Huber Value Research Articles

Hydraulics and non-structural carbohydrate contents of Ginkgo biloba under different environmental conditions in Shenyang City, China.

Ginkgo biloba is an important urban ornamental tree species, but poor growth and damages often occur in urban environments. As a street tree species, the decline and death of G. biloba is particularly frequent, with the relevant physiological mechanism being unclear. In this study, we compared hydraulic characteristics, non-structural carbohydrate (NSC) contents and health status between G. biloba trees growing along the streets and those in parks in Shenyang City. The results showed that G. biloba growing along the streets showed higher degrees of branch and leaf mortality than those growing in the parks. Branches of G. biloba growing in both conditions showed lower degrees of xylem embolism. Branch hydraulic vulnerable curves of G. biloba under the two growing conditions also showed no significant difference, with the average P50 being lower than -2.8 MPa. G. biloba growing along the streets had lower leaf area specific conductivity, smaller tracheid diameter, smaller hydraulic diameter, lower soluble sugar content and total NSC than those growing in parks. Hydraulic failure was not the direct reason for the decline and mortality of G. biloba growing along streets. Under the more stressed growth conditions along the streets, G. biloba had smaller tracheid diameters in stems and lower Huber values, which limited the ability of water transport and photosynthetic carbon assimilation at the whole branch level. In addition, in order to deal with more serious stress such as greater heat and drought stresses, G. biloba might need to invest more NSC to repair damage, which further decreaded NSC contents in branches and increased the risk of carbon imbalance. At the same habitat (street or park), xylem hydraulics and NSC contents of G. biloba also showed relatively large difference among sampling sites, which reflected large heterogeneity of urban environment for tree growth.

Urbanization intensifies tree sap flux but divergently for different tree species groups in China.

In recent years, positive and negative effects of urbanization on forest ecosystem have been reported by many studies, while some uncertainties about the impact of urbanization-induced spatial heterogeneity of environmental factors on forest systems still remain unclear. In this study, we analyzed the urbanization effects on sap flux of a common subtropical evergreen tree species Schima superba along an urban-rural gradient in Guangdong Province, South China, and identified the consistency of these results among different groups (evergreen, deciduous, and coniferous species) using data from 83 previously published studies in China. The mean sap flux density (Fd) of S. superba in Xiaoqingshan (XQS), Heshan (HS), Dinghushan (DHS), and Shimentai (SMT), along the urban-rural gradient was 40.9gm-2s-1, 32.1gm-2s-1, 17.0gm-2s-1, and 17.5gm-2s-1, respectively, presenting a decreasing trend with the diminishing urbanization. This pattern in Fd tended to enlarge with tree size and was well confirmed by the enhanced leaf transpiration rate (by 119%) and photosynthetic rate (by 8.8%) for the S. superba in another urbanization gradient from the urban (Hangzhou, denoted as "HZ") to rural sites (Jiande, denoted as "JD") in Zhejiang Province, East China, which has similar climatic condition and urbanization with Guangdong Province. We attributed such positive effects to the decreased sapwood density and specific leaf area (SLA), as well as the increased Huber value (sap wood area/leaf area) and the sap wood specific hydraulic conductivity (KS). We also found that pollutant emission exerted more impact on Fd than climatic factors change, since the variation of the latter was not large enough to cause significant change of Fd under the same climatic zone. In addition, we conducted a principal component analysis (PCA) based on the published 83 studies. Results showed Fd of evergreen tree species was related positively to principle 1 and negatively to principle 2, respectively, whereas the Fd of deciduous broadleaf and coniferous tree species was positively and negatively related to both principles, respectively. This study demonstrated the potential impact of urbanization-related pollutant emission changes on water use of forest trees and the growth among different groups.

