Slope position and water use by trees in a headwater catchment dominated by Japanese cypress: Implications for catchment‐scale transpiration estimates

AbstractWe assessed tree transpiration at three slope positions with different soil water conditions in a small Japanese cypress‐dominated catchment under a humid climate using the sap flow technique. Differences in canopy transpiration (EC) were explained by total sapwood area (AS_tree) of the stand rather than by sap flux density at the three slope positions. Although soil water storage nearly reached the lower limit of plant available water during the growing season at mid‐slope and ridge positions with shallow soils, we observed no clear reductions in EC. EC was maintained by canopy stomatal conductance (GC) in addition to large ratios of fine root to total root biomass in trees growing at mid‐slope and ridge positions. Conversely, in a riparian area characterized by high soil moisture throughout the growing season, EC was maintained by variations in GC and a lower ratio of fine root to total root biomass. Our results suggest that Japanese cypress trees adapted to variations in soil water conditions associated with different slope positions under a humid region, by short‐term stomatal regulation as well as long‐term morphological changes in traits such as AS_tree and fine root biomass. Because AS_tree was highly correlated with diameter at breast height (DBH), whole‐tree transpiration was also correlated with DBH across the three slope positions. Our results suggest that the distribution of DBH size classes in a catchment could be a useful parameter for estimating catchment‐scale transpiration irrespective of topographic gradients and associated variations in soil water conditions.