastics, typically, on to China for reprocessing. Unfortunately, the gathered plastics materials were not all equally useful, and the mixed plastics sent to China contained, along with “useful” (i.e. easily recyclable) plastics (like the #1 and #2 plastics), a large volume of “low quality” (i.e. harder to recycle) plastics (#’s 3,4,5,6 and 7 plastics). In 2017 China announced that it would no longer accept mixed plastics and now only 56% of the plastics once exported are accepted in foreign markets.

Many recycling centers around the country initially tried to get their contributors to only gather and donate #1 and #2 plastics. Detergent bottles (#3’s), grocery bags (#4’s), yogurt containers (#5’s) and Styrofoam packages (#6’s) were expressly forbidden to be included in the recycling materials. Recyclables also could not be packaged in trash bags or even the clear “recycling” bags (these are both #4 plastics)! For many recycling systems, though, the inability or unwillingness of their participants to heed these restrictions led a total ban on plastic collection. In most communities, plastics now can only go to trash incinerators or landfills.

Photo by Pexels

What can be done about this? Are we just going to bury ourselves in plastic waste? Each of us can take control of our own little ecosphere by not buying things packaged in plastic. Or, if that is too onerous, find a recycling center that takes #1 and #2 plastics (and, maybe also, #4’s) and only purchase things packaged in those polymers. It is time to do something positive! It is time to take some control!

Just a couple more points about environmental plastics: a paper published in the journal Nature Geoscience this past spring reported on a study conducted high up in the Pyrenees Mountains in France. Researchers took air samples in this remote area and found significant concentrations of microplastics. Microplastics are pieces of plastic that are less than 5 mm long (smaller than a sesame seed, to quote one researcher) and often thinner than a human hair. These plastics originate from the breakdown of macro-plastics in cities, landfills and farms (especially farms on which sewage sludge is sprayed). These micro-plastics have been shown to clog the digestive systems of insects, fish, and aquatic birds and mammals. Sea birds, sea turtles and whales have all been killed by the accumulation of these plastics in their stomachs and intestines.

These micro-plastics also attract a variety of toxic chemicals in the environment (including dioxins, PCB’s, DDT’s and PAH’s) and can deliver extremely high doses of these dangerous poisons to any organism (including humans) who ingest them. Bacteria (like E. coli) also are concentrated on these airborne and waterborne plastics.

Our plastic-covered world is more that just unpleasant to look at, it may have the potential to kill us all

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Signs of Winter 1: Natural History of a Black Cherry Tree!

Photo by N. Tonelli, FLickr

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Imagine that it’s late spring and we are standing on the North Country Trail in the Allegheny National Forest. In front of us is a large black cherry tree. The tree is more than sixty feet tall and almost two feet in trunk diameter. Its black, scaly, almost reptilian-looking bark is lumpy under our fingertips. Its tall, straight trunk rises twenty-five feet and then bifurcates into two nearly identical, vertical columns. The leaves are green and indistinct against a bright blue sky.

To our right and left, and also straight ahead and behind us are black cherry after black cherry after black cherry. Most of the trees are approximately the same size as the one we are touching. This entire stand of trees must have originated at approximately the same time. Some of the trees have significant woodpecker damage: great rectangular holes cut through the bark and deep into the wood. Several trees have piles of fine, red sawdust around their bases. Tiny holes in the trunks also seem to be leaking sawdust. These holes, drilled into or out of the trees, are possibly the cues the woodpeckers are following in their search for insect larvae. A few trees have been broken or thrown by winds. There are branches scattered about on the forest floor.

There are very few tree seedlings in the dense understory in between the older trees. The black cherry saplings and pole trees that are here almost all have dense masses of eastern tent caterpillars wedged into the forks of their branches and the crotches of their trunks. They also all have finger-shaped, “spindle galls”  protruding from upper surface of their leaves. The tent caterpillars’ preference for “wild” cherry trees may be explained by their ability to concentrate the cherry leaves’ toxins and regurgitate them. These regurgitated fluids are rich in hydrogen cyanide and  benzaldehyde and provide the caterpillars some level of protection against predaceous ants. The galls are caused by microscopic eriophyid mites feeding on the tissues of the leaves. They cause little damage to the leaflets or the trees.

Most of the space of forest floor is occupied by a nearly continuous mass of tall, hay scented ferns. The trees seems to be floating in a sea of ferns.

Black cherry trees are found throughout the eastern United States and southern Canada. They grow in a wide variety of landscapes, soils, and conditions. It is said that black cherry thrive in all but the very driest or the very wettest of sites. They grow in association with almost any other northern tree. They are, then, consummate generalists.

