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Tropical montane evergreen broad-leaved forests cover the majority of forest areas and have high carbon storage in Xishuangbanna, southwest China. However, stem radial growth dynamics and their correlations with climate factors have never been analyzed in this forest type. By combining bi-weekly microcoring and high-resolution dendrometer measurements, we monitored xylogenesis and stem radius variations of the deciduous species Betula alnoides Buch.-Ham. ex D. Don and the evergreen species Schima wallichii (DC.) Korth. We analyzed the relationships between weekly climate variables prior to sampling and the enlarging zone width or wall-thickening zone width, as well as weekly radial increments and climate factors during two consecutive years (2020 to 2021) showing contrasting hydrothermal conditions in the pre-monsoon season. In the year 2020, which was characterized by a warmer and drier pre-monsoon season, the onset of xylogenesis and radial increments of B. alnoides and S. wallichii were delayed by three months and one month, respectively, compared with the year 2021. In 2020, xylem formation and radial increments were significantly reduced for B. alnoides , but not for S. wallichii. The thickness of enlarging zone and wall-thickening zone in S. wallichii were positively correlated with relative humidity, and minimum and mean air temperature, but were negatively correlated with vapor pressure deficit during 2020 to 2021. The radial increments of both species showed significant positive correlations with precipitation and relative humidity, and negative correlations with vapor pressure deficit and maximum air temperature during two years. Our findings reveal that drier pre-monsoon conditions strongly delay growth initiation and reduce stem radial growth, providing deep insights to understand tree growth and carbon sequestration potential in tropical forests under a predicted increase in frequent drought events. [ABSTRACT FROM AUTHOR]

Tropical forests are experiencing increases in vapour pressure deficit (D), with possible negative impacts on tree growth. Tree‐growth reduction due to rising D is commonly attributed to carbon limitation, thus overlooking the potentially important mechanism of D‐induced impairment of wood formation due to an increase in turgor limitation. Here we calibrate a mechanistic tree‐growth model to simulate turgor limitation of radial stem growth in mature Toona cilitata trees in an Asian tropical forest. Hourly sap flow and dendrometer measurements were collected to simulate turgor‐driven growth during the growing season. Simulated seasonal patterns of radial stem growth matched well with growth observations. Growth mainly occurred at night and its pre‐dawn build‐up appeared to be limited under higher D. Across seasons, the night‐time turgor pressure required for growth was negatively related to previous midday D, possibly due to a relatively high canopy conductance at high D, relative to stem rehydration. These findings provide the first evidence that tropical trees grow at night and that turgor pressure limits tree growth. We suggest including turgor limitation of tree stem growth in models also for tropical forest carbon dynamics, in particular, if these models simulate effects of warming and increased frequency of droughts. [ABSTRACT FROM AUTHOR]

Karst and non-karst forests occur in the same region in south-west China, but the soil water and mineral nutrients availability are different between the forests. Our hypothesis was that the leaves of karst trees would be better adapted to dry, nutrient-poor conditions than those of trees in a nearby non-karst forest. We compared the gas exchange, anatomical characteristics and mineral nutrient concentrations in leaves from 21 tree species in a tropical karst forest and 19 species in a nearby non-karst forest in south-west China. We found that the leaves of karst trees had higher P concentrations, photosynthetic capacity and water use efficiency, and greater adaxial and abaxial epidermis thickness than leaves of non-karst forest trees. Evergreen and deciduous trees differed more significantly in leaf functional traits in the karst forest than in the non-karst forest. The leaf palisade : spongy mesophyll thickness ratio was positively correlated with stomatal conductance and negatively correlated with photosynthetic water use efficiency in the karst forest but not in the non-karst forest. Our findings indicate that karst forest trees are more conservative in water use, whereas soil P deficiency could be a major limiting factor for the growth of non-karst forest trees. Karst habitats cover extensive areas of south-west China; however, we know little about the adaptive mechanisms of plants there. We compared leaf functional traits between a karst forest and a nearby non-karst forest and found that karst trees had more conservative water use and higher leaf P concentrations. Our results show that trees in the karst forest are structurally and physiologically acclimated to the dry, nutrient-rich soil. [ABSTRACT FROM AUTHOR]

• We studied stem growth and xylogenesis of Toona ciliata and their climate drivers. • Stem radial increment of T. ciliata mainly occurred during March to October. • T. ciliata trees at a wetter site showed higher rates and longer duration of growth. • Stem growth and xylogenesis of T. ciliata were mainly controlled by precipitation. • We highlights the strong influence of soil water capacity on tropical tree growth. Climate extremes will increasingly influence ecosystem productivity of tropical forests, but little is known about their effects on tree stem radial growth, a major component of forest productivity. To analyze the role of climatic drivers and local site differences on tree growth at sub-annual timescales, we studied the timing, magnitude and climatic drivers of stem growth and xylem formation at two tropical forest sites with contrasting soil water availability in southwestern China. We combined high-resolution point dendrometer recordings and microcoring to investigate intra-annual stem radius variation and xylem formation of Toona ciliata over three consecutive years (2018–2020). Stem radial increment of T. ciliata derived from dendrometer data showed distinct seasonal patterns, with growth mainly occurring during March to October. The start day of stem radial growth coincided with the occurrence of the cell enlarging phase, and its end day occurred concomitantly with the cessation of cell enlarging as identified from microcores. Toona ciliata trees at the tropical ravine rainforest site with higher soil water availability showed higher mean annual stem radial increment, higher daily growth rates, and longer duration of xylem productivity than that at the tropical karst forest site. Weekly stem radial growth rates increased with precipitation and air temperature at the wetter ravine rainforest site, while relative humidity had a positive effect at the drier karst forest site. Our findings indicate that drought stress advances and shortens the growing season for T. ciliata. Our study highlights the strong influence of soil water capacity mediating the impact of climate, so response to drought is site-specific, which should be considered in growth models. [Display omitted] [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]

• Exploring the hydraulic-mortality relationships helps to understand forest dynamics. • Tree species with high embolism resistance survive droughts and increase abundance. • Tree species from tropical wet and dry forests exhibit contrasting hydraulic strategies. • Vulnerability segmentation predicts tree mortality in tropical dry forest. Because of climate change, extreme droughts have become more frequent and caused widespread tree mortality, and thus significant shifts in community structure and function in many forests. Hydraulic failure is a major cause of drought-induced tree mortality. However, its effects on tree dynamics during repeated extreme droughts in tropical forests with different soil water availability are poorly understood. In this study, we surveyed the mortality rate and change in abundance (basal area) for 29 tree species from a humid tropical ravine rainforest (TRF) and an adjacent dry tropical karst forest (TKF) between 2004 and 2015. During this period, two extreme droughts occurred. We measured leaf and stem hydraulic-related traits and analyzed the trait–demography relationships. Our results showed that repeated extreme droughts induced an increase in tree mortality, and thus significantly affected the change of species abundance in the two forest communities. In both forests, the tree species with higher stem embolism resistance were more likely to survive extreme droughts, and increased in basal area during the census period. Moreover, the tree species in the two forests exhibited contrasting hydraulic strategies to deal with extreme drought. The lower mortality rate was associated with larger leaf and stem hydraulic safety margins in the TRF, but with more negative leaf hydraulic safety margin and stronger vulnerability segmentation (larger difference in embolism resistance between leaf and stem) in the TKF. This study expands our understanding of the hydraulic mechanisms to cope with extreme droughts in tropical tree species grown in different substrates under the present and future climatic conditions. [ABSTRACT FROM AUTHOR]