If temperatures continue to rise and drought conditions worsen, more stress could be put on trees in the Southwest and limit their growth according to a recent study by a group of scientists.
Los Alamos National Laboratory, the U.S. Geological Survey, the University of Arizona and other organizations recently developed a new Forest Drought-Stress Index.
Researchers studied about 13,000 tree core samples from the Southwest that had moisture and temperature data. Combined with climate data since the 1800s, scientists found two climate variables, which include the total winter precipitation and the average summer-fall atmospheric evaporative demand.
A. Park Williams of Los Alamos National Laboratory in New Mexico said, "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."
Scientists are projecting that by 2050 a megadrought-type stress conditions will be exceeded regularly.
The most recent megadroughts struck the Southwest in the second half of the 1200s and again in the late 1500s. The first is said to have driven out the ancient Puebloan culture.
"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."
According to the study, drought stress during more than 30 percent of the last 13 years was equivalent or exceeded levels during the last two megadroughts.
The study makes it known that the Southwest may already be adapting to a more open and drought-tolerant ecosystem as a result of the 21st-century warmth and drought conditions.
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