祁连山北坡主要木本植物功能性状及其海拔分异

PDF HTML阅读 XML下载 导出引用 引用提醒 祁连山北坡主要木本植物功能性状及其海拔分异 DOI: 10.5846/stxb202110212974 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点研发计划（2019YFC0507405） The main woody plant functional traits and altitude differences on the northern slope of Qilian Mountains Author: Affiliation: Fund Project: National Key R&D Program of China（2019YFC0507405） 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:植物功能性状是植物对生境长期适应所形成的可测量特征，受植物遗传特性和环境因子的共同影响。祁连山是我国干旱区的一条重要山脉，北坡海拔介于2000-5000 m，分布着青海云杉、祁连圆柏等10余种木本植物。深入了解祁连山区木本植物功能性状随海拔梯度分异特征对认识山地植物的适应性和植被垂直地带分布具有重要意义。选取祁连山北坡天然分布的11种木本植物，其中灌木9种，分别是金露梅（Potentilla fruticosa）、银露梅（Potentilla glabra）、水栒子（Cotoneaster multiflorus）、猫儿刺（Ilex pernyi）、吉拉柳（Salix gilashanica）、鬼箭锦鸡儿（Caragana jubata）、刚毛忍冬（Lonicera hispida）、高山绣线菊（Spiraea alpina）和鲜黄小檗（Berberis diaphana），乔木2种，分别是青海云杉（Picea crassifolia）和祁连圆柏（Juniperus przewalskii），调查了木质密度、胡伯尔值、叶面积、叶干物质含量、叶含水率和比叶面积6个枝、叶功能性状。结果表明：（1）灌木的木质密度、胡伯尔值、叶含水率和比叶面积显著高于乔木，但叶面积和叶干物质含量显著低于乔木；（2）木质密度和叶干物质含量随海拔升高显著下降，其他功能性状沿海拔分异不显著；（3）生长季气温与叶干物质含量正相关，与胡伯尔值和叶含水率负相关，生长季降水与各功能性状相关关系不显著。说明乔木和灌木植物采取不同的枝、叶资源利用策略来适应高山生境；随海拔升高，降低结构性碳投资可能限制了高山植被的分布上限；生长季气温是引起性状沿海拔分异的重要环境因子，但不能完全解释性状沿海拔分异规律。 Abstract:Plant functional traits are the measurable characteristics of plants which are formed by long-term adaptation to habitat environmental conditions. The characterization of plant functional traits is mainly influenced by both the plants specific genetic basis and the shifted environmental factors. Qilian Mountains, with an elevation ranging from 2000 to 5000 m, is one of the major mountains in the dry land in Northwest China, where naturally distributed more than 10 kinds of woody plants such as Picea crassifolia and Juniperus przewalskii on the northern slope. It is of great academic and practice significance to understand the acclimation strategy and the vertical distribution pattern of alpine woody plants by deeply investigating the woody plant functional traits variation and the behind driving mechanism along with the altitude gradients in the Qilian Mountains. In the present study, we investigated 11 naturally distributed woody plant species along with the altitude in the Qilian Mountains, which included 9 shrub species (Potentilla fruticosa, Potentilla glabra, Cotoneaster multiflorus, Ilex pernyi, Salix gilashanica, Caragana jubata, Lonicera hispida, Spiraea alpine, and Berberis diaphana, respectively) and 2 tree species (Picea crassifolia and Juniperus przewalskii, respectively). A total of six plant functional traits of shoot and leaf were investigated in this study, and which include woody density, huber value, leaf area, leaf dry matter content, leaf water content, and specific leaf area, respectively. The results showed that:(1) woody density, huber value, leaf water content, and specific leaf area of shrub species were significantly greater than those of tree species, and leaf area and leaf dry matter content of shrub species were significantly lower than those of tree species. (2) Woody density and leaf dry matter content declined significantly along with the altitude gradients increased, and the other functional traits changed insignificantly along with the altitude gradients increased. (3) Growing-season air temperature was positively correlated with leaf dry matter content, and was negatively correlated with huber value and leaf water content; however, non-significant relationships between growing-season precipitation and plant functional traits were found. The results showed that tree and shrub plants adapted to the alpine habitats by adopting different branch and leaf resources utilization strategies; the decreasing structure carbon investment with the altitude gradients increased possibly has limited the upshift of alpine plants; the growing-season air temperature was one of the key environmental factors tuning woody plant functional traits variation pattern along with the altitudes gradients but could not explain all the variance. 参考文献 相似文献 引证文献

