Seasonal drought may alter N availability but not water use efficiency of dominant trees in a subtropical forest

Drought has had considerable effects on the ecological functions and processes of terrestrial ecosystems. However, little is known about the impacts of seasonal drought on the water use efficiency (WUE) and plant nitrogen (N) availability of dominant trees under field conditions. In this study, a field throughfall exclusion experiment simulating short-term drought (∼67% throughfall exclusion during the dry season from October to March) and prolonged drought (∼67% throughfall exclusion prolonging the dry season from October to May) was designed to understand the effects of seasonal drought on two dominant tree species (Michelia macclurei and Schima superba) in subtropical forest. The C and N content and isotopes in the deeper but not the surface soils in the forest were significantly affected by the seasonal drought. Neither the short-term nor the prolonged drought significantly altered δ13C values in the leaves and fine roots of the two species, but the short-term drought caused significant changes in δ15N values in the leaves and fine roots of S. superba. The WUE of the two dominant trees, indicated by the leaf δ13C values, was not significantly changed by the short-term and the prolonged drought. However, the N availability of S. superba, indicated by the δ15N values, was significantly altered by the seasonal short-term drought. Long-term influence of seasonal drought on the forest hydrological and N cycling and the potential shifts of species dominance induced by the changed N availability in subtropical forests should be assessed in the future research. Keywords: Nitrogen availability, Seasonal drought, Subtropical forest, Throughfall exclusion, Water use efficienc