Summary Increased droughts impair tree growth worldwide. This study analyzes hydraulic and carbon traits of conifer species, and how they shape species strategies in terms of their growth rate and drought resilience. We measured 43 functional stem and leaf traits for 28 conifer species growing in a 50‐yr‐old common garden experiment in the Netherlands. We assessed: how drought‐ and carbon‐related traits are associated across species, how these traits affect stem growth and drought resilience, and how traits and drought resilience are related to species’ climatic origin. We found two trait spectra: a hydraulics spectrum reflecting a trade‐off between hydraulic and biomechanical safety vs hydraulic efficiency, and a leaf economics spectrum reflecting a trade‐off between tough, long‐lived tissues vs high carbon assimilation rate. Pit aperture size occupied a central position in the trait‐based network analysis and also increased stem growth. Drought recovery decreased with leaf lifespan. Conifer species with long‐lived leaves suffer from drought legacy effects, as drought‐damaged leaves cannot easily be replaced, limiting growth recovery after drought. Leaf lifespan, rather than hydraulic traits, can explain growth responses to a drier future.

Conifers face increased drought mortality risks because of drought-induced embolism in their vascular system. Variation in embolism resistance may result from species differences in pit structure and function, as pits control the air seeding between water transporting conduits. This study quantifies variation in embolism resistance and hydraulic conductivity for 28 conifer species grown in a 50-year-old common garden experiment and assesses the underlying mechanisms. Conifer species with a small pit aperture, high pit aperture resistance and large valve effect were more resistant to embolism, as they all may reduce air seeding. Surprisingly, hydraulic conductivity was only negatively correlated with tracheid cell wall thickness. Embolism resistance and its underlying pit traits related to pit size and sealing were stronger phylogenetically controlled than hydraulic conductivity and anatomical tracheid traits. Conifers differed in hydraulic safety and hydraulic efficiency, but there was no trade-off between safety and efficiency because they are driven by different xylem anatomical traits that are under different phylogenetic control.

Stem xylem‐specific hydraulic conductivity (KS) represents the potential for plant water transport normalized by xylem cross section, length, and driving force. Variation in KS has implications for plant transpiration and photosynthesis, growth and survival, and also the geographic distribution of species. Clarifying the global‐scale patterns of KS and its major drivers is needed to achieve a better understanding of how plants adapt to different environmental conditions, particularly under climate change scenarios. Here, we compiled a xylem hydraulics dataset with 1,186 species‐at‐site combinations (975 woody species representing 146 families, from 199 sites worldwide), and investigated how KS varied with climatic variables, plant functional types, and biomes. Growing‐season temperature and growing‐season precipitation drove global variation in KS independently. Both the mean and the variation in KS were highest in the warm and wet tropical regions, and lower in cold and dry regions, such as tundra and desert biomes. Our results suggest that future warming and redistribution of seasonal precipitation may have a significant impact on species functional diversity, and is likely to be particularly important in regions becoming warmer or drier, such as high latitudes. This highlights an important role for KS in predicting shifts in community composition in the face of climate change.

The long-term contribution of global forest restoration to support multiple dimensions of biodiversity and ecosystem function remains largely illusive across contrasting climates and forest types. This hampers the capacity to predict the future of forest rewilding under changing global climates. Here, 120 studies are synthesized across five continents, and it is found that forest restoration promotes multiple dimensions of biodiversity and ecosystem function such as soil fertility, plant biomass, microbial habitat, and carbon sequestration across contrasting climates and forest types. Based on global relationship between stand age and soil organic carbon stock, planting 350 million hectares of forest under the UN Bonn Challenge can sequester >30 Gt soil C in the surface 20 cm over the next century. However, these findings also indicate that predicted increases in temperature and reductions in precipitation can constrain the positive effects of forest rewilding on biodiversity and ecosystem function. Further, important tradeoffs are found in very old forests, with considerable disconnection between biodiversity and ecosystem function. Together, these findings provide evidence of the importance of the multidimensional rewilding of forests, suggesting that on-going climatic changes may dampen the expectations of the positive effects of forest restoration on biodiversity and ecosystem function.

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