Drought is a major ecological threat to grassland ecosystems. Impacts of extreme drought on grassland biomass production are likely depend on the timing of occurrence because extreme drought occurs during important phenological periods of dominant species has much severe impacts on production. Moreover, long term chronic global change factors, such as N deposition may interact with short term extreme climate events to increase ecosystem vulnerability to shifts in environmental conditions. To test the combined effects of discrete climate extremes and chronic environmental changes on ecosystem processes and functioning, we imposed the spring or summer drought treatment in both fertilized and unfertilized plots in a meadow steppe in northeast China. We surveyed the vegetation and measured the aboveground biomass (AGB) and belowground biomass (BGB) before and after the drought treatment. We hypothesized that (1) AGB in Leymus chinensis meadow steppe is more sensitive to the spring drought than the summer drought because spring is a critical period of L. chinensis growth, and (2) N addition wound impact plant biomass allocation, and then alter sensitivity of aboveground biomass production to the manipulated extreme drought.
Results/Conclusions
The spring drought caused significant reduction in AGB. In contrast, the summer drought did not significantly affect AGB. The drought effects on AGB and BGB were negligible in 2017 (one year after the drought treatment). L. chinensis is the dominant species (90%) in this study site; thus, the growth strategy of L. chinensis and its adaptive strategy to drought events played a key role on the response of AGB at the community level. During the spring drought event, L. chinensis have completed the majority of their growth with quick growth rate, thus photosynthates were primarily used for growth of shoots to accumulate photosynthetic leaf area. In this period, plant growth and transpiration need consume large quantities of water. N addition induced an increase in AGB and a decrease in root-shoot ratio, eventually reduced ecosystem resistance to drought stress. Our results suggest that the response of AGB at the community level to extreme drought depend on the developmental stage of the dominant species, highlighting the importance of the timing of drought. Drought resistance was positively correlated with BGB. These findings enable us to better predict response of L. chinensis ecosystem productivity to the projected combined influence of discrete climate extremes and chronic environmental changes.