The optimal or ideal conditions for black cherry, though, are found right here on the Allegheny Plateau of northwestern Pennsylvania. The cool, moist conditions of the plateau sustain not only the densest growth of black cherry in North America but also some of the largest individual specimens. The economic value of these trees, for veneer and for furniture, is staggering.

Historically, though, black cherry made up only a very small percentage of the primary forest of the Allegheny Plateau. Researchers looking at “witness tree” data from original, pre-settlement surveys of this area, estimate that black cherry made up only 0.09% of the trees and existed primarily as isolated, individual trees scattered about in a nearly continuous forest of American beech, hemlock, maple, white pine, and birch.

Photo by D. Sillman

Looking around at the forest alongside this trail, though, we see black cherry and very little else. How did these trees get here? These trees are around a hundred years old. So, to understand how this formerly beech and hemlock forest became a forest approaching a black cherry monoculture, we need to go back to the last decades of the nineteenth century and the first decades of the twentieth century and visualize the biological and human forces acting on this area’s ecosystems.

Prior to 1880 logging on the Allegheny Plateau was confined to sites with access to streams that were sufficiently large to allow the water transport of logs. The economically valuable, and also easily floated, white pine was cut first followed by the less preferred, but still easily transported hemlock. Further, cutting hemlocks yielded valuable bark from which tannins could be extracted for processing of leather.

Around 1880, though, advances in railroad technology allowed logging to occur in areas that were previously inaccessible. By 1920, amazingly, almost all of the Allegheny Plateau was clear cut. The hemlocks and the rich mix of hardwoods were all removed. Lumber, tannins, paper, wood chemicals, charcoal, and more were generated in vast quantities. Some sites logged in the late nineteenth century were cut a second and maybe even a third time to remove the dense stands of young hardwood trees. These young, “pole” trees were processed at wood chemical plants for their acetates and alcohols and cooked into vast quantities of charcoal.

This site, then, was probably an old-growth beech and hemlock forest with a rich mix of other hardwood species. Clear cutting in the 1890’s would have disturbed the forest floor and opened up the canopy so that any hemlock seedlings would have been destroyed. The beech seedlings and their root and stump sprouts might have started to re-grow here, but they are notoriously slow growing and would have been out competed by other, more rapidly growing hardwoods like the maples and birches and even some black cherry.

By 1920, these young hardwoods would have been large enough to harvest by the wood chemical factories. So let’s imagine that another clear cutting occurred. Once again, the fastest growing, most sun tolerant hardwoods would have come to dominate this site. Since black cherry is capable of producing seed as early as ten years of age, quite a few cherry seeds and seedlings could have formed in this very young forest along with a scattering of maples (red and sugar) and the fast growing white ash.

This forest dominated by the previously very uncommon black cherry, maple, and ash is referred to as the “Allegheny Hardwood Forest.” It is the product, at least in part, of the intense human manipulation of both the original forest and the initial forest re-growth process. This forest, though, is not just a human creation. There was also a biological force that had a huge influence on this forest’s structure. Admittedly, human influences altered and amplified this biological force, but the influences of this particular “tree predator” have to be taken into consideration.

The Allegheny Plateau was historically rich in wildlife. White-tail deer, elk, black bear, wolves, cougars, and more abounded in its dense forests. White-tail deer were a small, but particularly important part of this fauna. Deer hides and meat were essential to the lives of Native Americans, and, as European settlers increasingly came to dominate the area, deer were vigorously hunted for their meat. This hunting was intensive and went on year round and was alarmingly thorough. By the end of the nineteenth century deer were so uncommon that sightings were reported on the front pages of local newspapers.

Photo by D. Sillman

At the start of the twentieth century, regulations were imposed on deer hunting. Hunting seasons were established and prohibitions against killing does were enacted. Deer were also imported from other states in order to jump start the re-establishment of Pennsylvania’s herd. These events were coincident with the massive cutting of the state’s forests. So, as the deer were being protected and imported, there was also a great bounty of browse available in the young, regenerating forests. Absence of significant predation (since hunting was restricted and wolves and cougars totally extirpated) combined with  a plentiful food supply led to a population explosion of the white-tail deer. In a few decades, white-tail deer attained a state-wide herd population that greatly exceeded its pre-settlement size.

These deer exerted, and still exert, an extremely significant influence on tree survival and growth in Pennsylvania’s forests. They consume the young, vulnerable life stages of the re-growing forest. Tree species that were highly palatable (like sugar maple, red maple, and white ash) were erased from large sections of the forming forest. Trees that grew slowly and, so, remained within browsing reach of deer for many seasons (like hemlock and white oak) were also increasingly likely to be consumed.

Any trees that combined chemical constituents that reduced their palatability to deer and an ability to grow rapidly enough to rise up above the browse layer of the forest, would come to dominate this new, deer sculpted forest. The tree species that exhibited both of these features is the black cherry.

