Climatic anomaly and its impact on vegetation phenology, carbon sequestration and water-use efficiency at a humid temperate forest

Changing climate, especially extreme weather event, is exerting considerable impacts on the hydrological and biogeochemical processes in forests worldwide. A deep understanding of climate change-terrestrial feedbacks is essential to predict future regional/global carbon and water budgets, which can be used to develop potential strategies for forest management. In this study, totally 11 years of eddy covariance tower measurements of CO2 and H2O fluxes, as well as the relevant environmental variables were analyzed to reveal the effects of climate anomalies on vegetation phenology, carbon sequestration and ecosystem water-use efficiency (WUE) dynamics in a humid temperate deciduous forest. Warmer spring temperatures altered the phenological phases with the green-up date advanced approximately 3.5 days per °C, and the extended growing season of about 3 days per °C, reaching the peak in 2012. Because of spring temperature anomaly, the shift from carbon source to sink occurred nearly 40 days earlier than usual. But the abnormal carbon dynamics happened during the peak growth period. Correlation analyses indicated that the amount of precipitation dominantly controlled the capacity of forest carbon sequestration (NEP) in this area. Therefore, the subsequent water scarcity owing to the extremely dry summer together with heatwave severely decreased the forest NEP by about 64.1%. Further analyses implied that the sharp reduction in gross primary production (GPP) rather than ecosystem respiration (Re) resulted to the decrease in NEP. Both GPP and evatranspiration (ET) were larger during the springtime in 2012 than those at adjacent years. But severe summer drought reduced the ecosystem WUE and yielded the lowest GPP and ET. Further work must focus on improving the recognition of forest feedback to climate systems.