Listening to the Forest Breathe by Andrew Kerkhoff, associate professor of biology, was originally published in the BFEC Newsletter, Vol. 19/No. 2, Spring 2015.
Everyone knows that old saying about “missing the forest for the trees,” and really, from the point of view of a tree-hugger like myself, that is really understandable — trees are really beautiful living things! We learn from a young age that trees are critically important to life on Earth as we know it.
In their lives, trees use sunlight to convert atmospheric carbon dioxide into food, releasing in the process the oxygen that is absolutely essential to animals like us. They literally use light to spin air into sugar! And in the process they give us the air we breathe. It seems too good to be true — a child’s dream, indeed.
But a forest is more than simply a collection of trees. Birds, bats, moths, and innumerable smaller insects flit through the canopy. Squirrels, foxes, and deer lurk, cavort, hide and stalk among the trunks, while woodmice and beetles dodge and clamber among the ferns, forbs, and leaf litter of the forest floor. The forest even extends out of sight belowground, where networks of fungal hyphae lace the soil, joining the roots of all the trees into one highly-connected, pulsating “wood wide web.” When you walk in a forest, you are truly immersed, buoyed by the ground between the soaring branches above your head and the burrowing roots beneath your feet.
This sense of immersion, of being totally enveloped, make forests my favorite place to walk on the BFEC: from the multilayered collection of oaks, cherries, elms, ashes, and hornbeams of the Fern Trail, to the quiet, shady cathedral of the pine plantation, and even the stand of optimistic young sycamores slowly reclaiming the old field along the River Trail. All of them are beautiful, but even though they are all forests, they are strikingly different.
In late summer, the sycamore grove is sunny and crowded. Uniformly straight and slender trunks of characteristically flaking green grey bark rise out of a head-high crop of wingstem, all scabrous leaves and yellow, daisy-like flowers. Wingstem is usually a prairie plant — you can find it in the BFEC wildlife garden and along roadsides as well — and it only grows in the sycamore forest because the young trees still let abundant sunlight through to the forest floor. As the trees grow and the canopy deepens, wingstem plants will slowly be pushed to the edges of the stand, to be replaced by other, more shade-tolerant species. This is a “successional” forest, recolonizing a narrow spit of land between the Kokosing River and the Kokosing Gap Trail. After many years of row crop agriculture, the plot was removed from cultivation by the BFEC in 1994.
The pine plantation came into being just a few years before that. In honor of Earth Day in 1990, Kenyon faculty, students, and community members planted 1,000 white pine seedlings on the hilltop above the BFEC farmhouse. Over the past 25 years, the plantation has slowly thrown the former pastureland into deep shade. The seedlings were planted only 10-15 feet apart, and as the pines grew, their crowns spread to close the gaps between the trees.
When I came to Kenyon in 2005, the pine plantation was still a largely impregnable coniferous thicket, punctuated with rambling vines of poison ivy. But as the trees grew taller, adding layered whorls of limbs and needles, the trees actually shaded out not just the ground vegetation, but their own lower branches. The boughs, most now more than a foot in diameter, are studded with skeletal and broken ghosts of their old crowns. The ground beneath the trees is a spongy bed of fallen needles; smaller plants only grow near the edge of the plantation and in the small gaps where individual trees succumbed, whether to disease or deer browsing. The resulting gallery, with row upon row of pillar-like pines extending in all directions, is so architectural that it is clearly a human construction, but it is still somehow a forest.
The pillars of the pine plantation, while impressive, are dwarfed by the tallest oaks and beeches of the forest along the Fern Trail. These rocky, steep slopes were never useful for pasture or crops, so the forest likely long served as a woodlot, with select trees being logged out for timber or firewood. As a result, the forest has developed a complex, layered canopy of multiple species and varied stature.
The greatest contrast with the pine forest and the sycamore grove is how open this more mature forest seems (pictured). The leaves rustle in the wind far above the hillside, and the height of the trees is matched with a much wider spacing between their trunks. The exception is in gaps created when one of the big trees dies and falls. There, younger trees crowd around the fallen log, racing each other upwards to fill the hole in the canopy. Other small species like hornbeams, ironwoods, and dogwoods don’t bother with such competitive jostling. Instead, they are adapted to thrive in the partial shade of the mid-story, capturing the light that passes through the sieve of the upper canopy.