地下水埋深和季节性干旱对古尔班通古特沙漠南缘梭梭生理和生长的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 地下水埋深和季节性干旱对古尔班通古特沙漠南缘梭梭生理和生长的影响 DOI: 10.5846/stxb202101190206 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 新疆维吾尔自治区天山青年计划项目（2020Q025）；国家自然科学基金项目（32171874）；新疆维吾尔自治区自然科学基金项目（2019D01A98） Effects of groundwater depth and seasonal drought on the physiology and growth of Haloxylon ammodendron at the southern edge of Gurbantonggut Desert Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:干旱区因降水稀少，地下水成为荒漠植被重要且稳定的水源。选取古尔班通古特沙漠南缘建群种植物梭梭（Haloxylon ammodendron）为研究对象，通过测量不同地下水埋深（3.45、9.08、10.47、13.27 m和15.91 m）下生长季前期和后期同化枝生理生化指标（黎明水势、正午水势、含水量、氯离子、钠离子、脯氨酸和非结构性碳水化合物）和生长与形态特征（生长速率和胡伯尔值），旨在认识荒漠植物对地下水埋深增加和季节性干旱的响应特征和调节适应机制。结果表明：（1）梭梭应对地下水埋深变化的生理调节对策，是采取先降低后升高黎明前同化枝水势、降低新枝形成期同化枝生长速率、增大胡伯尔值和积累非结构性碳水化合物的策略；（2）梭梭应对生长季大气干旱的生理调节对策，是通过降低黎明前同化枝水势、维持较高胡伯尔值、积累钠离子和消耗淀粉抵御季节性干旱；（3）在大气干旱与地下水水文干旱交互作用下，梭梭是采取降低正午同化枝水势、维持较高的同化枝含水量和积累可溶性糖的生态策略。综上所述，梭梭在响应地下水水文干旱和季节性大气干旱的生理特征间存在差异。研究结果丰富了水文和大气干旱对梭梭生理和生长影响的认知，可以为基于地下水资源管理的干旱区荒漠植被保育提供参考。 Abstract:Groundwater is an important and stable water source for desert vegetation due to the scarcity of precipitation in arid areas. Currently, anthropogenic withdraw of groundwater and climatic drought further limits the water availability for phreatophytic vegetation in arid regions and results in the decline of native vegetation. However, how phreatophytic plant response and adaptation to reduced water availability along the groundwater depth gradient are not clear. Haloxylon ammodendron was widely distributed in northwest arid zone of China and played an important role in desertification control. Groundwater was an important water resource for H. ammodendron. The declining groundwater depth has caused the partially decline even death of H. ammodendron in their native distribution area. In this study, we selected H. ammodendron, one of the main constructive species, at the southern edge of Gurbantonggut Desert as the taget species. In order to understand the response characteristics and regulatory adaptation mechanisms of desert plants to increasing groundwater depth and seasonal drought, we measured physiological and biochemical traits (including predawn water potential, midday water potential, water content, sodium ion, chloride ion, proline and non-structural carbohydrates), growth and morphological characteristics (including growth rate and Huber value) of assimilation branches in the early and late growing season under different groundwater depths (3.45, 9.08, 10.47, 13.27 and 15.91 m). The results showed that:(1) H. ammodendron adopted the strategy of first decreasing and then increasing predawn assimilation branch water potential, decreasing assimilation branch growth rate during new branch formation, increasing the Huber value and accumulating non-structural carbohydrates in response to increase in groundwater depth. (2) With the prolonged drought, H. ammodendron reduced predawn assimilation branch water potential, maintained high Huber values, accumulated sodium ions and consumed starch to resist seasonal drought during the growing season. (3) Reducing midday assimilation branch water potential, maintaining high assimilation branch water content and accumulating soluble sugars are ecological strategies in H. ammodendron to acclimate atmospheric and hydrological drought. In summary, there are differences in the physiological characteristics of H. ammodendron in response to groundwater hydrological drought and seasonally atmospheric drought. The results of the study enriched the knowledge of the effects of hydrological and atmospheric drought on the physiology and growth of H. ammodendron, and provided a reference for the conservation of desert vegetation based on groundwater resource management in arid areas. 参考文献 相似文献 引证文献

Impact of groundwater depth on hydraulic performance and growth of Haloxylon ammodendron in a desert region of central Asia

AbstractAnthropogenic withdraw of groundwater and climatic drought limits the water availability for phreatophytic vegetation in arid regions. In this study, a small xeric, phreatophytic tree Haloxylon ammodendron (C.A. Mey.) was investigated to understand the influence of depth to groundwater (DGW) on hydraulic traits and on the trade‐off between drought tolerance and leaf area increment. A suite of traits including leaf water potential, pressure–volume (P–V) curves, Huber value, assimilation branch growth and osmotic regulation substance was measured across five sites with DGW ranges from 3.45 to 15.91 m. Our results indicate that H. ammodendron was subject to greater water stress with increasing DGW, as indicated by decreased predawn (Ψpd) and midday (Ψmd) branch water potential. We also found that growth rate declined as Huber value increased with increasing DGW in the early growing season (EGS). Solute sugar, as a major osmotic substance, drives decreases in osmotic potential at full turgor and thus constrains assimilation branch growth with increasing DGW in EGS. Therefore, osmotic adjustment accompanied with water potential regulation (Ψpd–Ψmd) and plasticity of Huber value allows this phreatophyte to absorb water from deeper soil layers and tolerate drought. However, these adaptive adjustments cannot fully compensate for nonoptimal water conditions as growth rate continued to decrease as DGW increased in EGS and even became negative in the late growing season (LGS) at almost all sites. Greater depth of groundwater had a significant effect on H. ammodendron and may have similar effects for other non‐riparian phreatophytic plants in arid regions.

