Angiosperm wood structure: Global patterns in vessel anatomy and their relation to wood density and potential conductivity

Optimising height-growth predicts trait responses to water availability and other environmental drivers

Future changes in climate, together with rising atmosphericCO 2 , may reorganise the functional composition of ecosystems. Without long-term historical data, predicting how traits will respond to environmental conditions-in particular, water availability-remains a challenge. While eco-evolutionary optimality theory (EEO) can provide insight into how plants adapt to their environment, EEO approaches to date have been formulated on the assumption that plants maximise carbon gain, which omits the important role of tissue construction and size in determining growth rates and fitness. Here, we show how an expanded optimisation framework, focussed on individual growth rate, enables us to explain shifts in four key traits: leaf mass per area, sapwood area to leaf area ratio (Huber value), wood density and sapwood-specific conductivity in response to soil moisture, atmospheric aridity,CO 2 and light availability. In particular, we predict that as conditions become increasingly dry, height-growth optimising traits shift from resource-acquisitive strategies to resource-conservative strategies, consistent with empirical responses across current environmental gradients of rainfall. These findings can explain both the shift in traits and turnover of species along existing environmental gradients and changing future conditions and highlight the importance of both carbon assimilation and tissue construction in shaping the functional composition of vegetation across climates.

Mucosal epithelial homeostasis: Reference intervals for skin, gill lamellae and filament for Atlantic salmon and other fish species

Mucosal barriers are gatekeepers of health and exhibit homeostatic variation in relation to habitat and disease. Mucosal Mapping technology provides an in-depth examination of the dynamic mucous cells (MCs) in fish mucosal barriers on tangential sections, about 90° from the view of traditional histology. The method was originally developed and standardized in academia prior to the establishment of QuantiDoc AS to apply mucosal mapping, now trademarked as Veribarr™ for the analysis of skin, gills and gastrointestinal tracts. Veribarr™ uses design-based stereology for the selection and measurement of cell area (size) (μm2), the volumetric density of MCs in the epithelium (MCD, amount of the epithelia occupied by MCs, in %) and the calculated abundance of the MCs (barrier status or defence activity). MC production was mapped across the skin and gill epithelia in 12 species, discovering that gills consistently have two distinct groups of MCs, one on the lamellae where MCs are few and small and one on the filament where MCs are larger and more abundant. MCs were usually much larger in the skin than in the gills, with the latter requiring fewer and smaller cells for adequate respiration. The difference observed between MCs in gill lamella and gill filament is likely a result of functional demands. In addition, our findings also highlight a variation in the mucosal parameters between the species skin, which cannot be explained by the weight differences, and a potential link between MC distribution and species-specific lifestyles in the gill lamella. This diversity necessitates the development of species and tissue site-specific reference intervals for mucosal health evaluation. Mucosal bivariate reference intervals were developed for MC production, including size (trophy) and calculated defence activity (plasia) in the skin and gills of Atlantic salmon, to contrast new measurements against historical data patterns. The application of mucosal reference intervals demonstrates that stress from parasites and treatments can manifest as changes in mucosal architecture, as evidenced by MC hypertrophy and hyperplasia within the gill lamellae. These reference intervals also facilitate comparisons with wild Atlantic salmon, revealing a somewhat higher MC level in farmed salmon gill lamellae. These findings suggest that MC hyperplasia and hypertrophy in the gills are stress/environmental responses in aquaculture. They also advocate for developing specific mucosal bivariate homeostatic reference intervals in aquaculture to improve fish health and welfare across all farmed species.

Quality vs. Quantity: The Consequences of Elevated CO2 on Wood Biomaterial Properties

Since the late 1800s, anthropogenic activities such as fossil fuel consumption and deforestation have driven up the concentration of atmospheric CO2 around the globe by >45%. Such heightened concentrations of carbon dioxide in the atmosphere are a leading contributor to global climate change, with estimates of a 2-5° increase in global air temperature by the end of the century. While such climatic changes are mostly considered detrimental, a great deal of experimental work has shown that increased atmospheric CO2 will actually increase growth in various plants, which may lead to increased biomass for potential harvesting or CO2 sequestration. However, it is not clear whether this increase in growth or biomass will be beneficial to the plants, as such increases may lead to weaker plant materials. In this review, I examine our current understanding of how elevated atmospheric CO2 caused by anthropogenic effects may influence plant material properties, focusing on potential effects on wood. For the first part of the review, I explore how aspects of wood anatomy and structure influence resistance to bending and breakage. This information is then used to review how changes in CO2 levels may later these aspects of wood anatomy and structure in ways that have mechanical consequences. The major pattern that emerges is that the consequences of elevated CO2 on wood properties are highly dependent on species and environment, with different tree species showing contradictory responses to atmospheric changes. In the end, I describe a couple avenues for future research into better understanding the influence of atmospheric CO2 levels on plant biomaterial mechanics.

Xylem embolism induced by freeze-thaw and drought are influenced by different anatomical traits in subtropical montane evergreen angiosperm trees

Subtropical evergreen broadleaved forests distributed in montane zones of southern China experience seasonal droughts and winter frost. Previously, studies have recognized that xylem anatomy is a determinant of its vulnerability to embolism caused by drought and freezing events. We hypothesized that there is a coordination of xylem resistance to freeze-thaw and drought-induced embolism for the subtropical montane evergreen broadleaved tree species because they are influenced by common xylem structural traits (e.g., vessel diameter). We examined the branch xylem anatomy, resistance to drought-induced embolism (P50), and the percent loss of branch hydraulic conductivity after a severe winter frost (PLCwinter) for 15 evergreen broadleaved tree species in a montane forest in South China. Our results showed that P50 of the studied species ranged from -2.81 to -5.13 MPa, which was not associated with most xylem anatomical properties except for the axial parenchyma-to-vessel connectivity. These tree species differed substantially in PLCwinter, ranging from 0% to 76.41%. PLCwinter was positively related to vessel diameter and negatively related to vessel density, vessel group index, and vessel-to-vessel connectivity, but no coordination with P50. This study suggests that hydraulic adaptation to frost is important to determine the distributional limit of subtropical montane evergreen woody angiosperms.

Effects of functional composition on plant competitors, stress-tolerators, ruderals ecological strategies in forest communities across different climatic zones

Ecological strategies identified by plant functional traits are valuable descriptors for understanding species, populations, communities, and ecosystems in response to environmental conditions. Ecological strategies, in conjunction with the functional structure of plant communities, serve as crucial tools for investigating complex relationships among the environment, vegetation, and ecosystem functions. However, it remains unclear whether the functional structure (specifically, community-weighted mean [CWM] traits) accurately reflects the optimal ecological strategies in forest communities. Here, we gathered seven functional traits for each species from four distinct forest vegetation types across four climatic zones, including leaf area (LA), specific leaf area (SLA), leaf dry matter content (LDMC), leaf phosphorus concentration (LPC), leaf nitrogen concentration (LNC), wood density (WD) and maximum plant height (H). We based on CSR (Competitors, Stress-tolerators, Ruderals) theory and "StrateFy" ordination method utilizing LA, LDMC and SLA to position them within CSR triangle and categorize them into four ecological strategy groups: Competitive, Stress-tolerant, Intermediate, and Ruderal ecological strategy groups (C-group, S-group, Int-group, and R-group). We then determined the proportion of species in each group. Subsequently, we calculated the CWM trait values for the remaining four functional traits: WD (CWM-WD), LPC (CWM-LPC), LNC (CWM-LNC) and H (CWM-H). Non-metric multidimensional scaling and hierarchical partitioning revealed that CWM-WD, CWM-LPC, CWM-LNC and CWM-H significantly influenced the ecological strategies of forest communities. The synergistic interaction of CWM-WD and CWM-LPC had the most significant impact on ecological strategies within forest communities. Notably, CWM-WD emerged as the most crucial single CWM trait for explaining variation in ecological strategies within forest communities. In conclusion, our study demonstrates that CWM traits effectively reflect optimal CSR ecological strategies in forest communities across different climatic zones, with CWM-WD serving as a preferred indicator. This can improve our critical insights into key ecological processes in forest communities using trait-based approach.