So, lets’ go back to our tree standing beside the hiking trail and follow its life events.

Photo by Rasbak, Wikimedia Commons

This tree began as a seed inside of a fruit made by a parental tree. That parent tree could have started flowering as early as ten years of age, but most likely it would have been at least 30 and maybe as old as 100. The black cherry, unlike other cherries, flowers after it leafs out in the spring. Its flowers are white and perfect (they have both male and female parts), and they are pollinated by a variety of insects (including bees, flies, and even beetles). The fruit quickly sets and matures by mid-August.

The fruit and the seeds fall abundantly under the parental tree and can, via fruit eating birds and mammals, even be transported far from the parental tree. The seeds may persist in the forest soil and leaf litter for three or more years before germination. A forest soil, then, may accumulate very large numbers of black cherry seeds!

Germination occurs in the moist soil and leaf litter. The seedling develops very rapidly but will be inhibited by direct sunlight and dry conditions. In a forest, then, that contains mature black cherry trees, there will be an incredible bounty of black cherry seeds and seedlings throughout the forest floor. The seedlings can live in a growth suppressed state in the shade of the forest for up to five years. They may only be five or six inches tall as they wait for an opportunity to grow.

Our tree was probably one of many hundreds of tiny seedlings growing in this area of the forest. Its parental tree may have been right overhead or it may have been some distance away. It could have been a newly germinating seedling or it may have been slowly growing for several years. It may even have been a seedling for a longer period of time and had died back or was browsed by a deer then and re-sprouted from its roots. A disturbance event, though, had to occur to enable this tree to start its rapid growth phase. That disturbance had to cause the shading, over-story canopy of the forest to open up. Maybe a single tree died or was significantly damaged by wind or ice or some other stress. Maybe the disturbance was the second clear cutting of the entire forest. Whatever happened, our tree, possibly now in full sunlight, started to grow very rapidly. It may have increased in height by four to six feet each year until it stood over other tree species with which it was competing (like the slower growing sugar maple, red maple, white ash, and even any beech that might have survived). It  also over-topped the nearby individuals of its own species.

Photo by Rasbak, Wikimedia Commons

Our tree, then, somewhere around 1920, sprang upward from its seedling form and quickly passed through a sapling and pole stage on its way to becoming the large, canopy tree before us. Deer did not kill it: its toxic chemicals (“prunasin”, a cynaogenic glycoside), the large numbers of surrounding individuals available to the deer, and its rapid rate of growth gave it enough of an edge for survival that it was able to persist. It now makes abundant seed and fruit each year each of which face the same daunting odds against survival that this tree faced and against which it prevailed.

The deer herd in this area is still huge. The exploding population of the 1930’s crashed horribly possibly because the trees in the re-growing forests had reached heights that were above the browse line. Many thousands of deer were taken in hunting seasons in the late 1930’s and thousands more starved in the winter. The herd population has declined since this early twentieth century peak, but it is still far above even a generous estimate of their pre-settlement density. There are almost no seedlings or saplings in this forest of black cherry. What will happen when these cherry trees are logged  or when these relatively short-lived trees (mortality greatly increases when they reach 100 years of age!) begin to naturally senesce (a process which has begun and which will accelerate sharply over the next few decades)?

We are standing in a forest that has never existed on earth before, and there is no new forest coming up in the under-story. There are only ferns. What will happen next?



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Signs of Fall 11: Can We Talk to Plants?

Photo by Pixabay

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We humans are a successful species in large part because of our ability and willingness to communicate. We try to “talk” to each other via a wide variety of modalities, and through these lines of contact establish the intellectual and emotional interconnections that make us each much more than simply the sum of our parts.  Our brains, our lives, our dreams and our imaginations are all expanded because of the quality of these interconnections. Yuval Noah Harari in his wonderful books Sapiens and Homo Deus felt that this ability of humans to communicate with each other and form groups (along with our psychological capacity to “believe in” abstract entities) made humans the most formidable ecological and evolutionary force ever to exist on Earth!

So the drive to communicate with members of our species is hardwired in all of us! Some of us even try to communicate with other species. Back in March (2019) (Signs of Spring 4) I discussed the fading taboos against anthropomorphic explanations and descriptions in scientific discussions of animal behavior. Recognizing the full ranges of the levels of consciousness and emotion in non-human animals facilitates communication between those animals and humans. It is recognized, though, that these types of communication are profoundly difficult. As the philosopher Ludwig Wittgenstein put it, “If a lion could talk, we would not understand him.” The way a lion (or a blue jay or a white-footed mouse or a buffalo) thinks or feels, or how it organizes or prioritizes its reality is so different from any human reality that even if we could share words it would be nearly impossible to share understanding.

