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

Cavitation resistance is a critical determinant of drought tolerance in tropical tree species, but little is known of its association with life history strategies, particularly for seasonal dry forests, a system critically driven by variation in water availability. We analysed vulnerability curves for saplings of 13 tropical dry forest tree species differing in life history and leaf phenology. We examined how vulnerability to cavitation (P₅₀) related to dry season leaf water potentials and stem and leaf traits. P₅₀‐values ranged from ?0.8 to ?6.2 MPa, with pioneers on average 38% more vulnerable to cavitation than shade‐tolerants. Vulnerability to cavitation was related to structural traits conferring tissue stress vulnerability, being negatively correlated with wood density, and surprisingly maximum vessel length. Vulnerability to cavitation was negatively related to the Huber‐value and leaf dry matter content, and positively with leaf size. It was not related to SLA. We found a strong trade‐off between cavitation resistance and hydraulic efficiency. Most species in the field were operating at leaf water potentials well above their P₅₀, but pioneers and deciduous species had smaller hydraulic safety margins than shade‐tolerants and evergreens. A trade‐off between hydraulic safety and efficiency underlies ecological differentiation across these tropical dry forest tree species.     

Wood allocation trade-offs between fiber wall,fiber lumen,and axial parenchyma drive drought resistance in neotropical trees

Functional relationships between wood density and measures of xylem hydraulic safety and efficiency are ambiguous, especially in wet tropical forests. In this meta-analysis, we move beyond wood density per se and identify relationships between xylem allocated to fibers, parenchyma, and vessels and measures of hydraulic safety and efficiency. We analyzed published data of xylem traits, hydraulic properties and measures of drought resistance from neotropical tree species retrieved from 346 sources. We found that xylem volume allocation to fiber walls increases embolism resistance, but at the expense of specific conductivity and sapwood capacitance. Xylem volume investment in fiber lumen increases capacitance, while investment in axial parenchyma is associated with higher specific conductivity. Dominant tree taxa from wet forests prioritize xylem allocation to axial parenchyma at the expense of fiber walls, resulting in a low embolism resistance for a given wood density and a high vulnerability to drought-induced mortality. We conclude that strong trade-offs between xylem allocation to fiber walls, fiber lumen, and axial parenchyma drive drought resistance in neotropical trees. Moreover, the benefits of xylem allocation to axial parenchyma in wet tropical trees might not outweigh the consequential low embolism resistance under more frequent and severe droughts in a changing climate.     

Drought's legacy: multiyear hydraulic deterioration underlies widespread aspen forest die‐off and portends increased future risk

Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and carbon‐cycle feedbacks. Recent drought‐induced, widespread forest die‐offs highlight that climate change could accelerate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and precludes mechanistic modeling and prediction of forest die‐off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die‐off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumulated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2 years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavitation, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deterioration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vulnerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climate–vegetation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress‐induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.     

Testing the divergent adaptation of two congeneric tree species on a rainfall gradient using eco‐physio‐morphological traits

In tropical Africa, evidence of widely distributed genera transcending biomes or habitat boundaries has been reported. The evolutionary processes that allowed these lineages to disperse and adapt into new environments are far from being resolved. To better understand these processes, we propose an integrated approach, based on the eco‐physio‐morphological traits of two sister species with adjacent distributions along a rainfall gradient. We used wood anatomical traits, plant hydraulics (vulnerability to cavitation, wood volumetric water content, and hydraulic capacitance), and growth data from the natural habitat, in a common garden, to compare species with known phylogeny, very similar morphologically, but occupying contrasting habitats: Erythrophleum ivorense (wet forest) and Erythrophleum suaveolens (moist forest and forest gallery). We identified some slight differences in wood anatomical traits between the two species associated with strong differences in hydraulics, growth, and overall species distribution. The moist forest species, E. suaveolens, had narrower vessels and intervessel pits, and higher vessel cell‐wall reinforcement than E. ivorense. These traits allow a high resistance to cavitation and a continuous internal water supply of the xylem during water shortage, allowing a higher fitness during drought periods, but limiting growth. Our results confirm a trade‐off between drought tolerance and growth, controlled by subtle adaptations in wood traits, as a key mechanism leading to the niche partitioning between the two Erythrophleum species. The generality of this trade‐off and its importance in the diversification of the African tree flora remains to be tested. Our integrated eco‐physio‐morpho approach could be the way forward.     

