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.

Angiosperm tree species play a central role in temperate forest ecosystems, providing critical ecological and economic services, yet their physiological functioning and responses to environmental stress remain insufficiently understood compared with economically important conifer species. Current methods for studying angiosperm tree functioning such as physiological monitoring or wood formation analyses are often invasive, resource-demanding or difficult to scale, limiting our ability to integrate structural and functional perspectives over time. By contrast, wood anatomical data are abundant and relatively easy to collect across individuals, years and sites, offering a powerful opportunity to infer underlying physiological processes. Here, we synthesized current understanding of drivers of wood formation and resulting wood anatomy in European temperate angiosperms. We identified key knowledge gaps in wood formation, wood anatomy and tree physiology, and outline interdisciplinary methodological advances that could assist the integration of structural and functional data. Such integration will be essential for developing more robust and predictive frameworks of angiosperm tree growth under changing environmental conditions.

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.

The xylem and phloem anatomy of co-existing tree species provides valuable information on how different tree species face climate change and adjust their vascular structure to local weather conditions. We examined and compared annual ring widths and conduit size in earlywood and early phloem in Fraxinus ornus, Quercus pubescens and Ostrya carpinifolia in a sub-Mediterranean site during the period 2019-2021. The selected xylem and phloem traits were correlated with monthly weather conditions (precipitation and temperature). We found that phloem increment widths and conduits in earlywood and early phloem in the studied tree species showed different trends in terms of interannual variability and in relation to local weather conditions. In F. ornus, May conditions affected xylem traits, while June conditions phloem traits. In Q. pubescens, winter and March precipitation was related to phloem development. In O. carpinifolia, xylem ring width was positively correlated with June precipitation, while early phloem conduits were negatively affected by April temperature. Only two consistent patterns were detected across the species and years studied: wider xylem increments compared with phloem increments, and wider earlywood vessels compared with early phloem sieve tubes. Statistically significant differences were observed among species across all years for the size of xylem and phloem conduits and the hydraulic conductivity of earlywood vessels, which indicates great differences in the calculated hydraulic conductivity among the tree species. To summarize, hydraulic conductivity of earlywood vessels in Q. pubescens was on average for all 3 years 10.4-times and 114-times larger than in F. ornus and O. carpinifolia, respectively. High interannual variability and species-specific sensitivity of xylem and phloem traits to precipitation and temperature confirm high plasticity and different radial growth strategies of the studied tree species to ensure optimal functioning under local weather conditions.

Structural adaptations of wood to environmental conditions play a crucial role in shaping its mechanical and hydraulic properties, which are vital for the performance and survival of fir and beech. In this study, we investigated how site-specific climatic conditions influence tree-ring widths and wood-anatomical traits of fir and beech in the Carpathians. Increment cores were collected from three forest stands across the Carpathians, each characterized by distinct climate regimes. We developed chronologies for mean tree-ring width (MRW), mean lumen area of vessels/tracheids (MLA), cell density (CD), relative conductive tissue area (RCTA), and, for fir, mean tangential cell wall thickness (CWTTAN), covering the period from 1980 to 2016. By comparing MRW and wood-anatomical traits with climatic variables—daily minimum and maximum temperatures and daily precipitation sums from E-OBS climate data—we identified clear differences among the three sites. The relationships between tree-ring widths and wood-anatomical traits varied between fir and beech, reflecting species-specific responses to local climate conditions. Notably, beech appeared more sensitive to warm summer temperatures, while fir was comparatively less affected. Evaluating the variability in radial growth and wood anatomy is essential for understanding the plasticity of fir and beech under diverse environmental conditions, and represents a first step toward predicting their responses to future climate scenarios.

This review synthesizes recent advancements and identifies knowledge gaps in the tree growth phenology of both belowground and aboveground organs in extra-tropical forest ecosystems. Phenology, the study of periodic plant life cycle events, is crucial for understanding tree fitness, competition for resources, and the impacts of climate change on ecosystems. By examining the phenological processes of various tree organs, the review aims to provide a comprehensive understanding of how these processes are interconnected and how they influence overall tree growth and ecosystem dynamics. The review aims to provide a comprehensive overview of current knowledge, highlight recent technological advancements, and identify critical areas where further research is needed. The review highlights significant progress in monitoring leaf and canopy phenology, thanks to advancements in remote sensing and automated observation systems. These technologies have enhanced our ability to track seasonal changes in leaf development and canopy dynamics more accurately and over larger areas. There has also been a substantial increase in research on wood formation in stems, expanding beyond northern hemisphere conifers to include a broader range of functional groups. However, despite these efforts, identifying the precise drivers of wood formation remains challenging, necessitating further integration of molecular and eco-physiological insights. A critical area of focus is root phenology, encompassing both primary and secondary growth. Despite the fundamental role of roots in tree physiology and ecosystem dynamics, our understanding of root phenology remains limited, primarily due to the inherent difficulties in monitoring root growth. The review emphasizes the need for more detailed studies on root growth processes and the development of new methodologies and technologies to improve root phenology assessments. The review highlights the importance of incorporating eco-physiological insights into phenological assessments. Leaf and canopy phenology would benefit from more studies focusing on autumnal events. Indeed, compared to the onset of the growing season, much less is known about its end, despite its critical importance for understanding processes such as carbon uptake and nutrient cycle. Advancing knowledge of wood growth phenology will require greater focus on angiosperms, as research on xylogenesis has historically been centered on gymnosperms. This will likely necessitate the development of new, tailored methodologies to address the characteristics of angiosperm wood formation. Similarly, further exploration of phloem phenology is essential to better understand the links between phenological processes across different organs. Finally, compared to other organs, root growth remains less well understood, underscoring the need for deepening the investigation on root phenology in the coming years.