Photo by Priyanta 719. Wikimedia Commons

So, if we experience significant barriers in our communications with the members of our own biotic Kingdom (“Animalia”) with whom we share a significant number of drives and organ systems and genes, what is the likelihood that we can communicate with species outside of our kingdom? What are the chances that we can talk to plants and that they can talk to us? Given the even greater differences in our respective realities and needs, would the Wittgenstein dictum need to be written in boldfaced, capital letters? Come to think of it, do plants even know how to talk? Are plants conscious? Do plants have an emotional matrix with which we can connect? Do they love? Do they think and remember? Do they hate?

Amazingly, these questions are being discussed extensively in both the scientific and popular literature!

Monica Gagliano is a biologist at the University of Sydney in Australia. She has used a number of unconventional experiences (including the use of hallucinogens and immersion in the shamanistic cultures of Peru) to challenge prevailing scientific assumptions about plants. Dr. Gagliano is convinced that plants are intelligent and that they can (and do) communicate not only with each other but also with many of their pollinating and protective animals and with humans.

In her memoir Thus Spoke the Plant, Dr. Gagliano writes about her conversations with plants that have inspired some of her extremely creative experiments into plant consciousness, memory and communication. These experiments have been sufficiently relevant and rigorous to merit publication in a number of internationally recognized, peer reviewed scientific journals.

Mimosa pudica. Photo by J.M.Garg. Wikimedia Commons

In one of these experiments, Dr. Gagliano exposed a sensitive plant (Mimosa pudica) to a mild stress (being dropped from small height onto a foam pad). The plant initially responded to the fall by defensively folding its leaves. After several trials, though, the plant “learned” that this particular fall was not harmful and then stopped spending energy to fold its leaves. These same plants, though, continued to respond to other stimuli (like shaking) with active leaf folding which showed that the leaf folding mechanism was intact, but that it just not being employed in reaction to the “safe” stimuli. A month later, the habituated plants were dropped onto the foam pad, and they did not fold up their leaves. Dr. Gagliano’s conclusion was that these plants remembered  their previous experience and had learned that this particular stress was not harmful.

Pisum sativun flowers. Photo by Jamain, Wikimedia Commons

Dr. Galiano also trained pea plants (Pisum sativum) to anticipate sunlight when they were exposed to wind. Using these two stimuli in a classic Pavlovian learning experiment, peas were first exposed to wind and then light, and they would then grow toward the light source. After repeated exposure of the wind then light sequence, the plants exposed to just the wind would grow toward it as if they were anticipating the coming light.

In another experiment (inspired by a conversation with an Ayahuma tree in Peru) Dr. Gagliano showed that pea plant roots were capable of “hearing” the sound of water running. Planting a pea in a spilt-bottomed plant pot in which one of the bottom chambers had the sound of running water and the other did not, the pea preferentially grew its roots into the sound augmented chamber. Further, there was a suggestion that the pea plants’ ability to perceive the sound of water may be the mechanism by which plants in general find moist soil at a distance. Also, the experiment suggested, external sounds (like environmental noise pollution) could possibly disrupt a plant’s auditory perception systems and affect its ability to find water and survive in water stressed environments.

In other experiments, Dr. Gagliano measured a sound frequency of 220 hertz being generated by the roots of a plant. This sound (at the very low end of human sound perception) was produced as a crackling noise by the plant. Speculation is that these sound frequencies could be communication modalities between adjacent plants.

Photo by Pixabay

Dr. Gagliano and a growing number of researchers are exploring some unexpected areas in plant biology, and their use of terms like “consciousness” and “memory” and “learning” with regard to plants has drawn strong criticism from other plant biologists. Their focus on plants as thinking beings that can communicate with each other and with humans have also been much criticized and maligned.  This past summer, a multi-authored opinion piece in the journal Trends in Plant Science (July 3, 2019) entitled “Plants Neither Possess nor Require Consciousness” was published.  Fundamentally, the authors of this paper felt that consciousness (and cognition, emotional perception, memory and learning, and the ability to feel pain) requires a complex set of cells and tissues which forms the fundamental structure of a brain. Since plants lack these structures they cannot be conscious or display any of the other communicational, cognitive or emotional characteristics that are observed in complex animals.

Critics of this paper (which included Dr. Gagliano) focused on the authors’ definition of consciousness. Could there not be, these critics asserted, other types of consciousness (and cognition and emotional sensation) than what has thus far been described in animals?

To close this discussion, without taking sides because I am not sure what exactly I think about all of this, I would like to quote Dr. Gagliano (with a tiny bit of editing) from her New York Times interview of August 26, 2019:

“We should admit that … we know very little compared to what there is to know. To be open to explore and learn, I think is the sign of wisdom not madness.”








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Signs of Fall 10: Anthropomorphizing in the Woods!