Transgenic poplars with reduced lignin show impaired xylem conductivity, growth efficiency and survival

We studied xylem anatomy and hydraulic architecture in 14 transgenic insertion events and a control line of hybrid poplar (Populus spp.) that varied in lignin content. Transgenic events had different levels of down-regulation of two genes encoding 4-coumarate:coenzyme A ligase (4CL). Two-year-old trees were characterized after growing either as free-standing trees in the field or as supported by stakes in a greenhouse. In free-standing trees, a 20 to 40% reduction in lignin content was associated with increased xylem vulnerability to embolism, shoot dieback and mortality. In staked trees, the decreased biomechanical demands on the xylem was associated with increases in the leaf area to sapwood area ratio and wood specific conductivity (k(s)), and with decreased leaf-specific conductivity (k(l)). These shifts in hydraulic architecture suggest that the bending stresses perceived during growth can affect traits important for xylem water transport. Severe 4CL-downregulation resulted in the patchy formation of discoloured, brown wood with irregular vessels in which water transport was strongly impeded. These severely 4CL-downregulated trees had significantly lower growth efficiency (biomass/leaf area). These results underscore the necessity of adequate lignification for mechanical support of the stem, water transport, tree growth and survival.     

Poplar vulnerability to xylem cavitation acclimates to drier soil conditions

Xylem vulnerability to cavitation differs between tree species according to their drought resistance, more xerophilous species being more resistant to xylem cavitation. Variability in xylem vulnerability to cavitation is also found within species, especially between in situ populations. The origin of this variability has not been clearly identified. Here we analyzed the response of xylem hydraulic traits of Populus tremula×Populus alba trees to three different soil water regimes. Stem xylem vulnerability was scored as the xylem water potential causing 12, 50 and 88% loss of conductivity (P₁₂, P₅₀ and P₈₈). Vulnerability to cavitation was found to acclimate to growing conditions under different levels of soil water content, with P₅₀ values of ?1.82, ?2.03 and ?2.45 MPa in well‐watered, moderately water‐stressed and severely water‐stressed poplars, respectively. The value of P₁₂, the xylem tension at which cavitation begins, was correlated with the lowest value of midday leaf water potential (ψm) experienced by each plant, the difference between the two parameters being approximately 0.5 MPa, consistent with the absence of any difference in embolism level between the different water treatments. These results support the hypothesis that vulnerability to cavitation is a critical trait for resistance to drought. The decrease in vulnerability to cavitation under growing conditions of soil drought was correlated with decreased vessel diameter, increased vessel wall thickness and a stronger bordered pit field (t/b)². The links between these parameters are discussed.     

Cavitation and water storage capacity in bole xylem segments of mature and young Douglas-fir trees

Hydraulic specific conductivity, vulnerability to cavitation and water storage capacity of Douglas-fir sapwood was determined for samples from six young (1.0-1.5 m tall) and six mature trees (41-45 m tall). Measurements on samples from young trees showedthere were no effects of two contrasting sample types (entire stem segments vs sectors chiseled out of entire stems) on any of the calculated hydraulic parameters, for vulnerability to cavitation and water storage capacity. Measurements on mature trees were made on wood from four heights on the bole and from two sapwood depths. Outer and inner sapwood at the base of the tree had higher water storage capacities and were more vulnerable to cavitation than was sapwood from the tree top. At every height, old trees were more vulnerable to cavitation than at 1.0 m from the ground in young trees. The water storage capacities showed three distinct phases at the base of the trunk. Young trees had similar water storage capacity (per unit volume of sapwood) to the topof the mature trees, which was lower than the water storage capacity throughout the rest of the bole xylem. The way in which capacitance was calculated (on a volumetric basis vs a relative water content basis) affected the conclusion one would draw at the low water potentials (<-3 MPa). Within a tree, we found an apparent trade-off between having both hydraulic specific conductivity and stem water storage, and vulnerability to cavitation.     

