Extreme drought and extreme wet years are characteristic of arid and semiarid grasslands. Moreover, the frequency and magnitude of these extreme events is going to increase under climate change. The central objective of this talk is to assess how extreme precipitation events are translated into primary production. We hypothesize that ecosystem response to extreme events is modulated by: (1) duration of extreme precipitation event, (2) mean annual precipitation of the ecosystem, and (3) plant-functional group composition. We test these hypotheses using a combination of a precipitation manipulation experiment and long-term observations of aboveground net primary production. The experiment consisted of a 12-year manipulation of precipitation in a Chihuahuan semiarid grassland in New Mexico where we enhanced and reduced precipitation by 80% using rainout shelters and irrigation. Experimental results were complemented by analysis of 43 long-term (≥ 10 years) productivity data sets that cover a wide range of geographic and environmental spaces.
Results/Conclusions
We found that the effect of precipitation on primary production was not linear and was modulated by the duration of the extreme drought and wet conditions. Moreover, the effect of time showed distinct thresholds at 3 and 8 years. Small responses to drought occur in the first 3 years, followed by a plateau from years 3 to 8 and a sharp and apparently irreversible decline after year 8. The response to extreme wet conditions was also modulated by time but was not symmetrical to the response to drought. The response to dry and wet conditions was different for grasses and shrubs. Shrubs showed no response to drought treatment until year 9 when they started showing a positive response. Shrubs under drought treatment had higher production than controls and irrigated treatments suggesting that response to extreme precipitation events was modulated by competition between shrubs and grasses. At the regional scale, the response to extreme precipitation events was different for ecosystems with mean annual precipitation below or above 300mm/year of long-term precipitation. Arid ecosystems showed a larger response to wet conditions than to drought and mesic ecosystems presented the opposite pattern.