Douglas fir tree. Photo by Dog Walking Girl, Wikimedia Commons

(Click here to listen to an audio version of this blog!)

Peter Wohlleben is a German forester. He published an extremely popular book three years ago entitled The Hidden Life of Trees: What They Feel, How They Communicate- Discoveries From a Secret World.  Wohlleben described the natural community of trees in a forest and explored many ecological details of a number of common tree species. Instead of speaking in a careful, calculated scientific “voice,” though, he used a simple and very vivid anthropomorphic syntax to describe the trees’ “needs,” “feelings” and “desires.”

He described parental trees “loving” their offspring. He described healthy trees “pitying” their fallen or sickened companion trees (and also sending on some of their precious photosynthetic sugar production to keep them alive). He talked about trees being “friends” with each other and warning each other of dangers. He condensed the ecological matrices of several types of trees into descriptions of the trees being “sociable but sometimes bullies” (beeches), or “cold and unfeeling” (birches) or “loners” (willows).

Wohllenben states that he does not talk to trees, but the flow of his book suggests that he does listen to them very closely. He also states that in spite of their “feelings” and “emotions” trees should be cut down and their wood should be used for construction, for furniture and for fuel (not to mention for paper for his books!). The scientific foundations behind his anthropomorphisms are extensive, but this science gets overshadowed by the emotionality of his tree-creature images.

Giant Sequoia. Photo by D. Sillman

Wohllenben has been roundly criticized by ecologists and foresters for his imaginative but simplistic descriptions and discussions of trees and forest ecology. An online petition circulated by University of Gottingen scientists protesting Wohllenben’s appearance at a German literary festival called for “facts not fairy tales.” The petition stated: “It is very unfortunate … that, through this book, so many people obtain a very unrealistic understanding of forest ecosystems because the statements made here are a conglomerate of half-truths, biased judgments, and wishful thinking derived from very selective and unrepresentative sources of information.”

Wohllenben shrugs off these criticisms and claims, with some accuracy, that all of his summary descriptions of trees are based on published science. Many admirers of Wohllenben’s book have in turn heaped derision on his scientist critics. They referred to science or at least writing in science as “grey, dull and soulless” and praise books like Hidden Life of Trees as an antidote to the turgidity and lifelessness of scientific writing.

Since I have in my career as a teacher and, in retirement, as a writer always tried to express and explain scientific ideas not only clearly but with all of the energy and life they so richly deserve, I was really put off by the “grey, dull, soulless” description. I read Wohllenben’s book from the perspective that his sometimes fairy tale-like descriptions of the trees were ways to open the door into deeper discussions, and I trusted that he had a sound enough knowledge of forest ecology not to lose his way in his anthropomorphisms. I am afraid, though, that many readers come away from his chapters with the ideas that forests are social clubs of sentient trees rife with emotionally charged friendships and enmities.   There are incredibly elegant and much more interesting pictures of the living realities of these trees hidden beneath the simplified surface of his book. These would make a much richer story for scientists and non-scientists alike!

Douglas firs. Photo by Born 1945, Flickr

For example, Suzanne Simard is a professor of forest ecology at the University of British Columbia, and her work has been frequently cited by or alluded to by Wohllenben. Specifically, Dr. Simard uses organic molecules into which radioactive carbon atoms have been inserted to track the flow of nutrient molecules (like sugars) and communication molecules (like growth factors and hormones) between trees. Simard has shown that in a mixed forest of Douglas fir and paper-bark birch all of the trees are interconnected at their roots by a network of mycorrhizal fungi (a system that Wohllenben refers to as the “Wood Wide Web”), and that sugars are exchanged between firs and birches along this root/fungi network.

Most interestingly, this exchange occurs both from birch to fir and also from fir to birch depending on the season and on specific site conditions. In the fall, when the birches lose their leaves and stop photosynthesizing, sugars from the evergreen and, thus, still photosynthesizing fir trees are transferred to the birches. During the summer, though, sugars are transferred from the leafed out and more robustly photosynthesizing birches to the firs. This transfer of sugars especially goes to those firs that are photosynthetically inhibited because they are growing in the shade of other trees.

This is a coevolutionary association in which two different species of trees via their root interconnections are able to reduce seasonal or environmental stress! The sugars are probably moving under diffusion gradients from high concentration tissues into lower ones, and there is no “intention” involved on the part of either tree. The resulting reduction in stress, though, allows higher rates of survival and growth and increased rates of reproduction in both the firs and the birches.

Wohllenben would say, however, that these two tree species are “friends” and that they are “taking care of each other.” His descriptions leave out the incredible elegance of the ecology, physiology and evolution of this association and drapes a cover of human emotions over a fascinating interspecific symbiosis!

