Dominance-caused differences in transpiration of trees in a Karst broadleaved mixed forest

Abstract

Estimating stand transpiration of natural forests using traditional methods through up-scaling of sap flux density from sample trees based on stand sapwood area only is difficult because of the complexity of species, ages, and hierarchical structure of natural forests. To improve stand transpiration estimation, we developed an up-scaling method by considering the tree dominance effect based on the assumption that individual tree transpiration is affected by crown dominance and species, in addition to factors previously considered such as meteorological conditions, sapwood area, and soil moisture. In this study, the meteorological factors, soil moisture, and sap flux density of 15 sample trees of different species and dominance in a natural evergreen and deciduous broadleaved mixed forest were simultaneously monitored from March 2012 to February 2014 in the Karst mountain region in southwestern China. After establishing a single tree transpiration model which considers the effects of dominance and species, an up-scaling method was explored to estimate stand transpiration. The results show that the transpiration intensity increased exponentially with increasing tree dominance. The contribution to annual stand transpiration from a few dominant trees (5.4% of trees, 28.2% of basal area) was up to 65.0%. The corresponding contribution was 16.2% from sub-dominant trees (7.6% of trees, 16.2% of basal area) and 22.8% from middle- and lower-layer trees (87.0% of trees, 55.6% of basal area). The variation of individual tree transpiration was mainly (97.9%) explained by tree dominance, but very weakly by tree species. The estimated annual stand transpiration was 300.2 mm when using the newly developed method which considers tree dominance, 52.5 mm (14.9%) lower than the estimation (352.7 mm) of traditional method which considers only the sapwood area effect, and 8.5 mm (2.7%) lower than the estimation (308.6 mm) which considers the effects of both species composition and sapwood area. The main tree characteristics affecting stand transpiration are tree size (sapwood area) and dominance. Consideration of tree dominance will significantly improve stand transpiration estimation and provide a more solid basis for guiding integrated forest–water management at stand scale.