Southwestern plants are being forced to endure ever more stressful growing conditions as temperatures rise and droughts become more severe in the region, according to a new study from the University of Arizona.

The increasingly arid atmosphere draws more moisture from plants, leading to potentially higher stress, particularly at mid- and low elevations, the researchers report in a study to be published soon in the Journal of Geophysical Research Biogeosciences.

"We know the climate in the Southwest is getting warmer, but we wanted to investigate how the higher temperatures might interact with the highly variable precipitation typical of the region," said lead author Jeremy Weiss, a senior research specialist in the university's Department of Geosciences.

Weiss' team used a growing season index based on weather data to examine how plant growth is limited during times of drought. The index considers day length, cold temperature limits and a key metric called vapor pressure deficit to map and compare potential plant responses to two major Southwest droughts, one during 1953-1956 and the other between 2000-2003. Both led to widespread tree die-offs.

Vapor pressure deficit -- a key source of plant stress -- is defined as the difference between how much moisture the air can hold when it is saturated and the amount of moisture actually present in the air. A warmer atmosphere can hold more water vapor, and during droughts it acts like a sponge, absorbing available moisture from the ground, including plants.

Using the index allowed the researchers to model and map potential plant responses to drought under past, present and future conditions across the entire Southwest, said Julio Betancourt, a senior scientist with the U.S. Geological Survey and a co-author of the study along with Jonathan Overpeck, co-director of the University of Arizona Institute of the Environment.

"Our study helps pinpoint how vegetation might respond to future droughts, assuming milder winters and hotter summers, across the complex and mountainous terrain of the Southwest," Betancourt said.

Climate models suggest the region will see increasingly scorching summers and warmer winters in the coming decades.

"Our study suggests that as regional warming continues, drought-related plant stress associated with higher vapor pressure deficits will intensify and spread from late spring through summer to earlier and later parts of the growing season, as well to higher elevations," the authors write.

And the region's plants are already under stress, Weiss added.

"We're seeing climatic growing conditions already at an extreme level with just the relatively little warming we have seen in the region so far," he said. "Our concern is that vegetation will experience even more extreme growing conditions as anticipated further warming exacerbates the impacts of future droughts."

There is little that land managers can do to address the problem of drought-related tree mortality, but they can use the new information to try to better manage the regrowth of vegetation in the aftermath of large-scale ecological disturbances, including wildfires and drought-related tree die-offs, Betancourt suggested.

"Models like the one we developed can provide us with a road map of areas sensitive to future disturbances," he said. "The next step will be to start planning, determine the scale of intervention and figure out what can be done to direct or engineer the outcomes of vegetation change in a warmer world."

What actually causes physiological plant stress and tree death during drought is still unknown.

One theory is that the water column in a tree is disrupted as the tree struggles to pump moisture from the soil in the heat. Another is carbon starvation as leaves shut their openings, called stomates, to conserve leaf water, which slows down the uptake of carbon dioxide needed for photosynthesis.

"When the air is hotter and drier, it becomes more difficult for plants to conserve water while taking up carbon dioxide," Weiss said. "As plants become starved of carbon, it also weakens their defenses and renders them more susceptible to insect pests."

The size of the "atmospheric sponge" grows faster during increasingly hotter summers like those over the last 30 years, absorbing even more moisture from soil and vegetation, he added.

"When warmer temperatures combine with drought, relatively stressful growing conditions for a plant become even more stressful," Weiss said. "In a sense, it's a vicious circle. Warmer temperatures during droughts lead to even drier and hotter conditions."

Reese writes from Santa Fe, N.M.