Canopy conductance regulation and water use strategies in poplar plantations in North China under dry versus wet years

Canopy conductance ( g c ) is a key physiological and ecohydrological parameter controlling canopy transpiration, stomatal regulation and vegetation–atmosphere carbon–water exchange. However, the extent to which the environmental regulation of g c pathways shifts between contrasting hydroclimatic conditions remains poorly understood. This study investigates the adaptive responses of g c to interannual variability in atmospheric moisture availability, aiming to clarify whether stomatal strategies undergo significant transitions under contrasting hydrological conditions. The study was conducted in a plantation of Populus × euramericana (Dode) Guinier cv.'Zhonglin-46' in Minquan County, Yellow River Basin, China. Sap flow was monitored using the thermal dissipation method, with supporting meteorological data collected from an instrumented flux tower, during the growing seasons of 2022 (dry year) and 2025 (wet year). We systematically compared the temporal dynamics of g c and its differential responses to environmental drivers at both daily and half-hourly scales. Analytical methods included Pearson correlation analysis to examine linear associations, random forest to evaluate the relative importance of environmental factors, and structural equation modeling (SEM) to disentangle direct and indirect effects. The results showed that during the dry year, the peak g c remained relatively low, with daily maxima occurring predominantly in the morning and declining rapidly following canopy senescence. In contrast, during the wet year, the peak g c increased by approximately 70%, with maxima shifting toward the afternoon and followed by a substantially slower decline. In the dry year, relative humidity (RH) was the dominant driver, accounting for 43.2% of explained variation. Stomatal behavior showed an “early-morning opening and midday closure” drought avoidance strategy, with both g c -VPD and g c -PAR hysteresis loops exhibiting clockwise patterns. In the wet year, the regulation shifted to a more open, radiation-driven strategy, with VPD and R s acting as the primary direct drivers, while RH effects were largely indirect through VPD. Stomatal behavior shifted to a radiation-dominated, more open strategy, with hysteresis loops turning counterclockwise. Furthermore, structural equation modeling revealed that RH transitioned from a direct regulatory role in the dry year (direct effects: 0.36–0.64) to an indirect role mediated through VPD in the wet year. Overall, interannual variation in atmospheric moisture availability significantly shifts both the response pathways of g c and stomatal regulation strategies. These findings provide a mechanistic basis for improving transpiration models and developing climate-responsive water management strategies for plantation forests. However, because soil moisture was not directly measured, these interpretations are limited to atmospheric drivers and require further validation.