New research from the United States Department of Agriculture suggests plants' ability to adapt to extreme shifts in water availability, such as drought and flooding, may be greater than has been appreciated. However, scientists at the USDA's Agricultural Research Service said plants' ability to withstand these extreme patterns will be tested by future climate change

In a study published this week in the journal Nature ARS scientists led by Guillermo Ponce Campos and Susan Moran, working with colleagues in Australia, examined different ecosystems' ability to deal with the effects of climate change. The researchers conducted their investigation using measurements made during 2000-2009 at 29 sites in the United States, Puerto Rico, and Australia. This provided data about precipitation patterns in environments ranging from grasslands to forests.

Globally, the 2000-2009 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 establish approximate plant productivity at each site. Then they combined these approximations with field data of rainfall and estimates of plant water loss to generate indicators of plant water use efficiency. They observed that "ecosystem water-use efficiency increased in the driest years and decreased in the wettest years".

This suggests water demand is largely dependent on water availability and, the ARS researchers suggest, these systems have the capacity to tolerate reduced rainfall, then make the most of subsequent heavy rainfall. ARS spokesperson Edward B. Knipling commented on the findings, "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. These findings can help managers respond to the challenges of global climate change with effective strategies for maintaining agricultural productivity."

Scientists discover drought-resistance watershed

And the researchers' conclusions suggest that strategies will need to be developed. Although they saw plants' water use fluctuated according to availability, the researchers discovered a trend of "diminishing returns." Meaning plants will have 'water-use efficiency threshold' after which they will be severely affected by the effects of drought.

They warn that sensitive environments such as the arid grasslands in the South-western U.S. are already approaching this limit. Study author Susan Moran commented, "From grasslands to forests, plants can tolerate low precipitation, but if drought conditions continue past a certain point, this resilience will fail. 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."

She added, "Prolonged, warm drought makes a difference. 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. We know what the resilience was in the 1980s and 1990s, and we compared it to the early 21st century; that's how we know it's decreasing. We certainly found resilience, but it is approaching the threshold."  

The findings on ecosystem water use present a promising picture of greater resilience than had previously been appreciated, but the ARS scientists' second discovery suggests more needs to be done given the changes being wrought in the US, parts of which are just recovering from the effects of the country's worst drought in 50 years. Even now, the effects of the drought are causing concern for winter wheat on the Plains and in the Midwest.

Predictions for plants' ability to cope with climate change

The ARS scientists also used the data to develop predictions about future plant response to climate change. Their results suggest that ecosystems will become less resilient to the effects of drought over time, as warming increases, and with it the severity and frequency of drought. They suggest grasslands will be the first to feel the effects, "these plant communities are particularly sensitive to the hot and dry conditions of prolonged warm droughts."

Upon releasing their findings, the researchers urged agriculturalists to develop more resilient farming strategies, which will be better able to deal with changes in water availability envisaged in their models, to minimise the effect of reduced rainfall on food production.