Leaf trait modification in European beech trees in response to climatic and edaphic drought.

Leaf morphological and physiological traits control the carbon and water relations of mature trees and are determinants of drought tolerance, but it is not well understood how they are modified in response to water deficits. We analysed five sun-canopy leaf traits (mean leaf size (LS), specific leaf area (SLA), Huber value (HV), water potential at turgor loss point (Ψtlp ) and foliar carbon isotope signature (δ13 C)) in European beech (Fagus sylvatica L.) across three precipitation gradients sampled in moist (2010), dry (2019) and very dry (2018) summers, and tested their response to short-term water deficits (climatic water balance (CWB) preceding sample collection) and long-term water availability (mean annual precipitation (MAP), plant-available soil water capacity (AWC) and neighbourhood competition). Across the 34 sites, LS varied seven-fold (3.9-27.0 cm2 ), SLA four-fold (77.1-306.9 cm²·g-1 ) and HV six-fold (1.0-6.65 cm2 ·m-2 ). In the 2018 dataset, LS showed a negative and HV a positive relationship to MAP, which contradicts relations found in multi-species samples. Average Ψtlp ranged from -1.90 to -2.62 MPa and decreased across the sites with decreasing CWB in the month prior to measurement, as well as with decreasing MAP and AWC in 2019. Studied leaf traits varied considerably between years, suggesting that mast fruiting and the severe 2018 drought caused the formation of smaller leaves. We conclude that sun-canopy leaf traits of European beech exhibit considerable plasticity in response to climatic and edaphic aridity, and that osmotic adjustment may be an important element in the drought response strategy of this anisohydric tree species.

Selecting tree species with high transpiration and drought avoidance to optimise runoff reduction in passive irrigation systems

Rainfall in cities can generate large volumes of stormwater runoff which degrades receiving waterways. Irrigating trees with runoff (passive irrigation) has the potential to increase transpiration and contribute to stormwater management by reducing runoff received by downstream waterways, but the stochastic nature of rainfall may expose trees with high transpiration to drought stress. We hypothesized that for success in passive irrigation systems, tree species should exhibit i) high maximum transpiration rates under well-watered conditions, ii) drought avoidance between rainfall events, and iii) high recovery of transpiration with rainfall following a drought. We assessed 13 commonly planted urban tree species in Melbourne, Australia against three metrics representing these behaviours (crop factor, hydroscape area, and transpiration recovery, respectively) in a glasshouse experiment. To aid species selection, we also investigated the relationships between these three metrics and commonly measured plant traits, including leaf turgor loss point, wood density, and sapwood to leaf area ratio (Huber value). Only one species (Tristaniopsis laurina) exhibited a combination of high crop factor (>1.1 mm mm−1 d−1) indicating high transpiration, small hydroscape area (<3 MPa2) indicating drought avoidance, and high transpiration recovery (>85%) following water deficit. Hence, of the species measured, it had the greatest potential to reduce runoff from passive irrigation systems while avoiding drought stress. Nevertheless, several other species showed moderate transpiration, hydroscape areas and transpiration recovery, indicating a balanced strategy likely suitable for passive irrigation systems. Huber values were negatively related to crop factor and transpiration recovery and may therefore be a useful tool to aid species selection. We propose that selecting tree species with high transpiration rates that can avoid drought and recover well could greatly reduce stormwater runoff, while supporting broader environmental benefits such as urban cooling in cities.

Are leaf, stem and hydraulic traits good predictors of individual tree growth?

