Water limitation controls the pace of above- and belowground biological activity in drylands and is a defining characteristic of these ecosystems. Net ecosystem CO2 exchange (NEE) is affected by above- and belowground biophysical processes and is mediated by the balance between ecosystem CO2 uptake by photosynthesis (gross ecosystem productivity; GEP) and the efflux of CO2 by the respiration of plants and soil microbes (ecosystem respiration; Re). Understanding the carbon cycling consequences of precipitation variability in dryland ecosystems requires improved appreciation and accounting of how plant-mediated GEP and plant- and soil microbial respiration differ in their response to rainfall. Our objective was to contrast the sensitivity of dryland ecosystem GEP and Re in response to inter- and intra-annual precipitation variability in a nearby grassland, savanna, and shrubland ecosystems in southeastern Arizona. To do this, we modified the Richards-Baker Index, which quantifies the flashiness of a stream’s hydrograph, to calculate analogous indices of ecosystem metabolic flashiness. In this way, ecosystem metabolic flashiness describes the frequency and rapidity of short term fluctuations in CO2 exchange in response to precipitation using tower-based time-series of daily averaged GEP and Re. We calculated annual GEP and Re flashiness (GEPf and Ref respectively) using 6 years of daily-averaged GEP and Re estimated from eddy covariance at each site.
Results/Conclusions
In contrast to our prediction, annual GEPf was consistently greater than annual Ref. Furthermore, we predicted that increasing rooting depth would correlate with a decline in annual GEPf. In fact, annual GEPf was similar between the grassland, savanna, and shrubland. Whereas the response of annual GEPf to annual precipitation was plant community dependent and generally declined with increasing rainfall, annual Ref did not vary in response to precipitation. A decline in GEPf in response to increasing rainfall may be attributed to the saturating effect of additional rainfall on ecosystem photosynthesis. To better understand the influence of late summer monsoonal precipitation on ecosystem metabolic flashiness, we compared seasonal variation in GEPf and Ref. The effect of monsoonal storms on GEPf was plant community dependent such that shrubland GEPf strongly declined in response to rainfall whereas grassland and savanna GEPf was relatively unresponsive. We observed a similar pattern of plant community specific declines in Ref in response monsoonal precipitation. Conceptually similar to hydrologic flashiness, ecosystem metabolic flashiness may provide an additional lens through which to observe the influence of resource availability, shifts in community composition, and disturbance on ecosystem carbon cycling.