Signs of Fall 9: Natural History of a White Oak

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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).

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).

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).

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.

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.