In the fastest climate-induced boundary shift ever documented, the transition zone between two ecosystems moved by two kilometers in less than five years, scientists reported today in the Proceedings of the National Academy of Sciences.
The motion of the transition zone came as a result of a drought caused by global climate change.
The research was performed by environmental scientists at the Department of Energy's Los Alamos National Laboratory and the U.S. Geological Survey's Midcontinent Ecological Science Center.
"This research shows how rapidly vegetation can respond to climate," said Dave Breshears of the Environmental Science Group at Los Alamos. "It has significant implications for modeling how climate can decimate vegetation in droughts like the one we studied and for assessing the impacts of global climate change, which is a major scientific and social issue."
The shift of the boundary, known as an ecotone, between an ecosystem dominated by ponderosa pine forest and one dominated by pinon-juniper woodland is described in an article titled "Drought-inducted shift of a forest-woodland ecotone: rapid landscape response to climate variation."
Breshears and Craig Allen of the USGS field station in Bandelier National Monument, which is located on land just south of the Laboratory, studied the ecotone shift between the ponderosa and pinon-juniper ecosystems by poring over detailed aerial photographs taken from the 1930s through the 1970s. They also conducted a variety of field studies and reviewed historical information to verify their findings.
"Previous studies have documented shifts that take place over decades or even centuries, and they focus on birth and growth of vegetation," said Allen. "Our research shows that more attention should be paid to mortality because the rapidity of the shift resulted from the death of ponderosas as a result of the drought in the 1950s."
The ecotone not only moved rapidly over a relatively large distance, the researchers said, but the shift has persisted to the present, indicating that the drought may have pushed the vegetation pattern over a threshold from which it may be unlikely to recover.
Ecotones are important areas for study because the response of vegetation to variations in climate is expected to be the most extreme at the boundaries. The study area is also sensitive to change because it is in a semiarid climate.
"We focused on a specific site to better resolve and understand the spatial shift," Allen said. "It's also a relatively 'clean' wilderness area with little recent history of human activity, but it's representative of a much larger portion of the surrounding area."
The study site is a 5,874-acre area on Frijolito Mesa in the national monument. It ranges in elevation from 5,910 to 7,223 feet and has a mean precipitation of 16 inches a year. In 1954, the ponderosa pine forest covered about 37 percent of the site, but by 1958, this had been reduced to only 15 percent. The greatest change occurred at lower elevations.
The change coincided with the culmination of the drought, which was one of the most severe in the Southwest in the past 500 years. The researchers noted that the ecotone shift was amplified by an infestation of bark beetles and by competition with pinon and juniper that expanded in range after fires were suppressed, but they said the ponderosa pine mortality was "driven by the drought."
The researchers said in the paper that the "unprecedentedly rapid climate changes expected in coming decades" will produce rapid, extensive changes in the distribution of woody vegetation through rapid mortality. Moreover, they said, the resulting ecotone shifts are likely to occur globally because semiarid forests and woodlands are widespread and sensitive to change.
Breshears and Allen said one result of the change in vegetation in the study area has been a sharp increase in erosion, apparently due to the loss of herbaceous ground cover during the drought.
"We're watching this hill slope fall apart before our eyes," said Allen, describing an intensively studied two-and-half acre site inside the ecotone transition zone.
"This has become a very harsh micro-climate," added Breshears. "It's more extreme than some parts of the study site, but it's fairly typical."
In addition to its implications for global climate change, the work at the Bandelier site and a similar site on Laboratory property is helping the researchers understand basic environmental processes.
"This work also has site-specific significance," said Breshears. "For the national monument, it's important in helping to understand how to protect archaeological sites. For the Laboratory, it's related to our studies of water resources and the movement of contaminants through ecosystems."
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.
As the nation's largest water, earth and biological science, and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial scientific information to resource managers, planners, and other customers.
08-Dec-1998