Tree Physiology

Drought-induced embolism formation in conifers is associated with several tracheid and pit traits, which vary in parallel from stem apex to base. We tested whether this axial anatomical variability is associated with a progressive variation in embolism vulnerability along the stem from apex to base. We assessed the xylem pressure at 50% loss of conductivity (P50), the tracheid hydraulic diameter (Dh) and mean pit membrane area (PMA) on longitudinal stem segments extracted at different distances from the stem apex (DFA) in a Picea abies and an Abies alba tree. In both trees, Dh and PMA scaled with DFA0.2. P50 varied for more than 3 MPa from the treetop to the stem base, according to a scaling of -P50 with DFA-0.2. The largest Dh, PMA and P50 variation occurred for DFA<1.5 m. PMA and Dh scaled isometrically (exponent b=1). Pit traits vary proportionally with tracheid lumen diameter. Apex-to-base trends in tracheid and pit traits determine a large DFA-dependent P50 variability. Such a DFA effect on P50 did not receive sufficient attention so far, although analysing the relationships P50 vs. DFA is fundamental for the assessment of embolism vulnerability at the individual level. HighlightsO_LIConifer embolism vulnerability depends on pit properties, in agreement with published data. C_LIO_LIPit dimensions increase with tracheid lumen diameter, in agreement with published data C_LIO_LITracheid lumen diameter and pit dimensions increase progressively from the stem apex to base, in agreement with published data. C_LIO_LIXylem vulnerability to embolism formation (P50) varies for > 3 MPa from the stem apex to base, with the largest variation occurring within 1.5 m from the stem apex. C_LIO_LIAxial anatomical patterns should be accounted for when analyzing hydraulic properties at individual, intra- and inter-specific scales. C_LI

O_LIIsohydric species reduce water potential fluctuations through more stringent stomatal regulation. It is unclear whether isohydric behavior leads to smaller reversible diurnal stem shrinkage (i.e. greater nocturnal stem rehydration) during drought and a lower drought sensitivity of radial growth compared to anisohydric behavior. C_LIO_LIWith synchronous high-resolution sap flow and dendrometer measurements in mature anisohydric European beech and isohydric Douglas fir trees in pure and mixed stands, we quantified declines in stem water content (SWC), stem rehydration, sap flow and radial growth during soil dry-down and determined the critical soil moisture levels (expressed as Relative Extractable Water, REW) and elapsed desiccation time until 30-90% reductions in these traits. C_LIO_LISap flow and growth started to decline in both species at REW [~]0.6, preceding declines in stem rehydration. Water-spending Douglas fir approached 50% drops in SWC, sap flow, stem rehydration and growth during soil dry-down faster than beech, indicating higher drought sensitivity. In mixture, both species reached these reduction levels later than in monoculture, suggesting positive mixing effects on the species drought resistance. C_LIO_LIOur findings demonstrate that SWC, sap flow and radial growth decrease earlier than nocturnal stem rehydration, with isohydric and anisohydric species exhibiting different timelines of physiological downregulation during soil dry-down. C_LI

Understanding xylem embolism formation is challenging due to dynamic changes and multiphase interactions in conduits. If embolism spread involves gas movement in xylem, we hypothesise that it is affected by time. We measured hydraulic conductivity (Kh) in flow-centrifuge experiments over one hour at a given pressure and temperature for stem samples of three angiosperm species. Temporal changes in Kh at 5, 22, and 35{degrees}C, and at various pressures were compared to modelled gas pressure changes in a recently embolised vessel in the centre of a centrifuge sample. Temporal changes in Kh at 22{degrees}C showed maximum relative increases between 6% and 40%, and maximum decreases between 41% and 61% at low and high centrifugal speed, respectively. Logarithmic changes in Kh were species-specific, and most pronounced during the first 15 minutes. Embolism formation started near the edges of centrifuge samples and gradually increased at the centre. Moreover, measured decreases in Kh strongly correlated with modelled increases in gas concentration in a recently embolised vessel. Although embolism is mostly pressure-driven, our experimental and modelled data indicate that time, conduit characteristics, and temperature are involved due to their role in gas diffusion. Gas diffusion, however, does not cover the entire process of embolism spread.