Rootstocks affect the vulnerability to embolism and pit membrane thickness in Citrus scions

Embolism resistance of xylem tissue varies among species and is an important trait related to drought resistance, with anatomical attributes like pit membrane thickness playing an important role in avoiding embolism spread. Grafted Citrus trees are commonly grown in orchards, with the rootstock being able to affect the drought resistance of the whole plant. Here, we evaluated how rootstocks affect the vulnerability to embolism resistance of the scion using several rootstock/scion combinations. Scions of 'Tahiti' acid lime, 'Hamlin', 'Pera' and 'Valencia' oranges grafted on a 'Rangpur' lime rootstock exhibit similar vulnerability to embolism. In field-grown trees, measurements of leaf water potential did not suggest significant embolism formation during the dry season, while stomata of Citrus trees presented an isohydric response to declining water availability. When 'Valencia' orange scions were grafted on 'Rangpur' lime, 'IAC 1710' citrandarin, 'Sunki Tropical' mandarin or 'Swingle' citrumelo rootstocks, variation in intervessel pit membrane thickness of the scion was found. The 'Rangpur' lime rootstock, which is known for its drought resistance, induced thicker pit membranes in the scion, resulting in higher embolism resistance than the other rootstocks. Similarly, the rootstock 'IAC 1710' citrandarin generated increased embolism resistance of the scion, which is highly relevant for citriculture.

Stretched sapwood, ultra-widening permeability and ditching da Vinci: revising models of plant form and function

BACKGROUND

The mechanisms leading to dieback and death of trees under drought remain unclear. To gain an understanding of these mechanisms, addressing major empirical gaps regarding tree structure-function relations remains essential.

SCOPE

We give reasons to think that a central factor shaping plant form and function is selection simultaneously favouring constant leaf-specific conductance with height growth and isometric (1:1) scaling between leaf area and the volume of metabolically active sink tissues ('sapwood'). Sapwood volume-leaf area isometry implies that per-leaf area sapwood volumes become transversely narrower with height growth; we call this 'stretching'. Stretching means that selection must favour increases in permeability above and beyond that afforded by tip-to-base conduit widening ("ultra-widening permeability"), via fewer and wider vessels or tracheids with larger pits or larger margo openings. Leaf area-metabolically active sink tissue isometry would mean that it is unlikely that larger trees die during drought because of carbon starvation due to greater sink-source relationships as compared to shorter plants. Instead, an increase in permeability is most plausibly associated with greater risk of embolism, and this seems a more probable explanation of the preferential vulnerability of larger trees to climate change-induced drought. Other implications of selection favouring constant per-leaf area sapwood construction and maintenance costs are departure from the da Vinci rule expectation of similar sapwood areas across branching orders, and that extensive conduit furcation in the stem seems unlikely.

CONCLUSIONS

Because all these considerations impact the likelihood of vulnerability to hydraulic failure versus carbon starvation, both implicated as key suspects in forest mortality, we suggest that these predictions represent essential priorities for empirical testing.

Root topological order drives variation of fine root vessel traits and hydraulic strategies in tropical trees

Vessel traits contribute to plant water transport from roots to leaves and thereby influence how plants respond to soil water availability, but the sources of variation in fine root anatomical traits remain poorly understood. Here, we explore the variations of fine root vessel traits along topological orders within and across tropical tree species. Anatomical traits were measured along five root topological orders in 80 individual trees of 20 species from a tropical forest in southwestern China. We found large variations for most root anatomical traits across topological orders, and strong co-variations between vessel traits. Within species, theoretical specific xylem hydraulic conductivity (Kth) increased with topological order due to increased mean vessel diameter, size heterogeneity, and decreased vessel density. Across species, Kth was associated with vessel fraction in low-order roots and correlated with mean vessel diameter and vessel density in high-order roots, suggesting a shift in relative anatomical contributors to Kth from the second- to fifth-order roots. We found no clear relationship between Kth and stele: root diameter ratios. Our study shows strong variations in root vessel traits across topological orders and species, and highlights shifts in the anatomical underpinnings by varying vessel-related anatomical structures for an optimized water supply.

Adaptive strategies based on shrub leaf-stem anatomy and their environmental interpretations in the eastern Qaidam Basin

BACKGROUND

Water stress seriously affects the survival of plants in natural ecosystems. Plant resistance to water stress relies on adaptive strategies, which are mainly based on plant anatomy with following relevant functions: (1) increase in water uptake and storage; (2) reduction of water loss; and (3) mechanical reinforcement of tissues. We measured 15 leaf-stem anatomical traits of five dominant shrub species from 12 community plots in the eastern Qaidam Basin to explore adaptive strategies based on plant leaf-stem anatomy at species and community levels. and their relationship with environmental stresses were tested.

RESULTS

Results showed that the combination of leaf-stem anatomical traits formed three types of adaptive strategies with the drought tolerance of leaf and stem taken as two coordinate axes. Three types of water stress were caused by environmental factors in the eastern Qaidam Basin, and the established adaptive strategy triangle could be well explained by these environmental stresses. The interpretation of the strategic triangle was as follows: (1) exploitative plant strategy, in which leaf and stem adopt the hydraulic efficiency strategy and safety strategy, respectively. This strategy is mostly applied to plants in sandy desert (i.e., Nitraria tangutorum, and Artemisia sphaerocephala) which is mainly influenced by drought stress; (2) stable plant strategy, in which both leaf/assimilation branches and stem adopt hydraulic safety strategy. This strategy is mostly applied to plants in salty desert (i.e., Kalidium foliatum and Haloxylon ammodendron) which aridity has little effect on them; and (3) opportunistic plant strategy, in which leaf and stem adopt hydraulic safety strategy and water transport efficiency strategy. This strategy is mostly applied to plants in multiple habitats (i.e., Sympegma regelii) which is mainly affected by coldness stress.

CONCLUSION

The proposed adaptive strategy system could provide a basis for elucidating the ecological adaptation mechanism of desert woody plants and the scientific management of natural vegetation in the Qinghai-Tibet Plateau.

Functional traits of fossil plants

A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo-ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait-based ecology, to evaluate which well-established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait-based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi-quantitative ranking of 26 paleo-functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo-ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo-demography (life history), and Earth system history can be derived through the application of paleo-functional traits to fossil plants.

Linkages among stem xylem transport, biomechanics, and storage in lianas and trees across three contrasting environments

PREMISE

Stem xylem transports water and nutrients, mechanically supports aboveground tissues, and stores water and nonstructural carbohydrates. These three functions are associated with three types of cells-vessel, fiber, and parenchyma, respectively.

METHODS

We measured stem theoretical hydraulic conductivity (Kt), modulus of elasticity (MOE), tissue water content, starch, soluble sugars, cellulose, and xylem anatomical traits in 15 liana and 16 tree species across three contrasting sites in Southwest China.

RESULTS

Lianas had higher hydraulic efficiency and tissue water content, but lower MOE and cellulose than trees. Storage traits (starch and soluble sugars) did not significantly differ between lianas and trees, and trait variation was explained mainly by site, highlighting how environment shapes plant storage strategies. Kt was significantly positively correlated with vessel diameter and vessel area fraction in lianas and all species combined. The MOE was significantly positively correlated with fiber area fraction, wood density, and cellulose in lianas and across all species. The tissue water content was significantly associated with parenchyma area fraction in lianas. Support function was strongly linked with transport and storage functions in lianas. In trees, transport and support functions were not correlated, while storage function was tightly linked with transport and support functions.

