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The extent to which water availability can be used to predict the enlargement and final dimensions of xylem conduits remains an open issue. We reconstructed the time course of tracheid enlargement in Pinus sylvestris trees in central Spain by repeated measurements of tracheid diameter on microcores sampled weekly during a 2 yr period. We analyzed the role of water availability in these dynamics empirically through time-series correlation analysis and mechanistically by building a model that simulates daily tracheid enlargement rate and duration based on Lockhart's equation and water potential as the sole input. Tracheid enlargement followed a sigmoid-like time course, which varied intra- and interannually. Our empirical analysis showed that final tracheid diameter was strongly related to water availability during tracheid enlargement. The mechanistic model was calibrated and successfully validated (R-2 = 0.92) against the observed tracheid enlargement time course. The model was also able to reproduce the seasonal variations of tracheid enlargement rate, duration and final diameter (R-2 = 0.84-0.99). Our results support the hypothesis that tracheid enlargement and final dimensions can be modeled based on the direct effect of water potential on turgor-driven cell expansion. We argue that such a mechanism is consistent with other reported patterns of tracheid dimension variation.

Background Xylem, the most abundant tissue on Earth, is responsible for lateral growth in plants. Typical xylem has a radial system composed of ray parenchyma cells and an axial system of fusiform cells. In most angiosperms, fusiform cells comprise vessel elements for water transportation and libriform fibers for mechanical support, while both functions are performed by tracheids in other vascular plants such as gymnosperms. Little is known about the developmental programs and evolutionary relationships of these xylem cell types. Results Through both single-cell and laser capture microdissection transcriptomic profiling, we determine the developmental lineages of ray and fusiform cells in stem-differentiating xylem across four divergent woody angiosperms. Based on cross-species analyses of single-cell clusters and overlapping trajectories, we reveal highly conserved ray, yet variable fusiform, lineages across angiosperms. Core eudicots Populus trichocarpa and Eucalyptus grandis share nearly identical fusiform lineages, whereas the more basal angiosperm Liriodendron chinense has a fusiform lineage distinct from that in core eudicots. The tracheids in the basal eudicot Trochodendron aralioides , an evolutionarily reversed trait, exhibit strong transcriptomic similarity to vessel elements rather than libriform fibers. Conclusions This evo-devo framework provides a comprehensive understanding of the formation of xylem cell lineages across multiple plant species spanning over a hundred million years of evolutionary history.

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.

During the growing season, the cambium of conifer trees produces successive rows of xylem cells, the tracheids, that sequentially pass through the phases of enlargement and secondary wall thickening before dying and becoming functional. Climate variability can strongly influence the kinetics of morphogenetic processes, eventually affecting tracheid shape and size. This study investigates xylem anatomical structure in the stem of Picea abies to retrospectively infer how, in the long term, climate affects the processes of cell enlargement and wall thickening. Tracheid anatomical traits related to the phases of enlargement (diameter) and wall thickening (wall thickness) were innovatively inspected at the intra-ring level on 87-year-long tree-ring series in Picea abies trees along a 900 m elevation gradient in the Italian Alps. Anatomical traits in ten successive tree-ring sectors were related to daily temperature and precipitation data using running correlations. Close to the altitudinal tree limit, low early-summer temperature negatively affected cell enlargement. At lower elevation, water availability in early summer was positively related to cell diameter. The timing of these relationships shifted forward by about 20 (high elevation) to 40 (low elevation) d from the first to the last tracheids in the ring. Cell wall thickening was affected by climate in a different period in the season. In particular, wall thickness of late-formed tracheids was strongly positively related to August-September temperature at high elevation. Morphogenesis of tracheids sequentially formed in the growing season is influenced by climate conditions in successive periods. The distinct climate impacts on cell enlargement and wall thickening indicate that different morphogenetic mechanisms are responsible for different tracheid traits. Our approach of long-term and high-resolution analysis of xylem anatomy can support and extend short-term xylogenesis observations, and increase our understanding of climate control of tree growth and functioning under different environmental conditions.

Conifer tree rings are generally composed of large, thin-walled cells of light earlywood followed by narrow, thick-walled cells of dense latewood. Yet, how wood formation processes and the associated kinetics create this typical pattern remains poorly understood. We monitored tree-ring formation weekly over 3 yr in 45 trees of three conifer species in France. Data were used to model cell development kinetics, and to attribute the relative importance of the duration and rate of cell enlargement and cell wall deposition on tree-ring structure. Cell enlargement duration contributed to 75% of changes in cell diameter along the tree rings. Remarkably, the amount of wall material per cell was quite constant along the rings. Consequently, and in contrast with widespread belief, changes in cell wall thickness were not principally attributed to the duration and rate of wall deposition (33%), but rather to the changes in cell size (67%). Cell enlargement duration, as the main driver of cell size and wall thickness, contributed to 56% of wood density variation along the rings. This mechanistic framework now forms the basis for unraveling how environmental stresses trigger deviations (e. g. false rings) from the normal tree-ring structure