Water transport through tall trees: A vertically explicit,analytical model of xylem hydraulic conductance in stems

Trees grow by vertically extending their stems, so accurate stem hydraulic models are fundamental to understanding the hydraulic challenges faced by tall trees. Using a literature survey, we showed that many tree species exhibit continuous vertical variation in hydraulic traits. To examine the effects of this variation on hydraulic function, we developed a spatially explicit, analytical water transport model for stems. Our model allows Huber ratio, stem‐saturated conductivity, pressure at 50% loss of conductivity, leaf area, and transpiration rate to vary continuously along the hydraulic path. Predictions from our model differ from a matric flux potential model parameterized with uniform traits. Analyses show that cavitation is a whole‐stem emergent property resulting from non‐linear pressure‐conductivity feedbacks that, with gravity, cause impaired water transport to accumulate along the path. Because of the compounding effects of vertical trait variation on hydraulic function, growing proportionally more sapwood and building tapered xylem with height, as well as reducing xylem vulnerability only at branch tips while maintaining transport capacity at the stem base, can compensate for these effects. We therefore conclude that the adaptive significance of vertical variation in stem hydraulic traits is to allow trees to grow tall and tolerate operating near their hydraulic limits.     

Wood structure and function change with maturity: Age of the vascular cambium is associated with xylem changes in current‐year growth

Xylem vessel structure changes as trees grow and mature. Age‐ and development‐related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water‐stress‐induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current‐year growth of young (1–4 years), intermediate (2–7 years), and older (3–10 years) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding plant hydraulic function and architecture requires the sampling of multiple‐aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.     

Replicated throughfall exclusion experiment in an Indonesian perhumid rainforest: wood production,litter fall and fine root growth under simulated drought

Climate change scenarios predict increases in the frequency and duration of ENSO‐related droughts for parts of South‐East Asia until the end of this century exposing the remaining rainforests to increasing drought risk. A pan‐tropical review of recorded drought‐related tree mortalities in more than 100 monitoring plots before, during and after drought events suggested a higher drought‐vulnerability of trees in South‐East Asian than in Amazonian forests. Here, we present the results of a replicated (n = 3 plots) throughfall exclusion experiment in a perhumid tropical rainforest in Sulawesi, Indonesia. In this first large‐scale roof experiment outside semihumid eastern Amazonia, 60% of the throughfall was displaced during the first 8 months and 80% during the subsequent 17 months, exposing the forest to severe soil desiccation for about 17 months. In the experiment's second year, wood production decreased on average by 40% with largely different responses of the tree families (ranging from −100 to +100% change). Most sensitive were trees with high radial growth rates under moist conditions. In contrast, tree height was only a secondary factor and wood specific gravity had no influence on growth sensitivity. Fine root biomass was reduced by 35% after 25 months of soil desiccation while fine root necromass increased by 250% indicating elevated fine root mortality. Cumulative aboveground litter production was not significantly reduced in this period. The trees from this Indonesian perhumid rainforest revealed similar responses of wood and litter production and root dynamics as those in two semihumid Amazonian forests subjected to experimental drought. We conclude that trees from paleo‐ or neotropical forests growing in semihumid or perhumid climates may not differ systematically in their growth sensitivity and vitality under sublethal drought stress. Drought vulnerability may depend more on stem cambial activity in moist periods than on tree height or wood specific gravity.     

Tree hydraulic traits are coordinated and strongly linked to climate‐of‐origin across a rainfall gradient

Plant hydraulic traits capture the impacts of drought stress on plant function, yet vegetation models lack sufficient information regarding trait coordination and variation with climate‐of‐origin across species. Here, we investigated key hydraulic and carbon economy traits of 12 woody species in Australia from a broad climatic gradient, with the aim of identifying the coordination among these traits and the role of climate in shaping cross‐species trait variation. The influence of environmental variation was minimized by a common garden approach, allowing us to factor out the influence of environment on phenotypic variation across species. We found that hydraulic traits (leaf turgor loss point, stomatal sensitivity to drought [P_gs], xylem vulnerability to cavitation [P_x], and branch capacitance [C_branch]) were highly coordinated across species and strongly related to rainfall and aridity in the species native distributional range. In addition, trade‐offs between drought tolerance and plant growth rate were observed across species. Collectively, these results provide critical insight into the coordination among hydraulic traits in modulating drought adaptation and will significantly advance our ability to predict drought vulnerability in these dominant trees species.     