CONCLUSIONS

These findings enhance our understanding of the relationship between stem xylem structure and function in lianas and trees, providing valuable insights into how plants adapt to environmental changes and the distinct ecological strategies employed by lianas and by trees to balance the demands of hydraulic transport, mechanical support, and storage.

Contrasting water-use strategies revealed by species-specific transpiration dynamics in the Caatinga dry forest

In forest ecosystems, transpiration (T) patterns are important for quantifying water and carbon fluxes and are major factors in predicting ecosystem change. Seasonal changes in rainfall and soil water content can alter the sensitivity of sap flux density to daily variations in vapor pressure deficit (VPD). This sensitivity is species-specific and is thought to be related to hydraulic strategies. The aim of this work is to better understand how the sap flux density of species with low versus high wood density differ in their sensitivity to VPD and soil water content and how potentially opposing water-use strategies influence T dynamics, and ultimately, correlations to evapotranspiration (ET). We use hysteresis area analysis to quantify the sensitivity of species-specific sap flux density to changes in the VPD, breakpoint-based models to determine the soil water content threshold instigating a T response and multiscalar wavelet coherency to correlate T to ET. We found that low wood density Commiphora leptophloeos (Mart.) Gillett had a more dynamic T pattern, a greater sensitivity to VPD at high soil water content, required a higher soil water content threshold for this sensitivity to be apparent, and had a significant coherency correlation with ET at daily to monthly timescales. This behavior is consistent with a drought avoidance strategy. High wood density Cenostigma pyramidale  (Tul.)  E. Gagnon & G. P. Lewis, conversely, had a more stable T pattern, responded to VPD across a range of soil water content, tolerated a lower soil water content threshold to T, and had a significant coherency correlation with ET at weekly timescales. This behavior is consistent with a drought-tolerant strategy. We build on previous research to show that these species have contrasting water-use strategies that should be considered in large-scale modeling efforts.

Wood-density has no effect on stomatal control of leaf-level water use efficiency in an Amazonian forest

Forest disturbances increase the proportion of fast-growing tree species compared to slow-growing ones. To understand their relative capacity for carbon uptake and their vulnerability to climate change, and to represent those differences in Earth system models, it is necessary to characterise the physiological differences in their leaf-level control of water use efficiency and carbon assimilation. We used wood density as a proxy for the fast-slow growth spectrum and tested the assumption that trees with a low wood density (LWD) have a lower water-use efficiency than trees with a high wood density (HWD). We selected 5 LWD tree species and 5 HWD tree species growing in the same location in an Amazonian tropical forest and measured in situ steady-state gas exchange on top-of-canopy leaves with parallel sampling and measurement of leaf mass area and leaf nitrogen content. We found that LWD species invested more nitrogen in photosynthetic capacity than HWD species, had higher photosynthetic rates and higher stomatal conductance. However, contrary to expectations, we showed that the stomatal control of the balance between transpiration and carbon assimilation was similar in LWD and HWD species and that they had the same dark respiration rates.

Do root secondary xylem functional traits differ between growth forms in Fabaceae species in a seasonally dry Neotropical environment?

BACKGROUND AND AIMS

Whole-plant performance in water-stressed and disturbance-prone environments depends on a suitable supply of water from the roots to the leaves, storage of reserves during periods of shortage, and a morphological arrangement that guarantees the maintenance of the plants anchored to the soil. All these functions are performed by the secondary xylem of roots. Here, we investigate whether different growth forms of Fabaceae species from the seasonally dry Neotropical environment have distinct strategies for water transport, mechanical support and non-structural carbon and water storage in the root secondary xylem.

METHODS

We evaluated cross-sections of root secondary xylem from species of trees, shrubs and subshrubs. We applied linear models to verify the variability in secondary xylem anatomical traits among growth forms.

KEY RESULTS

Secondary xylem with larger vessels and lower vessel density was observed in tree species. Vessel wall thickness, vessel grouping index, potential hydraulic conductivity and cell fractions (vessels, fibres, rays and axial parenchyma) were not statistically different between growth forms, owing to the high interspecific variation within the groups studied.

CONCLUSION

Our results showed that the variability in anatomical traits of the secondary xylem of the root is species specific. In summary, the cellular complexity of the secondary xylem ensures multiple functional strategies in species with distinct growth forms, a key trait for resource use in an environment with strong water seasonality.

Tree species differ in plant economic spectrum traits in the tropical dry forest of Mexico

In tropical dry forests, studies on wood anatomical traits have concentrated mainly on variations in vessel diameter and frequency. Recent research suggests that parenchyma and fibers also play an important role in water conduction and in xylem hydraulic safety. However, these relationships are not fully understood, and wood trait variation among different functional profiles as well as their variation under different water availability scenarios have been little studied. In this work, we aim to (1) characterize a set of wood anatomical traits among six selected tree species that represent the economic spectrum of tropical dry forests, (2) assess the variation in these traits under three different rainfall regimes, and (3) determine the relationships between wood anatomical traits and possible functional trade-offs. Differences among species and sites in wood traits were explored. Linear mixed models were fitted, and model comparison was performed. Most variation occurred among species along the economic spectrum. Obligate deciduous, low wood density species were characterized by wood with wide vessels and low frequency, suggesting high water transport capacity but sensitivity to drought. Moreover, high cell fractions of carbon and water storage were also found in these tree species related to the occurrence of abundant parenchyma or septate fibers. Contrary to what most studies show, Cochlospermum vitifolium, a succulent tree species, presented the greatest variation in wood traits. Facultative deciduous, high wood density species were characterized by a sturdy vascular system that may favor resistance to cavitation and low reserve storage. Contrary to our expectations, variation among the rainfall regimes was generally low in all species and was mostly related to vessel traits, while fiber and parenchyma traits presented little variation among species. Strong functional associations between wood anatomical traits and functional trade-offs were found for the six tree species studied along the economic spectrum of tropical dry forests.

Functional trade-offs are driven by coordinated changes among cell types in the wood of angiosperm trees from different climates

Wood performs several functions to ensure tree survival and carbon allocation to a finite stem volume leads to trade-offs among cell types. It is not known to what extent these trade-offs modify functional trade-offs and if they are consistent across climates and evolutionary lineages. Twelve wood traits were measured in stems and coarse roots across 60 adult angiosperm tree species from temperate, Mediterranean and tropical climates. Regardless of climate, clear trade-offs occurred among cellular fractions, but did not translate into specific functional trade-offs. Wood density was negatively related to hydraulic conductivity (Kth ) in stems and roots, but was not linked to nonstructural carbohydrates (NSC), implying a functional trade-off between mechanical integrity and transport but not with storage. NSC storage capacity was positively associated with Kth in stems and negatively in roots, reflecting a potential role for NSC in the maintenance of hydraulic integrity in stems but not in roots. Results of phylogenetic analyses suggest that evolutionary histories cannot explain covariations among traits. Trade-offs occur among cellular fractions, without necessarily modifying trade-offs in function. However, functional trade-offs are driven by coordinated changes among xylem cell types depending on the dominant role of each cell type in stems and roots.

