Enhanced isohydric behavior decoupled the whole-tree sap flux response to leaf transpiration under nitrogen addition in a subtropical forest

Anthropogenic nitrogen deposition has the potential to change the leaf water-use strategy in the subtropical region of China. Nevertheless, the whole-tree level response crucial for ecosystem functions has not been well addressed over the past decades. In this study, the stem sap flux density (JS) was monitored for the whole-tree water transport capacity in two dominant species (Schima superba and Castanopsis chinensis) in a subtropical forest. To simulate the increased nitrogen deposition, the NH4NO3 solutions were sprayed onto the forest canopy at 25 kg ha-1 year-1 (CAN25) and 50 kg ha-1 year-1 (CAN50), respectively, since April 2013. The JS and microclimate (monitored since January 2014) derived from the whole-tree level stomatal conductance (GS) were used to quantify the stomatal behavior (GS sensitive to vapor pressure deficit, GS-VPD) in response to the added nitrogen. The maximum shoot hydraulic conductance (Kshoot-max) was also measured for both species. After one year of monito...