Photo by N. Tonelli, FLickr
(Click here to listen to an audio version of this blog!)
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?