Dynamicity of Stand‐Scale Transpiration and Its Controlling Factors: Insights From Mixed‐Conifer Mediterranean Headwater Catchments

The mixed‐conifer forests of Sierra Nevada create a complex system of water, energy and nutrient cycling. Thus, this study aims to understand the spatial and temporal variability of soil moisture and stand‐scale transpiration across three central Sierra headwater catchments and their controlling factors. The mean daily average sap flow of Abies concolor varies from 0.69 ± 0.48 to 1.27 ± 0.72 L/h. Higher spatial variability in soil moisture and stand‐scale transpiration was found in the snowmelt period, followed by the soil moisture recession period when vegetation growth is not water limited. The daily average sap flow was higher on south aspect than on north aspect, but this pattern was not consistent across all seasons. The diel pattern of the sap flow revealed a larger contribution of nocturnal sap flow during the dry period (22.3%–37.8%) and a smaller contribution during the recession period (16.3%–23.9%) across all catchments. Multiple linear regression analysis suggests that sap flow during summer (recession period) was governed by the ecosystem's energy, indicated in part by vapour pressure deficit and temperature. Soil moisture regulated the subsequent dry‐period transpiration. The energy‐limited condition in winter restricted the transpiration even when sufficient soil moisture was available. Topography, explained by topographic positioning index ( tpi ), played a major role in explaining stand‐scale transpiration, especially during higher atmospheric demand periods, whereas tree size (i.e., DBH) explained more than 10% sap flow variance in all seasons. The findings of this study will provide the basis for a better understanding of the eco‐hydrological processes of the Sierra Nevada under natural and anthropogenic‐induced changes.