Abstract A major foundation of trait‐based ecology is that traits have an impact on individual performance. However, trait–growth relationships have not been extensively assessed in trees, especially outside tropical ecosystems. In addition, measuring traits directly related to physiological processes remains difficult and the differences between inter‐ and intraspecific relationships are seldom explored. Here, we use individual‐level data on a set of hydraulic, leaf and stem traits to assess their ability to predict basal area increment (BAI) and growth efficiency (BAI per unit of tree leaf area, GE) among and within species for six dominant tree species along a water availability gradient under Mediterranean climate (Catalonia, NE Spain). Measured traits include: leaf mass per area (LMA), leaf nitrogen concentration (N), leaf C isotopic composition (δ13C), the leaf water potential at turgor loss (Ptlp), stem wood density (WD) and branch‐level estimates of the Huber value (Hv), sapwood‐ and leaf‐specific hydraulic conductivity (KS and KL) and resistance to xylem embolism (P50). Trait–growth associations were generally weak, particularly for BAI and within species. High values of both growth metrics were associated with ‘conservative’ leaf and hydraulic traits. In particular, BAI was negatively associated with KL (and wood density), while GE increased with LMA, allocation to sapwood relative to leaves (Hv) and resistance to xylem embolism (P50). Climate effects on BAI and GE were indirectly mediated by changes in traits, stand structure and tree basal area. Overall, these results suggest that maintaining functionality over extended periods of time may be more important than maximum gas exchange or hydraulic capacity to achieve high radial growth under Mediterranean climates. Our study reveals that widely used ‘functional traits’ may be poor predictors of tree growth variability along environmental gradients. Moreover, trait effects (when present) do not necessarily conform to simple hypotheses based on our understanding of organ‐level processes. An improved understanding of trait coordination along common axes of variation together with a revaluation of the variables that better reflect whole‐tree performance can greatly improve our understanding of trait–growth relationships. A free Plain Language Summary can be found within the Supporting Information of this article.

Coordination of plant hydraulic and photosynthetic traits: confronting optimality theory with field measurements.

Summary Close coupling between water loss and carbon dioxide uptake requires coordination of plant hydraulics and photosynthesis. However, there is still limited information on the quantitative relationships between hydraulic and photosynthetic traits.We propose a basis for these relationships based on optimality theory, and test its predictions by analysis of measurements on 107 species from 11 sites, distributed along a nearly 3000‐m elevation gradient.Hydraulic and leaf economic traits were less plastic, and more closely associated with phylogeny, than photosynthetic traits. The two sets of traits were linked by the sapwood to leaf area ratio (Huber value, v H). The observed coordination between v H and sapwood hydraulic conductivity (K S) and photosynthetic capacity (V cmax) conformed to the proposed quantitative theory. Substantial hydraulic diversity was related to the trade‐off between K S and v H. Leaf drought tolerance (inferred from turgor loss point, –Ψtlp) increased with wood density, but the trade‐off between hydraulic efficiency (K S) and –Ψtlp was weak. Plant trait effects on v H were dominated by variation in K S, while effects of environment were dominated by variation in temperature.This research unifies hydraulics, photosynthesis and the leaf economics spectrum in a common theoretical framework, and suggests a route towards the integration of photosynthesis and hydraulics in land‐surface models.

A multiple-trait analysis of ecohydrological acclimatisation in a dryland phreatophytic shrub

Water is the main limiting factor for groundwater-dependent ecosystems (GDEs) in drylands. Predicted climate change (precipitation reductions and temperature increases) and anthropogenic activities such as groundwater drawdown jeopardise the functioning of these ecosystems, presenting new challenges for their management. We developed a trait-based analysis to examine the spatiotemporal variability in the ecophysiology of Ziziphus lotus, a long-lived phreatophyte that dominates one of the few terrestrial GDEs of semiarid regions in Europe. We assessed morpho-functional traits and stem water potential along a naturally occurring gradient of depth-to-groundwater (DTGW, 2–25 m) in a coastal aquifer, and throughout the species-growing season. Increasing DTGW and salinity negatively affected photosynthetic and transpiration rates, increasing plant water stress (lower predawn and midday water potential), and positively affected Huber value (sapwood cross-sectional area per leaf area), reducing leaf area and likely, plant hydraulic demand. However, the species showed greater salt-tolerance at shallow depths. Despite groundwater characteristics, higher atmospheric evaporative demand in the study area, which occurred in summer, fostered higher transpiration rates and water stress, and promoted carbon assimilation and water loss more intensively at shallow water tables. This multiple-trait analysis allowed us to identify plant ecophysiological thresholds related to the increase in salinity, but mostly in DTGW (13 m), and in the evaporative demand during the growing season. These findings highlight the existence of tipping points in the functioning of a long-lived phreatophyte in drylands and can contribute to the sustainable management of GDEs in southern Europe, paving the way for further studies on phreatophytic species.

