Abstract

ABSTRACT 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 CO 2 exchange (NEE), ecosystem respiration (R eco ), 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 (R soil ) 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 R eco /GEP ratios followed plant mortality, suggesting increased available carbon after mortality was rapidly decomposed. R soil in low-mortality plots exceeded highmortality plots over drier summer periods, likely from more root respiration. However, total cumulative R soil 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 R soil plots was similar to that across the grassland, suggesting R soil from such areas underlay higher R eco . 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.