Most modern forests don't resemble what they replaced
The great forests that once covered the eastern United States were cut down almost completely by settlers who needed open land for farming and timber for construction. When some areas proved unsuitable for agriculture, such as flood plains and steep slopes, they were abandoned by the farmers who had cleared them, and the forests had a chance to grow back.
Indiana forests in 1820 (left) and 1992. Data gathered and graphics created by Sean P. Sweeney.
But what grew back was different from what had been destroyed, partly because there was a lot less of it.
"In general, once a landscape becomes degraded from high-intensity use, like agriculture or heavy cattle grazing, it takes a very long time for it to become valuable for other less-intense uses," said Kelly Caylor, assistant professor of geography at Indiana University Bloomington. "Effects of erosion associated with degradation can be long term as well, since soils take decades to recover and changes in soils can favor less commercially valuable species and more often invasives that are adapted to thrive in disturbed landscapes."
For example, comparing land cover in Indiana in 1820 and in 1992 shows that vast areas of forest were replaced by similarly vast open spaces. Letting trees grow back in small areas where farming efforts were unsuccessful did not undo the damage already done, Caylor said.
"Breaking up forests into smaller communities changes the composition of species," he explained. "In addition, clearing a forested slope leads to immediate changes in surface water movement and patterns of soil erosion that will have long-term impacts on subsequent landscape recovery."
This is as true for an area the size of Indiana's Hoosier National Forest, for example, as it is for the familiar patches of woods that have been allowed to grow here and there in urban areas.
"The ecosystem in Hoosier National Forest was also cleared," Caylor said. "When those who had cleared it tried farming and failed, they abandoned the land. The forest that grew back had some of the same species in it, but not like what was there before it was cleared."
The results of failed attempts at farming in unfavorable areas are just one aspect of Caylor's work, which has to do with disturbed or marginal ecosystems and how they are experiencing either degradation or recovery. Many of these ecosystems are particularly sensitive to changes in water movement associated with changes in either climate or vegetation.
"This is a wide-ranging topic for me that extends from issues of landscape change in southern Africa to impacts of suburban growth on water quality and flood frequency in central Indiana," he said. "I believe that all of these issues are related to the current discussion of global warming. There are real questions about how to manage marginal ecosystems for the future when the future is so uncertain."
Marginal areas of the globe that may seem at first glance to have no similarity may actually have some important things in common. One example is the striking resemblance between what is happening in some parts of southern Africa and similar developments in parts of the United States.
"Botswana in southern Africa is like the U.S. desert southwest in that much of the rangeland has experienced dramatic increases in woody vegetation as a result of land management and climate changes," Caylor said. "A major difference, however, is that there are a large number of pastoralists in southern Africa, who raise animals for a living and often use animals as the basis of their economy. They depend on the land for both fuel and food. When landscapes become over-grazed or degraded beyond use, the local communities don't have financial resources to invest in recovery. In the face of a changing climate, increasing demand and meager resources, it's unlikely that past management practices of marginal landscapes will continue to be successful in the future."
What will future landscapes look like?
"We don't know," Caylor said. To predict that, he and others need to be able to understand how future climates, particularly changes in patterns of rainfall, will interact with land use decisions to govern future changes in landscapes. What they do know is that after a certain threshold is crossed and an ecosystem changes, it's often very hard to go back to the way things were.
"Landscapes integrate the action of climate, vegetation and soils. If any of these changes, then we can expect changes in the other two. For example, it has been predicted that the future climate in Botswana will be drier and hotter, which will cause shifts in vegetation that will lead to increased soil erosion. Once this erosion begins, it's very difficult to get back to the original vegetation. Current practices of over-grazing only increase the potential impact of additional climate change. Patterns of land management in Botswana are based on what the landscape used to be like, not necessarily how it is today, and people are struggling to deal with today's degraded landscapes. In the context of climate change, they are extremely vulnerable when we consider the potential landscapes of the future," he said.
The great hope is that information from Earth-monitoring satellites will be able to detect degradation as it occurs in marginal landscapes, but nobody knows for sure what will happen to the satellites that Caylor and other scientists have relied on. While satellites such as Landsat have provided a continuous record of changes in Earth's surface since 1972, NASA's Earth-monitoring program started to decline in 2002, according to a recent report by the National Academy of Sciences. Since President Bush announced plans in 2004 to return astronauts to the moon and later send them to Mars, many involved with the NASA Earth science program have warned that their efforts are being cut back and will be further restricted in the future.
"NASA's Earth satellite program is so important for what we do," Caylor said. "Detecting changes in Earth's climate and the land surface requires both long-term data and constant innovation in the quality of our observations. The 30-year record from satellites demonstrates that many marginal landscapes have undergone rapid transitions. Understanding how these changes occurred is the key to determining what the future holds for these landscapes and the people who depend on them."
Partial funding for this research was provided by the National Science Foundation to the Center for the Study of Institutions, Population and Environmental Change at Indiana University Bloomington.
