PNNL

Abstract

Dryland ecosystems play a critical role in determining the trend and interannual variability of the global terrestrial carbon sink. The responses of such ecosystems to weather and environmental conditions remain as an important factor that limits accurate projections of carbon balance under future climate change. In this study, we investigate how shifts in vegetation phenology resulting from changes in weather and environmental conditions influences ecosystem carbon cycling at one semiarid ecosystem at the Hanford Area in central Washington, USA. We examined two years of eddy covariance, soil chamber, and phenology camera measurements from an upland sagebrush ecosystem primarily fed by precipitation without groundwater access. The two years had distinct diel and seasonal patterns of CO2 fluxes, driven by differences in vegetation phenology. While net ecosystem exchange of CO2 (NEE) and evapotranspiration (ET) in 2019 was strongly enlarged by shifted vegetation phenology due to a cold and snow-covered winter, warm and dry winter in 2020 resulted in constrained magnitudes of NEE and ET during the summer months. The annual gross primary productivity was much higher in 2019 than in 2020 (–211 vs. –112 gC m-2), whereas the ecosystem respiration was comparable in these two years (164 vs. 144 gC m-2). Thus, the annual NEE in 2019 was negative (–47 gC m-2) with the ecosystem functioning as a carbon sink, while the positive annual NEE in 2020 indicates that the ecosystem functioned as a carbon source. Our results demonstrated that winter snowpack can be a critical driver for annual carbon uptake in semiarid ecosystems.