Residual water losses after stomatal closure have recently been identified as key determinants of drought-induced hydraulic failure, particularly under heatwave conditions. However, little is known about the intraspecific variability of residual conductance (gres) and its thermal sensitivity. Here, we investigated the genetic and environmental sources of variation in gres and its associated thermal parameters (phase transition temperature T_, and temperature sensitivities Q10a and Q10b) in Abies alba Mill., together with vulnerability to xylem embolism (P50). Measurements were performed using the Drought-Box on seven French provenances grown in a common garden to assess genetic variability, and on trees growing across contrasting forest sites to quantify phenotypic plasticity. Seasonal dynamics and within-canopy microclimatic effects were also examined, and linked to needle biochemical traits. Residual conductance exhibited a marked seasonal decline, with high values in newly formed needles followed by a stabilization from late summer to the following spring, closely tracking the accumulation of cuticular waxes. In contrast, Klason lignin content showed little seasonal variation. Difference between provenances was weak for all investigated parameters, suggesting strong constraints on these safety-related traits. By contrast, gres showed significant environmental plasticity, with lower values at more climatically constrained sites, while thermal parameters and P50 remained relatively conserved. Our results identify gres as a developmentally dynamic and environmentally plastic trait in silver fir, potentially representing a key lever of acclimation to drought. Incorporating such variability into mechanistic models should improve predictions of tree vulnerability under future climates combining intensified droughts and heatwaves. Key message.Residual conductance in Abies alba is developmentally dynamic and environmentally plastic but genetically constrained, highlighting its key role in acclimation to drought and heatwave-driven hydraulic failure.

Centrifuges provide a fast and standard approach to quantify embolism resistance of xylem in vulnerability curves (VCs). Traditionally, embolism formation in centrifuge experiments is assumingly driven by centrifuge speed, and thus pressure, but unaffected by spin time. Here, we explore to what extent embolism resistance is not only pressure but also spin time dependent, and hypothesise that time-stable hydraulic conductivity (Kh) values could shift VCs. We quantified time-based shifts in flow- centrifuge VCs and their parameter estimations for six angiosperm species by measuring Kh at regular intervals over 15 minutes of spinning at a particular speed before a higher speed was applied to the same sample. We compared various VCs per sample based on cumulative spin time, and modelled the relationship between Kh, xylem water potential ({Psi}), and spin time. Time-based changes of Kh showed considerable increases and decreases at low and high centrifuge speeds, respectively, which generally shifted VCs towards more positive {Psi} values. Values corresponding to 50% loss of hydraulic conductivity (P50) increased up to 0.72 MPa in Acer pseudoplatanus, and on average by 8.5% for all six species compared to VCs that did not consider spin time. By employing an asymptotic exponential model, we estimated time-stable Kh, which improved the statistical significance of VCs in 5 of the 6 species studied. This model also revealed the instability of VCs at short spin times, and showed that embolism formation in flow-centrifuges followed a saturating exponential growth curve. Although pressure remains the major determinant of embolism formation, spin time should be considered in flow- centrifuge VCs to avoid overestimation of embolism resistance. This spin-time artefact is species- specific, and likely based on relatively slow gas diffusion associated with embolism spreading. It can be minimized by determining time-stable Kh values for each centrifuge speed, without considerably extending the experimental time to construct VCs.

Climate change is projected result in higher frequencies of drought events across the world and lead to reduced performance in many temperate tree species. However, many studies in this area focus specifically on adult tree drought responses and overlook how trees in other age classes might differ in their vulnerability. Evidence shows that seedling drought response can differ from that of adults and furthermore that demographic performance in the seedling age class will have disproportionately strong effects on the assembly dynamics of future forests, together suggesting that understanding seedling drought responses will be critical to our ability to predict how forests will respond to climate change. In this study, we measured four indices of hydraulic response to drought (leaf water potential, photosynthetic capacity, non-structural carbohydrate concentration, and hydraulic conductivity), as well as interaction effects with shade treatments, for seedlings of two temperate tree species that differ in their adult drought response: isohydric Acer saccharum and anisohydric Quercus rubra. We found a strong isohydric response in A. saccharum seedlings that included conservation of leaf water potentials (>-1.8 MPa) and reductions in non-structural carbohydrate concentrations consistent with reduction of stomatal conductance. Quercus rubra seedlings were able to survive to more negative water potentials, but only rarely, and they showed a similar reduction in photosynthetic capacity as was found for A. saccharum. Our results suggest that, although Q. rubra seedlings display some anisohydric responses to drought, they are more isohydric than adults. Both species seem to be relatively similar in their vulnerability to drought despite the differences predicted from adult drought response, and our results suggest that seedlings of both species will be similarly vulnerable to future drought events.

