Modeling climate change impacts can guide long-range, regional planning.
In the 19th and early 20th centuries, a mosaic of mixed forests flourished throughout East Tennessee. Areas that had been burned or otherwise disturbed often supported pines. Alongside pine trees grew specialty hardwoods. White oaks were prized for making barrels. Black oak, maple and hickory woods were ideal for kitchen cabinets and furniture. Basswood made fine hardwood floors.
A series of droughts since 1980 killed thousands of hardwood trees, which were replaced by fast-growing pines. As a result, pine emerged as the commercial wood of choice in much of Tennessee, especially for building construction and manufacture of paper products.
During this period, timber companies purchased thousands of acres of land and replaced the previous blend of mixed hardwood and pine forest lands with an environmentally less friendly tree monoculture. One consequence was that many animals that had adapted to native hardwood forests avoided pine plantations.
By 2003, pine plantations covered 80,000 acres, or about 13% of the land cover in five southern Cumberland Plateau counties between the Alabama border and Sewanee, Tenn.
Drought-weakened and densely planted pine plantations contain trees that are vulnerable to a deadly insect predator. In one of the largest pine beetle outbreaks in recorded history, infested trees rapidly died and fell over. In some locations, wildfires ripped through pine forests, leaving hundreds of acres of charred land.
In 2005, Bowater, a timber company with a paper mill located in Calhoun, sold more than 300,000 acres on the Cumberland Plateau to private developers and the state of Tennessee. On much of the privately owned portion, houses, condominiums and shopping centers are springing up where forests once flourished. For the state portion, Gov. Phil Bredesen has proposed planting native hardwood forests after the timber is harvested.
Disturbances and human intervention
Virginia Dale, a corporate fellow and environmental scientist at Oak Ridge National Laboratory, knows about disturbances and human intervention and their effects on forest growth. She has witnessed first-hand the disastrous results of planting non-native seeds instead of native vegetation after the Mount St. Helens volcanic eruption of 1980 destroyed the forest around the mountain. Her papers suggest the need for improved ecosystem management in the wake of such disturbances. Familiar with disturbances brought about in Tennessee by drought and insect infestation, Dale and her colleagues have modeled the future of forests, which cover one-third of U.S. land south of Canada. Concerning climate change and pine beetle outbreaks in Tennessee, Washington and Canada, she writes:
"As climate change leads to increased disturbances such as fire or drought, these disruptions are very likely to create the type of environment ripe for the spread of invasive species. Insects and pathogens, by virtue of their mobility and short reproduction times, can respond to climate change much more rapidly than tree populations. As a result, insects and disease are likely to cause some of the early impacts of climate change on forests."
Dale's team has been examining several scenarios in which a warmer, drier climate interacts with land use. In Brazil, the clearing and burning of rainforests to provide land for agriculture have released large amounts of the greenhouse gas, carbon dioxide, into the atmosphere. Through this and other similar examples, Dale has become acutely aware of the unintentional impacts well-meaning land-use policies can have on climate and the ecosystem.
At ORNL a recent climate change study of Tennessee divided Tennessee into five ecological zones. The study projected that southern mixed forests would change the most, with loblolly pines becoming much more dominant when drought occurs.
Dale and her colleagues have projected the effects of climate change in 2030 and 2080. Their climate projections are combined with expected changes in land cover and land use in the region from Oak Ridge to the Kentucky border, which includes the Cumberland Plateau. Using computer modeling, the researchers produced colorful maps in which "green" forests shrink, yielding to an expanding transitional area speckled with "red" urban, suburban and "exurban" communities with populations growing along roads. Enhanced by this graphic illustration, the potential effects of changing land use on water quantity and quality are striking.
"Scientists ideally should model entire regions to understand the key patterns," Dale says. "How various ecosystems around the world respond to drought will be different. We need to start with a system perspective, add an interface between scientific understanding and government policy decision making and place each region in the context of socioeconomics, human health and aspects of anticipated global change."
For example, a warmer climate could cause tree species to move away from the equator and toward more polar regions. Sugar maple trees, which are abundant in New England, are expected to shift gradually north to Canada, affecting in different ways the economies of both regions.
Southeastern mixed deciduous forests—oaks, hickory and pine trees—are projected to expand under moderately warm scenarios. Under hotter climate scenarios these forests could be transformed into savannas and grasslands.
Some projections suggest that today's atmospheric CO2 concentration may double by 2100. At least a portion of the consequences of this increase is not clear. According to ORNL researcher Paul Hanson, a synthesis of laboratory studies, field experiments and modeling indicates that forest productivity could actually rise in response to the fertilizing effect of elevated atmospheric CO2. Furthermore, changes in temperature and water availability could enhance plant growth. One difficulty in predicting effects of climate change on the southeastern United States is that some models project drier conditions while others project wetter weather.
Across a wide range of scenarios, the data indicate that modest warming will lengthen the growing season and boost carbon storage in most U.S. forest ecosystems. In the Southeast, however, greater warming could lead to losses of carbon, possibly exacerbated by more forest fires. Climate and ecological models suggest that overall forest productivity and yields for forest product industries could increase nationally, even as some regions experience declining productivity.
The possibility of a warmer, drier climate brings the possibility of other changes that are unclear. Current ozone levels have reduced wood production by 5% in southern pine plantations. In the Great Smoky Mountains National Park on the border of Tennessee and North Carolina, a negative impact on forest growth from increased ozone could be offset by additional CO2 and nitrous oxide from coal-fired power plants and vehicle emissions. The offset could supply plants with the nitrogen needed for photosynthesis.
Forests are essential to providing clean water, moderate stream flows and aquatic habitats. With increased temperatures, longer growing seasons and greater leaf area, vegetation would take up more water that would escape to the atmosphere as water vapor, even though elevated CO2 increases water-use efficiency. Increased transpiration reduces runoff.
Managing a disturbed ecosystem
"Managers typically try to manage the ecosystem after a disturbance, not before it happens," Dale says. "They often try to fix the problem by moving the ecosystem back to the way it was before the disturbance. What we have learned is that often we cannot move the ecosystem back to the original status."
Dale believes policymakers face a fundamental decision of whether to wait until the impacts of climate change occur or to attempt as best they can to manage the ecosystem or influence the change to produce a more desirable outcome.
"Think about climate, land use and invasive species in forests. Can we manage them as a system? Can we mitigate climate change by reducing carbon dioxide emissions? Should we adapt to a warmer, drier climate by harvesting trees that are not resistant to drought and invasive species, and planting trees that are?"
Dale believes policymakers should anticipate large infrequent disturbances while researchers continue to develop more sophisticated models for more powerful computers to project potential environmental effects of regional land-use changes in a warmer land. For both groups, the stakes could not be higher.—Carolyn Krause
Contact: Virginia H. Dale
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