Quantifying the relative contributions of evolutionary mechanisms to tree water-use strategies is critical for predicting species’ responses to climate change and supporting forest management strategies. Common garden experiments can explicitly address the contributions of genetics and plasticity in physiological-related traits. However, these experiments typically focus on young trees, and long-term physiological measurements from common garden experiments are largely lacking. Stable isotope analysis of tree rings bridges this gap by enabling the reconstruction of long-term water-use strategies of mature trees growing in long-term common garden experiments.In this study, we investigate the evolutionary mechanisms underlying long-term water-use strategies in Quercus petraea across its distribution range by analysing annually-resolved stable isotope ratios (δ¹³C, δ¹⁸O, δ²H) from tree-ring cellulose. We sampled 234 individuals originating from nine provenances grown in four European common gardens (Denmark, France, Poland, and the United Kingdom). For the period 2012–2021, we derived annual carbon isotope discrimination (∆¹³C), intrinsic water-use efficiency (iWUE), and isotopic enrichment relative to precipitation (∆¹⁸O and ∆²H). Linear mixed-effects models were used to quantify the contributions of genetic variation, phenotypic plasticity, and its interaction (i.e. genetically-based plasticity) to variation in iWUE, ∆¹⁸O, and ∆²H. The dual-isotope approach (δ¹³C and ∆¹⁸O) was applied to investigate the provenance-specific adjustments in photosynthetic rate and stomatal conductance across sites.Our results revealed significant genetic and genetically-based plasticity effects on all isotope ratios whereas phenotypic plasticity had a significant effect only on ∆²H. ∆¹⁸O and ∆²H exhibited distinct patterns related to genetics and phenotypic plasticity effects. Notably, ∆²H variability across sites exceeded provenance-level variation. These results could be indirectly related to the link of ∆²H to primary C metabolism. The dual-isotope analysis (δ¹³C and ∆¹⁸O) further identified adjustments in stomatal conductance as the main plastic response to contrasting environments. The provenance with the least plasticity (originally from the United Kingdom) also showed reductions in photosynthetic rates, indicating a limited capacity to adjust to contrasting environments. Overall, these findings highlight strong genetic and plastic control in water-use traits and demonstrate the potential of stable isotopes in tree rings to unravel evolutionary mechanisms in tree water-use strategies.

Understanding wood formation is critical for interpreting tree growth and carbon allocation under changing environmental conditions. While major progress has been made for gymnosperms, harmonized approaches for studying xylogenesis in angiosperms remain limited. Here, we present practical recommendations for observing and analysing xylogenesis in angiosperm trees, illustrated from examples from temperate and sub-Mediterranean forests. The perspective includes guidance on identifying xylem cell types in histological sections, defining developmental phenophases and establishing a workflow for data collection (and analysis). Annotated images are provided to support reproducibility and inter-observer consistency. We also discuss key challenges unique to angiosperms, including cell-type-specificities and wood type differences. Future research priorities include conserving histological images, extending xylogenesis to branches and coarse roots, enabling cross-biome comparisons and advancing kinetic analysis. This framework supports the coordinated expansion of angiosperm xylogenesis studies, enabling deeper insights into tree functioning in a changing world.

The persistence of organisms in changing climates depends on both phenotypic plasticity and adaptation. However, despite extensive research, it remains largely unclear how forest trees will genetically adapt or phenotypically acclimate to future climates. In this study, we investigated the genetic architecture of tree-ring and leaf traits in sessile oak (Quercus petraea) and European beech (Fagus sylvatica) and tested for signals of climate adaptation at loci controlling these traits. We combined trait data from common gardens established in the 1990s across the species' European ranges with climatic and genomic data. We conducted genome-wide association (GWA) analyses and tested the effects of temperature and precipitation transfer distances on associated loci. Our GWA analyses revealed a complex, multi-locus genetic architecture underlying functional traits in both species. Sessile oak displayed a greater number of trait associations than European beech and we found indications of pleiotropy, particularly in sessile oak. European beech showed limited plastic and adaptive responses to climate at the genomic level. In contrast, sessile oak showed signals of both adaptive plasticity and climate adaptation, particularly at loci associated with growth responses to extreme droughts and to long-term summer temperature. Our findings suggest that sessile oak possesses a stronger capacity to respond to climate change than European beech. This capacity may facilitate range expansion in sessile oak, while raising concerns about the sustainability of beech in European lowland forests under future climates.