Optimal balancing of xylem efficiency and safety explains plant vulnerability to drought

In vast areas of the world, forests and vegetation are water limited and plant survival depends on the ability to avoid catastrophic hydraulic failure. Therefore, it is remarkable that plants take hydraulic risks by operating at water potentials (ψ) that induce partial failure of the water conduits (xylem). Here we present an eco-evolutionary optimality principle for xylem conduit design that explains this phenomenon based on the hypothesis that conductive efficiency and safety are optimally co-adapted to the environment. The model explains the relationship between the tolerance to negative water potential (ψ50 ) and the environmentally dependent minimum ψ (ψmin ) across a large number of species, and along the xylem pathway within individuals of two species studied. The wider hydraulic safety margin in gymnosperms compared to angiosperms can be explained as an adaptation to a higher susceptibility to accumulation of embolism. The model provides a novel optimality-based perspective on the relationship between xylem safety and efficiency.

Tree growth is correlated with hydraulic efficiency and safety across 22 tree species in a subtropical karst forest

Karst forests are habitats in which access to soil water can be challenging for plants. Therefore, safe and efficient xylem water transport and large internal water storage may benefit tree growth. In this study, we selected 22 tree species from a primary subtropical karst forest in southern China and measured their xylem anatomical traits, saturated water content (SWC), hydraulic conductivity (Ks) and embolism resistance (P50). Additionally, we monitored growth of diameter at breast height (DBH) in 440 individual trees of various sizes over three consecutive years. Our objective was to analyze the relationships between xylem structure, hydraulic efficiency, safety, water storage and growth of karst tree species. The results showed significant differences in structure but not in hydraulic traits between deciduous and evergreen species. Larger vessel diameter, paratracheal parenchyma and higher SWC were correlated with higher Ks. Embolism resistance was not correlated with the studied anatomical traits, and no tradeoff with Ks was observed. In small trees (5-15 cm DBH), diameter growth rate (DGR) was independent of hydraulic traits. In large trees (>15 cm DBH), higher Ks and more negative P50 accounted for higher DGR. From lower to greater embolism resistance, the size-growth relationship shifted from growth deceleration to acceleration with increasing tree size in eight of the 22 species. Our study highlights the vital contributions of xylem hydraulic efficiency and safety to growth rate and dynamics in karst tree species; therefore, we strongly recommend their integration into trait-based forest dynamic models.

Root and branch hydraulic functioning and trait coordination across organs in drought-deciduous and evergreen tree species of a subtropical highland forest

Vessel traits are key in understanding trees' hydraulic efficiency, and related characteristics like growth performance and drought tolerance. While most plant hydraulic studies have focused on aboveground organs, our understanding of root hydraulic functioning and trait coordination across organs remains limited. Furthermore, studies from seasonally dry (sub-)tropical ecosystems and mountain forests are virtually lacking and uncertainties remain regarding potentially different hydraulic strategies of plants differing in leaf habit. Here, we compared wood anatomical traits and specific hydraulic conductivities between coarse roots and small branches of five drought-deciduous and eight evergreen angiosperm tree species in a seasonally dry subtropical Afromontane forest in Ethiopia. We hypothesized that largest vessels and highest hydraulic conductivities are found in roots, with greater vessel tapering between roots and equally-sized branches in evergreen angiosperms due to their drought-tolerating strategy. We further hypothesized that the hydraulic efficiencies of root and branches cannot be predicted from wood density, but that wood densities across organs are generally related. Root-to-branch ratios of conduit diameters varied between 0.8 and 2.8, indicating considerable differences in tapering from coarse roots to small branches. While deciduous trees showed larger branch xylem vessels compared to evergreen angiosperms, root-to-branch ratios were highly variable within both leaf habit types, and evergreen species did not show a more pronounced degree of tapering. Empirically determined hydraulic conductivity and corresponding root-to-branch ratios were similar between both leaf habit types. Wood density of angiosperm roots was negatively related to hydraulic efficiency and vessel dimensions; weaker relationships were found in branches. Wood density of small branches was neither related to stem nor coarse root wood densities. We conclude that in seasonally dry subtropical forests, similar-sized coarse roots hold larger xylem vessels than small branches, but the degree of tapering from roots to branches is highly variable. Our results indicate that leaf habit does not necessarily influence the relationship between coarse root and branch hydraulic traits. However, larger conduits in branches and a low carbon investment in less dense wood may be a prerequisite for high growth rates of drought-deciduous trees during their shortened growing season. The correlation of stem and root wood densities with root hydraulic traits but not branch wood points toward large trade-offs in branch xylem towards mechanical properties.

Anatomical traits related to leaf and branch hydraulic functioning on Amazonian savanna plants

Amazonian savannas are isolated patches of open habitats found within the extensive matrix of Amazonian tropical forests. There remains limited evidence on how Amazonian plants from savannas differ in the traits related to drought resistance and water loss control. Previous studies have reported several xeromorphic characteristics of Amazonian savanna plants at the leaf and branch levels that are linked to soil, solar radiation, rainfall and seasonality. How anatomical features relate to plant hydraulic functioning in this ecosystem is less known and instrumental if we want to accurately model transitions in trait states between alternative vegetation in Amazonia. In this context, we combined studies of anatomical and hydraulic traits to understand the structure-function relationships of leaf and wood xylem in plants of Amazonian savannas. We measured 22 leaf, wood and hydraulic traits, including embolism resistance (as P₅₀), Hydraulic Safety Margin (HSM) and isotope-based water use efficiency (WUE), for the seven woody species that account for 75% of the biomass of a typical Amazonian savanna on rocky outcrops in the state of Mato Grosso, Brazil. Few anatomical traits are related to hydraulic traits. Our findings showed wide variation exists among the seven species studied here in resistance to embolism, water use efficiency and structural anatomy, suggesting no unique dominant functional plant strategy to occupy an Amazonian savanna. We found wide variation in resistance to embolism (-1.6 ± 0.1 MPa and -5.0 ± 0.5 MPa) with species that are less efficient in water use (e.g. Kielmeyera rubriflora, Macairea radula, Simarouba versicolor, Parkia cachimboensis and Maprounea guianensis) showing higher stomatal conductance potential, supporting xylem functioning with leaf succulence and/or safer wood anatomical structures and that species that are more efficient in water use (e.g. Norantea guianensis and Alchornea discolor) can exhibit riskier hydraulic strategies. Our results provide a deeper understanding of how branch and leaf structural traits combine to allow for different hydraulic strategies among coexisting plants. In Amazonian savannas, this may mean investing in buffering water loss (e.g. succulence) at leaf level or safer structures (e.g. thicker pit membranes) and architectures (e.g. vessel grouping) in their branch xylem.

Irrigation with primary wastewater alters wood anatomy and composition in willow Salix miyabeana SX67

Traditional treatment of wastewaters is a burden for local governments. Using short rotation coppice willow (SRCW) as vegetal filter has several environmental and economic benefits. Here, we investigated the effect of primary wastewater irrigation on wood structure and composition of the willow cultivar Salix miyabeana 'SX67' following two years of growth. Compared to unirrigated plants (UI), stem sections of plants irrigated with primary wastewater (WWD) showed an unexpected decrease of hydraulic conductance (K_S) associated with a decrease in vessel density but not vessel diameter. The majority (86%) of vessels had diameters range groups [20-30[, [30-40[and [40-50[µm and contributed to > 75% of theoretical K_S, while the group class [50-60[µm (less than 10% of vessels) still accounted for > 20% of total K_S regardless irrigation treatments. WWD significantly alters the chemical composition of wood with an increase of glucan content by 9 to 16.4% and a decrease of extractives by 35.3 to 36.4% when compared to UI or to plants irrigated with potable water (PW). The fertigation did also increase the proportion of the tension wood which highly correlated with glucan content. In the context of energetic transition and mitigation of climate change, such results are of high interest since WWD effectively permit the phytofiltration of large amounts of organic contaminated effluents without impairing SRCW physiology.

