Drivers and thresholds of carbon and water flux dynamics in a semi-humid urban forest ecosystem

Abstract

Urban forests play a critical role in regulating microclimates, sequestering carbon, and providing various ecosystem services. Due to urban land use changes and human activities, urban water and thermal environments have been significantly altered. This has made urban ecosystems more complex than natural ecosystems. However, our understanding of the magnitude, driving factors, and environmental response thresholds of carbon and water fluxes in urban forests remains limited. This study measured carbon and water fluxes in a semi-humid urban forest in China using eddy covariance technology over four consecutive growing seasons (May–October) from 2020 to 2023. Multi-year averages of gross primary production (GPP), evapotranspiration (ET), and water use efficiency (WUE) during the growing seasons were 1177.1 g C m⁻² yr⁻¹, 520.5 mm yr⁻¹, and 2.2 g C kg⁻¹ H₂O, respectively. Under non-drought conditions, canopy conductance (Gc), GPP, and ET were significantly (p < 0.05) higher than under drought conditions. Higher soil water content (SWC) partially alleviated the negative effects of high net radiation (Rn), air temperature (Ta), and vapor pressure deficit (VPD) on GPP and ET during droughts, although it was not the primary driver of their variability. Structural equation modeling revealed that under drought conditions, GPP was primarily regulated by atmospheric demand (e.g., VPD), whereas ET was primarily controlled by energy availability (e.g., Rn and Ta), with SWC exerting a positive influence on both GPP and ET. In contrast, under non-drought conditions, energy availability dominated the regulation of GPP and ET. Threshold analyses further revealed that GPP and ET responded nonlinearly to environmental drivers, initially increasing with Rn, Ta, and VPD but declining after reaching specific thresholds. These findings enhance our understanding of the mechanisms underlying carbon and water flux dynamics in urban forest ecosystems, particularly in the context of drying and warming conditions.