Seasonal Precipitation Legacy Effects Determine the Carbon Balance of a Semiarid Grassland

Semiarid grasslands are water-limited ecosystems where precipitation (PPT) controls the onset and duration of the growing season; however, this variable does not fully explain interannual variability of productivity at temporal scales. We examined the relationship between PPT and carbon (C) fluxes in a semiarid grassland at both seasonal and interannual scales, as well as the role of lagged effects of PPT and asymmetric sensitivities of net ecosystem carbon exchange to PPT and its components (gross ecosystem exchange [GEE] and ecosystem respiration [ER]). Six years of continuous net ecosystem C exchange data measured with the eddy covariance technique and GEE estimated with 15 years of enhanced vegetation index and the gross primary productivity of Moderate Resolution Imaging Spectroradiometer were used. The semiarid grassland was a C source and a C sink among contrasting PPT years (114 to -107 g C.m(-2).year(-1)). At seasonal scale, PPT relationship with the 15 years of GEE derived from enhanced vegetation index and gross primary productivity was sigmoidal. Moreover, PPT legacies of the previous dry season determined the C balance of the grassland by affecting the C uptake and ecosystem respiration of the following growing season, but productivity was more sensitive to PPT changes than respiration. Models of climate change for semiarid grasslands in North America predict up to 30% reduction of winter-spring PPT and slight summer PPT decrease. Thus, our results suggest that future changes in PPT may have a strong impact on the C uptake capacity of this ecosystem due to weakened legacy effects in summer C uptake. Plain language summary Precipitation is the main driver of productivity in semiarid ecosystems; however, annual precipitation does not fully explain interannual variability of productivity. Semiarid grasslands influenced by monsoonal precipitation regimes exhibit 6 to 8 months of drought with less than 20% of annual precipitation. This small amount of rain when vegetation is dormant would seem to be unimportant in terms of imposing control on productivity or carbon (C) fluxes. Using ecosystem flux data, we tested whether this small precipitation amount in the dry season had a role in controlling C fluxes and productivity and how much this affects the annual C uptake. We demonstrated for a semiarid grassland with summer prevailing rains that the small dry-season precipitation highly determines the carbon balance of both the humid growing season and the annual C balance. Also, we found that dry seasons with large amounts of precipitation enhanced productivity more than ecosystem respiration in summer. Our study highlights the importance of the dry-season precipitation on the productivity of semiarid grasslands where it is forecasted a greater proportion of precipitation reduction for winter than for summer at the end of this century. In this sense, our study highlights possible impacts of climate change on the vulnerability of semiarid ecosystems.