The relative area of vessels in xylem correlates with stem embolism resistance within and between genera

The resistance of xylem conduits to embolism is a major factor defining drought tolerance and can set the distributional limits of species across rainfall gradients. Recent work suggests that the proximity of vessels to neighbors increases the vulnerability of a conduit. We therefore investigated whether the relative vessel area of xylem correlates with intra- and inter-generic variation in xylem embolism resistance in species pairs or triplets from the genera Acer, Cinnamomum, Ilex, Quercus and Persea, adapted to environments differing in aridity. We used the optical vulnerability method to assess embolism resistance in stems and conducted anatomical measurements on the xylem in which embolism resistance was quantified. Vessel lumen fraction (VLF) correlated with xylem embolism resistance across and within genera. A low VLF likely increases the resistance to gas movement between conduits, by diffusion or advection, whereas a high VLF enhances gas transport thorough increased conduit-to-conduit connectivity and reduced distances between conduits and therefore the likelihood of embolism propagation. We suggest that the rate of gas movement due to local pressure differences and xylem network connectivity is a central driver of embolism propagation in angiosperm vessels.

A unit pipe pneumatic model to simulate gas kinetics during measurements of embolism in excised angiosperm xylem

The pneumatic method has been introduced to quantify embolism resistance in plant xylem of various organs by applying a partial vacuum to cut-open xylem. Despite the similarity in vulnerability curves between the pneumatic and other methods, a modeling approach is needed to investigate if changes in xylem embolism during dehydration can be accurately quantified based on gas diffusion kinetics. Therefore, a unit pipe pneumatic (UPPn) model was developed to estimate gas extraction from intact conduits, which were axially interconnected by inter-conduit pit membranes to cut-open conduits. The physical laws used included Fick's law for diffusion, Henry's law for gas concentration partitioning between liquid and gas phases at equilibrium and the ideal gas law. The UPPn model showed that 91% of the extracted gas came from the first five series of embolized, intact conduits and only 9% from the aqueous phase after 15 s of simulation. Considering alternative gas sources, embolism resistance measured with a pneumatron device was systematically overestimated by 2-17%, which corresponded to a typical measuring error of 0.11 MPa for P50 (the water potential equivalent to 50% of the maximum amount of gas extracted). It is concluded that pneumatic vulnerability curves directly measure embolism of intact conduits due to the fast movement of gas across interconduit pit membranes, while gas extraction from sap and diffusion across hydrated cell walls is about 100 times slower. We expect that the UPPn model will also contribute to the understanding of embolism propagation based on temporal gas dynamics.

Xylem hydraulics strongly influence the niche differentiation of tree species along the slope of a river valley in a water-limited area

Xylem hydraulic characteristics govern plant water transport, affecting both drought resistance and photosynthetic gas exchange. Therefore, they play critical roles in determining the adaptation of different species to environments with various water regimes. Here, we tested the hypothesis that variation in xylem traits associated with a trade-off between hydraulic efficiency and safety against drought-induced embolism contributes to niche differentiation of tree species along a sharp water availability gradient on the slope of a unique river valley located in a semi-humid area. We found that tree species showed clear niche differentiation with decreasing water availability from the bottom towards the top of the valley. Tree species occupying different positions, in terms of vertical distribution distance from the bottom of the valley, showed a strong trade-off between xylem water transport efficiency and safety, as evidenced by variations in xylem structural traits at both the tissue and pit levels. This optimized their xylem hydraulics in their respective water regimes. Thus, the trade-off between hydraulic efficiency and safety contributes to clear niche differentiation and, thereby, to the coexistence of tree species in the valley with heterogeneous water availability.

Shifts in Community Vegetative Organs and Their Dissimilar Trade-Off Patterns in a Tropical Coastal Secondary Forest, Hainan Island, Southern China

The ecology of functional features highlights the importance of the leaf economic spectrum (LES) in understanding plant trade-offs between conservative and commercial resource use. However, it is still unclear whether changes in the plant attributes of various vegetative organs can be altered and whether the plant economic spectrum (PES) is categorized by multiple vegetative organs. We investigated a total of 12 functional features of 174 woody tree species, with leaf and stem attributes, on Hainan Island. We used principal component analysis (PCA) to analyze the changes in attributes and connections to understand how the plant trade-offs differ. We detected that stem organic matter (SOM) and stem organic carbon (SOC) contributed most to the first principal component, followed by leaf organic matter (LOM) and leaf organic carbon (LOC). Using Spearman correlation analysis, we determined that leaf total nitrogen (LTN) and specific leaf area (SLA), LTN and leaf total phosphorus (LTP), and finally stem total nitrogen (STN) and stem total phosphorus (STP) were positively significantly correlated. These significant variations in the traits of nutrients are regulated, while the morphological traits of aboveground vegetative organs are diverse. The coexistence of species and community assembly can increase our knowledge on the tropical coastal secondary forests. Furthermore, our outcomes can help us to better understand the restoration of habitats and green infrastructure design, suggesting that selecting different species across multiple trait axes can help ensure functionality at the maximum level.

Functional trade-offs in volume allocation to xylem cell types in 75 species from the Brazilian savanna Cerrado

BACKGROUND AND AIMS

Xylem is a crucial tissue for plant survival, performing the functions of water transport, mechanical support and storage. Functional trade-offs are a result of the different assemblages of xylem cell types within a certain wood volume. We assessed how the volume allocated to different xylem cell types can be associated with wood functional trade-offs (hydraulics, mechanical and storage) in species from the Cerrado, the Brazilian savanna. We also assessed the xylem anatomical characters linked to wood density across species.

METHODS

We analysed cross-sections of branches collected from 75 woody species belonging to 42 angiosperm families from the Cerrado. We estimated the wood volume fraction allocated to different cell types and performed measurements of vessel diameter and wood density.

KEY RESULTS

The largest volume of wood is allocated to fibres (0.47), followed by parenchyma (0.33) and vessels (0.20). Wood density is positively correlated to cell wall (fibre and vessel wall), and negatively to the fractions of fibre lumen and gelatinous fibres. We observed a trade-off between hydraulics (vessel diameter) and mechanics (cell wall fraction), and between mechanics and storage (parenchyma fraction). The expected positive functional relationships between hydraulics (vessel diameter) and water and carbohydrate storage (parenchyma and fibre lumen fractions) were not detected, though larger vessels are linked to a larger wood volume allocated to gelatinous fibres.

CONCLUSIONS

Woody species from the Cerrado show evidence of functional trade-offs between water transport, mechanical support and storage. Gelatinous fibres might be potentially linked to water storage and release by their positive relationship to increased vessel diameter, thus replacing the functional role of parenchyma and fibre lumen cells. Species can profit from the increased mechanical strength under tension provided by the presence of gelatinous fibres, avoiding expensive investments in high wood density.

Functional susceptibility of tropical forests to climate change

Tropical forests are some of the most biodiverse ecosystems in the world, yet their functioning is threatened by anthropogenic disturbances and climate change. Global actions to conserve tropical forests could be enhanced by having local knowledge on the forests' functional diversity and functional redundancy as proxies for their capacity to respond to global environmental change. Here we create estimates of plant functional diversity and redundancy across the tropics by combining a dataset of 16 morphological, chemical and photosynthetic plant traits sampled from 2,461 individual trees from 74 sites distributed across four continents together with local climate data for the past half century. Our findings suggest a strong link between climate and functional diversity and redundancy with the three trait groups responding similarly across the tropics and climate gradient. We show that drier tropical forests are overall less functionally diverse than wetter forests and that functional redundancy declines with increasing soil water and vapour pressure deficits. Areas with high functional diversity and high functional redundancy tend to better maintain ecosystem functioning, such as aboveground biomass, after extreme weather events. Our predictions suggest that the lower functional diversity and lower functional redundancy of drier tropical forests, in comparison with wetter forests, may leave them more at risk of shifting towards alternative states in face of further declines in water availability across tropical regions.