Douglas fir with saplings. Photo by BLM

Dr. Simard has also described “hub trees” in the Douglas fir forest. These trees are mature, older trees that are producing seed (and seedlings) and that have a dense extension of their roots and root -associated mycorrhizal fungi that radiate out through the surrounding soil. These roots and fungi make contact with almost all of the other trees growing in the forest, but in particular, they make extensive and quite robust interconnections with seedlings that are genetically similar to the hub tree. In other words, the “mother tree” makes contact with its “offspring” probably through recognition of similarities in cellular surface proteins!

A “mother” Douglas fir, then, is able to transfer significant amounts of its own photosynthetically generated sugar preferentially to its progeny seedlings via these root/mycorrhizal connections. The “mother” tree is thus able to assist the growth and survival of its offspring and accomplish a very fundamental step in Darwinian selection! Trees that are able to make these connections and “recognize” their offspring will be able to more efficiently pass along their own DNA to future generations and have their offspring and genes make up an increasingly large proportion of the future population of Douglas firs!

Douglas fir. Photo by Wildcat Dunny, Flickr

The elegance and intricate (and immensely logical) mechanisms by which this evolutionary hegemony is accomplished is quite lost when Wohllenben simply states that these fir trees “suckle” their young.

Wohllenben tells children stories about some very complex  realities, but, I am afraid that he leaves too much out! We have to read his book as a collection of hints about the true wonder of tree existence out in Nature. And besides, I could never think of beeches as “bullies” nor of birches as “cold and unfeeling.” They are trees! We should try to understand them for what they really are!



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Signs of Fall 9: Natural History of a White Oak

Photo by D. Sillman

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(I wrote an initial draft of this essay during my 2007 Penn State sabbatical. I think that the trees described here are still standing along the Penn State New Kensington Nature Trail, but the trail, sadly, has not been maintained since I retired from Penn State and is no longer usable.)

I am standing at the top of the Ravine Trail on the Nature Trail of Penn State New Kensington. To my left is a large white oak (Quercus alba L.). It is 30 inches in diameter and approximately 60 feet tall. I estimate based on similar area trees that it is 200 years old, although I have never taken a core sample to count its rings. There is another, equally large white oak 100 feet away. The surrounding forest cover is dominated by yellow poplar and sugar and red maple. There are many poplar and maple seedlings and saplings in the understory but there are no white oak saplings or seedlings anywhere to be seen.

The bole of the oak rises up into a compact crown. Its branches are stout and solid, and they spread straight out around the trunk. The bark is dark gray and organized into long, raised columns that are regularly fissured into separate plates. The view up the tree reminds me of the Georgia O’Keefe painting of the Lawrence pine in New Mexico. The trunks and branches have an animal like shape.

The white oak was the most abundant tree in the pre-settlement forests of Pennsylvania although several other types of trees competed with it for superlatives. The white pine was initially the most commercially valuable tree in our primal forests, and its abundance and distribution was greatly exaggerated by land companies (Whitney 1994, Abrams 2001). Analyses of survey data (and the “witness” trees recorded by the surveyors), however, indicate that white pine seldom made up more than 6% of Pennsylvania’s primary forest (Abrams 2001). The official state tree of Pennsylvania is the eastern hemlock. The vast hemlock forests of the northern and central sections of the state were, indeed, impressive. But, although the hemlock was found throughout the state in varying mixes of forests, it was never as abundant nor as widely distributed as the white oak. Hemlocks probably made up only 15% of trees in Pennsylvania. White oak made up 65% of the state’s forest cover (Whitney 1994), and the botanist Michaux in 1818 reported that western Pennsylvania was dominated by forests that were 90% white oak. Richard Smith noted while working in New York state in 1769 that “Beech is the Master Wood here as Oak is in Pennsylvania.” (Whitney 1994).

Photo by Msact. Wikimedia Commons

The abundance of the white oak reflects its ability to thrive in a very wide range of soil, moisture, aspect, and slope conditions. It also is found growing in association with a large number of other tree species (Eyre 1980). It is capable of growing throughout the eastern half of the United States in almost any location except for those that are excessively dry or have too thin a layer of soil or, at the other extreme, sites that are too wet or which are regularly flooded (Rogers 1990). Oaks are also capable of surviving and flourishing after fires, an observation which has led some researchers to speculate about the ecological importance of fire (possibly managed by the resident Native Americans) in the pre-settlement forests of Pennsylvania (Abrams 1992).

The white oak is an incredibly useful tree. It was used for the construction of boats and buildings and, preferentially, for barrel staves. The wood of the white oak is strong and durable and resistant to rot. White oak was also used extensively for mine props and for charcoal production. In the forest, the white oaks’ acorns are food for over 180 different species of mammals and birds (Rogers 1990), and humans, including both Native Americans and European settlers, utilized these acorns as a subsistence food supply (Whitney 1994). Later, domestic livestock of the Europeans (especially pigs) were turned out into the surrounding oak forests to forage and graze. The abundance of acorns added greatly to the quality of these forest “pastures” (Cronon 1983).

