Impacts of climate and CO₂ changes on the vegetation growth and carbon balance of Qinghai–Tibetan grasslands over the past five decades

Climate change has significantly influenced global and regional terrestrial carbon balances. After being systematically calibrated against eddy-covariance measurements, meteorological observation, soil inventory data and satellite observed LAI (Leaf Area Index) in the Qinghai–Tibetan Plateau (Tan et al., 2010), the process-based ecosystem model called ORCHIDEE (ORganizing Carbon and Hydrology In Dynamic EcosystEms) was used in this study to investigate climate change and rising atmospheric CO₂ concentration driven spatio-temporal changes in vegetation net primary production (NPP) and net ecosystem production (NEP) of Qinghai–Tibetan grasslands from 1961 to 2009. Overall, our simulation suggests that Qinghai–Tibetan grassland NPP significantly increased with a rate of 1.9Tg Cyr⁻² (1Tg=10¹²g) since 1961. At the regional scale, change in precipitation, temperature, and atmospheric CO₂ concentration accounts for 52%, 34%, 39% of the increase in NPP, respectively, but their relative roles are not constant across the study area. Increase in NPP over the central and southwestern Qinghai–Tibetan Plateau is primarily attributed to precipitation changes, while rising atmospheric CO₂ concentration is the main cause of NPP increase in eastern plateau. The model simulation also suggests that Qinghai–Tibetan grassland NEP increased from a net carbon source of −0.5Tg Cyr⁻¹ in the 1960s to a net carbon sink of 21.8Tg Cyr⁻¹ in the 2000s, mainly due to the rising atmospheric CO₂ concentration and precipitation change. Although recent climate warming benefited vegetation growth, rising temperature did not significantly accelerate net carbon uptake from Qinghai–Tibetan grassland ecosystems due to enhanced soil carbon decomposition accompanying increase in temperature.