Plants can go thirsty but only for so long

U. ARIZONA (US) — Many plants’ demand for water is flexible, but prolonged drought conditions have put their resilience to the test.

Sensitive environments such as the arid grasslands in the Southwestern US already are approaching the limit of this resilience, say scientists affiliated with the Agricultural Research Service (ARS), the United States Department of Agriculture’s chief scientific research agency.

Repeat photography documents how lands change. Top: In 1902, David Griffiths’ horse-drawn buggy is clearly visible in the open grassland, surrounded by scattered desert hackberry plants at the foot of Huérfano Butte. (Credit: D. Griffiths). Middle: By 1941, burro weed and cholla cactus pop up, along with velvet mesquite trees. Bottom: In 2007, the grass cover gives way to velvet mesquite trees, and prickly pear replaces cholla as the dominant cacti. (Photos courtesy of M. McClaran/U. Arizona)

“We found that plants have a capacity for resilience even in the face of the severe drought over the past decade,” says Guillermo Ponce Campos, lead author of the study published in Nature. Ponce Campos led the research as part of his doctoral work at the University of Arizona and is now a research associate working with study co-leader Susan Moran.

“From grasslands to forests, plants can tolerate low precipitation, but if drought conditions continue past a certain point, this resilience will fail,” says Moran, 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 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 explains.

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.

Top ten heat

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.

Near the threshold

“Prolonged, warm drought makes a difference,” Moran says. “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.”

The authors report that in some Australian grasslands, ecosystem resilience has decreased with the increasing aridity widely reported as a result of the prolonged warm drought over these biomes.

Moran cautioned that her team also saw the limit in some of the study areas in Utah, Arizona, and New Mexico.

“We know what the resilience was in the 1980s and 1990s, and we compared it to the early 21st century,” she says. “That’s how we know it’s decreasing. We certainly found resilience, but it is approaching the threshold.”

Work like the present study can help resource managers develop agricultural production strategies that incorporate changes in water availability linked to changing precipitation patterns.

“In the United States, much of our agricultural productivity has depended on long-term precipitation regimes. But those patterns are changing and we need information for managing the effects of those shifts,” says ARS Administrator Edward Knipling.

“These findings can help managers respond to the challenges of global climate change with effective strategies for maintaining agricultural productivity.”

Some of the research took place at the Santa Rita Experimental Range, which is managed by the College of Agriculture and Life Sciences. Established in 1902, the study area encompasses 52,000 acres, or about 80 square miles, on the western side of the Santa Rita Mountains south of Tucson.

Source: University of Arizona