Abstract:

Objective: This study aims to reveal the response patterns of stand daily transpiration to canopy structure and environmental factors and their differences across slope positions, so as to facilitate precise coordinated forest-water management at the slope-scale. Method: Three fixed plots were established at three slope positions (upper, middle, and lower slopes) on a typical hillslope fully covered by Larix gmelinii var. principis-rupprechtii plantations in the semi-humid area of Liupan Mountains, Ningxia. During the growing season (from June to October) in 2023, the sap flow of the tree trunk was monitored, and the daily stand transpiration was calculated. Simultaneously, the volumetric soil moisture (VSM) in the 0–80 cm soil layer, the effective leaf area index (LAI_e) of forest canopy, and the potential evapotranspiration (PET) outside forest were monitored. Subsequently, the response function relationship was determined based on the outer envelope of the scatter plot between daily stand transpiration and every single factor for each slope position. A daily stand transpiration model reflecting the influence of multiple factors for each slope position was established through multiplicative coupling. The model was calibrated and validated using measured data. The slope-position differences in the response of daily stand transpiration to each dominant factor were quantitatively analyzed. The stand daily transpiration changes under climate change scenarios were simulated, and the LAI_e regulation schemes required to maintain a target transpiration level were determined. Result: 1) The average daily transpiration of stands during the study period exhibited a clear difference among slope-positions, i.e., it at the middle slope (0.85 mm·d^?1) was not significantly higher than that at the upper slope (0.72 mm·d^?1), but significantly (P < 0.05) higher than that at the lower slope (0.67 mm·d^?1). 2) The response of daily stand transpiration to key factors includes a quadratic function that first increases and then decreases with rising PET, and an exponential function that firstly increases with rising VSM and LAI_e but later gradually levels-off. 3) A daily stand transpiration coupling model that simultaneously accounts for the effects of PET, VSM, and LAI_e across all slope-position plots explains approximately 85% (78%–89%) of the variation in daily stand transpiration, effectively reflecting the combined influence of multiple factors. 4) Model-based simulation analyses indicate that there are slope position differences in change slope (or sensitivity) of daily stand transpiration with varying key factors, and PET shows middle slope > upper slope > lower slope, VSM shows middle slope > upper slope > lower slope, and LAI_e shows middle slope > lower slope > upper slope. Under a scenario where future climate change causes a 15% increase in PET, the reduction of forest transpiration to its original level to mitigate ecological water consumption would require an slope position specific reduction of LAI_e (27.4% at upper slope, 25.8% at middle slope, and 16.6% at lower slope). Conclusion: There are differences in the stand daily transpiration and its response sensitivity to major driving factors among slope positions in L. gmelinii var. principis-rupprechtii plantations. Therefore, it is necessary to implement precise forest-water coordinated management with slope position differences.

Key words: Larix gmelinii var. principis-rupprechtii plantation hillslope, stand transpiration, potential evapotranspiration, volumetric soil moisture, canopy effective leaf area index