Sea level rise, one of the more certain consequences of climate change, has already had significant impacts on southeastern coastal areas. Vast stands of forests are dying along the Gulf of Mexico shoreline due to saltwater intrusion. Since 1852, when the first topographic maps of the region were developed, high tidal flood elevations have increased approximately 12 inches. Bald cypress and live oak mortality has occurred as far inland as 30 miles. The resulting stands of dead, sun-bleached trees are locally referred to as “ghost forests.” Rising sea level is one of several factors that have caused the loss of over one million acres of wetlands in Louisiana since 1900. Other processes contributing to these losses include subsidence due to ground-water withdrawal and natural sediment compaction, wetland drainage, and levee construction.

Expect Extreme Events

Prone to natural weather disasters, the Southeast will be further impacted by changes in temperature and precipitation that alter the frequency, intensity, duration, and timing of disturbances such as fire, drought, invasive species, insect and pathogen outbreaks, and hurricanes. Although disturbances are a natural and integral part of forest ecosystems, it is the changes in disturbance regimes that will have the greatest impact on forest systems. Climate change alters the interactions between the ecosystems and the disturbances by causing disturbances that exceed their natural range of variation. This variability can cause extreme changes in the structure and functions of our forests. “Climate variability causes much more damage than climate change,” says Steve McNulty, leader of the SRS Southern Global Change Team, “and this will likely be the case for the next several decades.”

Over the past 100 years, intense precipitation events have increased across the South. This trend is projected to continue, raising the likelihood of flooding and erosion. Climate and ecological models predict the seasonal severity of fire hazard will increase by 10 percent over much of the country, with larger increases in the South. A single disturbance such as fire has multiple effects on forest ecosystems, including the acceleration of nutrient cycling, mortality of individual trees, shifts in succession, loss of seed banks, loss of soil nutrients, changes in soil surface organic layers, and changes in underground plant root and reproductive tissues. It is difficult to predict the interactions between climate changes and multiple disturbances, partly because many disturbances are cascading. Drought often weakens tree health, leading to insect infestations or diseases; these in turn promote future fires by increasing fuel loads, then fires promote future infestations by weakening tree defenses.

We understand even less about climate change interactions when an ecosystem experiences multiple disturbances at once. When a new disturbance strikes a system that is still recovering from a previous disturbance, the compounded effects can lead the system to a new ecological state. Invasive species can further complicate these interactions due to their ability to modify existing disturbance regimes or introduce entirely new disturbances.

Biological Diversity

Currently considered among the highest in North America, the biodiversity of southern forests will clearly be impacted by climate change, although the effects are difficult to predict. Some species are likely to expand while others decline, and entirely new communities may form. Human activities have undeniably modified the quality, amount, and configuration of habitats in the South. The expansive ranges of forest community types encountered by settlers, including spruce-fir in the Southern Appalachians, Atlantic white-cedar in Virginia and North Carolina, and longleaf pine in the southeastern coastal plains, have been reduced to less than 2 percent of their presettlement ranges. Not surprisingly, many species that depend on these ecosystems, such as the longleaf pinedependent red-cockaded woodpecker, are currently in peril. At the same time, species associated with humandominated landscapes have greatly expanded and some, such as deer, are now so abundant that the primary concern is population control.

Ecological models using several different climate scenarios indicate changes in the location and area of potential habitat for many tree species and plant communities. According to one model, American beech and sugar maple are predicted to gradually shift north into Canada, leaving only a few isolated communities in the United States. Loblolly pine and sweetgum are also expected to shift as far north as New Hampshire and Maine. How well plant species adapt to changes in their potential habitat is largely dependent on their dispersal abilities. Invasive species that disperse rapidly are likely to find many opportunities in newly forming communities and expanding urban areas.

More or Less Forests

Between 1980 and 2000, population growth in the South was faster than for the Nation as a whole. Urbanization could eliminate about 12 percent of current forest land in the South by 2020. Research conducted by SRS scientist David Wear suggests that the total change in forest land is dependent on whether rural areas in the South experience increases in forest land. In the past, shifts in land use from agriculture to forest land have offset the net loss of forest land to urbanization. Although these changes have not been distributed evenly across the region, the overall forest land area has remained relatively stable since the 1930s. Future offsets will depend on prices of timber and agricultural products.

