Environmental controls on carbon fluxes in an urban forest in the Megalopolis of Beijing, 2012-2020

Afforestation projects in urban areas have generally prospered in China. Although urban forests play an important role in sequestering carbon (C) at local to regional scales, understanding the current role of C-sink strength in urban forests is generally absent. We used continuous eddy-covariance and hydrometeorological data from 2012 to 2020 acquired from an urban forest reserve in Beijing to assess the interannual variability (IAV) in C-fluxes, as well as their key controlling factors. Over the study period, the urban forest was mostly a C-sink, maintaining a mean net ecosystem exchange (NEE) of -195 ± 98 g C m−2 yr−1. Interannual variability in NEE was largely due to variation in ecosystem respiration (Re). Interannual variation in annual gross ecosystem production (GEP) and Re were controlled to a large extent by summer soil water content (SWC). Since the effects of SWC on GEP and Re partially offset their individual responses, IAV in annual NEE was seen as being less influenced by summer SWC. Annual NEE was constrained by changes in summer air temperature (Ta). Residuals based on a Re-to-SWC regression exhibited significant decreases with increasing summer Ta, appreciably more than the residuals based on GEP-to-SWC regressions. This underscored the importance of summer Ta in impacting annual NEE through the suppression of Re, rather than through GEP. Our analysis also revealed that Re was more affected by drought than GEP. Long-lasting drought, greater than one month, was seen to impact GEP and Re more than short-term drought. Our study might imply that irrigation by elevating SWC was not be an effective strategy improving NEE of urban forest. Under anticipated climate change, drought and attendant high temperatures would impact C-fluxes more in the future. Land-surface models should adequately address the inhibitive characteristics of high temperatures on C-fluxes to improve simulations.