Interannual variation in gross ecosystem production and evapotranspiration in a temperate semiarid grassland undergoing vegetation recovery

Abstract

Carbon gains (gross ecosystem production, GEP) and water losses (evapotranspiration, ET) are critical for assessing vegetation recovery potential in drylands. However, large uncertainties remain regarding the controlling factors that regulate GEP and ET in semiarid grasslands, especially during periods of vegetation re-establishment and succession. Using the eddy-covariance technique, we assessed the interannual variability and key biophysical controls of carbon (C) and water vapor (H2O) fluxes in a semiarid grassland ecosystem in northern China from 2012 to 2020. After years of being subjected to heavy grazing and with a recent ban, the shrub-to-herb ratio at the site increased from 0.5 in 2014 to 0.63 in 2020. Associated with decreasing soil water availability (∼4.0% year−1), canopy transpiration and light use efficiency also decreased at rates of about 3.3% and 6.2% year−1, respectively. Multi-year means of GEP and ET were 166.6 ± 20.1 g C m−2 (mean ± SD) and 190.2 ± 21.8 mm, respectively. Interannual variability in GEP was mostly driven by variation in summer GEP, which for the most part was limited by mid-summer air temperature (Ta), particularly when exceeding the plants’ optimal temperature for photosynthesis. Interannual variability in ET was predominantly caused by variation in spring ET, which was mostly affected by spring vapor pressure deficit (VPD) and associated reductions in canopy stomatal conductance. Synchronous changes in GEP and ET resulted in a relatively stable annual water use efficiency (WUE) of 0.97 ± 0.13 g C kg−1 H2O. This stability is indicative of the shrubs’ capabilities to acclimate to arid conditions. Our results suggest that temperature and atmospheric aridity have important roles in regulating C gains and H2O losses in recovering dryland vegetation, respectively. Future increases in climatic variability anticipated with global climate change may augment the regional transition from grass to shrub cover.