Forests In The Southwest Could Change Due To Climate Change
October 1, 2012

Climate Change Could Cause ‘Substantial Changes’ To Southwest US Forests

redOrbit Staff & Wire Reports - Your Universe Online

Rising temperatures and decreases in precipitation associated with climate change could cause "widespread tree death" in the forests of the southwestern US, which could lead to "substantial changes" both in the landscape and the creatures living there in the near future, experts warn in a new study.

Scientists from Los Alamos National Laboratory, the U.S. Geological Survey (USGS), the University of Arizona, Columbia University, and the University of Tennessee studied tree-ring growth records, historical data, climate records, and computer-model projections of future weather patterns, and concluded that warmer, drier conditions could drastically affect the distribution of both forests and species.

The data paints "a grim picture" for the future of the region's trees, according to a September 30 press release from the Tucson-based school. As part of their research, the scientists compiled a new Forest Drought-Stress Severity Index, which includes winter precipitation amounts, and late summer and fall temperatures and precipitation into a single statistic.

"The new 'Forest Drought-Stress Index'“¦ devised from seasonal precipitation and temperature-related variables matches the records of changing forest conditions in the Southwest remarkably well," Thomas W. Swetnam, director of the University of Arizona's Laboratory of Tree-Ring Research and co-author of the study, said in a statement.

"Among all climate variables affecting trees and forests that have ever been studied, this new drought index has the strongest correlation with combined tree growth, tree death from drought and insects, and area burned by forest fires that I have ever seen," he added.

The index was created by A. Park Williams, lead author of the study and a researcher with the Los Alamos National Laboratory. Williams and his colleagues reviewed several different variables to determine which had the greatest impact on forests.

They then isolated the two variables that appeared to "estimate annual southwestern tree-growth variability with exceptional accuracy: total winter precipitation and average summer-fall atmospheric evaporative demand, a measure of the overall dryness of the environment."

According to Williams, warmer air allows the atmosphere to hold more water vapor, and as a result, the earth and plant life tend to dry out faster under these conditions. Because of a gradual increase in temperature that has been occurring -- and is expected to continue occurring, due to greenhouse gases -- in the Southwest, the more frequent warm summers will counteract high-precipitation winters to limit the growth of trees in the region.

"We can use the past to learn about the future," Williams said. "For example, satellite fire data from the past 30 years show that there has been a strong and exponential relationship between the regional tree-ring drought-stress record and the area of southwestern forests killed by wildfire each year. This suggests that if drought intensifies, we can expect forests not only to grow more slowly, but also to die more quickly."

The researchers' used climate-model projections to predict that "megadrought-type forest drought-stress conditions will be exceeded regularly by the 2050s," according to the press release. Assuming those predictions are correct, "forest drought-stress levels during even the wettest and coolest years of the late 21st century will be more severe than the driest, warmest years of the previous megadroughts."

As for the drought the area has been experiencing since 2000, Williams' team says that forests in the Southwest experienced drought-related stress during 30% of the last 13 years that was equal to or greater than megadrought-level conditions experienced during the 13th and 16th century. Over the past 1,000 years, that frequency of drought-related stress has never occurred during regular drought conditions -- only during megadroughts.

The study, which was sponsored by the Los Alamos National Laboratory, the US Department of Energy, and the National Science Foundation (NSF), was published last week in the journal Nature Climate Change.