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Future climate presents conflicting implications for forest biomass. We evaluate how plant hydraulic traits, elevated CO2 levels, warming, and changes in precipitation affect forest primary productivity, evapotranspiration, and the risk of hydraulic failure. We used a dynamic vegetation model with plant hydrodynamics (FATES-HYDRO) to simulate the stand-level responses to future climate changes in a wet tropical forest in Barro Colorado Island, Panama. We calibrated the model by selecting plant trait assemblages that performed well against observations. These assemblages were run with temperature and precipitation changes for two greenhouse gas emission scenarios (2086-2100: SSP2-45, SSP5-85) and two CO2 levels (contemporary, anticipated). The risk of hydraulic failure is projected to increase from a contemporary rate of 5.7% to 10.1-11.3% under future climate scenarios, and, crucially, elevated CO2 provided only slight amelioration. By contrast, elevated CO2 mitigated GPP reductions. We attribute a greater variation in hydraulic failure risk to trait assemblages than to either CO2 or climate. Our results project forests with both faster growth (through productivity increases) and higher mortality rates (through increasing rates of hydraulic failure) in the neo-tropics accompanied by certain trait plant assemblages becoming nonviable.

Hydraulic failure due to xylem embolism has been identified as one of the main mechanisms involved in drought‐induced forest decline. Trees vulnerability to hydraulic failure depends on their hydraulic safety margin (HSM). While it has been shown that HSM globally converges between tree species and biomes, there is still limited knowledge regarding how HSM can adjust locally to varying drought conditions within species. In this study, we relied on three long‐term partial rainfall exclusion experiments to investigate the plasticity of hydraulic traits and HSM for three Mediterranean tree species (Quercus ilex L., Quercus pubescens Willd., and Pinus halepensis Mill.). For all species, a homeostasis of HSM in response to rainfall reduction was found, achieved through different mechanisms. For Q. ilex, the convergence in HSM is attributed to the adjustment of both the turgor loss point (Ψtlp) and the water potential at which 50% of xylem conductivity is lost due to embolism (P50). In contrast, the maintenance of HSM for P. halepensis and Q. pubescens is related to its isohydric behavior for the first and leaf area adjustment for the latter. It remains to be seen whether this HSM homeostasis can be generalized and if it will be sufficient to withstand extreme droughts expected in the Mediterranean region. Summary statement: Mediterranean species submitted to aggravated water stress have maintained their hydraulic safety margin (HSM). For Pinus halepensis and Quercus pubescens, HSM maintenance resulted from their capacity to escape water stress, while it results from the plasticity of Ψtlp and P50 for Quercus ilex. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Robbins et al. (2024), 244: 2239–2250. [ABSTRACT FROM AUTHOR]

Abstract Key message Winter reddening of young Douglas-fir (Pseudotsuga menziesii Mirb. Franco), triggered by large thermal fluctuations in late winter, is a critical problem for European forestry. A literature review identified certain climatic conditions that are characteristic of ‘reddening’ years, including warm daily temperatures, high daily temperature amplitude, low relative humidity, moderate wind speeds, as well as the occurrence of freeze-thaw cycles with cold night temperatures. By describing the triggering environmental and stand factors, we propose three hypotheses for the physiological processes leading to winter reddening, namely (i) hydraulic failure due to winter drought stress, (ii) photo-oxidative stress in shade-acclimated trees, and (iii) early cold deacclimation during warm periods. i) Low soil temperature, by reducing root water uptake, combined with anticyclonic conditions, by increasing water losses, can induce hydraulic failure in the xylem. Hydraulic failure may be further accelerated by night frosts. ii) Winter reddening can occur when low temperature and high irradiance coincide, disrupting photostasis. Overwhelming of winter photo-protection may lead to photodamage and subsequent reddening. iii) Warm periods, by inducing cold deacclimation, make trees susceptible to frost damage. Finally, the three processes may interact under atypical anticyclonic conditions in late winter (e.g. cold or dry soils, warm days, high irradiance and/or freezing nights). Indeed, trees under water stress would develop a higher sensitivity to freezing night and photooxidative stress. We therefore proposed mitigation actions to avoid exposing trees to stressful conditions based on e.g. stand characteristics, understorey vegetation and planting.