The ability of trees to survive and naturally regenerate in increasing drought conditions will depend on their capacity to vary key hydraulic and morphological traits that increase drought tolerance. Despite many studies investigating variability in these drought-tolerant traits, there has been limited investigation into this variability under recurrent severe drought conditions since the establishment phase. We investigated the long-term hydraulic and leaf trait adjustments of Scots pine trees settled in an abandoned slag quarry by comparing them across three different topographic positions inducing contrasted effects on growth and development. We measured xylem and foliar traits to compare the water status of trees according to tree location and to evaluate the respective risk for xylem hydraulic failure using the soil-plant hydraulic model SurEau. Compared to upslope and downslope trees, slope trees exhibited lower growth, vulnerability to embolism, specific hydraulic conductivity and photosynthetic pigment contents, as well as higher water potential at turgor loss point and midday water potentials. The hydraulic adjustments of trees settled on slag slopes reduced the risk for hydraulic failure and thus prevented an increase in embolism compared to downslope and upslope trees. These results suggest a prioritization of hydraulic safety over growth in Scots pine developed in a harsh environment, resulting in a dwarf phenotype.

O_LIIt is often stated that trees experiencing climate change may be more vulnerable to damaging effects of pest and disease, but experimental tests are still rare. In this study, we examined the separate and combined impact of experimentally applied xylem and phloem damage in an Australian Eucalypt species (Corymbia calophylla), which is increasingly suffering from phloem and xylem damage by a canker disease caused by Quambalaria coyrecup. C_LIO_LICut treatments were applied to saplings under controlled conditions by removing xylem only ([~]56%), phloem only ([~]70%) or both in the main stem under well-watered and drought treatments. C_LIO_LIAs expected, xylem damage reduced whole-plant conductance, stomatal conductance and photosynthesis, and hence growth. Phloem damage limited phloem transport, increased leaf non-structural carbohydrate concentrations (NSC), and reduced root NSC, photosynthesis and growth. Contrary to expectations, effects were larger in well-watered than droughted plants. The combined effects of damage to xylem and phloem were generally less than additive. Plants were remarkably resilient to significant loss of xylem and phloem, although xylem damage had a greater impact on most of the physiological parameters than phloem damage. C_LIO_LIThese results reveal potential consequences of xylem-phloem dysfunction due to biotic attack, leading to whole tree mortality, in association with drought stress. C_LI

Xylem embolism is a major threat to tree function and survival under drought, in natural and agricultural settings alike, with its impact increasing in light of global climate change. Conversely, potassium (K+) has been shown to increase xylem conductivity (Ks) in trees, and carbohydrates were reported to impact leaf gas exchange. In this study we examined the effects of K+ and carbohydrates on Ks in two divergent evergreen tree species that are regularly exposed to drought: pine (Pinus brutia) and lemon (Citrus x limon). Five-year-old trees were pretreated with zero, moderate, and high K+, and with ambient or elevated CO2, to experimentally increase their xylem K+ or carbohydrates levels, respectively. Trees were then monitored for Ks and embolism (using a microCT), along with leaf gas exchange and water potential, before and after a 1.5-2.5 month drought period. Potassium fertigation had a positive effect on Ks, in both species when irrigated, which was eliminated following drought. Drought decreased Ks about 10-fold in lemon, with little effect in pine. CO2-treated trees had the same Ks as control trees before and after drought. Our results indicate a positive effect of K+ on tree hydraulics, which was more pronounced in lemon than in pine, supporting the hypothesis of interaction with the angiosperm pit membrane, and not with the gymnosperm bordered pit. Yet, the elimination of this benefit following drought, and the lack of benefit from elevated carbohydrates following a short-term CO2 treatment, question the relevance of these components to tree drought resistance mechanisms. Key massagePotassium fertigation increases hydraulic conductivity and reduces xylem embolism in the gymnosperm pine, and more so in the angiosperm lemon tree, benefits which were eliminated following drought.

