Large lianas and trees in the forest canopy are challenged by hydraulic and mechanical failures and need to balance hydraulic conductivity, hydraulic safety and mechanical safety. Our study integrates these functions in canopy branches to understand the performance of canopy trees and lianas, and their difference. We sampled and measured branches from 22 species at a canopy crane in the tropical forest at Xishuangbanna, SW China. We quantified the hydraulic conductivity from the xylem-specific hydraulic conductivity (KS), hydraulic safety from the cavitation resistance (P50) and mechanical safety from the modulus of rupture (MOR) to evaluate trade-offs and differences between lianas and trees. We also measured a number of anatomical features that may influence these three functional traits. Our results suggest the following: trade-offs between hydraulic conductivity, hydraulic safety and mechanical safety are weak or absent; liana branches better resist external mechanical forces (higher MOR) than tree branches; and liana and tree branches were similar in hydraulic performance (KS and P50). The anatomical features underlying KS, P50 and MOR may differ between lianas and trees. We conclude that canopy branches of lianas and trees diverged in mechanical design due to fundamental differences in wood formation, but converged in hydraulic design.

Increasing drought pressure under anthropogenic climate change may jeopardize the potential of tropical forests to capture carbon in woody biomass and act as a long-term carbon dioxide sink. To evaluate this risk, we assessed drought impacts in 483 tree-ring chronologies from across the tropics and found an overall modest stem growth decline (2.5% with a 95% confidence interval of 2.2 to 2.7%) during the 10% driest years since 1930. Stem growth declines exceeded 10% in 25% of cases and were larger at hotter and drier sites and for gymnosperms compared with angiosperms. Growth declines generally did not outlast drought years and were partially mitigated by growth stimulation in wet years. Thus, pantropical forest carbon sequestration through stem growth has hitherto shown drought resilience that may, however, diminish under future climate change.

Extreme drought events are becoming frequent globally, resulting in widespread plant mortality and forest dieback. Although savanna vegetation cover ~20% of the earth’s land area, their responses to extreme drought have been less studied than that of forests. Herein, we quantified branch dieback, individual mortality and the associated physiological responses of four evergreen shrubs (Tarenna depauperate Hutch., Maytenus esquirolii (H. Lév.) C.Y. Cheng, Murraya exotica L., Jasminum nudiflorum Lindl.) in a savanna ecosystem in Southwest China to an incidence of extreme drought during 2019 and 2020. We found that 80–100% of the individuals of these species exhibited branch dieback, whereas individual mortality was only found in T. depauperate (4.5%). All species showed high resistance to stem embolism (P₅₀, water potential at 50% loss of hydraulic conductivity ranged from −5.62 to −8.6 MPa), whereas the stem minimum water potentials reached −7.6 to ca −10.0 MPa during the drought. The low water potential caused high native embolism levels (percentage loss of hydraulic conductivity (PLC) 23–65%) in terminal branches, and the remaining stems maintained 15–35% PLC at the end of the drought. Large within-individual variations in stem vulnerability to embolism were detected, and shedding of vulnerable branches could be a mechanism for shrubs to reduce water and carbon consumption. Overall, the content of total nonstructural carbohydrates (NSC) and their components in the stem were generally comparable to or higher than those in the rainy season in three of the four species. Because the leaves were turgor-less for most time during the drought, high NSC levels during the drought could be due to recycling of NSC from dead branches or translocation from roots. Our results suggest high tolerance of savanna shrub species to extreme drought, which could be facilitated by high embolism resistance in some stems and shedding of vulnerable branches to maintain individual water and carbon balance.