• Premise of the study: Xylem structure determines the hydraulic and mechanical properties of a stem, and its plasticity is fundamental for maintaining tree performance under changing conditions. Unveiling the mechanism and the range of xylem adjustment is thus necessary to anticipate climate change impacts on vegetation.• Methods: To understand the mechanistic process and the functional impact of xylem responses to warming in a cold-limited environment, we investigated the relationship between temperature and tracheid anatomy along a 312-yr tree-ring chronology of Larix sibirica trees from the Altay Mountains in Russia.• Key results: Climate-growth analyses indicated that warming favors wider earlywood cell lumen, thicker latewood walls, denser maximum latewood, and wider rings. The temperature signal of the latewood was stronger (r > 0.7) and covered a longer and more stable period (from June to August) than that of earlywood and tree-ring width. Long-term analyses indicated a diverging trend between lumen and cell wall of early- and latewood.• Conclusions: Xylem anatomy appears to respond to warming temperatures. A warmer early-growing season raises water conduction capacity by increasing the number and size of earlywood tracheids. The higher-performing earlywood tracheids promote more carbon fixation of the latewood cells by incrementing the rate of assimilation when summer conditions are favorable for growth. The diverging long-term variation of lumen and cell wall in earlywood vs. latewood suggests that xylem adjustments in latewood increase mechanical integrity and support increasing tree size under the ameliorated growing conditions.

Key message A better understanding of the influence of environmental conditions on wood formation should help to improve the radial growth of trees and to prepare for climate change. The cambial activity of trees is associated with seasonal cycles of activity and dormancy in temperate zones. The timing of cambial reactivation in early spring and dormancy in autumn plays an important role in determination of the cambial growth and the environmental adaptivity of temperate trees. This review focuses on the temperature regulation of the timing of cambial reactivation and xylem differentiation and highlights recent advances of bud growth in relation to cambial activity of temperate trees. In addition, we discuss relationships between the timing of cambial reactivation, start of xylem differentiation and changes in levels of storage materials to identify the source of the energy required for cell division and differentiation. We also present a summary of current understanding of the effects of rapid increases and decreases in temperature on cambial activity, by localized heating and cooling, respectively. Increases in temperature from late winter to early spring influence the physiological processes that are involved in the initiation of cambial reactivation and xylem differentiation both in localized heated stems and under natural conditions. Localized cooling has a direct effect on cell expansion, the thickening of walls of differentiating tracheids, and the rate of division of cambial cells. A rapid decrease in temperature of the stem might be the critical factor in the control of latewood formation and the cessation of cambial activity. Therefore, temperature is the main driver of cambial activity in temperate trees and trees are able to feel changes in temperature through the stem. The climate change might affect wood formation in trees.

This study investigated the variations in tracheid length of Pseudotsuga menziesii (Mirb.) Franco from three sites in Croatia in relation to cambium age, within- and between-site differences, and growth rate. Tracheids are the main structural element in P. menziesii wood, varying in length following different patterns that should be precisely determined. After the maceration procedure, earlywood tracheid length (EWTL), latewood tracheid length (LWTL), annual growth ring tracheid length (RTL), earlywood ring width (EWW), latewood ring width (LWW), and annual ring width (ARW) were measured in selected annual growth rings. The significant effect of annual growth rings and zone interaction for EWTL and LWTL, as well as of annual growth rings, trees, and sites for RTL, was determined. The results conclude on the differences between the trends in EWTL and LWTL from pith toward the bark. In addition, the correlation analysis between the tracheid length and different growth patterns was investigated, and very weak or no association between the variables was detected. This research contributes to better understanding the degree of wood uniformity of P. menziesii from the technological perspective, as well as the variability factor in the optimization of forest management with favoring overall wood quality.

Globally distributed extant conifer species must adapt to various environmental conditions, which would be reflected in their xylem structure, especially in the tracheid characteristics of earlywood and latewood. With an anatomical trait dataset of 78 conifer species growing throughout China, an interspecific study within a phylogenetic context was conducted to quantify variance of tracheid dimensions and their response to climatic and soil conditions. There was a significant difference in tracheid diameter between earlywood and latewood while no significant difference was detected in tracheid wall thickness through a phylogenetically paired t -test. Through a phylogenetic principle component analysis, Pinaceae species were found to be strongly divergent in their tracheid structure in contrast to a conservative tracheid structure in species of Cupressaceae, Taxaceae, and Podocarpaceae. Tracheid wall thickness decreased from high to low latitudes in both earlywood and latewood, with tracheid diameter decreasing for latewood only. According to the most parsimonious phylogenetic general least square models, environment and phylogeny together could explain about 21∼56% of tracheid structure variance. Our results provide insights into the effects of climate and soil on the xylem structure of conifer species thus furthering our understanding of the trees’ response to global change.