树木高生长限制的几个假说

树木生长到一定年龄后高生长停滞, 对这一现象的解释存在很多争议。成熟假说认为树木顶端分生组织分裂活性下降导致树木高生长减慢。营养限制假说认为土壤中营养元素(特别是氮素)在植物活体或枯落物中积累使土壤中可利用的养分含量降低, 细根生物量增加和叶片光合能力下降导致了地上部分生长的减缓。呼吸假说认为边材呼吸消耗随个体发育的增加使投入到高生长的碳减少。水力限制假说认为水分运输阻力随高度增加而增加导致了叶片总光合碳同化下降, 分配到高生长的生物量减少。树木发展假说认为植物用多种调节机制克服随个体发育增加的水力阻力, 包括叶片结构和生理特征的变化, 叶/边材面积比降低, 边材渗透性和树干储水能力的增加等。水力限制假说得到了较多的关注, 对不同高度树木的叶比导率、光合特征和树干生长量等测定结果支持这一假说。但对这一假说 也存在很多的争议, 主要表现在: 水力阻力是否确实随高度的增加而增加, 水力阻力的分布, 补偿机制的作用和生物量分配转变等。本文综述了树木高生长限制的4个假说以及争论的焦点, 并总结了目前研究的热点问题和今后的研究方向。     

树木高生长限制的几个假说

树木生长到一定年龄后高生长停滞,对这一现象的解释存在很多争议.成熟假说认为树木顶端分生组织分裂活性下降导致树木高生长减慢.营养限制假说认为土壤中营养元素(特别是氮素)在植物活体或枯落物中积累使土壤中可利用的养分含量降低,细根生物量增加和叶片光合能力下降导致了地上部分生长的减缓.呼吸假说认为边材呼吸消耗随个体发育的增加使投入到高生长的碳减少.水力限制假说认为水分运输阻力随高度增加而增加导致了叶片总光合碳同化下降,分配到高生长的生物量减少.树木发展假说认为植物用多种调节机制克服随个体发育增加的水力阻力,包括叶片结构和生理特征的变化,叶/边材面积比降低,边材渗透性和树干储水能力的增加等.水力限制假说得到了较多的关注,对不同高度树木的叶比导率、光合特征和树干生长量等测定结果支持这一假说.但对这一假说也存在很多的争议,主要表现在:水力阻力是否确实随高度的增加而增加,水力阻力的分布,补偿机制的作用和生物量分配转变等.本文综述了树木高生长限制的4个假说以及争论的焦点,并总结了目前研究的热点问题和今后的研究方向.     

Water stress induced cavitation and embolism in some woody plants

A comparison was made of the relative vulnerability of xylem conduits to cavitation and embolism in three species [ Thuja occidentalis L., Tsuga canadensis (L.) Carr. and Acer saccharum Marsh.]. Waterlogged samples of wood were air dehydrated while measuring relative water loss, loss of hydraulic conductance, cumulative acoustic emissions (= cavitations) and xylem water potential. Most cavitation events and loss of hydraulic conductance occurred while water potential declined from – 1 to –6 MPa. There were differences in vulnerability between species. Other people have hypothesized that large xylem conduits (e.g. vessels) should be more vulnerable to cavitations than small conduits (e.g. tracheids). Our findings are contrary to this hypothesis. Under water stress, the vessel bearing wood retained water better than tracheid bearing wood. However, within a species large conduits were more prone to cavitation than small conduits.     

