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Quantitative wood anatomy is critical for establishing climate reconstruction proxies, understanding tree hydraulics, and quantifying carbon allocation. Its accuracy depends upon the image acquisition methods, which allows for the identification of the number and dimensions of vessels, fibres, and tracheids within a tree ring. Angiosperm wood is analysed with a variety of different image acquisition methods, including surface pictures, wood anatomical micro-sections, or X-ray computed micro-tomography. Despite known advantages and disadvantages, the quantitative impact of method selection on wood anatomical parameters is not well understood. In this study, we present a systematic uncertainty analysis of the impact of the image acquisition method on commonly used anatomical parameters. We analysed four wood samples, representing a range of wood porosity, using surface pictures, micro-CT scans, and wood anatomical micro-sections. Inter-annual patterns were analysed and compared between methods from the five most frequently used parameters, namely mean lumen area ( ), vessel density ( ), number of vessels ( ), mean hydraulic diameter ( ), and relative conductive area ( ). A novel sectorial approach was applied on the wood samples to obtain intra-annual profiles of the lumen area ( ), specific theoretical hydraulic conductivity ( ), and wood density ( ). Our quantitative vessel mapping revealed that values obtained for hydraulic wood anatomical parameters are comparable across different methods, supporting the use of easily applicable surface picture methods for ring-porous and specific diffuse-porous tree species. While intra-annual variability is well captured by the different methods across species, wood density ( ) is overestimated due to the lack of fibre lumen area detection. Our study highlights the potential and limitations of different image acquisition methods for extracting wood anatomical parameters. Moreover, we present a standardized workflow for assessing radial tree ring profiles. These findings encourage the compilation of all studies using wood anatomical parameters and further research to refine these methods, ultimately enhancing the accuracy, replication, and spatial representation of wood anatomical studies.

• Vulnerability to cavitation and conductive efficiency depend on xylem anatomy. We tested a large range of structure-function hypotheses, some for the first time, within a single genus to minimize phylogenetic ‘noise' and maximize detection of functionally relevant variation. • This integrative study combined in-depth anatomical observations using light, scanning and transmission electron microscopy of seven Acer taxa, and compared these observations with empirical measures of xylem hydraulics. • Our results reveal a 2 MPa range in species' mean cavitation pressure (MCP). MCP was strongly correlated with intervessel pit structure (membrane thickness and porosity, chamber depth), weakly correlated with pit number per vessel, and not related to pit area per vessel. At the tissue level, there was a strong correlation between MCP and mechanical strength parameters, and some of the first evidence is provided for the functional significance of vessel grouping and thickenings on inner vessel walls. In addition, a strong trade-off was observed between xylem-specific conductivity and MCP. Vessel length and intervessel wall characteristics were implicated in this safety-efficiency trade-off. • Cavitation resistance and hydraulic conductivity in Acer appear to be controlled by a very complex interaction between tissue, vessel network and pit characteristics.

Flowering plants predominantly conduct water in tubes known as vessels, with vessel diameter playing a crucial role in plant adaptation to climate and reactions to climate change. The importance of vessels makes it essential to understand how and why vessel diameter, plant height, and other ecological factors are interrelated. Although shoot length is by far the main driver of variation in mean vessel diameter across angiosperms, much remains to be understood regarding the factors accounting for the abundant variation around the y-axis in plots of mean species vessel diameter against shoot length. Here, we explore the potential role of porosity types, wood density, leaf phenology, background imperforate tracheary element type, vasicentric tracheids, vascular tracheids, perforation plate type, and successive cambia in causing variation in the y-intercept or slope of the mean species vessel-diameter– and vessel-density–shoot-length associations at the shoot tip and base. We detected numerous cases of ecologically significant variation. For example, latewood vessels of ring porous species scale with a lower slope than earlywood, i.e., latewood vessels are relatively narrow in taller plants. This pattern is likely the result of selection favoring freezing-induced embolism resistance via narrow vessels. Wood density was negatively associated with vessel diameter, with low wood density plants having wider vessels for a given height. Species with scalariform perforation plates scale with a lower shoot base vessel-diameter–shoot-length slope, likely reflecting selection against scalariform plates in wide vessels. In other cases, functional groups scaled similarly. For example, species with successive cambia did not differ from those with conventional single cambia in their mean vessel-diameter–shoot-length scaling, rejecting our prediction that species with successive cambia should have narrower vessels for a given shoot length. They did, however, have fewer vessels per unit shoot cross-sectional area than plants of similar heights, likely because vessels have longer functional lifespans (and therefore are fewer) in species with successive cambia. Our methods illustrate how vessel diameter can be studied taking shoot length into account to detect ecologically important variation and construct theory regarding plant adaptation via the hydraulic system that includes plant size as a vital element.

