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

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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

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.

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).

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.

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.

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).

References:

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. http://www.iucnredlist.org. 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.” http://www.dcnr.state.pa.us/forestry/fpm_invasives_EAB.aspx. (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. (https://www.nrs.fs.fed.us/disturbance/invasive_species/eab/control_management/biological_control) (3 December, 2019)

USDA, NRCS. 2019. Fraxinus. The PLANTS Database (http://plants.usda.gov, 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.