Constraints of aboveground production by vegetation structure: Evidence from a water manipulation experiment in the Patagonian steppe.

YAHDJIAN, LAURA; SALA, OSVALDO

Portland, Oregon, USA

Congreso; 89th Annual Meeting of the ESA; 2004

Ecological Society of America

ABSTRACT- Aboveground net primary production (ANPP) increases linearly along spatial precipitation gradients in grasslands but the temporal models relating time series of ANPP and annual precipitation for single sites show lower slopes and regression coefficients than the spatial models. These differences could be explained by lags in the ecosystem response to increased water availability. These lags may result from structural and biogeochemical constraints that systems experienced after drought. Our objective was to explore the structural constraints of the ANPP response to rainfall variability in a semiarid ecosystem. We designed a rainfall exclusion experiment in the Patagonian steppe, South of Argentina, where we manipulated water input with rainout shelters, which included 3 levels of rainfall interception (30%, 55%, and 80%) and control. After two years of rainfall exclusion, we supplemented with water half of the 10 plots of each treatment to evaluate the recovery of the plant community cover (our surrogate for ANPP) at the different levels of drought imposed previously. Soil water content at 0-30-cm depth was significantly lower (p<0.05) in the sheltered plots than in the controls and was significantly (p<0.001) augmented after rainfall supplementation. Total percent cover of green biomass was significantly (p<0.05) reduced only by the 80% exclusion treatment and was mostly explained by a reduction in the cover of grasses. The recovery of grasses when we supplemented with rainfall was slower when plants had experienced 80% rainfall exclusion the previous years, which could be explained by a significantly (p<0.05) lower number of plants in the plots of that treatment at the end of the exclusion period. Thereby, the number of plants structurally constrains the recovery of grasses after they had experienced drought and these constraints determine time lags which limit the capacity of the ecosystem to take advantage of wet years after dry years.