Siberian evergreen conifers maintain stable starch reserves, while deciduous larch adopts a dynamic carbon strategy with autumn NSC accumulation to fuel spring growth, enhancing boreal forest resilience. Non-structural carbohydrates (NSC) play a fundamental role in tree growth, metabolism, and resilience by buffering temporal imbalances between carbon supply and demand. However, comparative studies integrating long-term growth records with stem NSC dynamics remain scarce in Siberian boreal forests. Here, we combine six decades of tree-ring width (TRW; 1960–2022) analyses with radial stem NSC profiling (1993–2022) and seasonal NSC measurements during the 2022 growing season to investigate growth responses and stem carbohydrate storage dynamics in three co-existing Siberian conifers (Pinus sylvestris L., Abies sibirica Ledeb., and Larix sibirica Ledeb.). Tree-ring chronologies reveal species-specific climate-growth relationships despite the common environment. P. sylvestris responds positively to warm autumn and late winter temperatures, A. sibirica to previous autumn and current summer precipitation, whereas L. sibirica shows greater sensitivity to current-year temperature. Stem NSC dynamics also differ markedly among species. The evergreen P. sylvestris and A. sibirica maintain relatively stable, starch-dominated NSC pools, whereas the deciduous L. sibirica exhibits pronounced seasonal fluctuations and substantially greater autumn reserve accumulation. Across all species, NSC concentrations increased towards the outermost sapwood. These findings demonstrate that co-existing boreal conifers differ in both climate-growth relationships and stem NSC storage dynamics. Contrasting stem storage patterns provide a physiological framework for interpreting species-specific growth responses and may contribute to understanding the resilience of mixed boreal forests under future climate change.

Cambium phenology is a crucial process in wood production and carbon sequestration of forest ecosystems. Although cambium phenology has been widely studied, research specifically focusing on the cessation of wood formation remains limited. To better understand the influence of environmental and intrinsic factors on the cessation of wood formation, we built and compared three ecophysiological models (temperature sum model, photoperiod-influenced temperature sum model and soil moisture-and photoperiod-influenced temperature sum model) in their ability to predict the date of cessation of xylem cell enlargement (cE) in three major Northern Hemisphere conifer species (Black spruce, Norway spruce and Scots pine). We developed these models based on xylogenesis data collected for 130 site-years across Europe and Canada. Our results demonstrate that the photoperiod-influenced temperature sum model is well-supported by data across all conifer species, with a RMSE of 9.2 days, suggesting that both temperature and photoperiod are critical drivers of wood growth cessation. However, incorporating soil moisture effects does not improve model performance. Our model effectively captures the inter-site variability in cE across a wide environmental gradient, with a fair model efficiency (ME = 0.51 +/- 0.22), but performed less well for annual anomalies (ME = 0.10 +/- 0.09). Additionally, we found that the total ring cell number also affected prediction accuracy. Using this model, we reconstructed historical trends in cE over the past six decades and found a trend to delayed cessation dates. This delay varied geographically, with slower shifts at higher latitudes and elevations, likely due to constrained cambial responses and conservative growth strategies in colder regions. Our model framework offers a simple yet accurate approach for predicting wood growth cessation at large spatial scales, providing a basis for integrating cambium phenology into land surface models and forest productivity assessments.

Wood density (WD) is a crucial anatomical trait influencing forest carbon storage. However, dynamic global vegetation models (DGVMs) typically assume a fixed species-level WD, neglecting environment-driven variability. In this proof-of-concept study, we explore the potential impact of dynamic WD on tree- and forest-level carbon storage by integrating a simple temperature-response function of WD into the DGVM LPJ-GUESS from Smith et al., 2014. Simulations along a temperature gradient show that incorporating environmentally responsive WD can substantially alter simulated stand structure and carbon stocks. Overall, our model experiments illustrated that sites with higher WD had more, but smaller trees, which stored less carbon compared to the standard model. The strongest effects were predicted to appear before canopy closure, where per-tree carbon deviated by up to 32%. This exploratory study suggests the need to represent a mechanism for dynamic WD to better assess ecological feedbacks to forest carbon storage predictions, particularly in young and regenerating forests.