The rare pit hypothesis predicts that the extensive inter-vessel pitting in large early-wood vessels of ring-porous trees should render many of these vessels extremely vulnerable to cavitation by air-seeding. This prediction was tested in Quercus gambelii. Cavitation was assessed from native hydraulic conductivity at field sap tension and in dehydrated branches. Single-vessel air injections gave air-seeding pressures through vessel files; these data were used to estimate air-seeding pressures for inter-vessel walls and pits. Extensive cavitation occurred at xylem sap tensions below 1 MPa. Refilling occurred below 0.5 MPa and was inhibited by phloem girdling. Remaining vessels cavitated over a wide range to above 4 MPa. Similarly, 40% of injected vessel files air-seeded below 1.0 MPa, whereas the remainder seeded over a wide range exceeding 5 MPa. Inter-vessel walls averaged 1.02 MPa air-seeding pressure, similar and opposite to the mean cavitation tension of 1.22 MPa. Consistent with the rare pit hypothesis, only 7% of inter-vessel pits were estimated to air-seed by 1.22 MPa. The results confirm the rare pit prediction that a significant fraction of large vessels in Q. gambelii experience high probability of failure by air-seeding.

Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water-transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for > 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade-offs between efficiency and safety of water transport. However, trade-offs in hydraulic function are influenced both by pit-level differences in structure (e.g. average porosity of pit membranes) and by tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of individual pits to the whole plant.

Background and AimsVarious correlations have been identified between anatomical features of bordered pits in angiosperm xylem and vulnerability to cavitation, suggesting that the mechanical behaviour of the pits may play a role. Theoretical modelling of the membrane behaviour has been undertaken, but it requires input of parameters at the nanoscale level. However, to date, no experimental data have indicated clearly that pit membranes experience strain at high levels during cavitation events.MethodsTransmission electron microscopy (TEM) was used in order to quantify the pit micromorphology of four tree species that show contrasting differences in vulnerability to cavitation, namely Sorbus aria, Carpinus betulus, Fagus sylvatica and Populus tremula. This allowed anatomical characters to be included in a mechanical model that was based on the Kirchhoff–Love thin plate theory. A mechanistic model was developed that included the geometric features of the pits that could be measured, with the purpose of evaluating the pit membrane strain that results from a pressure difference being applied across the membrane. This approach allowed an assessment to be made of the impact of the geometry of a pit on its mechanical behaviour, and provided an estimate of the impact on air-seeding resistance.Key ResultsThe TEM observations showed evidence of residual strains on the pit membranes, thus demonstrating that this membrane may experience a large degree of strain during cavitation. The mechanical modelling revealed the interspecific variability of the strains experienced by the pit membrane, which varied according to the pit geometry and the pressure experienced. The modelling output combined with the TEM observations suggests that cavitation occurs after the pit membrane has been deflected against the pit border. Interspecific variability of the strains experienced was correlated with vulnerability to cavitation. Assuming that air-seeding occurs at a given pit membrane strain, the pressure predicted by the model to achieve this mechanical state corresponds to experimental values of cavitation sensitivity (P50).ConclusionsThe results provide a functional understanding of the importance of pit geometry and pit membrane structure in air-seeding, and thus in vulnerability to cavitation.

The flow of xylem sap through conifer bordered pits, particularly through the pores in the pit membrane, is not well understood, but is critical for an understanding of water transport through trees. Models solving the Navier–Stokes equation governing fluid flow were based on the geometry of bordered pits in black spruce (Picea mariana) and scanning electron microscopy images showing details of the pores in the margo of the pit membrane. Solutions showed that the pit canals contributed a relatively small fraction of resistance to flow, whereas the torus and margo pores formed a large fraction, which depended on the structure of the individual pit. The flow through individual pores in the margo was strongly dependent on pore area, but also on the radial location of the pore with respect to the edge of the torus. Model results suggest that only a few per cent of the pores in the margo account for nearly half of the flow and these pores tend to be located in the inner region of the margo where their contribution will be maximized. A high density of strands in outer portions of the margo (hence narrower pores) may be more significant for mechanical support of the torus.