Effects of Provenance, Growing Site, and Growth on Quercus robur Wood Anatomy and Density in a 12-Year-Old Provenance Trial

Vessels are responsible for an efficient and safe water transport in angiosperm xylem. Whereas large vessels efficiently conduct the bulk of water, small vessels might be important under drought stress or after winter when large vessels are embolized. Wood anatomy can adjust to the environment by plastic adaptation, but is also modified by genetic selection, which can be driven by climate or other factors. To distinguish between plastic and genetic components on wood anatomy, we used a Quercus robur trial where trees from ten Central European provenances were planted in three locations in Austria along a rainfall gradient. Because wood anatomy also adjusts to tree size and in ring-porous species, the vessel size depends on the amount of latewood and thereby ring width, we included tree size and ring width in the analysis. We found that the trees' provenance had a significant effect on average vessel area (VA), theoretical specific hydraulic conductivity (Ks), and the vessel fraction (VF), but correlations with annual rainfall of provenances were at best weak. The trial site had a strong effect on growth (ring width, RW), which increased from the driest to the wettest site and wood density (WD), which increased from wet to dry sites. Significant site x provenance interactions were seen only for WD. Surprisingly, the drier site had higher VA, higher VF, and higher Ks. This, however, is mainly a result of greater RW and thus a greater proportion of latewood in the wetter forest. The average size of vessels > 70 μm diameter increased with rainfall. We argue that Ks, which is measured per cross-sectional area, is not an ideal parameter to compare the capacity of ring-porous trees to supply leaves with water. Small vessels (<70 μm) on average contributed only 1.4% to Ks, and we found no evidence that their number or size was adaptive to aridity. RW and tree size had strong effect on all vessel parameters, likely via the greater proportion of latewood in wide rings. This should be accounted for when searching for wood anatomical adaptations to the environment.

Parenchyma fractions drive the storage capacity of nonstructural carbohydrates across a broad range of tree species

PREMISE

Nonstructural carbohydrates (NSCs) play a key role in tree performance and functioning and are stored in radial and axial parenchyma (RAP) cells. Whether this relationship is altered among species and climates or is linked to functional traits describing xylem structure (wood density) and tree stature is not known.

METHODS

In a systematic review, we collated data for NSC content and the proportion of RAP in stems for 68 tree species. To examine the relationships of NSCs and RAP with climatic factors and other functional traits, we also collected climatic data at each tree's location, as well as wood density and maximum height. A phylogenetic tree was constructed to examine the influence of species' evolutionary relationships on the associations among NSCs, RAP, and functional traits.

RESULTS

Across all 68 tree species, NSCs were positively correlated with RAP and mean annual temperature, but relationships were only weakly significant in temperate species and angiosperms. When separating RAP into radial parenchyma (RP) and axial parenchyma (AP), both NSCs and wood density were positively correlated with RP but not with AP. Wood in taller trees was less dense and had lower RAP than in shorter trees, but height was not related to NSCs.

CONCLUSIONS

In trees, NSCs are stored mostly in the RP fraction, which has a larger surface area in warmer climates. Additionally, NSCs were only weakly linked to wood density and tree height. Our analysis of evolutionary relationships demonstrated that RAP fractions and NSC content were always closely related across all 68 tree species, suggesting that RAP can act as a reliable proxy for potential NSC storage capacity in tree stems.

Different Responses in Vascular Traits between Dutch Elm Hybrids with a Contrasting Tolerance to Dutch Elm Disease

The ascomycetous fungus Ophiostoma novo-ulmi is the causative agent of the current Dutch elm disease (DED) pandemic, which has ravaged many tens of millions of European and North American elm trees. Host responses in vascular traits were studied in two Dutch elm hybrids, 'Groeneveld' and 'Dodoens', which show different vascular architecture in the secondary xylem and possess contrasting tolerances to DED. 'Groeneveld' trees, sensitive to DED, possessed a high number of small earlywood vessels. However, these trees showed a poor response to DED infection for the earlywood vascular characteristics. Following infection, the proportion of least vessels with a vessel lumen area less than 2500 µm2 decreased from 65.4% down to 53.2%. A delayed response in the increasing density of vessels showing a reduced size in the latewood prevented neither the rapid fungal spread nor the massive colonisation of the secondary xylem tissues resulting in the death of the infected trees. 'Dodoens' trees, tolerant to DED, possessed a low number of large earlywood vessels and showed a prominent and fast response to DED infection. Vessel lumen areas of newly formed earlywood vessels were severely reduced together with the vessel size : number ratio. Following infection, the proportion of least vessels with a vessel lumen area less than 2500 µm2 increased from 75.6% up to 92.9%. A trend in the increasing density of vessels showing a reduced size was maintained not only in the latewood that was formed in the year of infection but also in the earlywood that was formed in the consecutive year. The occurrence of fungal hyphae in the earlywood vessels that were formed a year following the infection was severely restricted, as revealed by X-ray micro-computed tomography imaging. Possible reasons responsible for a contrasting survival of 'Groeneveld' and 'Dodoens' trees are discussed.

Above- and below-ground functional trait coordination in the Neotropical understory genus Costus

The study of plant functional traits and variation among and within species can help illuminate functional coordination and trade-offs in key processes that allow plants to grow, reproduce and survive. We studied 20 leaf, above-ground stem, below-ground stem and fine-root traits of 17 Costus species from forests in Costa Rica and Panama to answer the following questions: (i) Do congeneric species show above-ground and below-ground trait coordination and trade-offs consistent with theory of resource acquisition and conservation? (ii) Is there correlated evolution among traits? (iii) Given the diversity of habitats over which Costus occurs, what is the relative contribution of site and species to trait variation? We performed a principal components analysis (PCA) to assess for the existence of a spectrum of trait variation and found that the first two PCs accounted for 21.4 % and 17.8 % of the total trait variation, respectively, with the first axis of variation being consistent with a continuum of resource-acquisitive and resource-conservative traits in water acquisition and use, and the second axis of variation being related to the leaf economics spectrum. Stomatal conductance was negatively related to both above-ground stem and rhizome specific density, and these relationships became stronger after accounting for evolutionary relatedness, indicating correlated evolution. Despite elevation and climatic differences among sites, high trait variation was ascribed to individuals rather than to sites. We conclude that Costus species present trait coordination and trade-offs that allow species to be categorized as having a resource-acquisitive or resource-conservative functional strategy, consistent with a whole-plant functional strategy with evident coordination and trade-offs between above-ground and below-ground function. Our results also show that herbaceous species and species with rhizomes tend to agree with trade-offs found in more species-rich comparisons.

Evidence for distinct isotopic compositions of sap and tissue water in tree stems: consequences for plant water source identification

The long-standing hypothesis that the isotopic composition of plant stem water reflects that of source water is being challenged by studies reporting bulk water from woody stems with an isotopic composition that cannot be attributed to any potential water source. The mechanism behind such source-stem water isotopic offsets is still poorly understood. Using a novel technique to extract selectively sap water from xylem conduits, we show that, in cut stems and potted plants, the isotopic composition of sap water reflects that of irrigation water, demonstrating unambiguously that no isotopic fractionation occurs during root water uptake or sap water extraction. By contrast, water in nonconductive xylem tissues is always depleted in deuterium compared with sap water, irrespective of wood anatomy. Previous studies have shown that isotopic heterogeneity also exists in soils at the pore scale in which water adsorbed onto soil particles is more depleted in deuterium than unbound water. Data collected at a riparian forest indicated that sap water matches best unbound soil water from depth below -70 cm, while bulk stem and soil water differ markedly. We conclude that source-stem isotopic offsets can be explained by micrometre-scale heterogeneity in the isotope ratios of water within woody stems and soil micro-pores.