Leaf morphology of 40 evergreen and deciduous broadleaved subtropical tree species and relationships to functional ecophysiological traits

We explored potential of morphological and anatomical leaf traits for predicting ecophysiological key functions in subtropical trees. We asked whether the ecophysiological parameters stomatal conductance and xylem cavitation vulnerability could be predicted from microscopy leaf traits. We investigated 21 deciduous and 19 evergreen subtropical tree species, using individuals of the same age and from the same environment in the Biodiversity‐Ecosystem Functioning experiment at Jiangxi (BEF‐China). Information‐theoretic linear model selection was used to identify the best combination of morphological and anatomical predictors for ecophysiological functions. Leaf anatomy and morphology strongly depended on leaf habit. Evergreen species tended to have thicker leaves, thicker spongy and palisade mesophyll, more palisade mesophyll layers and a thicker subepidermis. Over 50% of all evergreen species had leaves with multi‐layered palisade parenchyma, while only one deciduous species (Koelreuteria bipinnata) had this. Interactions with leaf habit were also included in best multi‐predictor models for stomatal conductance (g_s) and xylem cavitation vulnerability. In addition, maximum g_s was positively related to log ratio of palisade to spongy mesophyll thickness. Vapour pressure deficit (vpd) for maximum g_s increased with the log ratio of palisade to spongy mesophyll thickness in species having leaves with papillae. In contrast, maximum specific hydraulic conductivity and xylem pressure at which 50% loss of maximum specific xylem hydraulic conductivity occurred (Ψ₅₀) were best predicted by leaf habit and density of spongy parenchyma. Evergreen species had lower Ψ₅₀ values and lower maximum xylem hydraulic conductivities. As hydraulic leaf and wood characteristics were reflected in structural leaf traits, there is high potential for identifying further linkages between morphological and anatomical leaf traits and ecophysiological responses.     

Selection for improved growth and wood quality in lodgepole pine: effects on phenology,hydraulic architecture and growth of seedlings

Selection for both growth and wood quality is the primary goal of many tree breeding programs. In order to investigate the impacts of such selection on adaptive traits, observations were made on phenology, drought resistance, xylem conductance and xylem vulnerability to cavitation of 2-year-old seedlings of lodgepole pine (Pinus contorta Dougl. Ex Loud var. latifolia Engelm) from four subpopulations: (1) fast height growth and high wood density (FH); (2) slow height growth and high density (SH); (3) fast height growth and low density (FL); and (4) slow height growth and low density (SL). Results showed that the impact of the selection on phenology was minor; differences in growth among subpopulations were mainly attributable to maximal growth rate and the duration of the period of rapid growth. The FL subpopulation showed a significantly stronger productivity decline in response to moderate drought compared to the two slower-growing subpopulations. The FH subpopulation showed significantly higher xylem specific conductivity than the two slower-growing subpopulations, and greater resistance to cavitation than the other three subpopulations, suggesting the possibility of selecting families that combine fast growth, high wood density and drought tolerance.     

Xylem hydraulic characteristics of subtropical trees from contrasting habitats grown under identical environmental conditions

Five evergreen subtropical tree species growing under identical environmental conditions were investigated to establish which hydraulic properties are genotypically rigid and which show phenotypic plasticity. Maximum xylem-specific conductivity ( k _s) correlated well with the anatomical characteristics (conduit diameter and density) for the four angiosperms Tecomaria capensis , Trichilia dregeana , Cinnamomum camphora and Barringtonia racemosa ; the anatomy of the gymnosperm Podocarpus latifolius was not assessed. Huber values (functional xylem cross-sectional area : leaf area) varied inversely with k _s among species. Maximum leaf-specific conductivity was similar in the five unrelated species. Vulnerability of xylem to cavitation differed between species, as did the relationship between transpiration and water potential. Models of these parameters and isolated midday readings confirm that these trees operate at similar maximum leaf-specific conductivity ( k _l) values. The data are consistent with the hypothesis that conductivity characteristics ( k _l, k _s) are influenced by environment, whereas vulnerability to cavitation is genetically determined.     

