COS 40-1
The ecological importance of seemingly insignificant small rainfall events in desert grassland
During the summer monsoon, small precipitation events of less than 3.8 mm have, on average, an insignificant effect on total seasonal precipitation in Chihuahuan Desert grasslands, New Mexico, USA. To explore the influence of these events on drought severity and ecosystem recovery from drought, we experimentally removed small events (< 3.8 mm) from desert grassland during a dry monsoon (2012), and added small rainfall events in 3.8 mm increments to the same experimental grassland during a subsequent wet monsoon (2013). To evaluate responses to these treatments, we measured vegetation aboveground production, soil moisture, soil carbon and nitrogen content, and soil enzymatic activity.
Results/Conclusions
We observed no difference in aboveground productivity between control and experimental grassland in 2012, although experimental grassland experienced lower soil moisture (p < 0.001) and nutrient availability than control grassland (p < 0.03). In 2013, experimental grassland had significantly lower aboveground productivity (p < 0.02) and soil moisture (p < 0.001) than control grassland. Aboveground vegetation biomass was similar between treatments at the beginning of the 2013 monsoon, suggesting that nutrient limitation and reduced belowground biomass inhibited experimental grassland recovery from drought in 2013, even under a supplemented rainfall regime. The dry and wet monsoons that occurred in our experiment are common in the Chihuahuan Desert, and the 2012 treatment provides an analog to the slightly drier conditions that are predicted in global climate change scenarios. Our results show that small rainfall events have a significant influence on ecosystem nutrient availability, and that the effects of even slightly drier conditions may inhibit future nutrient availability and grassland recovery from drought. In a warming climate, the effects of drought may be enhanced by the lower residence time of small rainfall events as soil moisture, with results that influence subsequent growing seasons.