Ecological differentiation in xylem cavitation resistance is associated with stem and leaf structural traits

Plant size traits are key contributors in the spatial variation of net primary productivity across terrestrial biomes in China

Understanding the spatial variability of ecosystem functions is an important step forward in predicting changes in ecosystems under global transformations. Plant functional traits are important drivers of ecosystem functions such as net primary productivity (NPP). Although trait-based approaches have advanced rapidly, the extent to which specific plant functional traits are linked to the spatial diversity of NPP at a regional scale remains uncertain. Here, we used structural equation models (SEMs) to disentangle the relative effects of abiotic variables (i.e., climate, soil, nitrogen deposition, and human footprint) and biotic variables (i.e., plant functional traits and community structure) on the spatial variation of NPP across China and its eight biomes. Additionally, we investigated the indirect influence of climate and soil on the spatial variation of NPP by directly affecting plant functional traits. Abiotic and biotic variables collectively explained 62.6 % of the spatial differences of NPP within China, and 28.0 %-69.4 % across the eight distinct biomes. The most important abiotic factors, temperature and precipitation, had positive effects for NPP spatial variation. Interestingly, plant functional traits associated with the size of plant organs (i.e., plant height, leaf area, seed mass, and wood density) were the primary biotic drivers, and their positive effects were independent of biome type. Incorporating plant functional traits improved predictions of NPP by 6.7 %-50.2 %, except for the alpine tundra on the Qinghai-Tibet Plateau. Our study identifies the principal factors regulating NPP spatial variation and highlights the importance of plant size traits in predictions of NPP variation at a large scale. These results provide new insights for involving plant size traits in carbon process models.

Survival, growth, and functional traits of tropical wet forest tree seedlings across an experimental soil moisture gradient in Puerto Rico

Droughts are predicted to become more frequent and intense in many tropical regions, which may cause shifts in plant community composition. Especially in diverse tropical communities, understanding how traits mediate demographic responses to drought can help provide insight into the effects of climate change on these ecosystems. To understand tropical tree responses to reduced soil moisture, we grew seedlings of eight species across an experimental soil moisture gradient at the Luquillo Experimental Forest, Puerto Rico. We quantified survival and growth over an 8-month period and characterized demographic responses in terms of tolerance to low soil moisture-defined as survival and growth rates under low soil moisture conditions-and sensitivity to variation in soil moisture-defined as more pronounced changes in demographic rates across the observed range of soil moisture. We then compared demographic responses with interspecific variation in a suite of 11 (root, stem, and leaf) functional traits, measured on individuals that survived the experiment. Lower soil moisture was associated with reduced survival and growth but traits mediated species-specific responses. Species with relatively conservative traits (e.g., high leaf mass per area), had higher survival at low soil moisture whereas species with more extensive root systems were more sensitive to soil moisture, in that they exhibited more pronounced changes in growth across the experimental soil moisture gradient. Our results suggest that increasing drought will favor species with more conservative traits that confer greater survival in low soil moisture conditions.

Water storage capacity is inversely associated with xylem embolism resistance in tropical karst tree species

Tropical karst habitats are characterized by limited and patchy soil, large rocky outcrops and porous substrates, resulting in high habitat heterogeneity and soil moisture fluctuations. Xylem hydraulic efficiency and safety can determine the drought adaptation and spatial distribution of woody plants growing in karst environments. In this study, we measured sapwood-specific hydraulic conductivity (Ks), vulnerability to embolism, wood density, saturated water content, and vessel and pit anatomical characteristics in the branch stems of 12 evergreen tree species in a tropical karst seasonal rainforest in southwestern China. We aimed to characterize the effects of structural characteristics on hydraulic efficiency and safety. Our results showed that there was no significant correlation between Ks and hydraulic safety across the tropical karst woody species. Ks was correlated with hydraulic vessel diameter (r = 0.80, P < 0.05) and vessel density (r = -0.60, P < 0.05), while the stem water potential at 50 and 88% loss of hydraulic conductivity (P50 and P88) were both significantly correlated with wood density (P < 0.05) and saturated water content (P = 0.052 and P < 0.05, respectively). High stem water storage capacity was associated with low cavitation resistance possibly because of its buffering the moisture fluctuations in karst environments. However, both Ks and P50/P88 were decoupled from the anatomical traits of pit and pit membranes. This may explain the lack of tradeoff between hydraulic safety and efficiency in tropical karst evergreen tree species. Our results suggest that diverse hydraulic trait combination may facilitate species coexistence in karst environments with high spatial heterogeneity.

Contrasting water-use strategies revealed by species-specific transpiration dynamics in the Caatinga dry forest

In forest ecosystems, transpiration (T) patterns are important for quantifying water and carbon fluxes and are major factors in predicting ecosystem change. Seasonal changes in rainfall and soil water content can alter the sensitivity of sap flux density to daily variations in vapor pressure deficit (VPD). This sensitivity is species-specific and is thought to be related to hydraulic strategies. The aim of this work is to better understand how the sap flux density of species with low versus high wood density differ in their sensitivity to VPD and soil water content and how potentially opposing water-use strategies influence T dynamics, and ultimately, correlations to evapotranspiration (ET). We use hysteresis area analysis to quantify the sensitivity of species-specific sap flux density to changes in the VPD, breakpoint-based models to determine the soil water content threshold instigating a T response and multiscalar wavelet coherency to correlate T to ET. We found that low wood density Commiphora leptophloeos (Mart.) Gillett had a more dynamic T pattern, a greater sensitivity to VPD at high soil water content, required a higher soil water content threshold for this sensitivity to be apparent, and had a significant coherency correlation with ET at daily to monthly timescales. This behavior is consistent with a drought avoidance strategy. High wood density Cenostigma pyramidale  (Tul.)  E. Gagnon & G. P. Lewis, conversely, had a more stable T pattern, responded to VPD across a range of soil water content, tolerated a lower soil water content threshold to T, and had a significant coherency correlation with ET at weekly timescales. This behavior is consistent with a drought-tolerant strategy. We build on previous research to show that these species have contrasting water-use strategies that should be considered in large-scale modeling efforts.

Variations in the plasticity of functional traits indicate the differential impacts of abiotic and biotic factors on the structure and growth of trees in tropical dry forest fragments

Abiotic and biotic factors have considerable impact on the plasticity of plant functional traits, which influences forest structure and productivity; however, their inter-relationships have not been quantified for fragmented tropical dry forest (TDF) ecosystems. We asked the following questions: (1) what are the variations in the plasticity of functional traits due to soil moisture availability in TDF fragments? (2) what are the roles of soil nutrients and forest disturbances in influencing variations in the plasticity of functional traits in the TDF fragments? and (3) how do the variations in the plasticity of functional traits influence the structure and productivity of TDF fragments? Based on linear mixed-effects results, we observed significant variations among tree species for soil moisture content (SMC) under the canopy and selected functional traits across forest fragments. We categorized tree species across fragments by principal component analysis (PCA) and hierarchical clustering on principal components (HCPC) analyses into three functional types, viz., low wood density high deciduous (LWHD), high wood density medium deciduous (HWMD), and high wood density low deciduous (HWLD). Assemblage of functional traits suggested that the LWHD functional type exhibits a drought-avoiding strategy, whereas HWMD and HWLD adopt a drought-tolerant strategy. Our study showed that the variations in functional trait plasticity and the structural attributes of trees in the three functional types exhibit contrasting affinity with SMC, soil nutrients, and disturbances, although the LWHD functional type was comparatively more influenced by soil resources and disturbances compared to HWMD and HWLD along the declining SMC and edge distance gradients. Plasticity in functional traits for the LWHD functional type exhibited greater variations in traits associated with the conservation of water and resources, whereas for HWMD and HWLD, the traits exhibiting greater plasticity were linked with higher productivity and water transport. The cumulative influence of SMC, disturbances, and functional trait variations was also visible in the relative abundance of functional types in large and small sized fragments. Our analysis further revealed the critical differences in the responses of functional trait plasticity of the coexisting tree species in TDF, which suggests that important deciduous endemic species with drought-avoiding strategies might be prone to strategic exclusion under expected rises in anthropogenic disturbances, habitat fragmentation, and resource limitations.