Key message: Winter reddening of young Douglas-fir (Pseudotsuga menziesii Mirb. Franco), triggered by large thermal fluctuations in late winter, is a critical problem for European forestry. A literature review identified certain climatic conditions that are characteristic of 'reddening' years, including warm daily temperatures, high daily temperature amplitude, low relative humidity, moderate wind speeds, as well as the occurrence of freeze-thaw cycles with cold night temperatures. By describing the triggering environmental and stand factors, we propose three hypotheses for the physiological processes leading to winter reddening, namely (i) hydraulic failure due to winter drought stress, (ii) photo-oxidative stress in shade-acclimated trees, and (iii) early cold deacclimation during warm periods. i) Low soil temperature, by reducing root water uptake, combined with anticyclonic conditions, by increasing water losses, can induce hydraulic failure in the xylem. Hydraulic failure may be further accelerated by night frosts. ii) Winter reddening can occur when low temperature and high irradiance coincide, disrupting photostasis. Overwhelming of winter photo-protection may lead to photodamage and subsequent reddening. iii) Warm periods, by inducing cold deacclimation, make trees susceptible to frost damage. Finally, the three processes may interact under atypical anticyclonic conditions in late winter (e.g. cold or dry soils, warm days, high irradiance and/or freezing nights). Indeed, trees under water stress would develop a higher sensitivity to freezing night and photooxidative stress. We therefore proposed mitigation actions to avoid exposing trees to stressful conditions based on e.g. stand characteristics, understorey vegetation and planting. [ABSTRACT FROM AUTHOR]

Knowledge of the physiological mechanisms underlying species vulnerability to drought is critical for better understanding patterns of tree mortality. Investigating plant adaptive strategies to drought should thus help to fill this knowledge gap, especially in tropical rainforests exhibiting high functional diversity. In a semi-controlled drought experiment using 12 rainforest tree species, we investigated the diversity in hydraulic strategies and whether they determined the ability of saplings to use stored non-structural carbohydrates during an extreme imposed drought. We further explored the importance of water- and carbon-use strategies in relation to drought survival through a modelling approach. Hydraulic strategies varied considerably across species with a continuum between dehydration tolerance and avoidance. During dehydration leading to hydraulic failure and irrespective of hydraulic strategies, species showed strong declines in whole-plant starch concentrations and maintenance, or even increases in soluble sugar concentrations, potentially favouring osmotic adjustments. Residual water losses mediated the trade-off between time to hydraulic failure and growth, indicating that dehydration avoidance is an effective drought-survival strategy linked to the 'fast–slow' continuum of plant performance at the sapling stage. Further investigations on residual water losses may be key to understanding the response of tropical rainforest tree communities to climate change. [ABSTRACT FROM AUTHOR]

Evaluate the structural damage and the changes in the photosynthesis and transpiration rates of aquatic lirium leaves caused by ultrasound (US) irradiation in search of environmentally friendly methodologies for the control of this weed. Aquatic lirium plants were extracted from Xochimilco water canals in Mexico City. A part of the group of plants was selected for irradiation, and the rest formed the control group. The irradiation plants group was exposed to US irradiation of 17 kHz frequency and 30 W × 4 output power for 2 h, at noon and 25 °C room temperature. The structural analysis was done with a MOTICAM 1 digital camera, 800 × 600 pixels, incorporated into the MOTIC PSM-1000 optical microscope and edited with Motic Images Plus 2.0 ML software. The total stomata density and the damaged stomata density were determined by dividing the numbers of total and damaged stomata by the visual field area (67,917 mm2), respectively. The leaves' photosynthesis and transpiration rates were measured using an LI-6400XT Portable Photosynthesis System. Significant damage was observed in the stomata and epidermal cells, finding that the average ratio between the damaged and total stomata densities as a function of time (days) showed an exponential increase described by a Box–Lucas equation with a saturation value near unity and a maximum rate of change of the density of damaged stomata on zero-day (immediately after irradiation), decreasing as the days go by. The transpiration rate showed a sudden increase during the first hour after irradiation, reaching a maximum of 36% of its value before irradiation. It then quickly fell during the next 6 days and more slowly until the 21st day, decreasing 79.9% of its value before irradiation. The photosynthetic rate showed similar behavior with a 37.7% maximum increment and a 73.6% minimum decrease of its value before irradiation. The results of structural stomata damage on the ultrasound-irradiated aquatic lirium leaves are consistent with an excessive ultrasound stimulation on stomata's mechanical operation by guard cells that produce the measured significant increase of the photosynthetic and transpiration rates during the first hour after irradiation. The initial high evaporation could alter the water potential gradient, with a possible generation of tensions in the xylem that could cause embolism in their conduits. The loss of xylem conductivity or hydraulic failure would be consistent with the observed significant fall in the photosynthesis and transpiration rates of the aquatic lirium leaves after its sudden rise in the first hour after irradiation. [ABSTRACT FROM AUTHOR]