Hydraulic variability of three temperate broadleaf tree species along a water availability gradient in central Europe

Plant hydraulic traits are key for understanding and predicting tree drought responses. Information about the degree of the traits' intra-specific variability may guide the selection of drought-resistant genotypes and is crucial for trait-based modelling approaches. For the three temperate minor broadleaf tree species Acer platanoides, Carpinus betulus and Tilia cordata, we measured xylem embolism resistance (P50 ), leaf turgor loss point (PTLP ), specific hydraulic conductivity (KS ), Huber values (HVs), and hydraulic safety margins in adult trees across a precipitation gradient. We further quantified trait variability on different organizational levels (inter-specific to within-canopy variation), and analysed its relationship to climatic and soil water availability. Although we observed a certain intra-specific trait variability (ITV) in safety-related traits (P50 , PTLP ) with higher within-tree and between-tree than between populations variability, the magnitude was small compared to inter-specific differences, which explained 78.4% and 58.3% of the variance in P50 and PTLP , respectively. In contrast, efficiency-related traits (KS , HV) showed a high ITV both within populations and within the crowns of single trees. Surprisingly, the observed ITV of all traits was neither driven by climatic nor soil water availability. In conclusion, the high degree of conservatism in safety-related traits highlights their potential for trait-based modelling approaches.

Climate and functional traits jointly mediate tree water-use strategies.

Tree water use is central to plant function and ecosystem fluxes. However, it is still unknown how organ-level water-relations traits are coordinated to determine whole-tree water-use strategies in response to drought, and whether this coordination depends on climate. Here we used a global sap flow database (SAPFLUXNET) to study the response of water use, in terms of whole-tree canopy conductance (G), to vapour pressure deficit (VPD) and to soil water content (SWC) for 142 tree species. We investigated the individual and coordinated effect of six water-relations traits (vulnerability to embolism, Huber value, hydraulic conductivity, turgor-loss point, rooting depth and leaf size) on water-use parameters, also accounting for the effect of tree height and climate (mean annual precipitation, MAP). Reference G and its sensitivity to VPD were tightly coordinated with water-relations traits rather than with MAP. Species with efficient xylem transport had higher canopy conductance but also higher sensitivity to VPD. Moreover, we found that angiosperms had higher reference G and higher sensitivity to VPD than did gymnosperms. Our results highlight the need to consider trait integration and reveal the complications and challenges of defining a single, whole-plant resource use spectrum ranging from 'acquisitive' to 'conservative'.

Coordinated variation in stem and leaf functional traits of temperate broadleaf tree species in the isohydric-anisohydric spectrum.

Stomatal regulation serves as an important strategy for plants to adapt to drought. However, the understanding of how complexes of plant-functional traits vary along the continuum from isohydry to anisohydry remains insufficient. In this study, we investigated a proxy of the degree of iso/anisohydry-the water potential at stomatal closure-and a series of functional traits of leaves and branches in 20 temperate broadleaf species planted in an arid limestone habitat in northern China. The results showed that the water potential at stomatal closure was significantly correlated with many functional traits. At the anisohydric end of the spectrum, species had a higher leaf carbon content and vein density, a greater stomatal length, a thicker lower leaf epidermis, higher embolism resistance, higher wood density, a greater Huber value, a greater ratio of fiber wall thickness to xylem lumen diameter, a larger proportion of total fiber wall area to xylem cross-sectional area, a lower water potential at the turgor loss point (TLP), a smaller relative water content at the TLP, a lower osmotic potential at full turgor and a smaller specific leaf area. It is concluded that a continuum of coordination and trade-offs among co-evolved anatomical and physiological traits gives rise to the spectrum from isohydry to anisohydry spanned by the 20 tree species, and the anisohydric species showed stronger stress resistance, with greater investment in stems and leaves than the isohydric species to maintain stomatal opening under drought conditions.

Vulnerability to xylem cavitation of Hakea species (Proteaceae) from a range of biomes and life histories predicted by climatic niche.

Extreme drought conditions across the globe are impacting biodiversity, with serious implications for the persistence of native species. However, quantitative data on physiological tolerance are not available for diverse flora to inform conservation management. We quantified physiological resistance to cavitation in the diverse Hakea genus (Proteaceae) to test predictions based on climatic origin, life history and functional traits. We sampled terminal branches of replicate plants of 16 species in a common garden. Xylem cavitation was induced in branches under varying water potentials (tension) in a centrifuge, and the tension generating 50 % loss of conductivity (stem P50) was characterized as a metric for cavitation resistance. The same branches were used to estimate plant functional traits, including wood density, specific leaf area and Huber value (sap flow area to leaf area ratio). There was significant variation in stem P50 among species, which was negatively associated with the species climate origin (rainfall and aridity). Cavitation resistance did not differ among life histories; however, a drought avoidance strategy with terete leaf form and greater Huber value may be important for species to colonize and persist in the arid biome. This study highlights climate (rainfall and aridity), rather than life history and functional traits, as the key predictor of variation in cavitation resistance (stem P50). Rainfall for species origin was the best predictor of cavitation resistance, explaining variation in stem P50, which appears to be a major determinant of species distribution. This study also indicates that stem P50 is an adaptive trait, genetically determined, and hence reliable and robust for predicting species vulnerability to climate change. Our findings will contribute to future prediction of species vulnerability to drought and adaptive management under climate change.

