The researchers looked at the impacts of the 2010 spring drought on primary productivity in southwestern China. This was the region's most severe and long-lasting spring drought in the last 50 years. Satellite observations on vegetation greenness together with climate data allowed the team to study how the drought affected both primary productivity and ecosystem carbon uptake.

"The spring and annual productivity in 2010 were among the lowest during the 11-year period 2000–2010 and as low as those seen in 2000, one of the driest years in the last five decades," said Li Zhang of the Center for Earth Observation and Digital Earth, Chinese Academy of Sciences.

The spring drought substantially reduced plant productivity during spring 2010. It also reduced the winter wheat yield in southwestern China in 2010 by 10.8% compared to 2009. Plant growth did not fully recover from the stress caused by the drought until August 2010. The drought reduced regional annual gross primary productivity and net primary productivity in 2010 by 65 and 46 Tg of carbon per year, respectively (1 Tg = 1012 grams or 1 megatonne).

"Drought has profound impacts on ecosystem carbon balance by reducing photosynthesis and altering ecosystem respiration," added Jingfeng Xiao of the Earth Systems Research Center at the University of New Hampshire, US. "Drought can also indirectly influence carbon fluxes because it may lead to fires. Trees can die and insect outbreaks, which harm plants, become more prevalent."

The spring drought lowered the water levels of rivers, reservoirs and lakes, and even dried up some water bodies, which further limited the amount of water available for irrigation. Agricultural production was adversely affected, as were drinking water supplies to the local population.

"During this drought, large water deficits occurred at the same time as higher air temperatures," said Zhang. "Higher air temperatures increased evaporation and further exacerbated water shortage."

"High air temperatures and solar radiation should promote plant growth," added Xiao, "but the negative effects of water shortage outweighed these positive effects."

The unique karst landform in large areas of southwestern China may also have exacerbated the surface water conditions, explained Zhang. "Surface water drained into the ground, leading to less water on the surface, which aggravated the drought even further," she said.

According to Xiao, future droughts will likely have larger impacts on plant growth and the terrestrial carbon cycle. "More frequent and severe droughts will partly offset the enhancement effects of rising atmospheric carbon-dioxide concentration, elevated air temperatures and nitrogen deposition on plant growth and ecosystem carbon sequestration," he said.

The scientists, who report their study in Environmental Research Letters (ERL) are now extending their analyses over larger areas to investigate the impacts of severe and long-lasting droughts on plant growth and the terrestrial carbon cycle across China, North America and the entire world.