Summary: Mixing species with contrasting resource use strategies could reduce forest vulnerability to extreme events. Yet, how species diversity affects seedling hydraulic responses to heat and drought, including mortality risk, is largely unknown.Using open‐top chambers, we assessed how, over several years, species interactions (monocultures vs mixtures) modulate heat and drought impacts on the hydraulic traits of juvenile European beech and pubescent oak. Using modeling, we estimated species interaction effects on timing to drought‐induced mortality and the underlying mechanisms driving these impacts.We show that mixtures mitigate adverse heat and drought impacts for oak (less negative leaf water potential, higher stomatal conductance, and delayed stomatal closure) but enhance them for beech (lower water potential and stomatal conductance, narrower leaf safety margins, faster tree mortality). Potential underlying mechanisms include oak's larger canopy and higher transpiration, allowing for quicker exhaustion of soil water in mixtures.Our findings highlight that diversity has the potential to alter the effects of extreme events, which would ensure that some species persist even if others remain sensitive. Among the many processes driving diversity effects, differences in canopy size and transpiration associated with the stomatal regulation strategy seem the primary mechanisms driving mortality vulnerability in mixed seedling plantations. See also the Commentary on this article by Wright, 241: 955–957. [ABSTRACT FROM AUTHOR]

Drought-induced plant mortality, resulting from either hydraulic failure or carbon starvation, is hypothesized to be modulated by the drought intensity. However, there is a paucity of research investigating the response strategies in desert shrubs under drought stress with different intensities. We transplanted potted Reaumuria soongorica (Pall.) Maxim. and Salsola passerina Bunge seedlings in the rain-out shelter, and implemented three water treatments: a control (well-watered, CK), a chronic drought (gradually less watered, CD), and a flash drought (not watered, FD). We then quantified plant physiological traits associated with water use and carbon assimilation. Both R. soongorica and S. passerina showed similar changes in water use and carbon characteristics under different drought treatments. Water use efficiency was not significantly changed, but embolism resistance was significantly lower in CD, and leaf specific conductivity and embolism resistance were significantly lower in FD compared to CK. Under the drought treatment, both shrubs had significantly lower hydraulic safety margins than CK, with FD being significantly lower than CD. Notably, FD had lower carbon assimilation and a lower leaf non-structural carbon concentration, but higher stem non-structural carbon concentration. The results of a principal component analysis showed that net photosynthetic rate, sapwood specific conductivity, embolism resistance, midday water potential, and leaf and stem soluble sugar concentration were the main axes of variation for R. soongorica traits. CK had the highest water use efficiency, net photosynthetic rate, and gas exchange rate, while FD had the lowest embolism resistance and highest osmoregulation. Midday water potential, leaf and stem soluble sugar concentration were the main axes of variation for S. passerina traits, and individual distribution under three water treatments was associated with drought tolerance traits. The findings suggest that species exhibit different response strategies for resistance to drought stress, with R. soongorica being drought-avoidant and S. passerina being drought-tolerant. These findings highlight the adaptive capacity of desert shrubs to water deficit and provide insights for assessing hydraulic failure and carbon starvation in desert shrubs. [ABSTRACT FROM AUTHOR]

