Scientists with the U.S. Department of Agriculture, or USDA, and their partners have determined that water demand by many plant communities can fluctuate in response to water availability, indicating a capacity for resilience even when changing climate patterns produce periodic droughts or floods.

But their research also suggests that a limit to this resilience ultimately could threaten the survival of these plant communities. Sensitive environments such as the arid grasslands in the Southwestern U.S. already are approaching this limit.

Results from this study were published in the journal Nature by a team of Agricultural Research Service, or ARS, scientists, including three scientists affiliated with the UA. ARS is USDA's chief scientific research agency.

The study was led by UA-affiliated ARS researchers Guillermo Ponce Campos and Susan Moran and an Australian team led by Alfredo Huete from the University of Technology, Sydney.

"We found that plants have a capacity for resilience even in the face of the severe drought over the past decade," said Ponce Campos, the study's lead author. Ponce Campos led the research as part of his doctoral work at the UA and now is a research associate working with Moran.

"From grasslands to forests, plants can tolerate low precipitation, but if drought conditions continue past a certain point, this resilience will fail," said Moran, who graduated from the UA and now is a researcher with the USDA ARS Southwest Watershed Research Center and an adjunct professor in the department of soil, water and environmental science in the UA College of Agriculture and Life Sciences.

Once that limit is reached, water-starved plants lose their ability to take advantage of increased precipitation, even if the drought makes way for wetter conditions, Moran explained.

The researchers conducted their investigation using measurements made during 2000-09 at 29 sites in the United States, Puerto Rico and Australia. This provided data about precipitation patterns in the various types of environments. Globally, the 2000-09 decade ranked as the 10 warmest years of the 130-year (1880-2009) record. The team compared these data with measurements taken from 1975 to 1998 at 14 sites in North America, Central America and South America.

To calculate ecosystem water use, the scientists used satellite observations to approximate above-ground net plant productivity at each site. Then they combined these approximations with field data of precipitation and estimates of plant water loss to generate indicators of plant water use efficiency.

The team observed that ecosystem water-use efficiency increased in the driest years and decreased in the wettest years. This suggests that plant water demand fluctuated in accordance with water availability and that there is a cross-community capacity for tolerating low precipitation and responding to high precipitation during periods of warm drought.

However, the team observed that the water-use efficiency data exhibited a trend of "diminishing returns." This suggests plant communities eventually will approach a water-use efficiency threshold that will disrupt plant water use and severely limit plant production when drought is prolonged.

"Prolonged, warm drought makes a difference," Moran said. "To date, it appears there is resilience, but in the more sensitive biomes like grasslands, we are starting to see evidence of decreasing resilience. And as more and more ecosystems increase in aridity, more will reach this threshold."