Interannual and Site-to-Site Variability in Biosphere-Atmosphere Water and Carbon Exchanges of the Southwest United States

How ecosystem vegetation shifts and climate change will alter biosphere-atmosphere exchanges of water vapor (H2O) and carbon dioxide (CO2) depends, in part, by how biotic activity is controlled by and alters water availability. To understand this issue, multiyear datasets are needed across multiple seasons and years with changing water availability, found in places like the semiarid southwest U.S. We analyzed multiyear (7 to 10 years) eddy covariance datasets collected across a network of semiarid Ameriflux core sites, consisting of a grassland, shrubland and savanna located in southern Arizona. We examined how precipitation (P) controlled evapotranspiration (ET) and net ecosystem production (NEP), and its component fluxes of ecosystem respiration (Reco) and gross ecosystem production (GEP). Although variability was high in seasonal to annual precipitation totals, we found strong overall relationships between P or ET and the eddy fluxes of NEP, Reco and GEP across the network. But we also observed several important site differences influenced by soils and plant functioning (grasses vs. woody plants) and prior seasonal “priming effects” on subsequent fluxes induced by extreme climatic events.