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• Leaf mass-based photosynthetic rate was more enhanced in evergreen trees with eCO2. • Branch hydraulic efficiency was more enhanced in deciduous trees with eCO2. • eCO2 strengthens the coordination of stem hydraulics with leaf gas exchange. Elevated CO 2 concentration (eCO 2) in the atmosphere is expected to impact plant water relations and growth in several ecosystems across the globe. However, we still know little about such impact on tree species in subtropical regions. The present study investigated the impact of eCO 2 on leaf gas exchange, nitrogen and phosphorus concentrations, leaf and stem hydraulic conductivity, and growth of seedlings of four evergreen and four deciduous tree species from a subtropical forest in Southwest China. We found that both evergreen and deciduous tree species at eCO 2 had higher leaf area-based photosynthetic rates and lower leaf stomatal conductance. Further, leaf mass-based photosynthetic rate was more enhanced in evergreen than in deciduous trees at eCO 2. Biomass of evergreen and deciduous species was significantly higher at eCO 2 , with large species-specific variation among the evergreen species. Leaf-specific hydraulic conductivity was more enhanced in deciduous tree species than that of evergreen tree species with eCO 2 , which was mainly driven by the increase of biomass at eCO 2. Interestingly, eCO 2 significantly strengthened the coordination of stem hydraulic conductivity with leaf-gas exchange, leaf phosphorus concentration, and plant biomass across evergreen and deciduous species. These results highlighted greater enhancement of photosynthesis and greater species-specific variation in biomass at eCO 2 for evergreen species compared to deciduous species, and stronger hydraulic-photosynthesis correlations at eCO 2 than at aCO 2 for tree species from subtropical forests. The present study provides important insights on the potential impacts of eCO 2 on plant eco-physiology, growth and forest succession in a subtropical forest under global climate change. [ABSTRACT FROM AUTHOR]

Many studies focus on the relationships between plant functional traits and tree growth performances. However, little is known about the ontogenetic shifts of the relationships between functional traits and tree growth. This study examined associations between stem and leaf functional traits and growth rates and their ontogenetic shifts across 20 tropical tree species in a tropical seasonal rainforest in Xishuangbanna, south-west China. For each species, physiological active branches of individual trees belonged to three size classes (i.e. small, diameter at breast height (DBH) 5–10 cm; middle, DBH 10–20 cm; big, DBH >20 cm) were sampled respectively. We measured 18 morphological and structural traits, which characterised plant hydraulic properties or leaf economic spectrum. Associations between diameter growth rates and functional traits were analysed across three size classes. Our results revealed that diameter growth rates of big-sized trees were mainly related to traits related to plant hydraulic efficiency (i.e. theoretical hydraulic conductivity (Ktheo) and leaf vein density (Dvein)), which suggests that the growth of large trees is limited mainly by their xylem water transport capacity. For middle-sized trees, growth rates were significantly related to traits representing leaf economic spectrum (i.e. specific leaf area (SLA), individual leaf mass (ILM), palisade thickness (PT) and spongy thickness (SP)). Diameter growth rates of small-sized trees were not correlated with hydraulic or leaf economic traits. Thus, the associations between tree growth rates and functional traits are size dependent. Our results suggest ontogenetic shift of functional traits which could potential contribute to different growth response to climate change. Plant functional traits can directly reflect plants performance under environmental. We show that traits growth relationships are strongly size-dependent in tropical rainforest in south-west China: diameter growth rates correlated significantly with leaf economic spectrum traits for middle sized trees, but for large sized trees hydraulic efficiency traits play a more important role on growth. This suggests ontogenetic shift of functional traits could potential contribute to different growth response to climate change. [ABSTRACT FROM AUTHOR]

• Tropical karst forests face increasing extreme droughts under climate change. • Hydraulic safety margins assess hydraulic risks and strategies. • Karst evergreen trees experience leaf hydraulic failure under extreme drought. • Brevi-deciduous trees and lianas maintain positive leaf hydraulic safety margin. • Strong vulnerability segmentation indicates low leaf hydraulic safety margin. Hydraulic safety margins (HSM) have been widely studied to assess the hydraulic risks and/or strategies of plant species under drought. However, calculations of HSM tend to vary between studies, leading to conflicting conclusions. In this study, HSM of 16 co-occurring woody species (including evergreen trees, brevi-deciduous trees, and lianas) in a tropical karst forest was investigated. They were expressed as: (1) the difference between minimum leaf water potential and water potential causing 50% loss of leaf hydraulic conductance (HSM leaf), (2) the difference between water potential at stomatal closure and that at 50% loss of branch hydraulic conductivity (HSM stomatal), and (3) the difference between water potential at 50% loss of leaf hydraulic conductance and that at 50% loss of branch hydraulic conductivity (HSM segmentation). We asked the following questions: were HSM leaf in the normal and extreme dry seasons across plant groups different? Were there correlations between different HSM -calculations? Results showed that, on average, the three plant groups showed similar and positive HSM leaf in the normal dry season, but evergreen trees declined to a more negative value in the extreme dry season while the other two groups remained positive. The massive loss of leaf hydraulic conductance in several evergreen tree species was consistent with their extensive shoot dieback under extreme droughts. Across species, there were no significant relationships between HSM stomatal and HSM leaf. Most species (mainly lianas and brevi-deciduous trees) showed negative HSM segmentation , which did not support the vulnerability segmentation hypothesis that branches are more resistant to cavitation than leaves. Moreover, more negative HSM segmentation tended to have lower HSM stomatal and larger HSM leaf in the extreme dry season. This study indicates that karst evergreen trees are more likely to experience leaf hydraulic failure under extreme droughts, and reveals potential correlations between branch and leaf hydraulic safety strategies. Further studies on HSM-demographic rate relationship in the tropical karst forests are recommended. [ABSTRACT FROM AUTHOR]