As I have hiked over the interconnected ridges of northern Westmoreland County, I have seen many white oaks. Most of these trees are of substantial sizes, like the ones here on the Nature Trail. There are, though, on most of these potential oak sites, very few young white oaks either in the form of seedlings or saplings growing in and around these older individuals. The forest cover on these ridges is dominated by black cherry,  yellow poplar, and sugar and red maple with a rich mix of other occasional species (on our Nature trail, for example, there are 34 different hardwood species (Hone, 2007 personal comment), but most individual trees are either white ash (now in serious decline), yellow poplar, or black cherry and, in the wetter ravines, American beech or red and sugar maple).

Photo by Adamantos, Wikimedia Commons

But, these ridges and even the valleys in between them used to be overwhelmingly white oak. When these forests were cut (and almost all of these forests were logged in the nineteenth century), white oaks returned but in numbers and densities far less than they previously had attained.

In order to better understand these observations, let’s examine the life of a single oak tree. Let’s look at the possible life of one of the white oaks here on the Nature Trail.

To get a white oak, we, of course, have to start with a white oak. The parental tree of this specimen in front of us was likely part of the pre-settlement forest of this ridge top. White oaks can produce acorns from about ages 50 to well over 250 years (Rogers 1990). So, we can imagine the parental tree as an individual much like the tree in front of us. A given white oak can in a good production year make on average 10,000 acorns (in a bad production year that same tree can make zero acorns! (Connor et al. 1976, Johnson 1975)). It’s a boom and bust process regulated by the weather conditions during the weeks around the tree’s spring flowering and post-pollination period (Sharp and Sprague 1967)).

The parental tree may have flowered in April of 1807. Its yellow, “male” flowers clustered in dangling catkins formed first and then, five to ten days later, the reddish, “female” flowers formed either as single blooms or in pairs. This staggered flowering helps to insure that a given tree would tend not to pollinate itself. Pollen from the male flowers spreads on the wind, and if any of the pollen grains finds a female flower, then that flower persists and develops an embryo and an acorn. If the female flower is not pollinated it withers away. So, our future white oak tree starts off as an embryo in the protective, nutrient package of its forming acorn.

The acorns grow and develop for 120 days. They change from green to a light brown as they mature and will fall from the parental tree over the next month (Bonner 1976). It’s now early October, 1807, and our acorn, almost as soon as it hit the forest floor, begins to germinate (Rogers 1990). The year 1807 had to be a “good” acorn production year, because only in years of acorn abundance do any acorns survive the heavy predation by acorn eating insects, mammals, and birds (Rogers 1990). Our acorn also had to fall into a spot on the forest floor that was only lightly covered with leaf litter and that had soil that was rich in humus and that was relatively porous and loose. There also had to be some sunlight filtering down through the over-story canopy, but not too much so as to avoid excessive drying. Our acorn may even have been picked up by a squirrel or a blue jay or a crow and carried some distance away from the parental tree and then abandoned in the foraging chaos of food abundance. It may even have been blown by a stout wind a short distance from the parental tree, or may have rolled down the gentle slope further off to our left. But, whatever happened, the parent tree was most likely quite close by.

The germinating acorn grew a slender root down into the humus rich soil. No above-ground shoot or leaves form until the next spring and summer (Farmer 1977). The acorn must contain enough energy to survive this first winter. After the first year, the seedling will be three or four inches tall and have a slender tap root that is up to twelve inches long and becoming more and more branched as it spreads through the dark upper soil layers (Rogers 1990).

Photo by Msact, Wikimedia Commons

At this point, many things can happen to the seedling. It might steadily grow until some event in the over-story allows an incoming blast of sunlight to reach down to the forest floor. With sufficient sunlight  the young white oak can begin to rapidly grow (up to two feet per year!) up into the fragmented canopy. Or, things can happen to the seedling. It might be eaten by a passing deer, it might be stepped on by a logger’s boot (the parental trees might have been logged very soon after our acorn germinated), it might be burned in a brush fire (a frequent post-logging event throughout Pennsylvania). The consequence of these calamities, though, would not have been the death of the white oak. Young white oaks can readily re-sprout from their extensive root system after shoot damage. In fact, in white oaks with “stumps” of 2 to 5 inches, re-sprouting occurs in 80% of the trees (Johnson 1977)! This ability to sprout declines with increasing size and age until there is finally no chance of sprouting at all in damaged white oaks that have sixteen inch or more diameter stumps (Johnson 1977).

So our white oak may have root or stump sprouted, and this may have occurred several times. But our tree persisted, and then had two hundred years of steady growth to reach the mass that we see before us.