Climatic and soil factors explain the two-dimensional spectrum of global plant trait variation

Plant functional traits can predict community assembly and ecosystem functioning and are thus widely used in global models of vegetation dynamics and land-climate feedbacks. Still, we lack a global understanding of how land and climate affect plant traits. A previous global analysis of six traits observed two main axes of variation: (1) size variation at the organ and plant level and (2) leaf economics balancing leaf persistence against plant growth potential. The orthogonality of these two axes suggests they are differently influenced by environmental drivers. We find that these axes persist in a global dataset of 17 traits across more than 20,000 species. We find a dominant joint effect of climate and soil on trait variation. Additional independent climate effects are also observed across most traits, whereas independent soil effects are almost exclusively observed for economics traits. Variation in size traits correlates well with a latitudinal gradient related to water or energy limitation. In contrast, variation in economics traits is better explained by interactions of climate with soil fertility. These findings have the potential to improve our understanding of biodiversity patterns and our predictions of climate change impacts on biogeochemical cycles.

Stem Trait Spectra Underpin Multiple Functions of Temperate Tree Species

A central paradigm in comparative ecology is that species sort out along a slow-fast resource economy spectrum of plant strategies, but this has been rarely tested for a comprehensive set of stem traits and compartments. We tested how stem traits vary across wood and bark of temperate tree species, whether a slow-fast strategy spectrum exists, and what traits make up this plant strategy spectrum. For 14 temperate tree species, 20 anatomical, chemical, and morphological traits belonging to six key stem functions were measured for three stem compartments (inner wood, outer wood, and bark). The trait variation was explained by major taxa (38%), stem compartments (24%), and species within major taxa (19%). A continuous plant strategy gradient was found across and within taxa, running from hydraulic safe gymnosperms to conductive angiosperms. Both groups showed a second strategy gradient related to chemical defense. Gymnosperms strongly converged in their trait strategies because of their uniform tracheids. Angiosperms strongly diverged because of their different vessel arrangement and tissue types. The bark had higher concentrations of nutrients and phenolics whereas the wood had stronger physical defense. The gymnosperms have a conservative strategy associated with strong hydraulic safety and physical defense, and a narrow, specialized range of trait values, which allow them to grow well in drier and unproductive habitats. The angiosperm species show a wider trait variation in all stem compartments, which makes them successful in marginal- and in mesic, productive habitats. The associations between multiple wood and bark traits collectively define a slow-fast stem strategy spectrum as is seen also for each stem compartment.

Anatomies, vascular architectures, and mechanics underlying the leaf size-stem size spectrum in 42 Neotropical tree species

The leaf size-stem size spectrum is one of the main dimensions of plant ecological strategies. Yet the anatomical, mechanical, and hydraulic implications of small versus large shoots are still poorly understood. We investigated 42 tropical rainforest tree species in French Guiana, with a wide range of leaf areas at the shoot level. We quantified the scaling of hydraulic and mechanical constraints with shoot size, estimated as the water potential difference (ΔΨ) and the bending angle (ΔΦ), respectively. We investigated how anatomical tissue area, flexural stiffness and xylem vascular architecture affect such scaling by deviating (or not) from theoretical isometry with shoot size variation. Vessel diameter and conductive path length were found to be allometrically related to shoot size, thereby explaining the independence between ΔΨ and shoot size. Leaf mass per area, stem length, and the modulus of elasticity were allometrically related to shoot size, explaining the independence between ΔΦ and shoot size. Our study also shows that the maintenance of both water supply and mechanical stability across the shoot size range are not in conflict.

The role of wood anatomical traits in the coexistence of oak species along an environmental gradient

Oaks (Quercus) are a dominant woody plant genus in the northern hemisphere, which occupy a wide range of habitats and are ecologically diverse. We analysed the wood anatomical traits, the variables derived and the relative hydraulic conductivity of 21 oak species to identify their performance according to abiotic factors, leaf phenological patterns and phylogenetic restrictions by analysing the interspecific variation along an environmental gradient. First, we determine the causes of anatomical trait variation in the oaks, analysing the functional trade-offs related to distribution along the environmental gradient. We measure the phenotypic plasticity of the anatomical traits to determine the role of environment and geographic distance in the range of phenotypic plasticity. Second, we examined if oaks co-occurred along the environmental gradient. Then we analysed if wood anatomical traits reflect differences among their phylogenetic section, leaf habit and a phylogenetic section/leaf habit category. Last, we tested the phylogenetic signal. Our results showed that vessel diameter, vessel frequency, wood density and relative hydraulic conductivity are the main axes of trait variation in the species analysed among leaf habit categories. The aridity index and seasonal precipitation drive the variation in the analysed traits. Higher environmental distance resulted in a higher relative distance plasticity index among traits. Co-occurrence of oak species with different leaf habits and phylogenetic trajectories may promote complementary resource acquisition. The phylogenetic signal in the oak species studied was low, which implies labile wood traits.

Differentiation in stem and leaf traits among sympatric lianas, scandent shrubs and trees in a subalpine cold temperate forest

The scandent shrub plant form is a variant of liana that has upright and self-supporting stems when young but later becomes a climber. We aimed to explore the associations of stem and leaf traits among sympatric lianas, scandent shrubs and trees, and the effects of growth form and leaf habit on variation in stem or leaf traits. We measured 16 functional traits related to stem xylem anatomy, leaf morphology and nutrient stoichiometry in eight liana, eight scandent shrub and 21 tree species co-occurring in a subalpine cold temperate forest at an elevation of 2600-3200 m in Southwest China. Overall, lianas, scandent shrubs and trees were ordered along a fast-slow continuum of stem and leaf functional traits, with some traits overlapping. We found a consistent pattern of lianas > scandent shrubs > trees for hydraulically weighted vessel diameter, maximum vessel diameter and theoretical hydraulic conductivity. Vessel density and sapwood density showed a pattern of lianas = scandent shrubs < trees, and lianas < scandent shrubs = trees, respectively. Lianas had significantly higher specific leaf area and lower carbon concentration than co-occurring trees, with scandent shrubs showing intermediate values that overlapped with lianas and trees. The differentiation among lianas, scandent shrubs and trees was mainly explained by variation in stem traits. Additionally, deciduous lianas were positioned at the fast end of the trait spectrum, and evergreen trees at the slow end of the spectrum. Our results showed for the first time clear differentiation in stem and leaf traits among sympatric liana, scandent shrub and tree species in a subalpine cold temperate forest. This work will contribute to understanding the mechanisms responsible for variation in ecological strategies of different growth forms of woody plants.

Coordination of plant hydraulic and photosynthetic traits: confronting optimality theory with field measurements

Close coupling between water loss and carbon dioxide uptake requires coordination of plant hydraulics and photosynthesis. However, there is still limited information on the quantitative relationships between hydraulic and photosynthetic traits. We propose a basis for these relationships based on optimality theory, and test its predictions by analysis of measurements on 107 species from 11 sites, distributed along a nearly 3000-m elevation gradient. Hydraulic and leaf economic traits were less plastic, and more closely associated with phylogeny, than photosynthetic traits. The two sets of traits were linked by the sapwood to leaf area ratio (Huber value, vH ). The observed coordination between vH and sapwood hydraulic conductivity (KS ) and photosynthetic capacity (Vcmax ) conformed to the proposed quantitative theory. Substantial hydraulic diversity was related to the trade-off between KS and vH . Leaf drought tolerance (inferred from turgor loss point, -Ψtlp ) increased with wood density, but the trade-off between hydraulic efficiency (KS ) and -Ψtlp was weak. Plant trait effects on vH were dominated by variation in KS , while effects of environment were dominated by variation in temperature. This research unifies hydraulics, photosynthesis and the leaf economics spectrum in a common theoretical framework, and suggests a route towards the integration of photosynthesis and hydraulics in land-surface models.

