Nitrogen (N) or phosphorus (P) bioavailability constrain the magnitude of terrestrial carbon uptake and plant growth in tropical and subtropical regions, while carbon metabolism is coupled with plant hydraulic systems; therefore, nutrient availability changes are likely to exacerbate or alleviate drought-induced plant mortality. Most studies have neglected the hydraulic failure risk degree and are limited to single species fertilization experiments. We aimed to fill this gap by examining how variation in the relative P availability to N, as reflected by foliar N:P ratios, influences plant hydraulic failure risk degree across a broad range of species by collecting published data and conducting field experiments. The results demonstrated that increasing relative P availability to N would decrease the hydraulic safety margin (increase hydraulic failure risk) by regulating the leaf area: stem area ratio and changing vessel wall reinforcement. This study suggests that forests would potentially face increasing drought-induced mortality if relative P availability is increased through human activities including fertilization, while the mortality risk would decrease with a decrease in relative P availability due to heavy N deposition in tropical and subtropical regions. Relative P availability should be considered when assessing the forests vulnerability to global change.

N or P availability constrains plant growth in tropical and subtropical regions, and may influence plant hydraulic system that mediated forest vulnerability to drought. Although the effects of N/P availability on plant hydraulic traits have drawn much attention, most studies neglected the hydraulic failure threshold, and were limited to single species using fertilization experiments. This study aimed to fill this gap by examining how variation in the relative availability of N/P, as reflected by foliar N:P ratios, influences plant hydraulic failure threshold across a broad range of species by collecting published data and doing field experiments. The results demonstrated that increasing P availability would decrease the hydraulic safety margin (increase hydraulic failure risk) by regulating the leaf area: stem area ratio and vessel wall reinforcement. Forest drought-induced mortality was predicted to increase with increasing temperature; however, this study suggests N deposition-driven decreases in P availability would potentially alleviate this trend.

Water availability in karst forests is temporally and spatially heterogeneous; thus, xylem hydraulic efficiency, safety and water storage are potential drivers of tree growth. We selected 21 tree species from a primary subtropical karst forest in southern China and analyzed the contribution of xylem hydraulic efficiency, safety, and water storage to the growth of karst trees. The results showed that large vessel diameter, strong connectivity between vessels and axial parenchyma, and high saturated water content (SWC) led to high xylem hydraulic conductivity (Ks). SWC traded off against embolism resistance (P50). Ks had the strongest relationship with the average diametral growth rate (DGR). In large trees, both high Ks and more negative P50 were associated with high DGR. Through a growth-size relationship model, DGR acceleration was positively correlated with embolism-resistance across species. This study shows that xylem hydraulic efficiency and safety influenced growth rate and growth dynamics in karst tree species.

The importance of fog for the survival and growth of tropical cloud forests is receiving increasing interest in the context of climate change. Current knowledge on this topic is almost entirely based on studies at the species level, which can hardly reflect the response of the whole ecosystem to climate change. We evaluated community-level responses of trees and epiphytes to seasonal drought in Hainan tropical cloud forest ecosystem. We found that fog induced suppressed photosynthetic water demand and foliar water uptake help these forests escape from drought. We infer that reduction in fog due to future climate change may result in high plant mortality in these ecosystems, and artificially simulating fog may help save tropical cloud forests from vanishing worldwide.