Understanding the intra-annual timing of carbon allocation into wood is critical for linking environmental variability to forest carbon sequestration. We quantified the seasonal dynamics of xylem width increase and xylem biomass production in European beech (Fagus sylvatica) in Belgium during two contrasting growing seasons, a year characterized by a severe spring drought and peak summer hot drought (2020) and a mesic year (2021), using high-resolution X-ray micro-computed tomography (XμCT). Point dendrometer and microtomy measurements were used for validation and comparison. Both xylem width increase and cumulated xylem biomass production followed a sigmoidal seasonal trajectory in both years. Contrary to patterns commonly reported for conifers with a clear early- to latewood transition, the temporal lag between xylem width increase and xylem biomass production was minimal (<3 days) and statistically insignificant, indicating near-synchronous development. XμCT-derived measurements of ring width, wood density and biomass production broadly matched dendrometer data, as well as independent regional biomass estimates from the literature. Seasonal onset and cessation of xylogenesis shifted between years, with earlier onset and cessation in the dry year, which was aligned to microtomy data. Our results demonstrate that, in semi diffuse- to ring-porous angiosperms like beech, xylem expansion and secondary wall formation are tightly coupled, even under contrasting water availability, and suggest that dendrometer data can serve as a proxy for intra-seasonal wood biomass production at the stand level. XμCT provides a reliable tool for quantifying xylogenesis and carbon allocation, offering new insights into wood formation dynamics under variable environmental conditions.

Evolutionary processes such as phenotypic plasticity and genetic adaptation are key mechanisms that have enabled tree species to cope with major changes in their environments and to colonize new areas over millennia. Forest populations are currently experiencing extremely rapid environmental changes due to anthropogenic climate change, challenging their adaptation and resilience over the coming decades. Tree ecophysiological traits do not vary independently but are rather coordinated; however, our understanding of whether these functional traits are governed by the same evolutionary processes is far from complete. In this study, we assessed the evolutionary drivers of functional traits of two major European tree species: sessile oak (Quercus petraea (Matt.) Liebl.) and European beech (Fagus sylvatica L.). We used multiple common garden experiments (four per species) established in the 1990s within the distribution area of these two species, each comprising 9 to 11 provenances. We measured the following traits: i) tree growth including diameter at breast height, height and basal area increments; ii) specific leaf area; iii) long-term responses to climate including the correlation between annual tree growth and climate; and iv) short-term responses to extreme drought. Individual traits were modelled as a response of environment (sites), genetic identity (provenance) and genetically based plasticity (its interaction). To explore the potential influence of climate conditions at seed origin, both, genetic identity and genetically based plasticity, were correlated with the 19 bioclimatic variables from the seed origin (1961–1990) using Pearson correlations. Associations between the climate of origin and multi-trait genetic effects and genetically based plasticity, as well as associations between the climate of the site and multi-trait plasticity were also explored for both species. Our results indicate that range-wide variation in the studied traits of oak and beech is markedly driven by phenotypic plasticity. At the individual trait level, sessile oak showed evidence for both genetic and plastic causes of trait variation. In contrast, the variability of traits of European beech seemed to have been mostly shaped by environmentally driven responses with no clear signs of genetic effects and small genetically based phenotypic plasticity. The results of the integral multi-trait phenotypes, however, suggested genetically driven differences along a resource-use gradient governed by temperature conditions in both species. The plasticity of coordinated traits also reflects the ability of all provenances to adjust to new environmental conditions by optimizing the integrated phenotype along a resource-use gradient. Our results suggest that mitigation strategies for climate change could be directed towards seeking provenances that are more plastic in their integral phenotype across the resource-use gradient, rather than typically searching for populations adapted to the current or future conditions at the target site.

Increasing aridity is a major threat to forests worldwide. Understanding tree functional constraints under drought and their impacts on resilience and mortality among species is crucial to assess the impacts of global change on forests. We analyzed the long-term drought and atmospheric CO2 responses in three Mediterranean co-occurring species with differing drought tolerances (Pinus pinaster < Pinus pinea < Juniperus oxycedrus). In this mixed forest, P. pinaster exhibited widespread mortality and mistletoe infection, P. pinea showed scattered mortality, and J. oxycedrus showed no decline. Using tree-ring data (1978-2016), we compared intrinsic water-use efficiency (iWUE) and xylem hydraulic traits in healthy and non-healthy individuals of both pine species and healthy junipers. Healthy P. pinaster trees produced a more hydraulically efficient xylem, with wider lumen tracheids, than non-healthy trees, whereas P. pinea showed no anatomical differences between health statuses. Healthy P. pinaster displayed greater anatomical plasticity, adjusting hydraulic conductivity and cell-wall thickness to water availability. Despite small differences in average iWUE, the response of iWUE to rising CO2 and drought differed between species and health statuses. J. oxycedrus and P. pinea showed steady iWUE increases, but P. pinea experienced periods of stagnation following an extreme drought, later recovering regardless of health status. In contrast, iWUE in P. pinaster plateaued for over 20 years after a decline-inducing drought, particularly in non-healthy, mistletoe-infected trees. Differences in iWUE response to CO2 and anatomical plasticity to drought may explain the contrasting mortality patterns among these coniferous species. Our results suggest a long-term decline spiral in P. pinaster induced by low hydraulic efficiency in drought-induced defoliated trees and limited physiological responses to rising CO2 and drought. Increasing drought stress makes pine recovery increasingly unlikely.