Aim: Understanding tree mortality under climate change‐induced droughts requires knowledge of hydraulic trait distribution across environmental gradients. The spatial distribution of embolism resistance (i.e., P50), a key trait linked to hydraulic failure, is currently unknown across the Amazon. Here, we tested how precipitation, water table depth (WTD) and soil fertility interact as filters modulating the geographic hydraulic trait distribution. Location: Four sites in three key regions across the Amazon basin: Central, Western peripheral area and Brazilian Shield. Time Period: Present. Major Taxa Studied: Tree species. Method: We measured hydraulic vulnerability curves of 64 tree species (165 individuals) using an orthogonal design contrasting seasonal precipitation gradients, soil fertility and WTD. To estimate the geographical distribution of hydraulic traits for the basin scale, we use the coefficients of the best generalized linear model, projected on a 1 km grid in each environmental layer. Results: We show that WTD and soil fertility are the main drivers of embolism resistance, while precipitation has a secondary effect. Trees in shallow WTD and fertile soils had riskier hydraulic strategies. Our spatial projection identified the Amazonian southern band and valleys across the basin as the most vulnerable areas. High soil fertility in these regions increases the variability of hydraulic traits and thus uncertainty in predictions of drought responses. Main Conclusion: At the scale of the Amazon basin, we show that dominant tree species traits converge toward hydraulic resistance when resource (water and nutrients) levels are low and towards hydraulic vulnerability (but with higher trait variability) when resources increase. These results highlight the importance of WTD and soil fertility in structuring hydraulic traits and their effects on tree mortality under climate change‐induced droughts. Further empirical tests covering a wider spatial range and more tree species will help validate our biogeographical distribution model. [ABSTRACT FROM AUTHOR]

The hydraulic death hypothesis suggests that fires kill trees by damaging the plant's hydraulic continuum in addition to stem cambium. A corollary to this hypothesis is that plants that survive fires possess 'pyrohydraulic' traits that prevent heat-induced embolism formation in the xylem and aid post-fire survival. We examine whether hydraulic segmentation within stem xylem may act as such a trait. To do so, we measured the percentage loss of conductance (PLC) and vulnerability to embolism axially along segments of branches exposed to heat plumes in two differing species, fire-tolerant Eucalyptus cladocalyx F. Muell and fire-sensitive Kiggelaria africana L. testing model predictions that fire-tolerant species would exhibit higher degrees of hydraulic segmentation (greater PLC in the distal parts of the branch than the basal) than fire-intolerant species (similar PLC between segments). Following exposure to a heat plume, K. africana suffered between 73 and 84% loss of conductance in all branch segments, whereas E. cladocalyx had 73% loss of conductance in whole branches, including the distal tips, falling to 29% in the most basal part of the branch. There was no evidence for differences in resistance segmentation between the species, and there was limited evidence for differences in distal vulnerability to embolism across the branches. Hydraulic segmentation in E. cladocalyx may enable it to resprout effectively post-fire with a functional hydraulic system. The lack of hydraulic segmentation in K. africana reveals the need to understand possible trade-offs associated with hydraulic segmentation in long-lived woody species with respect to drought and fire. [ABSTRACT FROM AUTHOR]

Masson pine (Pinus massoniana Lamb.) is a dominant coniferous species in southern China, known for its rapid growth, abundant yield, and extensive utilization. Despite the robust adaptability of Masson pine and the rich annual precipitation in its distribution areas, this species still faces the mortality risk caused by the recurrent high temperatures in summer and low precipitation in subtropical regions. The mortality risk of Masson pine may increase in the future when facing a more frequent or intensive drought threat due to climate change. In this study, we conducted a manipulated drought experiment accompanying high temperature (~32.3 ± 0.7 °C in daytime and 28 °C in nighttime) to simulate a flash drought, aiming to explore the composite physiological response (hydraulic, gas exchange, and nonstructural carbon (NSC) characteristics) of Masson pine seedlings to extreme drought characterized by a high intensity and long duration. We found that, as the drought developed, the leaf water potential and gas exchange traits (net photosynthesis rate, stomatal conductance, and transpiration) significantly decreased while the percentage loss of hydraulic conductivity (PLC) significantly increased. In contrast, NSC remained a more constant trend before it was significantly reduced on day 30 after the beginning of the drought. Except for NSC, all the other traits had significant correlations between them. Additionally, hydraulic dysfunction indicated by the increasing PLC preceded the NSC depletion, which may indicate a more significant role for hydraulic failure than carbon starvation in drought-induced mortality. Conclusively, hydraulic and gas exchange traits showed a coupling response to drought, but NSC displayed an independent dynamic. The findings may improve our understanding of drought-coping strategies of Masson pine and provide some theoretical basis for Masson pine forest management. [ABSTRACT FROM AUTHOR]