Coordination of stem and leaf traits define different strategies to regulate water loss and tolerance ranges to aridity.

Adaptation to drought involves complex interactions of traits that vary within and among species. To date, few data are available to quantify within-species variation in functional traits and they are rarely integrated into mechanistic models to improve predictions of species response to climate change. We quantified intraspecific variation in functional traits of two Hakea species growing along an aridity gradient in southeastern Australia. Measured traits were later used to parameterise the model SurEau to simulate a transplantation experiment to identify the limits of drought tolerance. Embolism resistance varied between species but not across populations. Instead, populations adjusted to drier conditions via contrasting sets of trait trade-offs that facilitated homeostasis of plant water status. The species from relatively mesic climate, Hakea dactyloides, relied on tight stomatal control whereas the species from xeric climate, Hakea leucoptera dramatically increased Huber value and leaf mass per area, while leaf area index (LAI) and epidermal conductance (gmin ) decreased. With trait variability, SurEau predicts the plasticity of LAI and gmin buffers the impact of increasing aridity on population persistence. Knowledge of within-species variability in multiple drought tolerance traits will be crucial to accurately predict species distributional limits.

The importance of intraspecific trait variability in promoting functional niche dimensionality

Both inter‐ and intraspecific trait variation are critical to species distribution along environmental gradients, but our understanding of these patterns predominantly relies upon species‐level trait means and variances. Trait integration, defined as how strongly multiple traits covary with one another, is a key indicator of the dimensionality of functional space for accommodating biodiversity in communities. As trait covariance can differ dramatically at the interspecific versus intraspecific levels, how intraspecific trait variability alters the strength of trait integration and eventually modulates biodiversity along environmental gradients has been rarely tested. Here, we measured nine functional traits (leaf area, specific leaf area, leaf and stem dry‐matter content, leaf nitrogen and phosphorus contents, specific stem length, Huber value and maximum height) paired with site‐specific soil fertility for 70 woody communities in subtropical Chinese forests. All species‐by‐site combinations were sampled to ensure a sufficient representation of intraspecific trait variation across sites. Community‐level trait integration was quantified from the variance of eigenvalues of the trait correlation matrix. The direct and/or indirect effects of soil fertility and trait integration on species richness and trait diversity were assessed through path analyses. Trait integration quantified from both inter‐ and intraspecific variances was on average 21.7% weaker than that from only interspecific variance, indicating a crucial role of intraspecific trait variability in promoting niche dimensionality. Whether accounting for intraspecific variation or not, less fertile sites had stronger trait integration, which in turn depressed both taxonomic and functional diversity, supporting the assumption that higher environmental stress demands stronger tradeoffs among multiple functions in viable strategies. Importantly, the negative association between trait integration and species richness became stronger when accounting for intraspecific variation, suggesting that species distribution and occurrence can be a consequence of intraspecific trait variability. This study highlights the importance of intraspecific trait variability in understanding functional tradeoffs underlying biodiversity patterns.

Phenotypic Plasticity of Drought Tolerance Traits in a Widespread Eucalypt (Eucalyptus obliqua)

Long-term studies of tree responses to drought stress help us to understand the capacity of species to adapt to their environment. In this study, we investigated how Eucalyptus obliqua adjusts physiological and morphological traits in response to seasonal and multi-year droughts. We monitored physiological and morphological traits over multiple years in undisturbed control and throughfall reduction plots in a eucalypt forest in south-eastern Australia. The throughfall reduction treatment did not induce significantly lower soil moisture in the throughfall reduction plots compared with the control plots. However, natural variability in precipitation and evaporative demand induced drought stress of varying intensity each summer in all plots. We observed a significant relationship between seasonal precipitation and leaf pre-dawn water potential (ΨPD), with less precipitation over summer, resulting in a decline in ΨPD and drought stress when ΨPD fell below −0.75 MPa. Eucalyptus obliqua responded to short-term summer drought through rapid leaf osmotic adjustment which lowered the leaf water potential at the turgor loss point beyond the minimum leaf water potential. Morphological adjustments, such as the reduction of leaf area to sapwood area (higher Huber Value) were moderate during the measurement period and only occurred under severe drought stress (pre-dawn water potential &lt; −1.2 MPa). Overall, E. obliqua responded to short-term mild drought stress through physiological trait plasticity, while morphological adjustment only occurred under a more severe water deficit.

