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 (J_S) 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 J_S and microclimate (monitored since January 2014) derived from the whole-tree level stomatal conductance (G_S) were used to quantify the stomatal behavior (G_S sensitive to vapor pressure deficit, G_S-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 (J_S-mid) and daily mean (J_S-mean) sap flux rates did not change under all the nitrogen addition treatments (p > 0.05). A consistent decline in the G_S-VPD indicated an enhanced isohydric behavior for both species. In addition, the G_S-VPD in the wet season was much lower than that in the dry season. S. superba had a lower G_S-VPD and decreased J_S-mid/J_S-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 G_S for S. superba decreased and the G_S-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 J_S 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.

Please see the README document ("README.md.txt") and the accompanying published article: Zhao Z, Zhao P, Zhang Z, et al. Enhanced Isohydric Behavior Decoupled the Whole-Tree Sap Flux Response to Leaf Transpiration under Nitrogen Addition in a Subtropical Forest[J]. Forests, 2022, 13(11): 1847. DOI: 10.3390/f13111847

Please see the README document ("README.md.txt") and the accompanying published article: Zhao Z, Zhao P, Zhang Z, et al. Enhanced Isohydric Behavior Decoupled the Whole-Tree Sap Flux Response to Leaf Transpiration under Nitrogen Addition in a Subtropical Forest[J]. Forests, 2022, 13(11): 1847. DOI: 10.3390/f13111847