U.S. Southwest changing with climate

TUCSON, March 19 (UPI) -- As the U.S. Southwest becomes hotter and drier, semi-arid grasslands are slowly being replaced by a landscape dominated by mesquite trees, researchers say.

Their extremely deep-reaching roots give mesquite trees an edge over grasses, University of Arizona researchers said.

The trees and other woody shrubs are better able to cope with heat and water stress across seasonal precipitation periods than many native grasses, they said.

"Our results show that even the smallest mesquites are better adapted for thriving under elevated temperatures and dry conditions -- the projections for our future climate -- suggesting that these woody plants are here to stay," researcher Greg Barron-Gafford said in a university release.

Native to the region, mesquites have been in Southwest deserts for a long time but not in today's abundance, he said.

Scientists say they believe woody plants began displacing grasslands as a result of overgrazing but the phenomenon has since been propelled by changing climate.

Mesquites have roots that reach down to 160 feet or more that can tap into groundwater not accessible to plants like grasses with shallow root systems, Baron-Gafford said.

"Their roots are always out there and they find [the water], allowing them to bypass the grasses' evolutionary advantage," he said. "These deep-rooting shrubs and trees are accessing deeper water that was previously unavailable to drive plant biology in this area."