Assessments conducted by Wear examine two scenarios. The base scenario uses population, income, and housing forecasts to provide an estimate of how population and economic growth will drive urban land use if there is no change in timber or agricultural prices. The base scenario predicts a loss of 31 million acres of forest land by 2040. The market scenario shows how the base would be altered by a moderate increase, 0.5 percent per year, in timber prices and no change in agricultural returns. This scenario predicts a similar loss of forest land to urban use, but it also predicts a conversion of marginal, or less productive, agricultural land to forest. This conversion nearly compensates for the loss of forest land, predicting a net loss of 3 million acres.

As in the past, land use changes will not be evenly distributed. The land conversion projected in the market scenario indicates the largest block of potential gain in forest land would be in the western third of the South. Forest loss will be concentrated in the eastern half of the South—the greatest loss will occur in the Southern Appalachian Piedmont.

Shift in Productivity

Climate change will impact forest productivity in several ways. Both ozone and carbon dioxide directly impact trees and forest processes. Ozone levels in the troposphere, the part of the atmosphere closest to land, have increased due to human activities. Ozone is taken up by plants through stomata, the small openings in the leaf through which water and gases pass into and out of the plant. A strong oxidant, ozone directly and immediately damages plant cell membranes. The net effect is a decline in photosynthesis, the process that converts the energy in sunlight to chemical forms of energy that can be used by plants. Analyses suggest that ozone levels have decreased production in southern pine plantations by 5 percent.

Elevated levels of carbon dioxide in the atmosphere are having quite a different impact on trees, often referred to as a fertilizing effect. In a 10-year experiment in central North Carolina, trees exposed to continuously elevated concentrations of carbon dioxide had a 20-percent increase in overall growth, compared to untreated trees. Several studies have produced this increased growth response—although the actual responses vary according to the species and the availability of water and nutrients. It is uncertain how long these responses can be sustained, but in the short term, it seems forest productivity is likely to increase over the next 50 years due to the fertilizing effect of atmospheric carbon dioxide. Unfortunately, new research results indicate that increased productivity does not necessarily equate to increased carbon sequestration.

Economic Impacts May Depend on Human Values

The South has demonstrated a strong advantage in producing a renewable timber resource, but recent changes in timber markets have raised questions about the future. An analysis conducted by SRS scientists Wear and Jeffrey Prestemon indicates that concerns about southern timber markets have shifted from a focus on supply issues to a focus on demand issues. The South has supported a more than doubling of timber production over the past 30 years, and forecasting models show the region can easily supply even more timber. The question is whether future demand will rise as fast as the available supply. Evidence does not support continuing strong demand for pulpwood, at least not for paper production. This may be due in part to an increase in the use of recycled fiber in many paper products. Customer demand for engineered wood products is increasing, but so far, rising demand for these products has not been sufficient to offset declines in demand for pulpwood by the paper sector.

A Way of Life

Climate change could impact many of the amenities, goods, and services from forests. Changes in forest species composition, growth, and mortality will alter recreational opportunities, wildlife habitat, and the supply of specific types of wood and products— particularly specialized markets. The majority of plants harvested for medicinal, edible, and floral products require very specific site conditions to grow and reproduce, giving other species that more readily disperse the demand for snow-related recreation. However, fewer cold days and snow events will reduce opportunities to provide cost-effective winter recreation. Summer recreation, on the other hand, is very likely to increase as more “flatlanders” are expected to flock to the mountains seeking refuge from the heat. The biggest single impact to recreation may be to fishing. Warmer water temperatures will cause the disappearance of cold water fish species such as trout. The greatest losses of trout are predicted to occur in the southern mountains where fishing is integral to rural culture and tradition.

What of Their Future?

Climate change and variability have altered and will continue to alter the composition, structure, and processes of forest ecosystems, the amount and quality of forest products and resources, and the social and economic values of southern forests. But humans—through their demands, preferences, choices, policies, and values—can alter the level of climate change and significantly reduce adverse impacts.

For more information:
Steven McNulty at 919.515.9489 or smcnulty@fs.fed.us
David Wear at 919.549.4011 or dwear@fs.fed.us
Jeffrey Prestemon at 919.549.4033 or jprestemon@fs.fed.us