A foundation species is a primary life form, one that’s abundant and influential in its ecosystem—coral in a coral reef, kelp in kelp forests, chestnut in chestnut forests. Studying the loss of a foundation species such as American chestnut is especially difficult when very little is left of the trees that once made up a quarter of forest hardwoods over 200 million acres from Maine to Florida. Today all that remains are decaying stumps and sprouting saplings—and Elliott and other scientists are seeing fewer of these small saplings in the woods.

“A large tulip tree or eastern hemlock may give you a sense of what an old hardwood forest looked like once upon a time,” Elliott says. “But nothing left today looks like the old photographs of American chestnut forests.”

Fortunately, Elliott and her colleagues at Coweeta have more to work with than old photographs. The Forest Service started taking inventories of tree species across 987 plots at Coweeta back in the 1930s; Elliott and other Coweeta researchers used this data as the basis for reconstructing what a Southern Appalachian chestnut forest was like before the blight struck.

The data fills in a poorly documented history of how the blight transformed southeastern forests. For most historical invasions like the chestnut blight, scientists don’t have much to go by—neither good baseline data on the distribution and abundance of the affected species nor information on basic ecosystem processes. Until now, what scientists have known about the loss of the American chestnut has come mostly from records of a few preblight plots in Connecticut, short-range studies (none longer than 20 years) of blighted regions in the mid-Atlantic and Northeast, and investigations limited to using existing chestnut stumps as a reference.

“Our study area, a basin of more than 4,000 acres, is larger and more environmentally varied than those examined previously, and the period covered by our observations—about 60 years—is longer than those in other studies,” says Elliott, who conducted the study with fellow Coweeta scientist Wayne Swank.

Elliott and Swank’s findings illustrate the unique role the American chestnut played in the Southern Appalachians. Mountain streams probably benefited most from chestnut forests, because decomposition of chestnut wood is much slower than other hardwoods, even slower than oak and hemlock. Chestnut logs remained in the streams longer, providing structure and habitat for fish. The species’ abundant, yearly production of nuts provided a reliable supply of food for wildlife. Additionally, because chestnut trees grow fast, chestnut forests quickly stored, or sequestered, carbon and nutrients.

Where oak grew up in place of chestnut, rapidly decaying chestnut leaves with high-nutritional quality for aquatic insects and other macroinvertebrates were replaced by more slowly decaying oak leaves with lower nutritional quality. As a consequence, leaf processing and consumption rates would have declined, decreasing growth rates and adult body mass in macroinvertebrate shredder communities.

This Time, Eastern Hemlock

Chemicals in the leaves of the American chestnut may have suppressed the growth of other riparian trees and shrubs, including eastern hemlock and rhododendron, according to research by another Coweeta scientist, Barry Clinton.

“Ironically, the loss of one foundation species—American chestnut—may have facilitated the establishment of another—eastern hemlock—which in turn is now threatened,” Elliott says. But history does have a way of repeating itself. The hemlock woolly adelgid, an exotic insect smaller than a poppy seed, threatens to bring another round of changes to Southern Appalachian forests. Despite an aggressive campaign to control the adelgids, the region’s hemlocks may be dead within the next decade, opening up the forest canopy and removing shade from cool mountain streams.

Elliott and others at Coweeta are looking closely at the effects of hemlock death on the riparian zones, including changes to soil moisture, stream temperature, and water quality. “When forests lose whole species of trees, the impact is profound,” Elliott says. “Foundation species are, by definition, irreplaceable. For example, without hemlocks, hemlock forests cease to exist. No other native conifer can do exactly what the hemlock does for the forest, particularly along riparian corridors.”

Circling Back to Chestnut

Elliott’s colleagues at Coweeta—Clinton, James Vose, and Jennifer Knoepp—are very interested in studying the possibility of planting American chestnut seedlings in areas opened up by dying hemlocks. The seedlings would have to be the blight-resistant hybrids TACF scientists are developing—big, fast growing, and adaptable like the original American chestnuts but with the blight resistance of Chinese chestnuts.

The question is whether even the hybridized chestnuts would ever grow tall enough to get past the dense thickets of rhododendron that have taken over many cove forests of the Southern Appalachians.

Coweeta studies implicate the demise of the chestnut in the spread of rhododendron. Because it tolerates shade and reproduces vegetatively, rhododendron has extended its influence far beyond streamsides and into upland forests, where its thick growth blocks the sunlight needed by many tree seedlings to establish and grow.

Worldwide Loss Prevention

“Hemlock and chestnut are only two examples of the many foundation species worldwide that provide fundamental structure and function to ecosystems,” Elliott says. “We have long-term data from inventoried tree plots. We have species-level measures of water use, and we have stand-level measurements of microclimates related to light, temperature, and soil moisture. Together these give us important findings about the dynamics of foundation species. If we develop relationships—or models—using forest composition, tree growth and water use, and microclimate, then we can identify possible future trends in forests. This kind of information has application worldwide.”

Studying the effects of this rapid and possibly dramatic ecological change could even lead to better predictions about the effects of disturbances—from not only insects and pathogens but also land use change, human population growth, and hurricanes, to name a few—at regional, national, and global levels.

“How do we respond to the loss of a tree species? How soon do we become aware that there is a problem—a serious threat? Look how quickly we lost the chestnut,” Elliott says. “Because foundation species are common and abundant, and they have a wide distribution, our responses to a potential threat often come late and are not enough. The introduction of a nonnative species such as the hemlock woolly adelgid can reduce or eliminate a species in a short time.”

For more information:
Katherine Elliott at 828–524–2128, x110 or kelliott@fs.fed.us

Recommended reading:

Elliott, Katherine J.; Swank, Wayne T. [In press]. Long-term changes in forest composition and diversity following early logging (1919–1923) and the decline of the American chestnut (Castanea dentata). Plant Ecology.

Ellison, Aaron M.; Bank, Michael S.; Clinton, Barton D. [and others]. 2005. Loss of foundation species: consequences for the structure and dynamics of forested ecosystems. Frontiers in Ecology and the Environment. 3(9): 470–486.

Ford, Chelcy R.; Vose, James M. 2007. Tsuga canadensis (L.) Carr, mortality will impact hydrologic processes in Southern Appalachian forest ecosystems. Ecological Applications. 17(4): 1156–1167.

Vandermast, D.B.; Van Lear, D.H.; Clinton, B.D. 2002. American chestnut as an allelopath in the Southern Appalachians. Forest Ecology and Management. 165: 173–181.