Wood formation is a crucial process for carbon sequestration, yet how variations in carbon supply affect wood formation and carbon dynamics in trees more generally remains poorly understood. To better understand the role of carbon supply in wood formation, we restricted phloem transport using girdling and compression around the stem of mature white pines and monitored the effects on local wood formation and stem CO2 efflux, as well as nonstructural carbon concentrations in needles, stems, and roots. Growth and stem CO2 efflux varied with location relative to treatment (i.e., above or below on the stem). We observed up to a two-fold difference in the number of tracheids formed above versus below the manipulations over the remaining growing season. In contrast, the treatments did not affect mean cell size noticeably and mean cell-wall area decreased only slightly below them. Surprisingly, nonstructural carbon pools and concentrations in the xylem, needles, and roots remained largely unchanged, although starch reserves declined and increased marginally below and above the girdle, respectively. Our results suggest that phloem transport strongly affects cell proliferation and respiration in the cambial zone of mature white pine, but has little impact on nonstructural carbon concentrations. These findings contribute to our understanding of how wood formation is controlled. HighlightRestrictions in phloem transport designed to affect carbon supply, lead to changes in wood formation and stem respiration of mature white pines without substantially changing local nonstructural carbon concentrations.

Spring freezing is an unforgiving stress for young leaves, often leading to death, with consequences for tree productivity and survival. While both the plant water transport system and living tissues are vulnerable to freezing, we do not know whether damage to one or both of these systems causes death in leaves exposed to freezing. Whole saplings of Liriodendron tulipifera were exposed to freezing and thawing trajectories designed to mimic natural spring freezes. We monitored the formation of freeze-thaw xylem embolism and damage to photosynthetic tissues to reveal a predictable progression of ice formation across the leaf surface that is strongly influenced by leaf vein architecture, notably the presence or absence of bundle sheath extensions. Our data also show that freeze-thaw embolism occurs only in the largest vein orders where mean vessel diameter exceeds 30{micro}m. With evidence of both freeze-thaw embolism and damage to photosynthetic tissue, we conclude that this dual-mode lethality may be common among other wide-vesseled angiosperm-leaves, potentially playing a role in limiting geographic distributions, and show that bundle sheath extensions may stall or even prevent freezing spread. HighlightBoth ice and air lead likely lead to death in young L.tulipfera leaves exposed to freezing, with the spread of both governed by physical characteristics of these leaves.

A long-established ecological paradigm predicts a functional relationship determining vulnerability to cavitation: vulnerability increases with vessel hydraulic efficiency and vessel diameter. Even within a species, big vessels cavitate before small ones. Some centrifuge methods for measuring vulnerability are prone to artifacts due to nano-particles seeding early embolism, as the particles are drawn into vessels during measurements. Both the Sperry and Cochard rotors are prone to early cavitation due to nano-particles drawn into long and wide vessels in Robinia pseudoacacia and Quercus acutissima, whereas extraction centrifuge methods produce vulnerability curves more resistant to cavitation. Sufficient nano-particles pass through the stems to seed early embolism in all rotor designs. For several years, people have thought that early embolism is induced by nano-particles present in laboratory water. One new hypothesis is that the origin of nano-particles is from cut-open living cells but a much bigger study including many species is required to confirm this idea. This paper confirms the hypothesis in comparisons between short-vesselled Acer, and long-vesselled Robinia, and Quercus. Our new results and a review of old results justifies bigger study. Hypothetical nano-particles might explain why different methods for measuring vulnerability curves cause different T50 = tensions causing 50% loss of hydraulic conductivity. Hence the hypothesis for future research should be that the open-vessel artifact is consistent with long vessels surrounded by cut open living cells. One sentence SummaryNano-particles induced early cavitation in species with vessel lengths about [1/4] the stem length used in all centrifuge rotors, and the origin of nano-particles might be from living cells nearby vessels

O_LIThe future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil moisture is largely unknown. C_LIO_LILeaf gas exchange measurements were performed within a long-term (17 years) irrigation experiment in a Scots pine-dominated forest in one of Switzerlands driest areas on trees in naturally dry (control), irrigated, and irrigation-stop (after 11 years of irrigation) conditions. C_LIO_LISeventeen years of irrigation increased photosynthesis (A) and stomatal conductance (gs) and reduced the gs sensitivity to increasing VPD but not to soil drying. Following irrigation-stop, gas exchange did not decrease immediately, but after three years, had decreased significantly in irrigation-stop trees. Vcmax and Jmax recovered after five years. C_LIO_LIThese results suggest that long-term release of soil drought reduces the sensitivity to atmospheric evaporative demand and that atmospheric constraints may play an increasingly important role in combination with soil drought. In addition, they suggest that structural adjustments lead to an attenuation of initially strong leaf-level acclimation to strong multiple-year drought. C_LI

