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Variability in xylem anatomy is of interest to plant scientists because of the role water transport plays in plant performance and survival. Insights into plant adjustments to changing environmental conditions have mainly been obtained through structural and functional comparative studies between taxa or within taxa on contrasting sites or along environmental gradients. Yet, a gap exists regarding the study of hydraulic adjustments in response to environmental changes over the lifetimes of plants. In trees, dated tree-ring series are often exploited to reconstruct dynamics in ecological conditions, and recent work in which wood-anatomical variables have been used in dendrochronology has produced promising results. Environmental signals identified in water-conducting cells carry novel information reflecting changes in regional conditions and are mostly related to short, sub-annual intervals. Although the idea of investigating environmental signals through wood anatomical time series goes back to the 1960s, it is only recently that low-cost computerized image-analysis systems have enabled increased scientific output in this field. We believe that the study of tree-ring anatomy is emerging as a promising approach in tree biology and climate change research, particularly if complemented by physiological and ecological studies. This contribution presents the rationale, the potential, and the methodological challenges of this innovative approach.

Forests are major terrestrial carbon (C) sinks and play a crucial role in climate change mitigation. Despite extensive studies on forest C sequestration, the relationship between seasonal C uptake and its allocation to woody biomass is poorly understood. Here we used a novel dendro-anatomical approach to investigate the relationships between climate variability, C uptake, and woody biomass growth in an 80 year-old eastern white pine ( Pinus strobus ) plantation forest in Ontario, Canada. We used eddy covariance (EC) gross primary productivity (GPP) from 2003–2018 and woody biomass estimated from chronologies of cell wall area (CWA, a proxy for C storage in individual wood cells) and ring wall area (RWA) for earlywood (EW) and latewood (LW) from 1970–2018. Warm temperatures in early spring and high precipitation in mid-spring and summer positively and strongly affected GPP, while high temperature and high vapor pressure deficit in the summer had a negative effect. From 2003 to 2018, there was a steady increase in both GPP and woody cell biomass. Moreover, we found strong positive correlations between GPP and CWA both in EW (May—July GPP, r = 0.65) and LW (July—August GPP, r = 0.89). Strong positive correlations were also found between GPP and RWA both in EW and LW (April—September, r = ⩾ 0.79). All these associations were stronger than the association between annual GPP and tree-ring width ( r = 0.61) used in previous studies. By increasing the resolution of tree-ring analysis to xylem-cell level, we captured intra-annual variability in biomass accumulation. We demonstrated a strong control of seasonal C assimilation (source) over C accumulation in woody biomass at this site. Coupling high-resolution EC fluxes (GPP) and wood anatomical measurements can help to reduce existing uncertainties on C source-sink relationships, opening new perspectives in the study of the C cycle in forests.

Climate change has been reported to affect shrub growth positively at several sites at high northern latitudes, including several arctic environments. The observed growth rates are, however, not uniform in space and time, and the mechanistic drivers of these patterns remain poorly understood. Here we investigated spatio-temporal interactions between climatic conditions, xylem anatomical traits, and annual growth of 21 Betula nana L. individuals from western Greenland for the period 2001–2011. Structural equation modeling showed that summer precipitation and winter temperature are affecting annual growth positively. Furthermore, vessel lumen area and vessel grouping, which are related to water conductivity and hydraulic connectivity of the xylem, respectively, positively influenced annual growth. To optimize growth B. nana was thus able to adjust its water transporting system. Annual variation in vessel lumen area seemed to be driven mostly by spring and summer temperatures, whereas annual variation in vessel grouping was driven by winter temperature. Linear models did not reveal a pattern in the spatial variation of xylem anatomical traits across the sampled climatic gradient. However, growth was positively correlated with local variation in insolation. Our results suggest that B. nana can adjust its hydraulic capacity to annual fluctuations in climatic conditions in order to optimize its total radial stem growth rate.