Woody species with higher hydraulic efficiency or lower photosynthetic capacity discriminate more against 13C at the global scale

Leaf carbon isotope composition (δ13C) provides an integrative record on the carbon and water balance of plants over long periods. Photosynthetic ability and hydraulic traits which are highly associated with stomatal behavior could affect leaf δ13C. Association between photosynthetic ability and leaf δ13C has been examined, however, how hydraulic traits influence leaf δ13C has not been fully understood. To fill this gap, we investigated the variations in leaf δ13C among 2591 woody species (547 shrub and 2044 tree species), and analyzed the link of leaf δ13C with leaf photosynthetic and xylem hydraulic traits. Our result showed that leaf δ13C was positively correlated to leaf photosynthetic ability and capacity. For hydraulic traits, leaf δ13C was negatively related to hydraulic conductivity (Ks), xylem pressure inducing 50 % loss of hydraulic conductivity (P50) and vessel diameter (Vdia). Associations of leaf δ13C with xylem hydraulic traits indicate woody species with stronger hydraulic safety discriminated less against 13C, while woody species with higher hydraulic efficiency had more negative leaf δ13C. Shrub species, which showed a lower Vdia and P50, had a significant less negative leaf δ13C than tree species. Furthermore, woody species inhabiting in dry regions discriminated less against 13C than those growing in humid regions. Moreover, leaf δ13C displayed a low phylogenetic signal based on Blomberg's K statistic. Overall, woody species with a higher leaf photosynthetic ability or stronger hydraulic safety system discriminated less against 13C and adopt the provident water use strategy.

Water-use strategies and functional traits explain divergent linkages in physiological responses to simulated precipitation change

As a part of global climate change, precipitation patterns in arid regions will change significantly, and the different responses of desert plants to these changes will lead to alterations in community composition, thereby impacting ecosystem stability. Thus, understanding the mechanism underlying the associations among physiological response variables considering changing precipitation is crucial. Here, water-use strategies, functional traits, and physiological processes (e.g., photosynthesis (An), transpiration (Tr), leaf water potential (Ψl), stomatal conductance (gs), and soil respiration (Rs)) were measured in a precipitation experiment with two coexisting desert riparian species to determine how water-use strategies and functional traits operate together in generating physiological response mechanisms. The results showed that the two species exhibited divergent response pathways of physiological processes following rainfall events, although both were identified as isohydric plants with stringent stomatal regulation. For the shallow-rooted species N. sphaerocarpa, gs was sensitive to changes in both surface soil moisture (Swc) and Ψl, and Swc was the primary factor influencing Rs. These results were supported by the preference for shallow water and predominance of functional traits associated with drought avoidance. For the deep-rooted species R. soongorica, variations in gs were decoupled from Swc and directly influenced by enhanced Ψl, An was the main factor affecting Rs, while Ψl negatively affected Rs. These correlations could be attributed to the preference for deep water and functional traits associated with drought tolerance. These findings suggest that R. soongorica had a stronger tolerance to environmental water deficits and may expand extensively under drier climatic conditions in the future.

Petiole XLA (xylem to leaf area ratio) integrates hydraulic safety and efficiency across a diverse group of eucalypt leaves

A theoretical trade-off between the efficiency and safety of water transport systems in plants is used to explain diverse ecological patterns, from tree size to community structure. Despite its pervasive influence, this theory has marginal empirical support. This may be partially due to obfuscation of associations by wide phylogenetic sampling or non-standard sampling between studies. To address this, we examine the coordination of structural and anatomical traits linked to hydraulic safety and efficiency in the leaves of an ecologically diverse group of eucalypts. We introduce a new trait for characterising leaf water transport function measured as the cross-sectional XA at the petiole divided by the downstream leaf area (XLApetiole ). Variation in XLApetiole revealed support for a safety-efficiency trade-off in eucalypt leaves. XLApetiole was negatively correlated with theoretical petiole xylem conductivity (Ks_petiole ) and strongly negatively correlated with leaf cavitation vulnerability (Ψ50leaf ). Species with lower Ψ50leaf exhibited petiole xylem with narrower vessels and greater fibre wall area fractions. Our findings highlight XLApetiole as a novel integrative trait that provides insights into the evolution of leaf form and function in eucalypts and holds promise for wider use among diverse species.

Intraspecific variations of leaf hydraulic, economic, and anatomical traits in Cinnamomum camphora along an urban-rural gradient

Urbanization brings numerous benefits to residents, but it also introduces complex, variable, and heterogeneous habitat conditions to urban plants, resulting in an arid and hot urban environment that decreases tree growth and the ecological service capacity of trees. In this study, we evaluated leaf hydraulic, economic, and anatomical traits and their covariations of Cinnamomum camphora along an urban-rural gradient in Hefei, Eastern China. We found that Cinnamomum camphora in urban adopted a conservative hydraulic strategy with low leaf turgor loss point (Tlp), leaf hydraulic conductance (Kleaf), and leaf water potential resulting in 50 % loss of hydraulic conductance (P50), as well as a quick investment-return economic strategy with low unit leaf dry matter content (LMA) and high leaf nitrogen content (Leaf N). P50, Kleaf and LMA were significantly positively correlated with the urban-rural gradient (PC1urban-rural gradient), while Leaf N exhibited a negative correlation with it. The results showed a trade-off between intraspecific safety and efficiency in leaf hydraulic traits along the urban-rural gradient and an intraspecific coordinated variation in leaf hydraulic and economic traits. In addition, based on the analysis of a trait coordination network, it was revealed that leaf mesophyll and stomata were key structures for trait adjustment and coordination. Furthermore, our findings offer a significant theoretical underpinning for the effective management of landscape trees and the strategic planning of urban tree species.

Plasticity of wood and leaf traits related to hydraulic efficiency and safety is linked to evaporative demand and not soil moisture in rubber (Hevea brasiliensis)

The predicted increase of drought intensity in South-East Asia has raised concern about the sustainability of rubber (Hevea brasiliensis Müll. Arg.) cultivation. In order to quantify the degree of phenotypic plasticity in this important tree crop species, we analysed a set of wood and leaf traits related to the hydraulic safety and efficiency in PB260 clones from eight small-holder plantations in Jambi province, Indonesia, representing a gradient in local microclimatic and edaphic conditions. Across plots, branch embolism resistance (P50) ranged from -2.14 to -2.58 MPa. The P50 and P88 values declined, and the hydraulic safety margin increased, with an increase in the mean annual vapour pressure deficit (VPD). Among leaf traits, only the changes in specific leaf area were related to the differences in evaporative demand. These variations of hydraulic trait values were not related to soil moisture levels. We did not find a trade-off between hydraulic safety and efficiency, but vessel density (VD) emerged as a major trait associated with both safety and efficiency. The VD, and not vessel diameter, was closely related to P50 and P88 as well as to specific hydraulic conductivity, the lumen-to-sapwood area ratio and the vessel grouping index. In conclusion, our results demonstrate some degree of phenotypic plasticity in wood traits related to hydraulic safety in this tropical tree species, but this is only in response to the local changes in evaporative demand and not soil moisture. Given that VPD may increasingly limit plant growth in a warmer world, our results provide evidence of hydraulic trait changes in response to a rising evaporative demand.

Upscaling the effect of traits in response to drought: The relative importance of safety-efficiency and acquisitive-conservation functional axes

We tested the idea that functional trade-offs that underlie species tolerance to drought-driven shifts in community composition via their effects on demographic processes and subsequently on shifts in species' abundance. Using data from 298 tree species from tropical dry forests during the extreme ENSO-2015, we scaled-up the effects of trait trade-offs from individuals to communities. Conservative wood and leaf traits favoured slow tree growth, increased tree survival and positively impacted species abundance and dominance at the community-level. Safe hydraulic traits, on the other hand, were related to demography but did not affect species abundance and communities. The persistent effects of the conservative-acquisitive trade-off across organizational levels is promising for generalization and predictability of tree communities. However, the safety-efficient trade-off showed more intricate effects on performance. Our results demonstrated the complex pathways in which traits scale up to communities, highlighting the importance of considering a wide range of traits and performance processes.

