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 the 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 monitoring in January 2015, the mid-day (JS-mid) and daily mean (JS-mean) sap flux rates did not change under all the nitrogen addition treatments (p > 0.05). A consistent decline in the GS-VPD indicated an enhanced isohydric behavior for both species. In addition, the GS-VPD in the wet season was much lower than that in the dry season. S. superba had a lower GS-VPD and decreased JS-mid/JS-mean, implying a stronger stomatal control under the fertilization, which might be attributed to the low efficient diffuse-porous conduits and a higher JS. In addition, the GS for S. superba decreased and the GS-VPD increased more under CAN50 than that under CAN25, indicating that the high nitrogen dose restrains the extra nitrogen benefits. Our results indicated that the JS for both species was decoupled from the leaf transpiration for both species due to an enhanced isohydric behavior, and a xylem anatomy difference and fertilization dose would affect the extent of this decoupling relation.