Stomata control plant water loss and photosynthetic carbon gain. Developing more generalized and accurate stomatal models is essential for earth system models and predicting responses under novel environmental conditions associated with global change. Plant optimality theories offer one promising approach, but most such theories assume that stomatal conductance maximizes photosynthetic net carbon assimilation subject to some cost or constraint of water. We move beyond this approach by developing a new, generalized optimality theory of stomatal conductance, optimizing any non-foliar proxy that requires water and carbon reserves, like growth, survival, and reproduction. We overcome two prior limitations. First, we reconcile the computational efficiency of instantaneous optimization with a more biologically meaningful dynamic feedback optimization over plant lifespans. Second, we incorporate non-steady-state physics in the optimization to account for the temporal changes in the water, carbon, and energy storage within a plant and its environment that occur over the timescales that stomata act, contrary to previous theories. Our optimal stomatal conductance compares well to observations from seedlings, saplings, and mature trees from field and greenhouse experiments. Our model predicts predispositions to mortality during the 2018 European drought and captures realistic responses to environmental cues, including the partial alleviation of heat stress by evaporative cooling and the negative effect of accumulating foliar soluble carbohydrates, promoting closure under elevated CO2. We advance stomatal optimality theory by incorporating generalized evolutionary fitness proxies and enhance its utility without compromising its realism, offering promise for future models to more realistically and accurately predict global carbon and water fluxes.

Quantitative wood anatomy (QWA), which involves measuring wood cell anatomical characteristics commonly on dated tree rings, is becoming increasingly important within plant sciences and ecology. This approach is particularly valuable for studies that require processing a large number of samples, such as those aimed at millennial-long climatic reconstructions. However, the field faces significant challenges, including the absence of a publicly available comprehensive protocol for efficiently and uniformly producing high-quality wood thin sections for QWA along dated tree-ring series. This issue is especially critical for more brittle subfossil wood, in addition to fresh material from living trees. Our manuscript addresses these challenges by providing a detailed protocol for producing thin anatomical sections of wood and digital images, specifically tailored for long chronologies of tree-ring anatomy with an emphasis on conifer wood. The protocol includes step-by-step procedures for sample preparation, sectioning, and imaging, ensuring consistent and high-quality results. By offering this well-tried-and-tested protocol, we aim to facilitate reproducibility and accuracy in wood anatomical studies, ultimately advancing research in this field. It aims to serve as a reference for researchers and laboratories engaged in similar work, promoting standardized practices and enhancing the reliability of QWA data.

Tree rings are crucial for reconstructing past climates, with maximum latewood density (MXD) as a key metric. However, wood integrity is critical for accurate MXD‐based reconstructions, raising concerns when using potentially degraded relict wood. Quantitative wood anatomy (QWA) provides a morphometric alternative. We compared X‐ray and QWA‐derived density measurements from recent and five‐millennia‐old relict wood from Siberia's Yamal region. We measured bulk density and holo‐cellulose‐to‐wood ratio and employed spectroscopic analyses to identify chemical factors affecting density loss. The findings indicate likely abiotic degradation in relict wood, evidenced by significantly lower bulk density, holo‐cellulose content, and MXD than recent samples, while anatomical density and maximum radial cell wall thickness appeared unchanged. MXD‐based estimates suggested 1.7°C colder June‐August (JJA) temperatures 4,700 years ago, while QWA‐based estimates indicated 0.4°C warmer JJA temperatures than the 20th‐century mean. For relict wood with potential mass loss due to degradation, QWA is recommended over traditional MXD methods.

Controlled experiments suggest that the seasonal build-up of nitrogen (N) limitation constrains the responses of forest autumn phenology to elevated temperatures. Therefore, rising soil N is expected to increase the delaying effects of elevated temperature on the end of the season, i.e., leaf senescence. However, the interactive effects of temperature, soil N, and aridity on xylem autumn phenology remain unknown. We conducted a wide spatial analysis from 75 conifer sites in the Northern Hemisphere and found that rising soil N increases the delaying effects of elevated temperature on the end of xylem cell wall thickening but reduced the delaying effects on the cessation of cell enlargement, especially in humid regions. The contrasting effects of elevated soil N on cell enlargement versus cell wall thickening could affect xylem cell anatomy, thereby induce changes in wood density, and induce a decoupling of stem size growth from photosynthate production. These analyses extend previous findings on forest autumn phenology by systematically investigating the spatial variation in the interactive effects of temperature and soil N on xylem autumn phenology at the cellular scale.