Changes in mass allocation play a more prominent role than morphology in resource acquisition of the rhizomatous Leymus chinensis under drought stress

BACKGROUND AND AIMS

Plants can respond to drought by changing their relative investments in the biomass and morphology of each organ. The aims of this study were to quantify the relative contribution of changes in morphology vs. allocation and determine how they affect each other. These results should help us understand the mechanisms that plants use to respond to drought events.

METHODS

In a glasshouse experiment, we applied a drought treatment (well-watered vs. drought) at early and late stages of plant growth, leading to four treatment combinations (well-watered in both early and late periods, WW; drought in the early period and well-watered in the late period, DW; well-watered in the early period and drought in the late period, WD; drought in both early and late periods, DD). We used the variance partitioning method to compare the contribution of organ (leaf and root) biomass allocation and morphology to the leaf area ratio, root length ratio and root area ratio, for the rhizomatous grass Leymus chinensis (Trin.) Tzvelev.

KEY RESULTS

Compared with the continuously well-watered treatment, the leaf area ratio, root length ratio and root area ratio showed increasing trends under various drought treatments. The contribution of leaf mass allocation to leaf area ratio differed among the drought treatments and was 2.1- to 5.3-fold greater than leaf morphology, and the contribution of root mass allocation to root length ratio was ~2-fold greater than that of root morphology. In contrast, root morphology contributed more to the root area ratio than biomass allocation under drought in both the early and late periods. There was a negative correlation between the ratio of leaf mass fraction to root mass fraction and the ratio of specific leaf area to specific root length (or specific root area).

CONCLUSIONS

This study suggested that organ biomass allocation drove a larger proportion of variation than morphological traits for the absorption of resources in this rhizomatous grass. These findings should help us understand the adaptive mechanisms of plants when they are confronted with drought stress.

Water-limited environments affect the association between functional diversity and forest productivity

The link between biodiversity and ecosystem function can depend on environmental conditions. This contingency can impede our ability to predict how biodiversity-ecosystem function (BEF) relationships will respond to future environmental change, causing a clear need to explore the processes underlying shifts in BEF relationships across large spatial scales and broad environmental gradients. We compiled a dataset on five functional traits (maximum height, wood density, specific leaf area [SLA], seed size, and xylem vulnerability to embolism [P50]), covering 78%-90% of the tree species in the National Forest Inventory from Italy, to test (i) how a water limitation gradient shapes the functional composition and diversity of forests, (ii) how functional composition and diversity of trees relate to forest annual increment via mass ratio and complementarity effects, and (iii) how the relationship between functional diversity and annual increment varies between Mediterranean and temperate climate regions. Functional composition varied with water limitation; tree communities tended to have more conservative traits in sites with higher levels of water limitation. The response of functional diversity differed among traits and climatic regions but among temperate forest plots, we found a consistent increase of functional diversity with water limitation. Tree diversity was positively associated with annual increment of Italian forests through a combination of mass ratio and niche complementarity effects, but the relative importance of these effects depended on the trait and range of climate considered. Specifically, niche complementarity effects were more strongly associated with annual increment in the Mediterranean compared to temperate forests. Synthesis: Overall, our results suggest that biodiversity mediates forest annual increment under water-limited conditions by promoting beneficial interactions between species and complementarity in resource use. Our work highlights the importance of conserving functional diversity for future forest management to maintain forest annual increment under the expected increase in intensity and frequency of drought.

Restored vegetation is more resistant to extreme drought events than natural vegetation in Southwest China

Large scale Ecosystem restoration projects (ERPs) have been implemented to restore vegetation and increase carbon stocks across China. However, whether restored vegetation is strongly resistant to Extreme drought events (EDEs) remains unclear, especially when compared to natural vegetation. Therefore, we used the standardized anomaly of 3-month Standard Precipitation-Evapotranspiration Index (SPEI) to characterize the spatial-temporal trends of EDEs, and figured out the capacity of restored vegetation to withstand the strongest EDE in Southwest China by analyzing their changes of Gross Primary Productivity (GPP) and Water Use Efficiency (WUE). The results showed that Southwest China had experienced six typical EDEs with increasing frequency and severity from 1982 to 2017, particularly the EDE during 2009-2010 (EDE 2009/2010) which had the longest duration and strongest severity. Overall, the EDE 2009/2010 substantially suppressed the vegetation GPP and ecosystem WUE in both restored and natural vegetation area. Compared with natural vegetation, the GPP and WUE of restored vegetation was relative higher and moreover, their GPP decreased more slowly during the EDE 2009/2010 and increased more quickly during the recovery period. This indicates that restored vegetation had a higher drought resistance to the EDE than natural vegetation. Additionally, karst landforms have a stronger negative impact on vegetation GPP and WUE during the EDE. Furthermore, the reduction in the afforestation areas was more obviously observed than that in natural forest areas. Therefore, we suggest that vegetation suitable for regional characteristics should be selected during vegetation restoration, such as afforestation in the non-karst areas.

Stem hydraulic conductivity and embolism resistance of Quercus species are associated with their climatic niche

The hydraulic traits of a plant species may reflect its climate adaptations. Southwest China is considered as a biodiversity hotpot of the genus Quercus (oak). However, the hydraulic adaptations of Asian oaks to their climate niches remain unclear. Ten common garden-grown oak species with distinct natural distributions in eastern Asia were used to determine their stem xylem embolism resistance (water potential at 50% loss of hydraulic conductivity, P50), stem hydraulic efficiency (vessel anatomy and sapwood specific hydraulic conductivity (Ks)) and leaf anatomical traits. We also compiled four key functional traits: wood density, hydraulic-weighted vessel diameter, Ks and P50 data for 31 oak species from previous literature. We analyzed the relationship between hydraulic traits and climatic factors over the native ranges of 41 oak species. Our results revealed that the 10 Asian oak species, which are mainly distributed in humid subtropical habitats, possessed a stem xylem with low embolism resistance and moderate hydraulic efficiency. The deciduous and evergreen species of the 10 Asian oaks differed in the stem and leaf traits related to hydraulic efficiency. Ks differed significantly between the two phenological groups (deciduous and evergreens) in the 41-oak dataset. No significant difference in P50 between the two groups was found for the 10 Asian oaks or the 41-oak dataset. The oak species that can distribute in arid habitats possessed a stem xylem with high embolism resistance. Ks negatively related to the humidity of the native range of the 10 Asian oaks, but showed no trend when assessing the entire global oak dataset. Our study suggests that stem hydraulic conductivity and embolism resistance in Quercus species are shaped by their climate niche. Our findings assist predictions of oak drought resistance with future climate changes for oak forest management.

Contributions of phenotypic integration, plasticity and genetic adaptation to adaptive capacity relating to drought in Banksia marginata (Proteaceae)

The frequency and intensity of drought events are predicted to increase because of climate change, threatening biodiversity and terrestrial ecosystems in many parts of the world. Drought has already led to declines in functionally important tree species, which are documented in dieback events, shifts in species distributions, local extinctions, and compromised ecosystem function. Understanding whether tree species possess the capacity to adapt to future drought conditions is a major conservation challenge. In this study, we assess the capacity of a functionally important plant species from south-eastern Australia (Banksia marginata, Proteaceae) to adapt to water-limited environments. A water-manipulated common garden experiment was used to test for phenotypic plasticity and genetic adaptation in seedlings sourced from seven provenances of contrasting climate-origins (wet and dry). We found evidence of local adaptation relating to plant growth investment strategies with populations from drier climate-origins showing greater growth in well-watered conditions. The results also revealed that environment drives variation in physiological (stomatal conductance, predawn and midday water potential) and structural traits (wood density, leaf dry matter content). Finally, these results indicate that traits are coordinated to optimize conservation of water under water-limited conditions and that trait coordination (phenotypic integration) does not constrain phenotypic plasticity. Overall, this study provides evidence for adaptive capacity relating to drought conditions in B. marginata, and a basis for predicting the response to climate change in this functionally important plant species.