The subtropical regions in China are prone to recurrent summer droughts induced by the Western Pacific Subtropical High-Pressure, which has induced the death of tens of millions of culms of Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), a widely distributed giant bamboo with high economic and ecological values. In the future, the intensity and frequency of the summer drought are projected to increase in these areas due to global climate change, which may lead to significant age-specific mortality of Moso bamboo. So far, it is still unclear about the age-specific response mechanisms of hydraulic traits and carbon balance of Moso bamboo when it is suffering to an ongoing summer drought. This study aimed to investigate the hydraulic and photosynthetic responses of newly sprouted (1 year old) and established (2-5 years old) culms of Moso bamboo to summer drought, which was manipulated by throughfall reduction in Lin'an of Zhejiang. The results showed that both newly sprouted and established culms had a gradually weakening hydraulic conductivity and photosynthesis during the whole drought process. In the early stage of the manipulated drought, the established culms had more loss of hydraulic conductivity than the newly sprouted culms. However, the newly sprouted culms had significant more loss of hydraulic conductivity and lower photosynthetic rates and stomatal conductance in the middle and late stages of the manipulated drought. The results suggest that the newly sprouted culms were more susceptible to summer drought than established culms due to the combined effects of hydraulic damage and photosynthetic restriction, explaining why the newly sprouted culms have higher mortality than elder culms when subjected to extreme drought. These findings provided insights into the mechanisms of Moso bamboo's age-specific drought-induced mortality, which will help for the anti-drought management of bamboo. [ABSTRACT FROM AUTHOR]

Sand boiling is a common and dangerous event that can cause human and financial losses, especially downstream of earthen dams, shoreline, and embankment erosion as well as to the coastal zone in general. This study aimed at investigating the heave potential downstream of the earthen dam using the 22 obtained data from boiling tests of Sufian and Firuzkuh sand in Iran. Laboratory techniques such as uniformity coefficients (Cu) and relative compaction (Rc) were used on the critical hydraulic gradient of the soil, while numerical techniques were used to determine the safety factor against sand boiling in addition to the amount of discharge. The results showed that both Cu and Rc improved sandy soils against boiling. The results further showed that well-graded sand soils not only performed well against hydraulic failure but also increased the safety factor of the dam against heave in the numerical model. By improving the soils of Firuzkuh and Sufian with geotextiles, the boiling results of these samples indicate that reinforcement of the samples and the increase in the number of geotextile layers increased the critical hydraulic gradient of the samples against hydraulic failure. [ABSTRACT FROM AUTHOR]

Summary: Relative sea level rise (SLR) increasingly impacts coastal ecosystems through the formation of ghost forests. To predict the future of coastal ecosystems under SLR and changing climate, it is important to understand the physiological mechanisms underlying coastal tree mortality and to integrate this knowledge into dynamic vegetation models.We incorporate the physiological effect of salinity and hypoxia in a dynamic vegetation model in the Earth system land model, and used the model to investigate the mechanisms of mortality of conifer forests on the west and east coast sites of USA, where trees experience different form of sea water exposure.Simulations suggest similar physiological mechanisms can result in different mortality patterns. At the east coast site that experienced severe increases in seawater exposure, trees loose photosynthetic capacity and roots rapidly, and both storage carbon and hydraulic conductance decrease significantly within a year. Over time, further consumption of storage carbon that leads to carbon starvation dominates mortality. At the west coast site that gradually exposed to seawater through SLR, hydraulic failure dominates mortality because root loss impacts on conductance are greater than the degree of storage carbon depletion.Measurements and modeling focused on understanding the physiological mechanisms of mortality is critical to reducing predictive uncertainty. [ABSTRACT FROM AUTHOR]