Interannual variability of wood density – an important plant functional trait and environmental proxy – in conifers is poorly understood. We therefore explored the anatomical basis of density. We hypothesized that earlywood density is determined by tracheid size and latewood density by wall dimensions, reflecting their different functional tasks. To determine general patterns of variability, density parameters from 27 species and 349 sites across the Northern Hemisphere were correlated to tree-ring width parameters and local climate. We performed the same analyses with density and width derived from anatomical data comprising two species and eight sites. The contributions of tracheid size and wall dimensions to density were disentangled with sensitivity analyses. Notably, correlations between density and width shifted from negative to positive moving from earlywood to latewood. Temperature responses of density varied intraseasonally in strength and sign. The sensitivity analyses revealed tracheid size as the main determinant of earlywood density, while wall dimensions become more influential for latewood density. Our novel approach of integrating detailed anatomical data with large-scale tree-ring data allowed us to contribute to an improved understanding of interannual variations of conifer growth and to illustrate how conifers balance investments in the competing xylem functions of hydraulics and mechanical support.

Quantitative wood anatomy analyzes the variability of xylem anatomical features in trees, shrubs, and herbaceous species to address research questions related to plant functioning, growth, and environment. Among the more frequently considered anatomical features are lumen dimensions and wall thickness of conducting cells, fibers, and several ray properties. The structural properties of each xylem anatomical feature are mostly fixed once they are formed, and define to a large extent its functionality, including transport and storage of water, nutrients, sugars, and hormones, and providing mechanical support. The anatomical features can often be localized within an annual growth ring, which allows to establish intra-annual past and present structure-function relationships and its sensitivity to environmental variability. However, there are many methodological challenges to handle when aiming at producing (large) data sets of xylem anatomical data. Here we describe the different steps from wood sample collection to xylem anatomical data, provide guidance and identify pitfalls, and present different image-analysis tools for the quantification of anatomical features, in particular conducting cells. We show that each data production step from sample collection in the field, microslide preparation in the lab, image capturing through an optical microscope and image analysis with specific tools can readily introduce measurement errors between 5 and 30% and more, whereby the magnitude usually increases the smaller the anatomical features. Such measurement errors-if not avoided or corrected-may make it impossible to extract meaningful xylem anatomical data in light of the rather small range of variability in many anatomical features as observed, for example, within time series of individual plants. Following a rigid protocol and quality control as proposed in this paper is thus mandatory to use quantitative data of xylem anatomical features as a powerful source for many research topics.

• Tree responses to altered water availability range from immediate (e.g. stomatal regulation) to delayed (e.g. crown size adjustment). The interplay of the different response times and processes, and their effects on long-term whole-tree performance, however, is hardly understood. • Here we investigated legacy effects on structures and functions of mature Scots pine in a dry inner-Alpine Swiss valley after stopping an 11-yr lasting irrigation treatment. Measured ecophysiological time series were analysed and interpreted with a system-analytic tree model. • We found that the irrigation stop led to a cascade of downregulations of physiological and morphological processes with different response times. Biophysical processes responded within days, whereas needle and shoot lengths, crown transparency, and radial stem growth reached control levels after up to 4 yr only. Modelling suggested that organ and carbon reserve turnover rates play a key role for a tree’s responsiveness to environmental changes. Needle turnover rate was found to be most important to accurately model stem growth dynamics. • We conclude that leaf area and its adjustment time to new conditions is the main determinant for radial stem growth of pine trees as the transpiring area needs to be supported by a proportional amount of sapwood, despite the growth-inhibiting environmental conditions.

Analysis of annual rings in the secondary root xylem is a relatively new approach to gain a posteriori insight into individual and population life history of perennial dicotyledonous herbs (herb‐chronology). Until now, herb‐chronology has involved manual analysis, which is limited by low reproducibility and comparability and which requires considerable time and expertise from the researcher. We have therefore developed an automated image analysis system (Root Xylem Analysis System [ROXAS]) to improve the standardization and efficiency of conventional herb‐chronological analysis. Digital images of stained root cross sections are used by ROXAS to automatically extract xylem vessels according to morphometric criteria. Annual rings are detected by pattern algorithms that analyze the local anatomical context of each vessel. Besides growth parameters, such as annual ring width and area, parameters related to functional root anatomy, such as vessel area or vessel density, are automatically calculated. We evaluated the results produced by ROXAS, using six individuals from each of five perennial plant species representing a variety of taxonomic groups and anatomical root patterns. All species were conducive to automated analysis, which was six times faster than the manual method. Overall, 95% of all annual rings and 98% of all vessels were correctly identified. Accuracy of automated ring width measurements tended to be slightly higher than that of manual analysis, ranging from 2.1% to 5.3% deviation from reference data for all species. Further anatomical parameters, such as vessel area or vessel density, varied substantially between species, indicating anatomical adaptations of these perennial forbs to the constraints of their specific habitats. Automated herb‐chronology using ROXAS may therefore prove to be useful for efficient and reproducible analysis of annual growth increments in the roots of forbs and for investigations into their functional root anatomy.

