Kailiang Yu’s research while affiliated with Princeton University and other places

Wind is an important ecological factor for plants as it can increase evapotranspiration and cause dehydration. However, the impact of wind on plant hydraulics at a global scale remains unclear. Here we compiled plant key hydraulic traits, including water potential at 50% loss of hydraulic conductivity (P50), xylem-specific hydraulic conductivity (KS), leaf area to sapwood area ratio (AL/AS) and conduit diameter (D) with 2,786 species-at-site combinations across 1,922 woody species at 469 sites worldwide and analysed their correlations with wind speed. Even with other climatic factors controlled (for example, moisture index, temperature and vapour pressure deficit), wind speed clearly affected plant hydraulics; for example, on average, species from windier sites constructed sapwood with smaller D and lower KS that was more resilient to drought (more negative P50), deploying less leaf total area for a given sapwood cross-section. Species with these traits may be at an advantage under future climates with higher wind speeds.

Anthropogenic nitrogen (N) deposition has increased N availability in forests close to human settlements, potentially causing N-limited forests to become N saturated, and influencing forest productivity and future climate. However, the global patterns of N-saturated forests have remained unclear, hindering effective N management. In N-saturated forests, organisms use N extravagantly, and a high proportion of the supplied N is lost in forms such as N2O emissions. Here, we used experimental N addition data to derive the sensitivity of soil N2O emissions to N deposition (sN). Using field observations of forest N status, the global patterns of N-saturated forests indicated by sN show an accuracy of 81%. Globally, 47.5% of forests are N saturated, especially tropical and temperate forests affected by human activity. The spatially explicit map of forest N status is useful for predicting forest greenhouse gas emissions and productivity and for implementing region-specific environmental management practices.

Ecosystem restoration is a critical nature-based solution to mitigate climate change. However, the carbon sequestration potential of restoration, defined as the maximum achievable carbon storage, has likely been overestimated because previous studies have not adequately accounted for the competition between ecosystem water demands for maximizing carbon sequestration and human water needs. Here we used a comprehensive process-based model combined with extensive land-use data and evaporation recycling accounting for land–atmosphere feedback to estimate the water requirements associated with ecosystem restoration. We found that achieving the carbon sequestration potential of restoration would significantly reduce global water availability per capita by 26%, posing considerable risks to water security in water-stressed and highly populated regions. If human water use is safeguarded, the achievable carbon sequestration potential would be reduced by a third (from 396 PgC to 270 PgC). Brazil, the United States and Russia have the largest achievable potentials. Future projections accounting for changes in climate, atmospheric CO2, land use and human population under the shared socioeconomic pathway (SSP) scenarios SSP119, SSP245 and SSP585 suggest an increase in this achievable potential to 274–302 PgC by the end of the century, with China expected to have the largest potential. Our findings provide a nuanced understanding of the trade-offs and synergies between carbon sequestration goals and water security, offering an empirical framework to guide the sustainable implementation of ecosystem restoration strategies.

Leaf area to sapwood area ratio ( A L / A S ) influences carbon sequestration, community composition, and ecosystem functioning in terrestrial vegetation and is closely related to leaf economics and hydraulics. However, critical predictors of A L / A S are not well understood. We compiled an A L / A S data set with 1612 species‐site combinations (1137 species from 285 sites worldwide) from our field experiments and published literature. We found the global mean A L / A S to be 0.63 m ² cm ⁻² , with its variation largely driven by growing‐season precipitation ( P gs ), which accounted for 18% of the variation in A L / A S . Species in tropical rainforests exhibited the highest A L / A S (0.82 m ² cm ⁻² ), whereas desert species showed the lowest A L / A S (0.16 m ² cm ⁻² ). Soil factors such as soil nitrogen and soil organic carbon exhibited positive effects on A L / A S , whereas soil pH was negatively correlated with A L / A S . Tree density accounted for 7% of the variation in A L / A S . All biotic and abiotic predictors collectively explained up to 45% of the variation in A L / A S . Additionally, A L / A S was positively correlated to the net primary productivity (NPP) of the ecosystem. Our study provides insights into the driving factors of A L / A S at the global scale and highlights the importance of A L / A S in ecosystem productivity. Given that P gs is the most critical driver of A L / A S , alterations in global precipitation belts, particularly seasonal precipitation, may induce changes in plant leaf area on the branches.

Nutrient addition, particularly nitrogen, often increases plant aboveground biomass but causes species loss. Asymmetric competition for light is frequently assumed to explain the biomass-driven species loss. However, it remains unclear whether other factors such as water can also play a role. Increased aboveground leaf area following nitrogen addition and warming may increase transpiration and cause water limitation, leading to a decline in diversity. To test this, we conducted field measurements in a grassland community exposed to nitrogen and water addition, and warming. We found that warming and/or nitrogen addition significantly increased aboveground biomass but reduced species richness. Water addition prevented species loss in either nitrogen-enriched or warmed treatments, while it partially mitigated species loss in the treatment exposed to increases in both temperature and nitrogen. These findings thus strongly suggest that water limitation can be an important driver of species loss as biomass increases after nitrogen addition and warming when soil moisture is limiting. This result is further supported by a meta-analysis of published studies across grasslands worldwide. Our study indicates that loss of grassland species richness in the future may be greatest under a scenario of increasing temperature and nitrogen deposition, but decreasing precipitation.