Drought and the interannual variability of stem growth in an aseasonal,everwet forest

Linking drought to the timing of physiological processes governing tree growth remains one limitation in forecasting climate change effects on tropical trees. Using dendrometers, we measured fine‐scale growth for 96 trees of 25 species from 2013 to 2016 in an everwet forest in Puerto Rico. Rainfall over this time span varied, including an unusual, severe El Niño drought in 2015. We assessed how growing season onset, median day, conclusion, and length varied with absolute growth rate and tree size over time. Stem growth was seasonal, beginning in February, peaking in July, and ending in November. Species growth rates varied between 0 and 8 mm/year and correlated weakly with specific leaf area, leaf phosphorus, and leaf nitrogen, and to a lesser degree with wood specific gravity and plant height. Drought and tree growth were decoupled, and drought lengthened and increased variation in growing season length. During the 2015 drought, many trees terminated growth early but did not necessarily grow less. In the year following drought, trees grew more over a shorter growing season, with many smaller trees showing a post‐drought increase in growth. We attribute the increased growth of smaller trees to release from light limitation as the canopy thinned because of the drought, and less inferred hydraulic stress than larger trees during drought. Soil type accounted for interannual and interspecific differences, with the finest Zarzal clays reducing tree growth. We conclude that drought affects the phenological timing of tree growth and favors the post‐drought growth of smaller, sub‐canopy trees in this everwet forest. Abstract in Spanish is available with online material.     

Effects of wildfire on growth,transpiration and hydraulic properties of Pinus pinaster Aiton forest

In the last decades, forests have been seriously threatened by the effects of climate change. In Mediterranean regions, increasing fire occurrence and drought have a substantial impact on forests, possibly pushing them towards tipping points. Indeed, although trees could survive to fire events of medium severity in the short term, severe defoliation and environmental stressors could affect the capacity to transport water and to fix and use carbon, amplifying the possibility of a delayed post fire mortality. Therefore, it is urgent to better understand trees eco-physiological processes in response to wildfire in order to plan suitable forest management strategies. In this research an innovative continuous monitoring system of sap flow and micro-stationary climatic conditions of trees was combined with dendro-anatomical analyses and xylem cavitation vulnerability studies to assess fire impact on a Pinus pinaster forest affected by a severe fire and compared to an unburned control site. Xylem analyses showed that the forest-fire did not influence the xylem hydraulic parameters and did not increased vulnerability to embolism. However, burned trees with high defoliation rate, presented reduced assimilation capacity and carbon availability for growth, as well as a limited development of thicker xylem conduit walls compared to unburned trees. Furthermore, continuous monitoring highlighted an increase in sap flow of defoliated trees compared to the control ones during the hottest and driest months of the year indicating a higher stomatal conductance. Overall, our results suggested that although fire did not affect the xylem hydraulics properties of the vascular system, the severe crown damage caused growth reduction and transpiration dysfunction in the monitored period. Those effect could negatively affect the survival of burned trees, especially in areas subjected to hot and dry periods.     

Wood anatomical traits in black spruce reveal latent water constraints on the boreal forest

The effects of climate change on high‐latitude forest ecosystems are complex, making forecasts of future scenarios uncertain. The predicted lengthening of the growing season under warming conditions is expected to increase tree growth rates. However, there is evidence of an increasing sensitivity of the boreal forest to drought stress. To assess the influence of temperature and precipitation on the growth of black spruce (Picea mariana), we investigated long‐term series of wood anatomical traits on 20 trees from four sites along 600 km, the latitudinal range of the closed boreal forest in Quebec, Canada. We correlated the anatomical traits resolved at intraring level with daily temperature, vapor pressure deficit (VPD), and precipitation during the 1943–2010 period. Tree‐ring width, number of cells per ring and cell wall thickness were positively affected by spring and summer daily mean and maximum temperature at the northern sites. These results agree with the well‐known positive effect of high temperatures on tree ring formation at high latitudes. However, we captured, for the first time in this region, the latent impact of water availability on xylem traits. Indeed, in all the four sites, cell lumen area showed positive correlations with daily precipitation (mostly at low latitude), and/or negative correlations with daily mean and maximum temperature and VPD (mostly at high latitude). We inferred that drought, due to high temperatures, low precipitations, or both, negatively affects cell enlargement across the closed boreal forest, including the northernmost sites. The production of tracheids with narrower lumen, potentially more resistant to cavitation, could increase xylem hydraulic safety under a warmer and drier climate. However, this would result in lower xylem conductivity, with consequent long‐term hydraulic deterioration, growth decline, and possibly lead to tree dieback, as observed in other forest ecosystems at lower latitudes.