ρ-MtreeRing: A Graphical User Interface for X-ray Microdensity Analysis

Wood microdensitometry provides an integrated measurement of inter and intra-annual changes in wood anatomy and lignification. Although it can be acquired through a wide array of techniques, X-ray-based techniques are still the standard. Conversion of a grayscale X-ray image to density and annual ring boundaries delimitation is performed through image analysis software. Proprietary software has dominated these applications, albeit Free Open Source Software (FOSS) has been developed recently. We present ρ-MtreeRing, a user-friendly FOSS that streamlines the entire microdensitometry analysis process through a graphical user interface based on Shiny R Software without any programming knowledge. We compared the results of this program with the most widely used commercial software (WinDendro), showing the validity of the results. ρ-MtreeRing can be personalized and developed by the microdensitometry research community.

An integrated framework of plant form and function: the belowground perspective

Plant trait variation drives plant function, community composition and ecosystem processes. However, our current understanding of trait variation disproportionately relies on aboveground observations. Here we integrate root traits into the global framework of plant form and function. We developed and tested an overarching conceptual framework that integrates two recently identified root trait gradients with a well-established aboveground plant trait framework. We confronted our novel framework with published relationships between above- and belowground trait analogues and with multivariate analyses of above- and belowground traits of 2510 species. Our traits represent the leaf and root conservation gradients (specific leaf area, leaf and root nitrogen concentration, and root tissue density), the root collaboration gradient (root diameter and specific root length) and the plant size gradient (plant height and rooting depth). We found that an integrated, whole-plant trait space required as much as four axes. The two main axes represented the fast-slow 'conservation' gradient on which leaf and fine-root traits were well aligned, and the 'collaboration' gradient in roots. The two additional axes were separate, orthogonal plant size axes for height and rooting depth. This perspective on the multidimensional nature of plant trait variation better encompasses plant function and influence on the surrounding environment.

Beyond leaf habit: generalities in plant function across 97 tropical dry forest tree species

Leaf habit has been hypothesized to define a linkage between the slow-fast plant economic spectrum and the drought resistance-avoidance trade-off in tropical forests ('slow-safe vs fast-risky'). However, variation in hydraulic traits as a function of leaf habit has rarely been explored for a large number of species. We sampled leaf and branch functional traits of 97 tropical dry forest tree species from four sites to investigate whether patterns of trait variation varied consistently in relation to leaf habit along the 'slow-safe vs fast-risky' trade-off. Leaf habit explained from 0% to 43.69% of individual trait variation. We found that evergreen and semi-deciduous species differed in their location along the multivariate trait ordination when compared to deciduous species. While deciduous species showed consistent trait values, evergreen species trait values varied as a function of the site. Last, trait values varied in relation to the proportion of deciduous species in the plant community. We found that leaf habit describes the strategies that define drought avoidance and plant economics in tropical trees. However, leaf habit alone does not explain patterns of trait variation, which suggests quantifying site-specific or species-specific uncertainty in trait variation as the way forward.

Plant functional types broadly describe water use strategies in the Caatinga, a seasonally dry tropical forest in northeast Brazil

In seasonally dry tropical forests, plant functional type can be classified as deciduous low wood density, deciduous high wood density, or evergreen high wood density species. While deciduousness is often associated with drought-avoidance and low wood density is often associated with tissue water storage, the degree to which these functional types may correspond to diverging and unique water use strategies has not been extensively tested.We examined (a) tolerance to water stress, measured by predawn and mid-day leaf water potential; (b) water use efficiency, measured via foliar δ13C; and (c) access to soil water, measured via stem water δ18O.We found that deciduous low wood density species maintain high leaf water potential and low water use efficiency. Deciduous high wood density species have lower leaf water potential and variable water use efficiency. Both groups rely on shallow soil water. Evergreen high wood density species have low leaf water potential, higher water use efficiency, and access alternative water sources. These findings indicate that deciduous low wood density species are drought avoiders, with a specialized strategy for storing root and stem water. Deciduous high wood density species are moderately drought tolerant, and evergreen high wood density species are the most drought tolerant group.Synthesis. Our results broadly support the plant functional type framework as a way to understand water use strategies, but also highlight species-level differences.

Genetic Regulation of Vessel Morphology in Populus

During secondary growth, forest trees can modify the anatomy of the wood produced by the vascular cambium in response to environmental conditions. Notably, the trees of the model angiosperm genus, Populus, reduce the risk of cavitation and hydraulic failure under water stress by producing water-conducting vessel elements with narrow lumens, which are more numerous and more interconnected with each other. Here, we determined the genetic architecture of vessel traits affecting hydraulic physiology and resilience to water stress. Vessel traits were measured for clonally replicated genotypes of a unique Populus deltoides x nigra population carrying genomically defined insertions and deletions that create gene dosage variation. We found significant phenotypic variation for all traits measured (mean vessel diameter, height-corrected mean vessel diameter, vessel frequency, height-corrected vessel frequency, vessel grouping index, and mean vessel circularity), and that all traits were under genetic control and showed moderate heritability values, ranging from 0.32 to 0.53. Whole-genome scans of correlations between gene dosage and phenotypic traits identified quantitative trait loci for tree height, mean vessel diameter, height-corrected mean vessel diameter, height-corrected vessel frequency, and vessel grouping index. Our results demonstrate that vessel traits affecting hydraulic physiology are under genetic control, and both pleiotropic and trait-specific quantitative trait loci are found for these traits.

Trade-offs among transport, support, and storage in xylem from shrubs in a semiarid chaparral environment tested with structural equation modeling

Plant vascular systems play a central role in global water and carbon cycles and drought resistance. These vascular systems perform multiple functions that affect the fitness of plants, and trade-offs are present among these functions. Some trade-offs are well established, but studies have not examined the full suite of functions of these complex systems. Here, we used a powerful multivariate method, structural equation modeling, to test hypotheses about the trade-offs that govern this vital and globally important tissue. We show that xylem traits are broadly governed by trade-offs related to transport, mechanical support, and storage, which are rooted in cellular structure, and that the level of dehydration experienced by plants in the field exerts a strong influence over these relationships.

The xylem in plants is specialized to transport water, mechanically support the plant body, and store water and carbohydrates. Balancing these functions leads to trade-offs that are linked to xylem structure. We proposed a multivariate hypothesis regarding the main xylem functions and tested it using structural equation modeling. We sampled 29 native shrub species from field sites in semiarid Southern California. We quantified xylem water transport (embolism resistance and transport efficiency), mechanical strength, storage of water (capacitance) and starch, minimum hydrostatic pressures (P_min), and proportions of fibers, vessels, and parenchyma, which were treated as a latent variable representing “cellular trade-offs.” We found that xylem functions (transport, mechanical support, water storage, and starch storage) were independent, a result driven by P_min. P_min was strongly and directly or indirectly associated with all xylem functions as a hub trait. More negative P_min was associated with increased embolism resistance and tissue strength and reduced capacitance and starch storage. We found strong support for a trade-off between embolism resistance and transport efficiency. Tissue strength was not directly associated with embolism resistance or transport efficiency, and any associations were indirect involving P_min. With P_min removed from the model, cellular trade-offs were central and related to all other traits. We conclude that xylem traits are broadly governed by functional trade-offs and that the P_min experienced by plants in the field exerts a strong influence over these relationships. Angiosperm xylem contains different cell types that contribute to different functions and that underpin trade-offs.