Summary Non‐structural carbohydrates (NSC) play a crucial role in tree resistance and resilience to drought. Stem sapwood parenchyma is among the largest storage tissue for NSC in mature trees. However, there is a limited mechanistic understanding of how NSC reserves, stem parenchyma abundance and growth rates are interrelated, and how they respond to changing water availability. We quantified NSC, ray parenchyma abundance and ring width along four successive 5‐year radial sapwood segments of the stem of 40 mature Pinus sylvestris trees from a 10‐year irrigation experiment conducted at a xeric site in Switzerland. Percentage of ray volume (PERPAR) varied from 3·75 to 8·94% among trees, but showed low intra‐individual variability. PERPAR responded positively to irrigation with a lag of several years, but was unrelated to %NSC. %NSC was lower in wider rings. However, wider rings still contained a larger NSC pool that was positively related to next year's ring growth. Our results suggest that stem ray parenchyma does not limit NSC storage capacity, but responds to long‐term environmental drivers with years of delay. The observed carbon allocation patterns indicate a prioritization of storage over growth independent of growth conditions, likely as a mechanism to ensure long‐term survival. Furthermore, NSC pool size proved to be a determinant for the inter‐annual autocorrelation in tree‐ring growth. Our study highlights the importance of long‐term multi‐parameter studies to better understand tree responses to environmental variability at different time‐scales. A lay summary is available for this article. Lay Summary

Understanding which species are able to recover from drought, under what conditions, and the mechanistic processes involved, will facilitate predictions of plant mortality in response to global change. In response to drought, some species die because of embolism-induced hydraulic failure, whilst others are able to avoid mortality and recover, following rehydration. Several tree species have evolved strategies to avoid embolism, whereas others tolerate high embolism rates but can recover their hydraulic functioning upon drought relief. Here, we focus on structures and processes that might allow some plants to recover from drought stress via embolism reversal. We provide insights into how embolism repair may have evolved, anatomical and physiological features that facilitate this process, and describe possible trade-offs and related costs. Recent controversies on methods used for estimating embolism formation/repair are also discussed, providing some methodological suggestions. Although controversial, embolism repair processes are apparently based on the activity of phloem and ray/axial parenchyma. The mechanism is energetically demanding, and the costs to plants include metabolism and transport of soluble sugars, water and inorganic ions. We propose that embolism repair should be considered as a possible component of a ‘hydraulic efficiency-safety’ spectrum. We also advance a framework for vegetation models, describing how vulnerability curves may change in hydrodynamic model formulations for plants that recover from embolism.

Trees continuously adjust their axial xylem structure to meet changing needs imposed by ontogenetic and environmental changes. These axial structure–function responses need to be coordinated among competing biophysical constraints to avoid failure of the xylem system. Here, we investigated if ontogeny or experimental manipulation of CO2 and soil temperature influence these structure–function responses. We performed detailed xylem cell anatomical quantification along the axis of 40‐year‐old Larix decidua trees planted at the Swiss tree line and exposed to a combination of elevated CO2 (+200 ppm) and soil warming (+4°C) between 2001 and 2012. We assessed how mean hydraulic tracheid diameter (Dh), the cell wall reinforcement ((t/b)2), tracheid wall thickness (CWT) and the percent area of ray parenchyma (PERPAR)—proxies for hydraulic efficiency, hydraulic safety, biomechanical support and metabolic xylem functions, respectively—covary along the tree axis. Dh increased from the stem apex to base, strictly following a power function (R2=0.81), independent from ontogeny and experimental treatments. In contrast, axial trends of (t/b)2 and CWT were either influenced by treatment and/or ontogeny, or showed no axial trend (PERPAR). Additionally, we found that a larger Dh only at the stem apex promoted primary and secondary growth. Our approach of analysing xylem anatomical traits along the tree axis and across tree rings provides novel insights into xylem functional architecture and allows reconstructing xylem function over time. We conclude that the maintenance of hydraulic efficiency during ontogeny is very robust, that the tracheid diameter undergoes a strong apical control, and plays a fundamental role for assimilation and tree growth. Instead, the other functional traits more plastically vary with ontogeny and environmental changes. A plain language summary is available for this article. Plain Language Summary