Experimental evidence for weakened tree nutrient use and resorption efficiencies under severe drought in a subtropical monsoon forest

Aims: The functions of global forests are threatened by the increasing frequency of severe drought. Due to drought inducing reductions in soil nutrient availability, efficiencies of nutrient use and resorption of trees become crucial for forest functions and biogeochemical cycles. However, understanding the dynamics of responses of foliar nutrient use and resorption efficiencies to drought, especially in tropical or subtropical forests, is still limited. Our goal was to detect whether and how the importance of leaf nutrient use and resorption changes across different species in the hot and wet forests when suffering drought stress in different months. Methods: Based on a 70% throughfall exclusion experiment in a subtropical forest, we collected green and senesced leaves of Schima superba and Lithocarpus glaber in different months from October 2016 to May 2019, to estimate the effects of drought on leaf nitrogen (N) and phosphorus (P) use and resorption efficiencies (i.e. NUE and PUE, NRE and PRE). Important Findings: The effects of drought on nutrient use and resorption efficiencies varied between species and months. Based on a 2-year observation, drought had no effect on S. superba, but significantly decreased NUE, NRE and PRE of L. glaber by 3.4%, 20.2% and 7.1%, respectively. Furthermore, the negative drought effects were aggravated by the natural summer drying in 2017. As a result, NUE and PUE of L. glaber were significantly depressed by 17.2% and 58.1%, while NRE and PRE were significantly reduced by 56.5% and 53.8% in August 2017. Moreover, the responses of NRE, PRE and NUE to drought were related with soil moisture (SM) for L. glaber, and when SM decreased to a threshold near 9 v/v%, drought effects were shifted from unresponsive to negative. Our results highlight a species-specific threshold response of nutrient use under drought in a subtropical forest.