Mesquite trees, on the other hand, were found to be capable of running their photosynthetic metabolism at half their maximum rate regardless of whether it was hot and dry.
"Grasses can only function in a very narrow temperature range," Barron-Gafford said. "While the they are drying out, it's like a hot day by the pool for the mesquite."
"A heavy monsoon downpour saturates the soil, but the surface dries off fast because it's a hot and dry environment," Barron-Gafford said. "But deeper down, the water remains in the soil longer, where the mesquite can access it, but not the grasses. That is a really important implication because our predicted precipitation patterns over the next century are going to favor deeper rooted mesquite shrubs over shallow-rooted grasses."
Because they rely much less on rainfall, mesquites should benefit from the forecasted increasing precipitation variability, offering a possible explanation for the observed encroachment of mesquite plants even in areas no longer subjected to heavy grazing.
To monitor the interactions among plants, water, the soil and the atmosphere on an ecosystem-wide scale, researchers with Biosphere 2 and The U.S. Department of Agriculture operate so-called eddy covariance towers in areas such as the San Pedro River Riparian Area, the Santa Rita Mountains and the Catalina Mountains, which can measure temperature, moisture and gases at defined heights above ground.
"With the towers, we can measure carbon, water and energy flow across an entire ecosystem," Barron-Gafford said.
In addition, his team has transplanted grasses and mesquite saplings into large, highly instrumented drums within Biosphere 2, where the researchers can put the plants through intentional periods of drought and temperature stress. Visitors to Biosphere 2 can see this science in action, right along the tour route, and learn more from members of the research team
|Contact: Daniel Stolte|
University of Arizona