Differential effects of extreme drought on production and respiration: synthesis and modeling analysis

[摘要] Extremes in climate may severely impact ecosystem structure and function,with both the magnitude and rate of response differing among ecosystem typesand processes. We conducted a modeling analysis of the effects of extremedrought on two key ecosystem processes, production and respiration, and, toprovide a broader context, we complemented this with a synthesis of publishedresults that cover a wide variety of ecosystems. The synthesis indicated that across abroad range of biomes, gross primary production (GPP) was generally moresensitive to extreme drought (defined as proportional reduction relative toaverage rainfall periods) than was ecosystem respiration (ER). Furthermore,this differential sensitivity between production and respiration increased asdrought severity increased; it occurred only in grassland ecosystems, and notin evergreen needle-leaf and broad-leaf forests or woody savannahs. Themodeling analysis was designed to enable a better understanding of the mechanisms underlyingthis pattern, and focused on four grassland sites arrayed across the GreatPlains, USA. Model results consistently showed that net primary productivity(NPP) was reduced more than heterotrophic respiration (R_h) byextreme drought (i.e., 67% reduction in annual ambient rainfall) at allfour study sites. The sensitivity of NPP to drought was directly attributableto rainfall amount, whereas the sensitivity of R_h to drought wasdriven by soil drying, reduced carbon (C) input and a drought-inducedreduction in soil C content – a much slower process. However, differences inreductions in NPP and R_h diminished as extreme drought continued,due to a gradual decline in the soil C pool leading to further reductions inR_h. We also varied the way in which drought was imposed in themodeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or byreducing rainfall event number (REN). Modeled NPP and R_hdecreased more by ESR than REN at the two relatively mesic sites but less soat the two xeric sites. Our findings suggest that responses of production andrespiration differ in magnitude, occur on different timescales, and areaffected by different mechanisms under extreme, prolonged drought.

[学科分类] 地球化学与岩石