Woody plant encroachment is a significant ecological challenge for grassland ecosystems worldwide. Soil water is the major limiting factor for plant growth in arid and semiarid grasslands, which are highly vulnerable to woody plant encroachment. Xylem vessels and soil pores are highly associated between aboveground and belowground systems in relation to water utilization of shrubs. Despite their significant role in water processes, how soil pores and vessels are linked associated with water, is unclear. To address this issue, we quantified structures of soil pores and shrub xylem vessels under different shrub encroachment stages and different soil water conditions (low water, moderate water and field capacity conditions) using the X-ray computed tomography. Results showed that proportions of embolized vessel number peaked in the moderate water state of all water conditions (55.71%). Vessels 10–50 μm in size accounted for over 90% of total vessel numbers, and vessels >20 μm had high water conductivity and were vulnerable to water changes. Irregular pores and pores <30 μm retained water, whereas elongated pores and pores >80 μm were conducive to water movement. Soil porosity was positively correlated with vessel diameter, and the correlation was primarily mediated by root development. Positive correlations occurred between water-filled irregular pores and water-filled vessels, especially those <20 μm. Overall, plants primarily took up water stored in irregular soil pores, and this water was held stably within vessels <20 μm. In the context of climate change, the amplified woody plant encroachment might facilitate the development of xylem vessels and soil porosity, which would accelerate the soil drought.

The flow of xylem sap in conifers is strongly dependent on the presence of a low resistance path through bordered pits, particularly through the pores present in the margo of the pit membrane. A computational fluid dynamics approach was taken, solving the Navier–Stokes equation for models based on the geometry of pits observed in tracheids from stems and roots of Picea mariana (black spruce) and Picea glauca (white spruce). Model solutions demonstrate a close, inverse relationship between the total resistance of bordered pits and the total area of margo pores. Flow through the margo was dominated by a small number of the widest pores. Particularly for pits where the margo component of flow resistance was low relative to that of the torus, pore location near the inner edge of the margo allowed for greater flow than that occurring through similar-sized pores near the outer edge of the margo. Results indicate a surprisingly large variation in pit structure and flow characteristics. Nonetheless, pits in roots have lower resistance to flow than those in stems because the pits were wider and consisted of a margo with a larger area in pores.

Key messageBy combining dendrochronological time-series analysis with radial vessel features, we show that the reconstruction of hydraulic properties improves our understanding of tree species’ acclimation potential to climate change.The vascular architecture plays a crucial role in the productivity and drought tolerance of broadleaf trees, but it is not yet fully understood how the hydraulic system is acclimating to a warmer and drier climate. Because vessel features may record temporal and spatial variability in climatic signals of the past better than tree-ring width, we combined dendrochronological time-series analysis with the calculation of stem hydraulic properties derived from radial vessel features. We aimed to reconstruct the development and sensitivity of the hydraulic system over six decades and to identify climatic control of xylem anatomy for five co-existing broad-leaved diffuse- and ring-porous tree species (genera Acer, Fagus, Fraxinus and Quercus) across three sites covering a precipitation gradient from 548 to 793 mm. We observed a significant influence of the climatic water balance (CWB) on the vessel features of all species, but the time lag, magnitude and direction of the response was highly species-specific. All diffuse-porous species suffered a decline in vessel diameter in dry years, and increase in vessel density in dry years and the year following. However, F. sylvatica was the only species with a significant long-term change in anatomical traits and a significant reduction in potential hydraulic conductivity (Kp) after dry winters and in dry summers, accompanied with the largest long-term decline in tree-ring width and the largest growth reduction in and after years with a more negative CWB. In contrast, the comparison across the precipitation gradient did not reveal any significant vessel-climate relationships. Our results revealed considerable plasticity in the hydraulic system especially of F. sylvatica, but also evidence of the drought-sensitivity of this species in accordance with earlier dendroecological and physiological studies. We conclude that the long-term reconstruction of hydraulic properties can add substantially to the understanding of the acclimation potential of different tree species to climate change.