O_LIStemflow of forest trees can contribute a significant fraction of water to forests water fluxes; however, it is still unclear if and to what extent trees use stemflow for water supply and how much stemflow is lost by percolating below the root zone. C_LIO_LIWe applied deuterium-enriched stemflow equivalent to a throughfall depth of 23 mm to adult Fagus sylvatica trees to trace stemflow through the soil and trees. We continuously measured in-situ water stable isotope compositions in soil and xylem water and destructively sampled xylem water in the crowns of adult labelled (n=18), unlabelled F. sylvatica (n=15) and unlabeled Picea abies (n=9), complemented by destructive xylem water sampling of neighboring juvenile F. sylvatica (n=45). C_LIO_LIStemflow water supported 3.9 - 14.0% of daily sap flux of adult labelled F. sylvatica trees. In the soil, deuterium-enriched stemflow was detectable at a max. of 0 - 0.40 m to the labelled tree. However, unlabeled juvenile trees within a distance of [~]2 m showed label water uptake, indicating rooting into soil compartments affected by stemflow label. C_LIO_LIWe demonstrate the importance of stemflow as a water source for both adult and neighboring juvenile F. sylvatica, strongly profiting from stemflow infiltration. C_LI

O_LIWe investigated the potential of wood anatomical traits to improve the reconstruction of masting events--variable and synchronized patterns of seed production --which are key to understanding tree species responses to current and predicted climate variability. Traditional reliance on tree-ring width as a proxy for reproduction is limited, as growth reductions can also result from drought and other stressors. C_LIO_LIWe analyzed 12 beech cores from North-East Germany, building a 52-year dataset. A wide range of wood anatomical traits was assessed to disentangle the effects of masting and drought. We used multivariate regression and developed a random forest model to evaluate the predictive power of these traits compared to tree ring width alone. C_LIO_LIResults suggested a complex mechanism of carbon reallocation towards reproduction, while reflecting a compensatory strategy to maintain hydraulic function and mechanical stability under resource-constrained conditions. Number of parenchyma cells, vessel density, and lignin content estimates emerged as key predictors for masting, outperforming tree-ring width in capturing the reproductive signal. C_LIO_LIOur findings establish a novel link between wood anatomy and masting events, demonstrating that quantitative wood anatomical traits offer a more accurate and ecologically relevant approach for reconstructing past masting dynamics. C_LI

Intensive European forestry practices contribute to soil degradation and nutrient depletion, compromising tree health and ecosystem stability. However, the influence of soil nutrient scarcity on tree physiological responses during drought remains unclear, particularly in Pinus radiata D. Don plantations, where it may impair needle function and overall tree health. We investigated how soil nutrient availability influences leaf-level physiological strategies during drought by comparing same-aged needles from two P. radiata stands with contrasting management stages and ages: (i) trees ({approx}20 years), likely to be clear-cut at 30-35 years (herein managed), and (ii) trees (>45 years) not clear-cut at the typical rotation age (herein abandoned). During the severe summer drought of 2022, both stands exhibited downregulation of the photosynthetic apparatus, indicating impaired photosynthetic performance under drought. However, their leaf-physiological responses to nutrient scarcity diverged. Managed trees exhibited dependence on soil nutrients, with reduced photosynthetic performance under nutrient-poor conditions. In contrast, abandoned trees showed relative independence from soil nutrients, maintaining photosynthetic function even under nutrient-poor conditions. This highlights a trade-off: younger, managed trees may enhance photosynthesis under optimal nutrient conditions but are more susceptible to nutrient imbalances and environmental stress. In turn, older, abandoned trees appeared to buffer the effects of drought and nutrient scarcity through age-related physiological traits. Our findings underscore the physiological value of mature stands. Furthermore, higher soil organic carbon and vegetation diversity in abandoned stands suggest that management cessation may enhance long-term ecosystem resilience. These results emphasise the importance of integrating soil-leaf interactions into sustainable forest management.

- Norway spruce (Picea abies) dominates many European mountain forests, yet their seasonal water uptake strategies in high-elevation mono-specific natural stands remain poorly understood. We quantified contributions of shallow (0-10 cm) and deep (50-70 cm) soil layers to tree water uptake over three consecutive growing seasons (2020-2022) using stable water isotopes and Bayesian mixing analysis. - Contrary to the prevailing view of spruce as a shallow-rooted species relying primarily on water from the upper 10-20 cm of soil, our results showed more than 50% water uptake from deeper soil (50-70 cm), with deeper soil contributions crossing 80% in 2020. - During the dry and warm summer of 2022, positive soil recharge and elevated atmospheric demand increased evapotranspiration, with spruce trees taking up recently infiltrated rainfall from different soil depths, including >50% uptake from deeper layers. - Spruce water uptake shifted from cold-season-recharged soil water early in the growing season to warm-season precipitation in late summer. The timing of this shift in mid-summer can be explained by soil water recharge from recent rainfall infiltrated into the entire soil profile. This reliance on summer precipitation increases vulnerability of mono-specific spruce stands to more frequent droughts and heat waves under future climate change.

