Changing Climate to Alter Landscape of South-Western Forests

Based on the tree ring growth record with historical information, climate records, and computer-model projections of future climate trends, the team of scientists from Los Alamos National Laboratory, the U.S. Geological Survey, the University of Arizona, and other partner organizations have depicted a very grip scenario for the future of trees in the southwestern United States. 

The Southwestern forests grow well when winter precipitation is high and summer is not to hot and dry. 

If warmer and dry climate persists in the future in the Southwest, then widespread tree death is likely that would trigger changes in the distribution of forests and of species.

The details of these findings were published in the journal Nature Climate Change.

"The new 'Forest Drought-Stress Index' that Williams devised from seasonal precipitation and temperature related variables matches the records of changing forest conditions in the Southwest remarkably well," said co-author Thomas W. Swetnam, director of the UA Laboratory of Tree-Ring Research.

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

The paper "Temperature as a potent driver of regional forest drought stress and tree mortality" was led by A. Park Williams of Los Alamos National Laboratory in New Mexico.

In order to trace which climate variables affect forests, the researchers aligned some 13,000 tree core samples with known temperature and moisture data. They compared tree-ring record to climate data collected in the Southwest since the late 1800s. They noticed two climate variables that estimate annual southwestern tree-growth variability with exceptional accuracy. It is the total winter precipitation and average summer-fall atmospheric evaporative demand, a measure of the overall dryness of the environment.

"Atmospheric evaporative demand is primarily driven by temperature. When air is warmer, it can hold more water vapor, thus increasing the pace at which soil and plants dry out. The air literally sucks the moisture out of the soil and plants," said Williams.

"Finding that summer-fall atmospheric evaporative demand is just as important as winter precipitation has critical implications for the future of southwestern forests," he said.

According to the researchers there trends are already visible in the Southwest.

"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."

"When we look at our tree-ring record, we see this huge dip in the 1580s when all the tree rings are really tiny," Williams said. "Following the 1500s megadrought, tree rings get wider, and there was a major boom in new trees. Nearly all trees we see in the Southwest today were established after the late-1500s drought, even though the species we evaluated can easily live longer than 400 years. So that event is a benchmark for us today. If forest drought stress exceeds late 1500 levels, we expect that a lot of trees are going to be dying."

UA co-author Daniel Griffin said, "This research is distinctly different from work done in a similar vein in two ways: One, it puts these projections for the future in a concrete historical context, and two, it shows that the impacts on the forests will not be restricted to one species or one site at low elevation, but in fact will take place at forests across the landscape.