Turgor loss point and vulnerability to xylem embolism predict species-specific risk of drought-induced decline of urban trees

Increasing frequency and severity of drought events is posing risks to trees' health, including those planted in urban settlements. Drought-induced decline of urban trees negatively affects ecosystem services of urban green spaces and implies cost for maintenance and removal of plants. We aimed at identifying physiological traits that can explain and predict the species-specific vulnerability to climate change in urban habitats. We assessed the relationships between long-term risk of decline of different tree species in a medium-sized town and their key indicators of drought stress tolerance, i.e. turgor loss point (TLP) and vulnerability to xylem embolism (P₅₀ ). Starting from 2012, the study area experienced several summer seasons with positive anomalies of temperature and negative anomalies of precipitation. This trend was coupled with increasing percentages of urban trees showing signs of crown die-back and mortality. The species-specific risk of decline was higher for species with less negative TLP and P₅₀ values. The relationship between species-specific risk of climate change-induced decline of urban trees and key physiological indicators of drought tolerance confirms findings obtained in natural forests and highlights that TLP and P₅₀ are useful indicators for species selection for tree plantation in towns, to mitigate negative impacts of climate change.

Crown dieback and mortality of urban trees linked to heatwaves during extreme drought

Cities have been described as 'heat islands' and 'dry islands' due to hotter, drier air in urban areas, relative to the surrounding landscape. As climate change intensifies, the health of urban trees will be increasingly impacted. Here, we posed the question: Is it possible to predict urban tree species mortality using (1) species climate envelopes and (2) plant functional traits? To answer these, we tracked patterns of crown dieback and recovery for 23 common urban tree and shrub species in Sydney, Australia during the record-breaking austral 2019-2020 summer. We identified 10 heat-tolerant species including five native and five exotic species, which represent climate-resilient options for urban plantings that are likely to continue to thrive for decades. Thirteen species were considered vulnerable to adverse conditions due to their mortality, poor health leading to tree removal, and/or extensive crown dieback. Crown dieback increased with increasing precipitation of the driest month of species climate of origin, suggesting that species from dry climates may be better suited for urban forests in future climates. We effectively grouped species according to their drought strategy (i.e., tolerance versus avoidance) using a simple trait-based framework that was directly linked with species mortality. The seven most climate-vulnerable species used a drought-avoidance strategy, having low wood density and high turgor loss points along with large, thin leaves with low heat tolerance. Overall, plant functional traits were better than species climate envelopes at explaining crown dieback. Recovery after stress required two mild, wet years for most species, resulting in prolonged loss of cooling benefits as well as economic losses due to replacement of dead/damaged trees. Hotter, longer, and more frequent heatwaves will require selection of more climate-resilient species in urban forests, and our results suggest that future research should focus on plant thermal traits to improve prediction models and species selection.

Age-specific and species-specific tree response to seasonal drought in tropical dry forests

Millions of people depend on ecosystem services provided by Tropical Dry Forests (TDFs), yet their proximity to population centers, seasonally dry climate, and the ease at which they are converted to agriculture has left only 10 % of their original extent globally. As more TDFs become protected, basic information relating TDF age to subsurface water resources will help guide forest recovery. Severe deforestation and recent reforestation around Bahía de Caráquez, Ecuador produced a mosaic of different successional stages ideal for exploring relationships between TDF age, subsurface water availability and species-specific responses to seasonal drought. Over one year, we measured gravimetric water content, predawn and midday leaf water potential, and the stable isotope composition of xylem and source waters in two regenerating and one primary forest. Over the transition from wet to dry season, we discovered a sharper decrease in predawn water potential in younger successional forests than in the primary forest. Growing in degraded subsurface environments under increased competition, successional forest trees accessed deeper sources of moisture from unsaturated weathered bedrock and groundwater through the dry season; however, different species employed distinct water use strategies. Ceiba trichistandra maintained midday water potentials above -1.27 MPa through a drought avoidance strategy dependent on groundwater. Sideroxylon celastrinum tolerated drought by lowering predawn and midday water potential through the early dry season but took up greater proportions of saprolite moisture and groundwater as the dry season progressed. Contrastingly, Handroanthus chrysanthus maintained access to shallow soil and saprolite moisture by dropping midday water potential to -4.30 MPa, reflecting drought tolerance. Our results show that limited subsurface water resources in regenerating TDF's lead to species-specific adaptations reliant on deeper sources of moisture. The recovery of soil and saprolite hydrologic properties following disturbances is likely to exceed 100 years, highlighting the importance of forest conservation.

Strategies of tree species to adapt to drought from leaf stomatal regulation and stem embolism resistance to root properties

Considerable evidences highlight the occurrence of increasing widespread tree mortality as a result of global climate change-associated droughts. However, knowledge about the mechanisms underlying divergent strategies of various tree species to adapt to drought has remained remarkably insufficient. Leaf stomatal regulation and embolism resistance of stem xylem serves as two important strategies for tree species to prevent hydraulic failure and carbon starvation, as comprising interconnected physiological mechanisms underlying drought-induced tree mortality. Hence, the physiological and anatomical determinants of leaf stomatal regulation and stems xylem embolism resistance are evaluated and discussed. In addition, root properties related to drought tolerance are also reviewed. Species with greater investment in leaves and stems tend to maintain stomatal opening and resist stem embolism under drought conditions. The coordination between stomatal regulation and stem embolism resistance are summarized and discussed. Previous studies showed that hydraulic safety margin (HSM, the difference between minimum water potential and that causing xylem dysfunction) is a significant predictor of tree species mortality under drought conditions. Compared with HSM, stomatal safety margin (the difference between water potential at stomatal closure and that causing xylem dysfunction) more directly merge stomatal regulation strategies with xylem hydraulic strategies, illustrating a comprehensive framework to characterize plant response to drought. A combination of plant traits reflecting species' response and adaptation to drought should be established in the future, and we propose four specific urgent issues as future research priorities.

Seasonal adjustment of leaf embolism resistance and its importance for hydraulic safety in deciduous trees

Embolism resistance is often viewed as seasonally stable. Here we examined the seasonality in the leaf xylem vulnerability curve (VC) and turgor loss point (Ψ_TLP ) of nine deciduous species that originated from Mediterranean, temperate, tropical, or sub-tropical habitats and were growing on the Volcani campus, Israel. All four Mediterranean/temperate species exhibited a shift of their VC to lower xylem pressures (Ψ_x ) along the dry season, in addition to two of the five tropical/sub-tropical species. In three of the species that exhibited VC seasonality, it was critical for avoiding embolism in the leaf. In total, seven out of the nine species avoided embolism. The seasonal VC adjustment was over two times higher as compared with the seasonal adjustment of Ψ_TLP , resulting in improved hydraulic safety as the season progressed. The results suggest that seasonality in the leaf xylem vulnerability is common in species that originate from Mediterranean or temperate habitats that have large seasonal environmental changes. This seasonality is advantageous because it enables a gradual seasonal reduction in the Ψ_x without increasing the danger of embolism. The results also highlight that measuring the minimal Ψ_x and the VC at different times can lead to erroneous estimations of the hydraulic safety margins. Changing the current hydraulic dogma into a seasonal dynamic in the vulnerability of the xylem itself should enable physiologists to understand plants' responses to their environment better.