Canopy height is pivotal in sustaining carbon cycling and upholding ecological functions, especially in dryland forests where massive short-forests exist primarily due to insufficient water supply. Here, we divided global dryland forests into the tall-forests (36%) and short-forests (64%) and mapped their distributions separately for plantations and natural forests. Short-forests are ubiquitously distributed in global drylands, constituting 54% planted forests and 65% natural forests, with broader distribution thresholds across environmental gradients compared to tall-forests. Notably, the key ecological determinants of canopy height for both planted and natural short-forests are similar, involving topography (elevation), soil (soil moisture content), climate (mean temperature of warmest quarter and aridity index), and anthropogenic (population density) factors, but divergent between planted and natural tall-forests. The canopy height of planted tall-forests is predominantly influenced by precipitation, while natural tall-forests additionally depend on temperature, disturbance, and soil. Under all climate change scenarios, we projected that some dryland forests (more than 70%) cannot sustain current forest canopy heights, with a more pronounced decline in harsher climates, and some dryland tall forests may even degrade into short-forests or non-forests. With many dryland regions being marked as potential areas for forestation, our study offers critical insights for preserving dryland forests' carbon sequestration potential and guiding decision-making in dryland forestation initiatives.

It is well‐known that the mycorrhizal type of plants correlates with different modes of nutrient cycling and availability. However, the differences in drought tolerance between arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) plants remains poorly characterized. We synthesized a global dataset of four hydraulic traits associated with drought tolerance of 1457 woody species (1139 AM and 318 EcM species) at 308 field sites. We compared these traits between AM and EcM species, with evolutionary history (i.e. angiosperms vs gymnosperms), water availability (i.e. aridity index) and biomes considered as additional factors. Overall, we found that evolutionary history and biogeography influenced differences in hydraulic traits between mycorrhizal types. Specifically, we found that (1) AM angiosperms are less drought‐tolerant than EcM angiosperms in wet regions or biomes, but AM gymnosperms are more drought‐tolerant than EcM gymnosperms in dry regions or biomes, and (2) in both angiosperms and gymnosperms, variation in hydraulic traits as well as their sensitivity to water availability were higher in AM species than in EcM species. Our results suggest that global shifts in water availability (especially drought) may alter the biogeographic distribution and abundance of AM and EcM plants, with consequences for ecosystem element cycling and ultimately, the land carbon sink.

Woody plant encroachment (WPE), a global phenomenon documented across various biomes and continents, has the potential to significantly impact ecosystem carbon and water cycling. However, the impacts of WPE on carbon and water cycling in wetland ecosystems of middle and high latitudes are still lacking. In this study, the interannual and seasonal impacts of WPE on gross primary production (GPP) and evapotranspiration (ET), as well as their underlying mechanisms, within boreal wetland ecosystems located in middle-high latitude regions, were examined using remote sensing datasets spanning the period 2001–2016. Our results demonstrated that WPE enhanced annual GPP, ET, Normalized Difference Vegetation Index (NDVI), and solar-induced chlorophyll fluorescence (SIF) in boreal wetlands with impacts increasing over time. The multi-year average GPP and ET in fully encroached wetland ecosystems were approximately 31% and 3% higher, respectively, compared to pure wetland ecosystems. Prominent increases in wetland GPP occurred during the early growing season, while an exacerbation of ET was observed during the peak growing season. The impacts of WPE on wetland GPP and ET were predominantly attributed to increased vegetation greenness followed by secondary contributions from climate change. Climate change not only directly influenced the responses of GPP and ET to WPE but also exerted indirect effects by regulating vegetation greenness and the degree of encroachment. Our findings offer valuable insights into the understanding of the interaction between WPE and climate change, highlighting the importance of considering WPE effects and their drivers for accurate predictions of carbon and water cycles, as well as atmosphere–biosphere feedbacks in boreal wetlands.

An overlooked phenomenon is a potential increase in the distribution and abundance of plants with the highly water-usage-efficient crassulacean acid metabolism (CAM). In the present article, we critically analyze recent research to investigate to what extent and why CAM plants may have recently expanded their range and abundance under global change. We discuss the ecophysiological and evolutionary mechanisms linked with CAM succulence and the drivers underlying potential CAM expansion, including drought, warming, and atmospheric carbon dioxide enrichment. We further map the biogeographic pattern of CAM expansion and show that some CAM plants (e.g., Cylindropuntia, Opuntia, and Agave) are expanding and encroaching within dryland landscapes worldwide. Our results collectively highlight the recent expansion of CAM plants, a trend that could be sustained under increasing aridity with climate change. We recommend that CAM expansion be evaluated in a data-model integrated framework to better understand and predict the ecological and socioeconomic consequences of CAM expansion during the Anthropocene.