Predicted reductions of cool-season rainfall may expand and accelerate drought-induced plant mortality currently unfolding across the Southwest US. To assess how repeated plant mortality affects ecosystem functional attributes, we quantified net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), and gross ecosystem photosynthesis (GEP) responses to precipitation (P) at a semidesert grassland over spring (Feb 1–Apr 30) and summer (June 15–Oct 1) plant-active periods across eight years, including two with distinct patterns of extensive species-specific mortality. In addition, we quantified daily soil respiration (Rsoil) in high- (56–88%) and low-mortality (8–27%) plots the summer following the most recent mortality event. Plant mortality coincided with severely dry cool-season conditions (Dec 1–Apr 30). We found a positive relationship between springtime P and GEP, and that springtime conditions influenced GEP response to summer rainfall. High springtime Reco/GEP ratios followed plant mortality, suggesting increased available carbon after mortality was rapidly decomposed. Rsoil in low-mortality plots exceeded high-mortality plots over drier summer periods, likely from more root respiration. However, total cumulative Rsoil did not differ between plots, as variation in surviving plant conditions resulted in high and low C-yielding plots within both plot types. Vegetation status in high C-yielding Rsoil plots was similar to that across the grassland, suggesting Rsoil from such areas underlay higher Reco. This, coupled to springtime drought constraints to GEP, resulted in positive NEE under summer P accumulations that previously supported C-sink activity. These findings indicate that predicted lower cool-season precipitation may strongly and negatively affect summer season productivity in these semiarid grasslands.