The forest floor is as varied as the canopy above. Trillium and other wildflowers coexist with a collection of tree seedlings, ferns, and shrubs. Many of the largest oaks exceed three feet in diameter, and have likely been here for nearly two centuries. The largest may have been here when Philander Chase himself wandered across these hills, seeking a site for his college.
The woodlots, plantation and successional forests of the BFEC are representative of the forests in rural landscapes across eastern North America more generally. Instead of the largely unbroken expanse of forested land that greeted the earliest European colonists, our modern landscape is a patchy mosaic of different forest types, nestled among our fields and villages, our towns, suburbs and cities. Mirroring patterns across much of the U.S., Ohio’s forests have been reduced by more than two-thirds over the past two centuries, but the remaining patches (and potentially, the new plantations and successional areas) still provide valuable ecosystem services, including the provision of wildlife habitat, the filtration of surface and subsurface run-off, erosion control and carbon sequestration.
Given the reality of climate change caused by human greenhouse gas emissions, the exchange of carbon dioxide between forests and the atmosphere is particularly important. As mentioned above, every living tree takes up carbon dioxide, the most important greenhouse gas. Some of that carbon becomes part of the tree, locked away in the complex molecules that make up the wood and leaves of its body, its flowers and seeds. Even when parts of the tree die (remember all of those needles and dead branches in the pine plantation, or the leaf litter and fallen trees of the Fern Trail forest) much of the carbon remains in the litter and soil. The carbon that is bound up in the trunks, leaves and soil of the forest is sequestered. It is kept out of the atmosphere, at least for a time.
Globally, forest carbon sequestration strongly influences the overall cycling of carbon between the Earth surface and the atmosphere, and deforestation is itself a significant component of how humans cause climate change. Likewise, carbon sequestration through reforestation and the conservation of existing forests represents one of the most powerful tools human have for mitigating our climate impacts.
Just as the amount of money in your checking account is determined by your income and your expenses, the amount of carbon sequestered by a forest (the standing stock of carbon in the leaves, trunks, soil, etc.) represents the balance of carbon inputs (the total CO2 taken up by the leaves) and carbon outputs (the total CO2 released through respiration by all of the plants, animals, fungi and microbes). Together, these stocks, inputs and outputs are known as the carbon budget of the forest.
In my Ecology Laboratory class, my students and I study how the differences we so readily observe when walking through the BFEC forests influence their carbon budgets. Using protocols developed by the USGS Forest Service and other foresters and ecosystem scientists, we quantitatively estimate all of the major components of the carbon budget.
The methods are time consuming and range from primitive (cutting, drying and weighing vegetation) to positively high-tech (using tree ring records and statistical models to estimate tree growth). But over the course of the semester, the students build up a scientific picture of the forest carbon budgets that is comparable to those used by natural resource managers, ecosystem scientists and biogeochemists.
So what do we find out about those three BFEC forests that we were walking through earlier? Not surprisingly, the trees themselves make up a huge fraction of the carbon stock of any forest, and large, old trees like those oaks along the Fern Trail represent particularly large stocks of sequestered carbon. What the students are always a bit more shocked to realize is that the soil beneath our feet also represents a very large carbon stock. For example, in the sycamore grove, just the top 10 cm of soil contains almost as much carbon as all the trees, shrubs and herbaceous vegetation visible above ground — and the floodplain soils of that area are in fact much deeper.
Finally, we find that despite the striking differences we see when walking through them, the three forests are actually fairly similar in terms of their rate of carbon uptake. The huge trees of the Fern Trail are proportionally productive, but what the sycamores currently lack in size, they largely make up for in numbers. There are certainly differences in carbon uptake between the forests (for their size, the pines grow a bit more slowly than the broadleaves for example), but all of them take up significant amounts of carbon. Thus, keeping our few remaining older forests intact, and letting our young forests mature is a significant investment in carbon sequestration.
By learning to do all of this careful carbon accounting, Ecology Lab students are contributing to a project with implications well beyond a Kenyon course, and beyond the boundaries of the BFEC itself. Our results provide concrete data to inform future discussions of sustainability at Kenyon, and they contribute to a growing body of knowledge about how diverse forest types contribute to the carbon balance of terrestrial ecosystems more generally. One recent graduate was even able to put our same methods to use in helping to develop a sustainable forestry initiative in rural India. Bringing authentic research into undergraduate courses helps students to understand the process of science, and the forests of the BFEC provide a beautiful and inspiring setting for (literally) getting our hands dirty and trying to figure out how the natural world actually works.