Contrasting responses of gross primary productivity to precipitation events in a water-limited and a temperature-limited grassland ecosystem

The impact of global climate change on precipitation regimes may bring about significant consequences to the productivity of grassland ecosystems. This study reports the effects of the characteristics of precipitation events (PEC, e.g., size, frequency, interval, and seasonal distribution of precipitation events) on gross primary productivity (GPP) based on long-term measurements of net ecosystem CO2 exchange and evapotranspiration by the eddy covariance technique in a water-limited temperate steppe in Inner Mongolia and a humid but temperature-limited alpine meadow in Tibet, China. We determined that the predominant characteristics of precipitation events that affected GPP differed remarkably between these two ecosystems. The number of heavy precipitation events (>10 mm day−1) was the most important PEC to impact GPP in the temperate steppe. Years with more frequent heavy precipitation events favored higher GPP. However, in contrast with the case in the temperate steppe, precipitation interval was the factor that affected GPP most in the alpine meadow. GPP was higher as the precipitation intervals lengthened or their distribution shifted to fewer but longer intervals (concentrated distribution). Additionally, the mechanisms for the effects of PEC on GPP also differed between these two ecosystems. Compared with small events, heavy precipitation events recharged deeper soil layers in the temperate steppe, which was biologically more meaningful for plant transpiration (i.e., an increase in the ratio of transpiration to evapotranspiration), relief from drought stress (an increase in the duration of high soil water content), and thereby higher GPP. In contrast, mean air temperature was higher in the alpine meadow when the precipitation intervals increased or their distribution was more concentrated, leading to a longer period of higher temperature that was favorable to higher GPP. Our results imply that soil water is not the exclusive means of revealing the mechanism of precipitation affecting productivity; the corresponding changes in temperature along with precipitation events may also play a substantial role in temperature-limited grasslands.