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

Extremes in climate may severely impact ecosys- tem structure and function, with both the magnitude and rate of response differing among ecosystem types and processes. We conducted a modeling analysis of the effects of extreme drought on two key ecosystem processes, production and res- piration, and, to provide a broader context, we complemented this with a synthesis of published results that cover a wide variety of ecosystems. The synthesis indicated that across a broad range of biomes, gross primary production (GPP) was generally more sensitive to extreme drought (defined as pro- portional reduction relative to average rainfall periods) than was ecosystem respiration (ER). Furthermore, this differen- tial sensitivity between production and respiration increased as drought severity increased; it occurred only in grassland ecosystems, and not in evergreen needle-leaf and broad-leaf forests or woody savannahs. The modeling analysis was de- signed to enable a better understanding of the mechanisms underlying this pattern, and focused on four grassland sites arrayed across the Great Plains, USA. Model results consis- tently showed that net primary productivity (NPP) was re- duced more than heterotrophic respiration (Rh) by extreme drought (i.e., 67 % reduction in annual ambient rainfall) at all four study sites. The sensitivity of NPP to drought was di- rectly attributable to rainfall amount, whereas the sensitivity of Rh to drought was driven by soil drying, reduced carbon (C) input and a drought-induced reduction in soil C content - a much slower process. However, differences in reductions in NPP and Rh diminished as extreme drought continued, due to a gradual decline in the soil C pool leading to further re- ductions in Rh. We also varied the way in which drought was imposed in the modeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or by re- ducing rainfall event number (REN). Modeled NPP and Rh decreased more by ESR than REN at the two relatively mesic sites but less so at the two xeric sites. Our findings suggest that responses of production and respiration differ in magni- tude, occur on different timescales, and are affected by dif- ferent mechanisms under extreme, prolonged drought.

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