Phenotypic plasticity and genetic adaptation are key mechanisms that enable species to respond to changing environments. Tree traits do not vary independently but rather in coordination. However, our understanding of whether functional traits are governed by the same mechanism is still uncomplete. Thus, we aim at assessing the drivers of trait variability of sessile oak and European beech provenances across their distribution ranges. We estimated growth-related and leaf morphological traits from 9 and 11 provenances of oak (Quercus petraea) and beech (Fagus sylvatica), respectively, grown in four different common gardens distributed across their respective distribution areas. Overall, phenotypic plasticity played a dominant role in explaining individual trait variability. For most oak traits, variation among provenances and genetically based plasticity were correlated with the climate of origin, whereas for beech both provenance-related variation and plasticity showed fewer significant associations with the climate of origin. In oak, climate-transfer distance analyses revealed that some trait measures decreased when provenances were moved away from their local precipitation regime. In beech, significant climate-transfer distances were fewer and primarily related to temperature-related parameters. The pattern of multi-trait phenotypes indicates that resource-use strategies among provenances covary with the temperatures of origin in both species. Although beech shows adaptive potential through genetic differentiation among populations, most trait variation is plastic, which may not suffice long term to cope with extreme climatic events. Oak, by contrast, appears more responsive through adaptive mechanisms. Our study enhances understanding of the interplay between genetic adaptation and phenotypic plasticity in long-lived forest trees.

In the temperate zone, deciduous trees exhibit clear above-ground seasonality, marked by a halt in wood growth that represents the completion of wood formation in autumn and reactivation in spring. However, the growth seasonality of below-ground woody organs, such as coarse roots, has been largely overlooked. Here we use tree monitoring data and pot experiments involving saplings to examine the late-season xylem development of stem and coarse roots with leaf phenology in four common deciduous tree species in Western Europe. Coarse-roots wood growth continued throughout the winter whereas stem wood growth halted in autumn, regardless of the tree species, experimental setting or location. Our results do not indicate a clear temperature constraint on below-ground wood growth, even during prolonged periods with soil temperatures lower than 3 °C. The continuous differentiation of xylem root cells in autumn and winter suggests that the non-growing season does not exist sensu stricto for all woody organs of angiosperm deciduous tree species of the temperate zone. Our findings hold implications for understanding tree functioning, in particular the seasonal wood formation, the environmental controls of tree growth and the carbon reserves dynamics. As wood growth in deciduous tree stems halts during winter, it has been assumed that wood growth in coarse roots follows the same pattern. This study on the growth of stem and coarse roots of four European tree species challenges the assumption of winter halt in below-ground wood growth of temperate deciduous trees.

Climate change is increasing the frequency and intensity of drought episodes, altering regional aridity levels. Previous studies have focused on the temporary impacts of seasonal drought on nocturnal sap flow, but how long-term site aridity levels and species-specific stomatal behaviors influence nocturnal sap flow remains unclear. Here, we monitored sap flux density and relevant environmental factors for 16 isohydric Qinghai spruce (Picea crassifolia) trees on the north-facing slope and 14 anisohydric Qilian juniper (Juniperus przewalskii) trees on the south-facing slope at four arid to semi-arid sites on the northeastern Tibetan Plateau. We assessed their nocturnal sap flux density (F-n), nocturnal sap flow volume (Q(n)), and the proportion of Q(n) to daily sap flow volume (Q(n)/Q) over the growing season (May through September) in 2019. Our results show that Q(n) on non-rainy days accounted for about 83 % of total Q(n) on all days in the arid region and about 78 % in the semi-arid region. The average Q(n)/Q on the non-rainy days was 8 % for Qilian juniper and 5 % for Qinghai spruce at the arid sites, while it was 15 % and 14 %, respectively, at the semi-arid sites. Differences in F-n and Q(n)/Q were more pronounced between aridity levels (2-4 times) than between species (< 1.3 times). In the semi-arid region, atmospheric vapor pressure deficit dominated F-n for both species (r = 0.86 for juniper and 0.78 for spruce), while soil water potential had a more significant impact on F-n in the arid region (r = 0.26 for juniper and 0.57 for spruce). Our findings suggest that site aridity levels have a stronger impact on nocturnal sap flow and its environmental response than species-specific stomatal behaviors in high-elevation dryland ecosystems.