Survival and growth of woody species in the Mediterranean are mainly restricted by water availability. We tested the hypothesis that Mediterranean species acclimate their xylem vulnerability and osmotic potential along a precipitation gradient.We studied five predominant co‐occurring Mediterranean species; Quercus calliprinos, Pistacia palaestina, Pistacia lentiscus, Rhamnus lycioides and Phillyrea latifolia, over two summers at three sites. The driest of the sites is the distribution edge for all the five species. We measured key hydraulic and osmotic traits related to drought resistance, including resistance to embolism (Ψ50) and the seasonal dynamics of water and osmotic potentials.The leaf water potentials (Ψl) of all species declined significantly along the summer, reaching significantly lower Ψl at the end of summer in the drier sites. Surprisingly, we did not find plasticity along the drought gradient in Ψ50 or osmotic potentials. This resulted in much narrower hydraulic safety margins (HSMs) in the drier sites, where some species experienced significant embolism.Our analysis indicates that reduction in HSM to null values put Mediterranean species in embolism risk as they approach their hydraulic limit near the geographical dry edge of their distribution. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Canada's boreal forests, which occupy approximately 30% of boreal forests worldwide, play an important role in the global carbon budget. However, there is little quantitative information available regarding the spatiotemporal changes in the drought‐induced tree mortality of Canada's boreal forests overall and their associated impacts on biomass carbon dynamics. Here, we develop spatiotemporally explicit estimates of drought‐induced tree mortality and corresponding biomass carbon sink capacity changes in Canada's boreal forests from 1970 to 2020. We show that the average annual tree mortality rate is approximately 2.7%. Approximately 43% of Canada's boreal forests have experienced significantly increasing tree mortality trends (71% of which are located in the western region of the country), and these trends have accelerated since 2002. This increase in tree mortality has resulted in significant biomass carbon losses at an approximate rate of 1.51 ± 0.29 MgC ha−1 year−1 (95% confidence interval) with an approximate total loss of 0.46 ± 0.09 PgC year−1 (95% confidence interval). Under the drought condition increases predicted for this century, the capacity of Canada's boreal forests to act as a carbon sink will be further reduced, potentially leading to a significant positive climate feedback effect. [ABSTRACT FROM AUTHOR]

The characterisation of ecological strategies to predict drought response is still lacking for Caribbean dry forest seedlings. This study documents growth characteristics and tolerance to drought via xylem hydraulic and leaf cell properties of three dominant native species of the Caribbean dry forest. Twenty morphological and physiological traits were assessed in Citharexylum spinosum, Guaiacum officinale and Guapira fragrans in greenhouse conditions. The seedlings displayed contrasting growth rates, which were positively correlated with the capacity to quickly develop a large leaf area and root fraction. The three species had a similar xylem tolerance to embolism (P50: −4 MPa) but differed in leaf cell tolerance to dehydration, which was negatively correlated with RGR (R2 > 0.87). The slowest-growing, G. officinale, had high leaf tolerance to cell dehydration due to low ΨTLP and πo, but displayed a narrow hydraulic safety margin. The leaves of the fast-growing C. spinosum were sensitive to leaf dehydration but exhibited a surprisingly wide stem hydraulic safety margin. G. fragrans had intermediate traits. Our results showed that dry forest seedling growth in similar environments can exhibit distinct carbon growth strategies as well as contrasting water-use strategies, primarily as they relate to drought resistance, due to variation in root development and leaf cell resistance to dehydration. Our study thus provides an approach to estimate species performance under drought conditions. [ABSTRACT FROM AUTHOR]

Xylem anatomy may change in response to environmental or biotic stresses. Vascular occlusion, an anatomical modification of mature xylem, contributes to plant resistance and susceptibility to different stresses. In woody organs, xylem occlusions have been examined as part of the senescence process, but their presence and function in leaves remain obscure. In grapevine, many stresses are associated with premature leaf senescence inducing discolorations and scorched tissue in leaves. However, we still do not know whether the leaf senescence process follows the same sequence of physiological events and whether leaf xylem anatomy is affected in similar ways. In this study, we quantified vascular occlusions in midribs from leaves with symptoms of the grapevine disease esca, magnesium deficiency and autumn senescence. We found higher amounts of vascular occlusions in leaves with esca symptoms (in 27% of xylem vessels on average), whereas the leaves with other symptoms (as well as the asymptomatic controls) had far fewer occlusions (in 3% of vessels). Therefore, we assessed the relationship between xylem occlusions and esca leaf symptoms in four different countries (California in the USA, France, Italy and Spain) and eight different cultivars. We monitored the plants over the course of the growing season, confirming that vascular occlusions do not evolve with symptom age. Finally, we investigated the hydraulic integrity of leaf xylem vessels by optical visualization of embolism propagation during dehydration. We found that the occlusions lead to hydraulic dysfunction mainly in the peripheral veins compared with the midribs in esca symptomatic leaves. These results open new perspectives on the role of vascular occlusions during the leaf senescence process, highlighting the uniqueness of esca leaf symptoms and its consequence on leaf physiology. [ABSTRACT FROM AUTHOR]

