Native shrubland and managed buffelgrass savanna in drylands: implications for ecosystem carbon and water fluxes.
Land cover and land-use change (LCLUC) between woody- and grass-dominated ecosystems in drylands comprise one of the largest uncertainties in the land CO2 sink. This is especially true for the widespread transition from shrublands to grasslands/savannas caused by the establishment of exotic C4 grass species for grazing or through biological invasion of these species, where information about its impacts on ecosystem CO2 fluxes is limited. For studying this, we used three years of eddy covariance measurements of net ecosystem production (NEP), gross primary production (GPP), ecosystem respiration (Reco) and evapotranspiration (ET) over a Sonoran Desert shrubland and an adjacent grazing savanna of buffelgrass (Cenchrus ciliaris L.), established 35 years ago. At monthly, seasonal and annual time scales, we assessed whether between-site differences in CO2 fluxes were related to differences in ecosystem water use, measured as the fraction ET to precipitation, water use efficiency (WUE, i.e. the ratio between GPP and ET) and/or to the relation between Reco and GPP. Although the savanna had higher WUE than the shrubland, its summer NEP was limited by water use, due to limitations in leaf area index and likely rooting patterns. Conversely, the savanna had higher NEP than the shrubland during fall to spring due to increased WUE; possibly due to activity of buffelgrass and remaining woody species using (summer) water from deeper soil layers. However, differences across these seasons compensated each other at the inter-annual scale, and both sites had comparable net carbon sinks over the three-year study period. Further studies are needed to understand and reduce the uncertainty associated with carbon and water fluxes associated with LCLUC in dryland ecosystems.