Woody stems conduct both photosynthetic assimilation and respiration. The two processes work in concert, as stem photosynthesis helps refix CO2 released by stem respiration, thereby increasing carbon-use efficiency and generating a local pool of non-structural carbohydrates supporting cambial growth and stem hydraulic function. Despite its importance, little is known about seasonal variation in stem photosynthesis and the factors underlying its activity throughout the season. To fill this gap, we measured stem gas exchange together with growth activity, water status and photosynthetic pigment contents in two temperate species, Acer platanoides L. and Prunus avium L., over the season. In both species, gross photosynthetic rates (Pg) and dark respiration (Rd) changed significantly over the season in a similar pattern, indicating strong coordination between the two processes. Both Pg and Rd reached the highest values in May, during the period of rapid leaf expansion and secondary growth, and declined later in the growing season. At each measurement date, Rd exceeded Pg, resulting in a net CO2 efflux from the stems. The seasonal changes in Pg and Rd translated into seasonal variability in relative refixation of CO2, ranging from 3 to 59% and gradually decreasing towards the end of the season. Additionally, the Pg corresponded with the tissue hydration and increased significantly with increasing stem water potential. In contrast, total chlorophyll content showed less pronounced seasonal variation and thus explained substantially lower seasonal variability in Pg, except for the chlorophyll a/b ratio, which changed dynamically over the season and reached a minimum during the peak of the growing season. Overall, our results reveal that stem photosynthesis varies seasonally in accord with stem growth and water status, while the chlorophyll content has a lower impact on the seasonal changes. These findings are important for our understanding of the carbon relations of trees.

O_LIEmbolism spreading in angiosperm xylem occurs via mesoporous pit membranes between vessels. Here, we investigate how the size of pore constrictions in pit membranes is related to pit membrane thickness and embolism resistance. C_LIO_LIIn three models, pit membranes are modelled as multiple layers to investigate how pit membrane thickness and the number of intervessel pits per vessel determine pore constriction sizes, the probability of encountering large pores, and air-seeding. These estimations were complemented by measurements of pit membrane thickness, embolism resistance, and number of intervessel pits per vessel (n = 31, 31, and 20 species, respectively). C_LIO_LIConstriction sizes in pores decreased with increasing pit membrane thickness, which agreed with the measured relationship between pit membrane thickness and embolism resistance. The number of pits per vessel affected constriction size and embolism resistance much less than pit membrane thickness. A strong relationship between estimated air-seeding pressures and measured embolism resistance was observed. C_LIO_LIPore constrictions provide a mechanistic explanation why pit membrane thickness determines embolism resistance, and suggest that hydraulic safety can be uncoupled from hydraulic efficiency. Although embolism spreading remains puzzling and encompasses more than pore constriction sizes, angiosperms are unlikely to have leaky pit membranes, which enables tensile transport of water. C_LI

Lianas are increasing in relative abundance and biomass, mainly in seasonally dry forests, but it is unclear if this is associated with their hydraulic strategy. Here, we ask whether liana of seasonally dry forests are safer and more efficient in water transport than those of rainforest, which could explain liana distribution patterns and their recent increases. We measured hydraulic traits on five pairs of congeneric liana species (tribe Bignonieae) on one seasonal dry Atlantic forest and one Amazon rainforest. The predawn and minimum water potential, and the water potential at which 50% of the maximum gas amount was discharged were, on average, more negative in the liana species of the seasonal forest. However, these patterns were not constant at the genus level. The positive hydraulic safety margins and hydraulic efficiency were similar among species congeners across sites. The Bignonieae lianas studied likely experience equally low levels of embolism during drought, and maintain a high conductive capacity with efficient use of xylem space, which may favor survival and growth across tropical forests. The likely evolutionary convergence of high hydraulic safety associated with the opportunistic strategy of rapid growth, especially in disturbed areas can favor the abundant liana species in seasonal forests. HighlightTropical forest liana species have high hydraulic efficiency and high interspecific variability in hydraulic safety. Despite this variability, some seasonal forest liana species have greater hydraulic safety than rainforest lianas, indicating an evolutionary convergence across lineages.