Living stumps reported in many tree species were the first, circumstantial evidence of root grafting. Although stump overgrowth has been extensively studied in various tree species, anatomical observations have mainly focused on the xylem part. Here, we investigated the xylem and phloem structure in living stumps of two conifer species: Norway spruce (Picea abies (L.) Karst.) and silver fir (Abies alba Mill.). The mechanism of wound closure was similar in the two species, however, there were differences in the structure of wound tissues in wood and bark. Immediately after the injury, a callus consisting of parenchymal cells was formed, followed by wound-wood with wide increments. In addition, the wood structure and morphology of tracheids were changed. Tracheids lost their typical elongated and tapering structure; instead, numerous crooked and forked tracheids were present. Traumatic resin canals were present in both species but were more abundant in P. abies, which also had normal resin canals. While the structure of the xylem growth rings eventually normalized in both species, the width of the annual xylem increments was diminished. In both species, the typical structure of annual phloem increment was lost in the non-collapsed phloem and sclereids were very abundant in the collapsed phloem. In A. alba, the collapsed phloem was rich in swollen parenchyma cells containing blue-stained contents, while in P. abies, numerous large traumatic resin canals were conspicuous. The study showed that similar patterns of vascular tissue restoration exist in A. alba and P. abies in response to tree felling. However, there are certain species-specific peculiarities in the wood and phloem anatomical response, which might affect longterm stump functioning and survival.

IntroductionAssessing wood growth phenology over multiple years is essential for understanding the environmental drivers of forest growth and improving large-scale predictions of the carbon cycle. Xylogenesis methods facilitate the assessment of the timing and rate of xylem cell wall thickening, the primary sink of carbon in wood. In angiosperm trees, where wood anatomy is complex, significantly less is known about the factors controlling growth cessation in autumn due to indirect, sinteracting, and lag effects, in contrast to growth resumption in spring. Furthermore, both branch and stem growth must be considered to account for the total aboveground phenology. MethodsIn this study, we focused on European beech (Fagus sylvatica) and silver birch (Betula pendula) in a mild temperate region (Northern Belgium). We examined the progress of cell wall thickening in autumn and the seasonal timing of xylem growth cessation for these species’ stems over five years and for their branches over one year in mature trees. In addition, we investigated the same variables in the stems and branches of potted saplings for two years and for oak (Quercus robur) and aspen (Populus tremula) saplings over one year.ResultsOur results demonstrate a considerable variation in the progression and cessation of wood growth, with differences of up to a month and a half in growth cessation (early September to late October), predominantly driven by climatic variables. Early cessation of xylem growth in stems was strongly associated with high temperatures in April and August, elevated vapour pressure deficit, and severe soil drought in August. The progression of cell wall thickening in late summer was generally synchronized between branches and stems for every species. However, branches sustained a higher percentage of growth (approximately 2 weeks) in early autumn during non-drought years. DiscussionThese findings provide valuable insights for refining models of forest growth and carbon storage, enabling a more comprehensive representation that encompasses the entire tree under different climatic scenarios.

Dry and warm climate conditions in southern Europe represent clear limits for European beech (Fagus sylvatica) growth near the species southern distribution limit, but it is unclear how aridification and changes in seasonal precipitation regimes will affect these forests at the individual level. We explored climate-growth relationships and the seasonality of peak climate signals in European beech using daily climate data and a large collection of tree-ring width series from southern and southeastern Europe through Generalised Linear Mixed Models (GLMMs). In most cases we found a positive and significant influence of precipitation on tree growth, and a significant negative effect of maximum temperature. Predictions from the GLMMs revealed a positive impact of precipitation during an 88 day window from spring to early summer (mid-April to mid-July), for an average tree across our network. This critical growing time window ranged from 75 days in warmer and drier conditions, and extended up to 100 days in areas with mild temperatures and moderate summer precipitation. Maximum temperatures negatively affected trees for an average of 27 day window in summer (June-July). This period was reduced to <10 days in locations with wetter and colder summers, rising up to 45 days in sites with drier and warmer summers. The positive effect of precipitation on growth was stronger and commenced earlier in larger trees. Similarly, the negative effects of maximum temperatures were more pronounced for larger trees. The use of daily climate data and a tree-centred approach allowed for capturing critical temporal dynamics in climate-growth relationships that are often overlooked by conventional methods. These insights significantly enhance our understanding of climatic factors influencing individual beech growth at the edge of its distribution range and their seasonal variations.

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.

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.