Carbon acquisition and transpiration water consumption strategies of trees in different canopy layers of a cold-temperate Betula platyphylla secondary forest

The dynamics of carbon acquisition and water consumption are critical to forest ecosystem stability. However, their roles within single-species canopy niches remain unclear. This study focuses on a cold-temperate secondary Betula platyphylla forest in the Greater Khingan Mountains. By integrating eddy covariance and sap flow measurements, trees were categorized into three canopy niches (canopy, subcanopy, and understory). This was used to quantify transpiration (T), estimated gross primary productivity (GPPm), carbon use efficiency (CUE), and regulatory mechanisms dominated by photosynthetically active radiation (PAR). The results showed that canopy trees contributed the largest proportions of total T (41.62%) and GPPm (50.84%), whereas subcanopy trees exhibited the highest CUE (0.40). PAR was the primary driver of both T and GPPm and modulated the GPPm response to vapor pressure deficit (VPD) in a canopy-dependent manner. The negative effect of VPD on GPPm strengthened with increasing PAR in canopy and subcanopy trees, whereas understory GPPm was significantly suppressed by VPD only under the lowest PAR conditions (<200 μmol m −2 s −1 ). A soil volumetric water content (VWC) threshold of approximately 24% was identified, below which VWC positively influenced T and GPPm and above which it exerted a negative effect, with the magnitude varying across canopy positions. In this study, we elucidate how the light environment mediates tree responses to VPD and VWC, thereby driving canopy niche–specific carbon–water trade-offs. These findings suggest that under drought conditions, prioritizing the retention of canopy trees, maintaining stand vertical structure, and considering soil moisture thresholds may enhance the climate resilience of cold-temperate B. platyphylla secondary forests.