Divergence of hydraulic traits among tropical forest trees across topographic and vertical environment gradients in Borneo

Fine-scale topographic-edaphic gradients are common in tropical forests and drive species spatial turnover and marked changes in forest structure and function. We evaluate how hydraulic traits of tropical tree species relate to vertical and horizontal spatial niche specialization along such a gradient. Along a topographic-edaphic gradient with uniform climate in Borneo, we measured six key hydraulic traits in 156 individuals of differing heights in 13 species of Dipterocarpaceae. We investigated how hydraulic traits relate to habitat, tree height and their interaction on this gradient. Embolism resistance increased in trees on sandy soils but did not vary with tree height. By contrast, water transport capacity increased on sandier soils and with increasing tree height. Habitat and height only interact for hydraulic efficiency, with slope for height changing from positive to negative from the clay-rich to the sandier soil. Habitat type influenced trait-trait relationships for all traits except wood density. Our data reveal that variation in the hydraulic traits of dipterocarps is driven by a combination of topographic-edaphic conditions, tree height and taxonomic identity. Our work indicates that hydraulic traits play a significant role in shaping forest structure across topographic-edaphic and vertical gradients and may contribute to niche specialization among dipterocarp species.

Hurricanes increase tropical forest vulnerability to drought

Rapid changes in climate and disturbance regimes, including droughts and hurricanes, are likely to influence tropical forests, but our understanding of the compound effects of disturbances on forest ecosystems is extremely limited. Filling this knowledge gap is necessary to elucidate the future of these ecosystems under a changing climate. We examined the relationship between hurricane response (damage, mortality, and resilience) and four hydraulic traits of 13 dominant woody species in a wet tropical forest subject to periodic hurricanes. Species with high resistance to embolisms (low P₅₀ values) and higher safety margins ( SMP50 ) were more resistant to immediate hurricane mortality and breakage, whereas species with higher hurricane resilience (rapid post-hurricane growth) had high capacitance and P₅₀ values and low SMP50 . During 26 yr of post-hurricane recovery, we found a decrease in community-weighted mean values for traits associated with greater drought resistance (leaf turgor loss point, P₅₀ , SMP50 ) and an increase in capacitance, which has been linked with lower drought resistance. Hurricane damage favors slow-growing, drought-tolerant species, whereas post-hurricane high resource conditions favor acquisitive, fast-growing but drought-vulnerable species, increasing forest productivity at the expense of drought tolerance and leading to higher overall forest vulnerability to drought.

Functional traits and their plasticity shift from tolerant to avoidant under extreme drought

Under climate change, extreme droughts will limit water availability for plants. However, the species-specific responses make it difficult to draw general conclusions. We hypothesized that changes in species' abundance in response to extreme drought can be best explained by a set of water economic traits under ambient conditions in combination with the ability to adjust these traits towards higher drought resistance. We conducted a four-year field experiment in temperate grasslands using rainout shelters with 30% and 50% rainfall reduction. We quantified the response as the change in species abundance between ambient conditions and the rainfall reduction. Abundance response to extreme drought was best explained by a combination of traits in ambient conditions and their functional adjustment, most likely reflecting plasticity. Smaller leaved species decreased less in abundance under drought. With increasing drought intensity, we observed a shift from drought tolerance, i.e. an increase in leaf dry matter content, to avoidance, i.e. a less negative turgor loss point (TLP) in ambient conditions and a constancy in TLP under drought. We stress the importance of using a multidimensional approach of variation in multiple traits and the importance of considering a range of drought intensities to improve predictions of species' response to climate change.

Recent changes in high-mountain plant community functional composition in contrasting climate regimes

High-mountain plant communities are strongly determined by abiotic conditions, especially low temperature, and are therefore susceptible to effects of climate warming. Rising temperatures, however, also lead to increased evapotranspiration, which, together with projected shifts in seasonal precipitation patterns, could lead to prolonged, detrimental water deficiencies. The current study aims at comparing alpine plant communities along elevation and water availability gradients from humid conditions (north-eastern Alps) to a moderate (Central Apennines) and a pronounced dry period during summer (Lefka Ori, Crete) in the Mediterranean area. We do this in order to (1) detect relationships between community-based indices (plant functional leaf and growth traits, thermic vegetation indicator, plant life forms, vegetation cover and diversity) and soil temperature and snow duration and (2) assess if climatic changes have already affected the vegetation, by determining directional changes over time (14-year period; 2001-2015) in these indices in the three regions. Plant community indices responded to decreasing temperatures along the elevation gradient in the NE-Alps and the Apennines, but this elevation effect almost disappeared in the summer-dry mountains of Crete. This suggests a shift from low-temperature to drought-dominated ecological filters. Leaf trait (Leaf Dry Matter Content and Specific Leaf Area) responses changed in direction from the Alps to the Apennines, indicating that drought effects already become discernible at the northern margin of the Mediterranean. Over time, a slight increase in vegetation cover was found in all regions, but thermophilisation occurred only in the NE-Alps and Apennines, accompanied by a decline of cold-adapted cushion plants in the Alps. On Crete, xeromorphic shrubs were increasing in abundance. Although critical biodiversity losses have not yet been observed, an intensified monitoring of combined warming-drought impacts will be required in view of threatened alpine plants that are either locally restricted in the south or weakly adapted to drought in the north.

Functional Diversity in Woody Organs of Tropical Dry Forests and Implications for Restoration

Tropical dry forests (TDFs) represent one of the most diverse and, at the same time, most threatened ecosystems on earth. Restoration of TDFs is thus crucial but is hindered by a limited understanding of the functional diversity (FD) of original communities. We examine the FD of TDFs based on wood (vessel diameter and wood density) and bark traits (total, inner, and outer bark thicknesses) measured on ~500 species from 24 plant communities and compare this diversity with that of seven other major vegetation types. Along with other seasonally dry sites, TDFs had the highest FD, as indicated by the widest ranges, highest variances, and largest trait hypervolumes. Warm temperatures and seasonal drought seem to drive diverse ecological strategies in these ecosystems, which include a continuum from deciduous species with low-density wood, thick bark, and wide vessels to evergreen species with high-density wood, thin bark, and narrow vessels. The very high FD of TDFs represents a challenge to restoring the likely widest trait ranges of any habitat on earth. Understanding this diversity is essential for monitoring successional changes in minimal intervention restoration and guiding species selection for resilient restoration plantings in the context of climate change.

Photosynthetic and hydraulic traits influence forest resistance and resilience to drought stress across different biomes

Drought events lead to depressions in gross primary productivity (GPP) of forest ecosystems. Photosynthetic and hydraulic traits are important factors governing GPP variation. However, how these functional traits affect GPP responses to drought has not been well understood. We quantified the capacity of GPP to withstand changes during droughts (GPP_resistance) and its post-drought responses (GPP_resilience) using eddy covariance data from the FLUXNET2015 dataset, and investigated how functional traits of dominant tree species that comprised >80% of the biomass (or composition) influenced GPP_resistance or GPP_resilience. Light-saturated photosynthetic rate of dominant tree species was negatively related to GPP_resistance, and was positively correlated with GPP_resilience. Forests dominated by species with higher hydraulic safety margins (HSM), smaller vessel diameter (V_dia) and lower sensitivity of canopy stomatal conductance per unit land area (G_s) to droughts had a higher GPP_resistance, while those dominated by species with lower HSM, larger V_dia and higher sensitivity of G_s to droughts exhibited a higher GPP_resilience. Differences in functional traits of forests located in diverse climate regions led to distinct GPP sensitivities to droughts. Forests located in humid regions had a higher GPP_resilience while those in arid regions exhibited a higher GPP_resistance. Forest GPP_resistance was negatively related to drought intensity, and GPP_resilience was negatively related to drought duration. Our findings highlight the significant role of functional traits in governing forest resistance and resilience to droughts. Overall, forests dominated by species with higher hydraulic safety were more resistant to droughts, while forests containing species with higher photosynthetic and hydraulic efficiency recovered better from drought stress.