So, why are there not more white oaks on these ridges? There must have been many abundant years of acorn production, and by the classic model, these years should have seen a small number of these seeds dodging the filter of the acorn eaters and germinating into seedlings and on to saplings and so on. Possibly, because these white oaks are so widely spaced, the “perception” by the many acorn eating organisms is of a continuous acorn shortage even in the years of abundance. The light stocking density of the acorn producers may not allow the serendipitous survival of any of their seeds.

There is also the impact of competing tree species. The dense growth of sun tolerant, fast growing trees (like yellow poplar and white ash) generates a thick covering of leaf litter on the soil surface. Acorns must have bare patches of soil or at least places where the leaf litter cover is quite thin into which they can grow their extensive root system (Rogers 1990). The litter covering (which is quite extensive and also quite thick throughout these young ridge forests) may be blocking successful acorn germination. Also, the extent and persistence of the over-story shading may have blocked too much sunlight and thus inhibited acorn germination. A forest fire would remove these potential obstacles to oak growth, and, since the oaks both tolerate fire and vigorously root sprout after fire damage, an ecosystem fire would strongly shift the successional direction to favor an oak forest. Fires, though, have been strongly suppressed throughout these ridge forests.

Photo by D. Sillman

And, finally, there are large numbers of whitetail deer in this habitat (and in most habitats of Pennsylvania). These deer extensively browse the forest understory and have been clearly shown to be major effectors in determining which tree species can survive or persist in developing forests. The white oak seedlings are very palatable to deer (Blymer and Mosby 1977) and even their prodigious abilities to stump and root sprout are not able to counter the rates of deer predation.

On this same Nature Trail, near the entrance, is a dead white oak that has fallen off to the left of the Pine Trail path. This tree is also 30 inches in diameter and has 202 countable growth rings. This tree experienced two very significant white oak “events.” One was a lightning strike (the lightning scar is still visible along the fallen trunk). Lightning and fire scars can open the heartwood of a white oak to fungal invasion which can then lead to heartwood decay (Hepting 1977). The other event was a sequence of gypsy moth infestations. White oaks are aggressively attacked by gypsy moths (Rogers 1990). In the late 1980’s and early 1990’s there were three years of extensive defoliation of the oaks out on the Nature Trail. This oak, possibly because of its underlying weakening due to heartwood rot, was not able to withstand the repeated defoliation and, stood dead for several years until a 1999 summer microburst sent it to the forest floor.

References on White Oak:

Abrams, M. 1992. Fire and the development of oak forests. Bioscience 42(5): 346-353.

Abrams, M. D. 2001. Eastern white pine versatility in the pre-settlement forest. Bioscience 51(11): 967-979.

Blymer, M.J. and H.S. Mosby. 1977. Deer utilization of clearcuts in southeastern Virginia. Southern Journal of Applied Forestry 1(3): 10-13.

Bonner, F. T. 1976. Maturation of Shumard and white oak acorns. Forest Science 22(2): 148-154.

Connor, K.,  P.P.Feret, and R.E. Adamas. 1976. Variation in Quercus mast production. Virginia Journal of Science 27(2): 54

Cronon, William. 1983. Changes in the land : Indians, colonists, and the ecology of New England. Hill and Wang. New York.


Eyre, F.H. (ed) 1980. Forest cover types of the United States and Canada. Society of American Foresters. Washington, D.C. 148 p.

Farmer, Robert E., Jr. 1977. Epicotyl dormancy in whiteand chestnut oaks. Forest Science 23(3):329-332


Hepting, George H. 1971. Diseases of forest and shade trees of the United States. US Department of Agiculture, Agriculture Handbook 386. Washington, D.C. 658 p.

Johnson, Forrest L. 1975. White oak production in the upland streamside forest of central Illinois. University of Illinois Agriculture Experiment Station, Research Report 75-3. Urbana-Champaign. 2 p.

Johnson, Paul S. 1977. Predicting oak stump sprouting and sprout development in the Missouri Ozarks. USDA Forest Service, Research Paper NC-149. North Central Forest Experiment Station. St. Paul, MN. 11 p.

Rogers, R. 1990. Quercus alba L.: White Oak, pp 605-617, In, Burns, R. M. and B. H. Honkala (tech coord) “Silvics of North America: Volume 2, Hardwoods.” U.S. Department of Agriculture, Agriculture Handbook 654. Washington, D. C.877 p.


Sharp, W.M. and V.G. Sprague.  1967. Flowering and fruiting in the white oaks. Pistillate flowering, acorn development, weather, and yields. Ecology 48(2): 243-251.

Whitney, Gordon Graham. 1994. From coastal wilderness to fruited plain : a history of environmental change in temperate North America, 1500 to the present . Cambridge University Press.


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