Correlations between leaf economics, hydraulic, and shade-tolerance traits among co-occurring individual trees

Leaf economics, hydraulic, and shade-tolerance traits involve different resource-use strategies, and are critical for interactive plants, yet it remains unclear whether co-occurring individuals modulate the coupling relationships between these trait dimensions. To address this knowledge gap, we measured four leaf economics, five hydraulic, and two shade-tolerant traits in leaf and wood tissues across 90 individuals from 26 tree species in two subtropical forests in Eastern China. Confirmatory factor analysis (CFA) and Principal Component Analysis (PCA) were employed to explore multi-trait relationships. With respect to leaf economics traits, leaf nitrogen concentrations were positively associated with leaf stomatal density and conductance, leaf-specific hydraulic conductivity, and twig vessel diameter. However, leaf phosphorus concentrations were negatively associated with leaf-specific hydraulic conductivity. The specific leaf area was negatively associated with the light exposure index, light saturation point, and leaf Huber value. Regarding hydraulic traits, twig vessel diameters were positively correlated with the light saturation point and light exposure index. Stomatal conductance was positively associated with the light saturation point, whereas leaf-specific hydraulic conductivity was positively associated with the light exposure index. The first two PCA axes explained 54.23% of the total trait variation. The first axis of the multi-trait variation was mainly explained by leaf habits and represented the trade-offs between acquisitive hydraulic economics and cheap leaf economics. The second axis of the multi-trait variation was mainly explained by plant growth form, and defined the trade-offs between weak shade tolerance and cheap leaf area construction. In conclusion, our results provide evidence for the coupling relationships between leaf economics, hydraulic, and shade-tolerant traits among co-occurring individuals. This study also highlights that bivariate trait relationships based on large-scale interspecific surveys may be the opposite of that within-community scales.

Aridity drives coordinated trait shifts but not decreased trait variance across the geographic range of eight Australian trees.

Large intraspecific functional trait variation strongly impacts many aspects of communities and ecosystems, and is the medium upon which evolution works. Yet intraspecific trait variation is inconsistent and hard to predict across traits, species and locations. We measured within-species variation in leaf mass per area (LMA), leaf dry matter content (LDMC), branch wood density (WD), and allocation to stem area vs leaf area in branches (branch Huber value (HV)) across the aridity range of seven Australian eucalypts and a co-occurring Acacia species to explore how traits and their variances change with aridity. Within species, we found consistent increases in LMA, LDMC and WD and HV with increasing aridity, resulting in consistent trait coordination across leaves and branches. However, this coordination only emerged across sites with large climate differences. Unlike trait means, patterns of trait variance with aridity were mixed across populations and species. Only LDMC showed constrained trait variation in more xeric species and drier populations that could indicate limits to plasticity or heritable trait variation. Our results highlight that climate can drive consistent within-species trait patterns, but that patterns might often be obscured by the complex nature of morphological traits, sampling incomplete species ranges or sampling confounded stress gradients.

Species‐specific transpiration and water use patterns of two pioneer dominant tree species under manipulated rainfall in a low‐subtropical secondary evergreen forest

AbstractLike many other ecosystems, subtropical forests are suffering more intense and longer droughts with the ongoing climate change. This study aimed to explore the seasonal transpiration and physiological responses of two dominant tree species, Schima superba and Michelia macclurei, to manipulated precipitation patterns in a subtropical evergreen forest of South China, in which an ambient control treatment (BC), a drier dry and wetter wet season treatment (DD) and an extended dry and wetter wet season treatment (ED) were applied. Tree water use and associated ecophysiological characters, such as the daily whole‐tree transpiration (EL), intrinsic water use efficiency (WUEi), Huber values (As:Al) and utilization proportions from different water sources were determined during the period from October 2012 to September 2013. For both tree species, no significant difference in transpiration among the three treatments was observed in the wet season, and a relatively stronger decrease of transpiration occurred under DD and ED treatments during the later dry season. Moreover, the higher transpiration of M. macclurei and its advantage of utilizing the shallow water derived from light rainfall under dry condition suggested that it has more survival and growth advantages in this subtropical forest. Therefore, under the seasonal drought caused by uneven distribution of rainfall in the future, M. macclurei that inclines to use shallow soil water would adopt a drought‐avoidance strategy, whereas S. superba that could uptake deeper soil water would be drought tolerant. The different spatial and temporal patterns of water use, together with the contrasting water use strategies, could reduce competition of the two species and facilitate their coexistence under potential precipitation distribution changes.