Above- and below-ground trait coordination in tree seedlings depend on the most limiting resource: a test comparing a wet and a dry tropical forest in Mexico

The study of above- and below-ground organ plant coordination is crucial for understanding the biophysical constraints and trade-offs involved in species' performance under different environmental conditions. Environmental stress is expected to increase constraints on species trait combinations, resulting in stronger coordination among the organs involved in the acquisition and processing of the most limiting resource. To test this hypothesis, we compared the coordination of trait combinations in 94 tree seedling species from two tropical forest systems in Mexico: dry and moist. In general, we expected that the water limitation experienced by dry forest species would result in stronger leaf-stem-root coordination than light limitation experienced by moist forest species. Using multiple correlations analyses and tools derived from network theory, we found similar functional trait coordination between forests. However, the most important traits differed between the forest types. While in the dry forest the most central traits were all related to water storage (leaf and stem water content and root thickness), in the moist forest they were related to the capacity to store water in leaves (leaf water content), root efficiency to capture resources (specific root length), and stem toughness (wood density). Our findings indicate that there is a shift in the relative importance of mechanisms to face the most limiting resource in contrasting tropical forests.

Lack of phenotypic plasticity in leaf hydraulics for 10 woody species common to urban forests of North China

The survival and performance of urban forests are increasingly challenged by urban drought, consequently compromising the sustainability and functionality of urban vegetation. Plant-water relations largely determine species drought tolerance, yet little is known about the hydraulics of urban forest species. Here, we report the leaf hydraulic and carbon traits that govern plant growth and drought resistance, including vulnerability to embolism, hydraulic conductivity and leaf gas exchange characteristics, as well as morphological traits that are potentially linked with these physiological attributes, with the aim of guiding species selection and management in urban forests. Plant materials were collected from mature shrubs and trees on our university campus in Beijing, representing 10 woody species common to urban forests in north China. We found that the leaf embolism resistance, represented by the water potential inducing 50% loss of hydraulic conductivity (P50), as well as the hydraulic safety margin (HSM) defined by P50 and the water potential threshold at the inception of embolism (P12), varied remarkably across species, but was unrelated to growth form. Likewise, stem and leaf-specific hydraulic conductivity (Kstem and kl) was also highly species-specific. Leaf P50 was positively correlated with hydraulic conductivity. However, neither P50 nor hydraulic conductivity was correlated with leaf gas exchange traits, including maximum photosynthetic rate (Amax) and stomatal conductance (gs). Plant morphological and physiological traits were not related, except for specific leaf area, which showed a negative relationship with HSM. Traits influencing plant-water transport were primarily correlated with the mean annual precipitation of species climatic niche. Overall, current common woody species in urban forest environments differed widely in their drought resistance and did not have the capacity to modify these characteristics in response to a changing climate. Species morphology provides limited information regarding physiological drought resistance. Thus, screening urban forest species based on plant physiology is essential to sustain the ecological services of urban forests.

Climate and hydraulic traits interact to set thresholds for liana viability

Lianas, or woody vines, and trees dominate the canopy of tropical forests and comprise the majority of tropical aboveground carbon storage. These growth forms respond differently to contemporary variation in climate and resource availability, but their responses to future climate change are poorly understood because there are very few predictive ecosystem models representing lianas. We compile a database of liana functional traits (846 species) and use it to parameterize a mechanistic model of liana-tree competition. The substantial difference between liana and tree hydraulic conductivity represents a critical source of inter-growth form variation. Here, we show that lianas are many times more sensitive to drying atmospheric conditions than trees as a result of this trait difference. Further, we use our competition model and projections of tropical hydroclimate based on Representative Concentration Pathway 4.5 to show that lianas are more susceptible to reaching a hydraulic threshold for viability by 2100.

Leaf trait association in relation to herbivore defense, drought resistance, and economics in a tropical invasive plant

PREMISE

Exploring how functional traits vary and covary is important to understand plant responses to environmental change. However, we have limited understanding of the ways multiple functional traits vary and covary within invasive species.

METHODS

We measured 12 leaf traits of an invasive plant Chromolaena odorata, associated with plant or leaf economics, herbivore defense, and drought resistance on 10 introduced populations from Asia and 12 native populations from South and Central America, selected across a broad range of climatic conditions, and grown in a common garden.

RESULTS

Species' range and climatic conditions influenced leaf traits, but trait variation across climate space differed between the introduced and native ranges. Traits that confer defense against herbivores and drought resistance were associated with economic strategy, but the patterns differed by range. Plants from introduced populations that were at the fast-return end of the spectrum (high photosynthetic capacity) had high physical defense traits (high trichome density), whereas plants from native populations that were at the fast-return end of the spectrum had high drought escape traits (early leaf senescence and high percentage of withered shoots).

CONCLUSIONS

Our results indicate that invasive plants can rapidly adapt to novel environmental conditions. Chromolaena odorata showed multiple different functional trait covariation patterns and clines in the native and introduced ranges. Our results emphasize that interaction between multiple traits or functions should be considered when investigating the adaptive evolution of invasive plants.

High safety margins to drought-induced hydraulic failure found in five pasture grasses

Determining the relationship between reductions in stomatal conductance (g_s ) and leaf water transport during dehydration is key to understanding plant drought responses. While numerous studies have analysed the hydraulic function of woody species, minimal research has been conducted on grasses. Here, we sought to characterize hydraulic vulnerability in five widely-occurring pasture grasses (including both C3 and C4 grasses) and determine whether reductions in g_s and leaf hydraulic conductance (K_leaf ) during dehydration could be attributed to xylem embolism. Using the optical vulnerability (OV) technique, we found that all species were highly resistant to xylem embolism when compared to other herbaceous angiosperms, with 50% xylem embolism (P_X50 ) occurring at xylem pressures ranging from -4.4 to -6.1 MPa. We observed similar reductions in g_s and K_leaf under mild water stress for all species, occurring well before P_X50 . The onset of xylem embolism (P_X12 ) occurred consistently after stomatal closure and 90% reduction of K_leaf . Our results suggest that factors other than xylem embolism are responsible for the majority of reductions in g_s and K_leaf during drought and reductions in the productivity of pasture species under moderate drought may not be driven by embolism.

Variations in leaf water status and drought tolerance of dominant tree species growing in multi-aged tropical forests in Thailand

Large-scale abandoned agricultural areas in Southeast Asia resulted in patches of forests of multiple successions and characteristics, challenging the study of their responses to environmental changes, especially under climatic water stress. Here, we investigated seasonal variation in leaf water status and drought tolerance of dominant tree species in three multi-aged tropical forests, ranging from 5 to > 200 years old, with contrasting soil moisture in Thailand. Seasonal variation in leaf water status differed among the forests with trees in young and intermediate sites demonstrating larger differences between seasons than the old-growth forest. Although vulnerability to embolism curves revealed that trees in old-growth forest were potentially more sensitive to declining leaf water status than others, they were predicted to lose < 5% of their hydraulic capacity as opposed to 13% for the trees in the younger sites. Our results suggest that the responses to water stress of tree species in different forest ages greatly vary with a tendency of trees in younger sites to be more resilience than those in older sites. Such information would benefit the selection of tree species that could adapt well to specific environments, thus improving the strategies for managing forests of different ages under a warmer future.

Unlocking Drought-Induced Tree Mortality: Physiological Mechanisms to Modeling

Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models.

Small and slow is safe: On the drought tolerance of tropical tree species

Understanding how evolutionary history and the coordination between trait trade-off axes shape the drought tolerance of trees is crucial to predict forest dynamics under climate change. Here, we compiled traits related to drought tolerance and the fast-slow and stature-recruitment trade-off axes in 601 tropical woody species to explore their covariations and phylogenetic signals. We found that xylem resistance to embolism (P50) determines the risk of hydraulic failure, while the functional significance of leaf turgor loss point (TLP) relies on its coordination with water use strategies. P50 and TLP exhibit weak phylogenetic signals and substantial variation within genera. TLP is closely associated with the fast-slow trait axis: slow species maintain leaf functioning under higher water stress. P50 is associated with both the fast-slow and stature-recruitment trait axes: slow and small species exhibit more resistant xylem. Lower leaf phosphorus concentration is associated with more resistant xylem, which suggests a (nutrient and drought) stress-tolerance syndrome in the tropics. Overall, our results imply that (1) drought tolerance is under strong selective pressure in tropical forests, and TLP and P50 result from the repeated evolutionary adaptation of closely related taxa, and (2) drought tolerance is coordinated with the ecological strategies governing tropical forest demography. These findings provide a physiological basis to interpret the drought-induced shift toward slow-growing, smaller, denser-wooded trees observed in the tropics, with implications for forest restoration programmes.