Modern analytical tools are essential for advancing research and facilitating interdisciplinary collaboration. The R software serves as a comprehensive solution for statistical computing and graphics in all scientific disciplines, including dendrochronology. Beyond managing traditional tasks like data processing, analysis, and results visualization, R is pivotal in integrating innovative techniques, such as multi-proxy datasets, artificial intelligence or machine learning, to address emerging challenges in tree-ring research. However, a comprehensive overview of R’s functionalities in dendrochronology is lacking, despite its growing importance and increasing role in interdisciplinary research. Here we present an overview of 38 R packages relevant to tree-ring research, categorized by functionality. For each R package, concise descriptions and examples of usage are provided to facilitate the identification and selection of suitable tools for researchers, academicians, and students within and outside the field. We further discuss the transformative potential of R in building a centralized, open-access ecosystem, emphasizing its role in standardizing workflows, enhancing reproducibility, and expanding dendrochronology’s integration with other scientific disciplines in a digital era. We propose that these advancements not only streamline dendrochronological workflows but also provide valuable insights for addressing global environmental and ecological challenges.

Arid and semiarid forests are vulnerable to climate change. Whether rising temperatures would increase wood production by extending the growing season in these relatively dry forests remains unclear. Here, we document a decoupling of duration versus the amount of wood production. Despite the prolonged duration of xylem cell production induced by warming, we did not observe a consistent increase in the amount of xylem growth, based on a two‐year (2018–2019) monitoring of wood formation in Siberian spruce ( Picea obovata Ledeb.) along a natural hydrothermal gradient in the southern Altai Mountains of central Asia. The duration of cell production ranged from 43 to 93 days, with the total number of xylem cells between 37 and 185. Notably, the warmest Altai low‐altitude site (AL) had the longest duration (82.6 ± 13.3 days) but the lowest number of xylem cells (63.88 ± 15.6 cells) in 2018, indicating a decoupling between growing season length and xylem growth. However, this decoupling weakened in 2019 when spring temperatures were cooler and summer precipitation was higher than in 2018. Warmer spring temperatures can extend the duration of wood cell production by triggering an earlier onset of cambium activity. Additionally, sufficient summer precipitation provides the water necessary for turgor‐driven cell division and expansion, enhancing the cell production rate, which plays a dominant role in determining wood growth in arid and semiarid forests. Thus, warmer spring temperatures may amplify the observed decoupling, whereas sufficient summer precipitation may narrow it. Synthesis . This decoupling highlights the critical role of seasonal climates in regulating wood formation dynamics in arid and semiarid forests. With ongoing climate change, longer and warmer growing seasons may amplify the importance of water availability, further constraining forest growth and carbon sequestration when growing‐season water conditions are unfavourable.

Local hydroclimatic conditions influenced black spruce xylem anatomy in boreal treed fens. Earlywood cells were sensitive to early summer temperature, indicating that future warmer climates could negatively impact the xylem structure. Black spruce (Picea mariana) growing in treed boreal fens in North America face significant challenges due to anticipated warming and moisture deficits. To assess site and climate influence on black spruce xylem anatomy, we investigated two treed fens at different elevations (740 m and 320 m a.s.l.) in Alberta, Canada. We examined key xylem traits – cell number, cell lumen area, and cell wall thickness – comparing the two sites to assess the effects of local conditions. Additionally, we correlated these anatomical features with long-term temperature, precipitation, and vapor pressure deficit (VPD) data to evaluate the impact of inter-annual climate variability. We observed larger cell lumens and thinner cell walls in trees at the lower-elevation fen. Xylem climate responses were clearer at the higher-elevation fen, characterized by a more stable water table. Here, previous year summer temperatures were negatively associated with cell number. High temperatures and VPD during the current year late spring and early summer reduced earlywood lumen size. Precipitation showed marginal associations at both sites. This study demonstrates the utility of quantitative wood anatomy in understanding environmental influences on tree xylem anatomy in treed fens. Differences in climatic responses between nearby sites highlight the importance of local hydroclimatic conditions in shaping xylem structure. Notably, observed sensitivity of cell lumen area to spring and early summer temperature and VPD suggests that climate warming could significantly impact the xylem structure and water transport efficiency of black spruce in treed boreal fens.