Differential Adaptive Potential and Vulnerability to Climate-Driven Habitat Loss in Brazilian Mangroves

Geographic and environmental differences have been identified as factors influencing Brazilian mangrove trees' genetic diversity. Geographically, distinct species have convergent spatial genetic structures, indicating a limited gene flow between northern and southern populations. Environmentally, genomic studies and common garden experiments have found evidence of local adaptations along the latitudinal gradient of the Brazilian coast. However, little is known about how such adaptive heterogeneity could be affected by a rapidly changing climate in the coming decades, and the combination of deforestation and climate-induced habitat loss may affect these forests and their genetic diversity. Here, we applied two genomic-environmental association methods to model the turnover of potentially adaptive alleles for two dominant mangrove trees: Avicennia germinans and A. schaueriana. We analyzed a total of 134 individuals from six populations of A. germinans and 10 populations of A. schaueriana spanning the Brazilian coast from 1 °S to 28 °S. Gradient forest models identified temperature-related variables as the most important predictors for A. germinans outlier loci, whereas both temperature and precipitation were important for A. schaueriana. We modeled allele frequencies and projected them for future climatic scenarios to estimate adaptively driven vulnerability. We assessed climate-driven habitat loss through climate-only distribution models and calculated annual deforestation rates for each sampled region. Finally, to assess the vulnerability of individual populations, we combined the environmental suitability, deforestation data, and adaptive vulnerability projections. For both species, subtropical populations presented a higher vulnerability than equatorial populations to climate-driven habitat loss. We also identified deforestation rates at the sampled sites that were alarmingly higher than the global average mangrove deforestation rate. Our results provide improved estimates of the impacts of ongoing climate change and human-caused habitat loss on the distribution of mangroves and highlight the importance of site-based conservation strategies that consider individual subtropical and equatorial mangrove forests.

Divergent stem hydraulic strategies of Caragana korshinskii resprouts following a disturbance

Resprouting plants are distributed in many vegetation communities worldwide. With increasing resprout age post-severe-disturbance, new stems grow rapidly at their early age, and decrease in their growth with gradually decreasing water status thereafter. However, there is little knowledge about how stem hydraulic strategies and anatomical traits vary post-disturbance. In this study, the stem water potential (Ψstem), maximum stem hydraulic conductivity (Kstem-max), water potential at 50% loss of hydraulic conductivity (Kstem  P50) and anatomical traits of Caragana korshinkii resprouts were measured during a 1- to 13-year post-disturbance period. We found that the Kstem-max decreased with resprout age from 1-year-old resprouts (84.2 mol m-1 s-1 MPa-1) to 13-year-old resprouts (54.2 mol m-1 s-1 MPa-1) as a result of decreases in the aperture fraction (Fap) and the sum of aperture area on per unit intervessel wall area (Aap). The Kstem  P50 of the resprouts decreased from 1-year-old resprouts (-1.8 MPa) to 13-year-old resprouts (-2.9 MPa) as a result of increases in vessel implosion resistance (t/b)2, wood density (WD), vessel grouping index (GI) and decreases in Fap and Aap. These shifts in hydraulic structure and function resulted in an age-based divergence in hydraulic strategies i.e., a change from an acquisitive strategy to a conservative strategy, with increasing resprout age post-disturbance.

Biochar and alternate wetting-drying cycles improving rhizosphere soil nutrients availability and tobacco growth by altering root growth strategy in Ferralsol and Anthrosol

Biochar has been advocated as a sustainable and eco-friendly practice to improve soil fertility and crop productivity which could aid in the mitigation of climate change. Nonetheless, the combined effects of biochar and irrigation on tobacco growth and soil nutrients in diverse soil types have been incompletely explored. We applied a split-root experiment to investigate the impacts of amendment with 2% softwood- (WBC) and wheat-straw biochar (SBC) on growth responses and rhizosphere soil nutrients availability of tobacco plants grown in a Ferralsol and an Anthrosol. All plants within same soil type received same amount of water daily by either conventional deficit irrigation (CDI) or alternate wetting-drying cycles irrigation (AWD). Compared to the un-amended controls, SBC addition enhanced biomass, carbon (C)-, phosphorus (P)- and potassium (K)-pool in the aboveground organs especially in Anthrosol, despite a negative effect on aboveground nitrogen (N)-pool. Regardless of soil type, biochar combined with AWD lowered root diameter while increased root tissue mass density to engage the plant in an acquisitive strategy for resources, therefore altered leaves stoichiometry as exemplified by lowered N/K, C/P and N/P and increased C/N. The addition of SBC induced a liming effect by increasing Anthrosol soil pH which was further amplified by AWD, but was unaffected on Ferralsol. Moreover, compared to the controls, SBC and AWD increased available P and K, and total C, total N and C/N ratio in the rhizosphere soil which coincided with the lowered soil C and N isotope composition (δ13C and δ15N), though a slight reduction in C and N stocks under AWD. However, such effects were not evident with WBC might be associated with its natures. Thus, combined SBC/AWD application might be an effective strategy to synergistically overcome nutrients restriction and improve tobacco productivity by intensifying nutrients cycling and optimizing plant growth strategies.

Non-destructive Measurements of Toona sinensis Chlorophyll and Nitrogen Content Under Drought Stress Using Near Infrared Spectroscopy

Drought is a climatic event that considerably impacts plant growth, reproduction and productivity. Toona sinensis is a tree species with high economic, edible and medicinal value, and has drought resistance. Thus, the objective of this study was to dynamically monitor the physiological indicators of T. sinensis in real time to ensure the selection of drought-resistant varieties of T. sinensis. In this study, we used near-infrared spectroscopy as a high-throughput method along with five preprocessing methods combined with four variable selection approaches to establish a cross-validated partial least squares regression model to establish the relationship between the near infrared reflectance spectroscopy (NIRS) spectrum and physiological characteristics (i.e., chlorophyll content and nitrogen content) of T. sinensis leaves. We also tested optimal model prediction for the dynamic changes in T. sinensis chlorophyll and nitrogen content under five separate watering regimes to mimic non-destructive and dynamic detection of plant leaf physiological changes. Among them, the accuracy of the chlorophyll content prediction model was as high as 72%, with root mean square error (RMSE) of 0.25, and the RPD index above 2.26. Ideal nitrogen content prediction model should have R ² of 0.63, with RMSE of 0.87, and the RPD index of 1.12. The results showed that the PLSR model has a good prediction effect. Overall, under diverse drought stress treatments, the chlorophyll content of T. sinensis leaves showed a decreasing trend over time. Furthermore, the chlorophyll content was the most stable under the 75% field capacity treatment. However, the nitrogen content of the plant leaves was found to have a different and variable trend, with the greatest drop in content under the 10% field capacity treatment. This study showed that NIRS has great potential for analyzing chlorophyll nitrogen and other elements in plant leaf tissues in non-destructive dynamic monitoring.

Climatic and soil factors explain the two-dimensional spectrum of global plant trait variation

Plant functional traits can predict community assembly and ecosystem functioning and are thus widely used in global models of vegetation dynamics and land-climate feedbacks. Still, we lack a global understanding of how land and climate affect plant traits. A previous global analysis of six traits observed two main axes of variation: (1) size variation at the organ and plant level and (2) leaf economics balancing leaf persistence against plant growth potential. The orthogonality of these two axes suggests they are differently influenced by environmental drivers. We find that these axes persist in a global dataset of 17 traits across more than 20,000 species. We find a dominant joint effect of climate and soil on trait variation. Additional independent climate effects are also observed across most traits, whereas independent soil effects are almost exclusively observed for economics traits. Variation in size traits correlates well with a latitudinal gradient related to water or energy limitation. In contrast, variation in economics traits is better explained by interactions of climate with soil fertility. These findings have the potential to improve our understanding of biodiversity patterns and our predictions of climate change impacts on biogeochemical cycles.

Effects of Water Availability on the Relationships Between Hydraulic and Economic Traits in the Quercus wutaishanica Forests

Water availability is a key environmental factor affecting plant species distribution, and the relationships between hydraulic and economic traits are important for understanding the species' distribution patterns. However, in the same community type but within different soil water availabilities, the relationships in congeneric species remain ambiguous. In northwest China, Quercus wutaishanica forests in the Qinling Mountains (QM, humid region) and Loess Plateau (LP, drought region) have different species composition owing to contrasting soil water availability, but with common species occurring in two regions. We analyzed eight hydraulic traits [stomatal density (SD), vein density (VD), wood specific gravity (WSG_branch), lower leaf area: sapwood area (Al: As), stomatal length (SL), turgor loss point (Ψ_Tlp), maximum vessel diameter (Vd_max) and height (Height)] and five economic traits [leaf dry matter content (LDMC), leaf tissue density (TD), leaf dry mass per area (LMA), Leaf thickness (LT) and maximum net photosynthetic rate (P_max)] of congeneric species (including common species and endemic species) in Q. wutaishanica forests of QM and LP. We explored whether the congeneric species have different economic and hydraulic traits across regions. And whether the relationship between hydraulic and economic traits was determined by soil water availability, and whether it was related to species distribution and congeneric endemic species composition of the same community. We found that LP species tended to have higher SD, VD, WSG_branch, Al: As, SL, Ψ_Tlp and Vd_max than QM species. There was a significant trade-off between hydraulic efficiency and safety across congeneric species. Also, the relationships between hydraulic and economic traits were closer in LP than in QM. These results suggested that relationships between hydraulic and economic traits, hydraulic efficiency and safety played the role in constraining species distribution across regions. Interestingly, some relationships between traits changed (from significant correlation to non-correlation) in common species across two regions (from LP to QM), but not in endemic species. The change of these seven pairs of relationships might be a reason for common species' wide occurrence in the two Q. wutaishanica forests with different soil water availability. In drought or humid conditions, congeneric species developed different types of adaptation mechanisms. The study helps to understand the environmental adaptive strategies of plant species, and the results improve our understanding of the role of both hydraulic and economic traits during community assembly.

Functional recovery of secondary tropical forests

One-third of all Neotropical forests are secondary forests that regrow naturally after agricultural use through secondary succession. We need to understand better how and why succession varies across environmental gradients and broad geographic scales. Here, we analyze functional recovery using community data on seven plant characteristics (traits) of 1,016 forest plots from 30 chronosequence sites across the Neotropics. By analyzing communities in terms of their traits, we enhance understanding of the mechanisms of succession, assess ecosystem recovery, and use these insights to propose successful forest restoration strategies. Wet and dry forests diverged markedly for several traits that increase growth rate in wet forests but come at the expense of reduced drought tolerance, delay, or avoidance, which is important in seasonally dry forests. Dry and wet forests showed different successional pathways for several traits. In dry forests, species turnover is driven by drought tolerance traits that are important early in succession and in wet forests by shade tolerance traits that are important later in succession. In both forests, deciduous and compound-leaved trees decreased with forest age, probably because microclimatic conditions became less hot and dry. Our results suggest that climatic water availability drives functional recovery by influencing the start and trajectory of succession, resulting in a convergence of community trait values with forest age when vegetation cover builds up. Within plots, the range in functional trait values increased with age. Based on the observed successional trait changes, we indicate the consequences for carbon and nutrient cycling and propose an ecologically sound strategy to improve forest restoration success.

Contrasting Hydraulic Efficiency and Photosynthesis Strategy in Differential Successional Stages of a Subtropical Forest in a Karst Region

Understanding the successional process from a disturbed forest to a mature forest is essential for species recovery and conservation initiatives. The resource acquisition and drought tolerance of plants can be instructive to predictions of species abundance and distribution for different forests. However, they have not been adequately tested at different successional stages in karst regions. Here, we selected seven dominant species in an early-succession forest and 17 species in a late-succession forest in a karst region of southwestern China. Resource acquisition-related traits such as hydraulic conductivity and photosynthetic rate, and drought tolerance-related traits, including turgor loss point and wood density, were measured. We found that species in the early-succession forest had a higher hydraulic conductance and photosynthetic rate than those in the late-succession forest, while leaf water potential at turgor loss point and wood density showed nonsignificant differences between the two forests. In addition, we observed a significant negative relationship between photosynthetic rate and drought tolerance in the early-succession forest, which was not identified in late-succession forests. Our study indicates that resource acquisition rather than drought tolerance was the key factor explaining plant distributions in forests at different successional stages in karst regions. We also suggest that the resource acquisition and drought tolerance trade-off hypothesis is not always supported for karst region species. Our study could inform about the design of species replacements in successional forests and provide forest management and restoration guidelines for karst regions.

Differentiation in stem and leaf traits among sympatric lianas, scandent shrubs and trees in a subalpine cold temperate forest

The scandent shrub plant form is a variant of liana that has upright and self-supporting stems when young but later becomes a climber. We aimed to explore the associations of stem and leaf traits among sympatric lianas, scandent shrubs and trees, and the effects of growth form and leaf habit on variation in stem or leaf traits. We measured 16 functional traits related to stem xylem anatomy, leaf morphology and nutrient stoichiometry in eight liana, eight scandent shrub and 21 tree species co-occurring in a subalpine cold temperate forest at an elevation of 2600-3200 m in Southwest China. Overall, lianas, scandent shrubs and trees were ordered along a fast-slow continuum of stem and leaf functional traits, with some traits overlapping. We found a consistent pattern of lianas > scandent shrubs > trees for hydraulically weighted vessel diameter, maximum vessel diameter and theoretical hydraulic conductivity. Vessel density and sapwood density showed a pattern of lianas = scandent shrubs < trees, and lianas < scandent shrubs = trees, respectively. Lianas had significantly higher specific leaf area and lower carbon concentration than co-occurring trees, with scandent shrubs showing intermediate values that overlapped with lianas and trees. The differentiation among lianas, scandent shrubs and trees was mainly explained by variation in stem traits. Additionally, deciduous lianas were positioned at the fast end of the trait spectrum, and evergreen trees at the slow end of the spectrum. Our results showed for the first time clear differentiation in stem and leaf traits among sympatric liana, scandent shrub and tree species in a subalpine cold temperate forest. This work will contribute to understanding the mechanisms responsible for variation in ecological strategies of different growth forms of woody plants.

Nutrient cycling drives plant community trait assembly and ecosystem functioning in a tropical mountain biodiversity hotspot

Community trait assembly in highly diverse tropical rainforests is still poorly understood. Based on more than a decade of field measurements in a biodiversity hotspot of southern Ecuador, we implemented plant trait variation and improved soil organic matter dynamics in a widely used dynamic vegetation model (the Lund-Potsdam-Jena General Ecosystem Simulator, LPJ-GUESS) to explore the main drivers of community assembly along an elevational gradient. In the model used here (LPJ-GUESS-NTD, where NTD stands for nutrient-trait dynamics), each plant individual can possess different trait combinations, and the community trait composition emerges via ecological sorting. Further model developments include plant growth limitation by phosphorous (P) and mycorrhizal nutrient uptake. The new model version reproduced the main observed community trait shift and related vegetation processes along the elevational gradient, but only if nutrient limitations to plant growth were activated. In turn, when traits were fixed, low productivity communities emerged due to reduced nutrient-use efficiency. Mycorrhizal nutrient uptake, when deactivated, reduced net primary production (NPP) by 61-72% along the gradient. Our results strongly suggest that the elevational temperature gradient drives community assembly and ecosystem functioning indirectly through its effect on soil nutrient dynamics and vegetation traits. This illustrates the importance of considering these processes to yield realistic model predictions.