Tree mortality rates appear to be increasing in moist tropical forests (MTFs) with significant carbon cycle consequences. Here, we review the state of knowledge regarding MTF tree mortality, create a conceptual framework with testable hypotheses regarding the drivers, mechanisms and interactions that may underlie increasing MTF mortality rates, and identify the next steps for improved understanding and reduced prediction. Increasing mortality rates are associated with rising temperature and vapor pressure deficit, liana abundance, drought, wind events, fire and, possibly, CO2 fertilization-induced increases in stand thinning or acceleration of trees reaching larger, more vulnerable heights. The majority of these mortality drivers may kill trees in part through carbon starvation and hydraulic failure. The relative importance of each driver is unknown. High species diversity may buffer MTFs against large-scale mortality events, but recent and expected trends in mortality drivers give reason for concern regarding increasing mortality within MTFs. Models of tropical tree mortality are advancing the representation of hydraulics, carbon and demography, but require more empirical knowledge regarding the most common drivers and their subsequent mechanisms. We outline critical datasets and model developments required to test hypotheses regarding the underlying causes of increasing MTF mortality rates, and improve prediction of future mortality under climate change.

Observations of woody plant mortality in coastal ecosystems are globally widespread, but the overarching processes and underlying mechanisms are poorly understood. This knowledge deficiency, combined with rapidly changing water levels, storm surges, atmospheric CO2, and vapor pressure deficit, creates large predictive uncertainty regarding how coastal ecosystems will respond to global change. Here, we synthesize the literature on the mechanisms that underlie coastal woody‐plant mortality, with the goal of producing a testable hypothesis framework. The key emergent mechanisms underlying mortality include hypoxic, osmotic, and ionic‐driven reductions in whole‐plant hydraulic conductance and photosynthesis that ultimately drive the coupled processes of hydraulic failure and carbon starvation. The relative importance of these processes in driving mortality, their order of progression, and their degree of coupling depends on the characteristics of the anomalous water exposure, on topographic effects, and on taxa‐specific variation in traits and trait acclimation. Greater inundation exposure could accelerate mortality globally; however, the interaction of changing inundation exposure with elevated CO2, drought, and rising vapor pressure deficit could influence mortality likelihood. Models of coastal forests that incorporate the frequency and duration of inundation, the role of climatic drivers, and the processes of hydraulic failure and carbon starvation can yield improved estimates of inundation‐induced woody‐plant mortality. [ABSTRACT FROM AUTHOR]

Considerable evidences highlight the occurrence of increasing widespread tree mortality as a result of global climate change-associated droughts. However, knowledge about the mechanisms underlying divergent strategies of various tree species to adapt to drought has remained remarkably insufficient. Leaf stomatal regulation and embolism resistance of stem xylem serves as two important strategies for tree species to prevent hydraulic failure and carbon starvation, as comprising interconnected physiological mechanisms underlying drought-induced tree mortality. Hence, the physiological and anatomical determinants of leaf stomatal regulation and stems xylem embolism resistance are evaluated and discussed. In addition, root properties related to drought tolerance are also reviewed. Species with greater investment in leaves and stems tend to maintain stomatal opening and resist stem embolism under drought conditions. The coordination between stomatal regulation and stem embolism resistance are summarized and discussed. Previous studies showed that hydraulic safety margin (HSM, the difference between minimum water potential and that causing xylem dysfunction) is a significant predictor of tree species mortality under drought conditions. Compared with HSM, stomatal safety margin (the difference between water potential at stomatal closure and that causing xylem dysfunction) more directly merge stomatal regulation strategies with xylem hydraulic strategies, illustrating a comprehensive framework to characterize plant response to drought. A combination of plant traits reflecting species' response and adaptation to drought should be established in the future, and we propose four specific urgent issues as future research priorities. [ABSTRACT FROM AUTHOR]

Summary: Increasing seawater exposure is killing coastal trees globally, with expectations of accelerating mortality with rising sea levels. However, the impact of concomitant changes in atmospheric CO2 concentration, temperature, and vapor pressure deficit (VPD) on seawater‐induced tree mortality is uncertain.We examined the mechanisms of seawater‐induced mortality under varying climate scenarios using a photosynthetic gain and hydraulic cost optimization model validated against observations in a mature stand of Sitka spruce (Picea sitchensis) trees in the Pacific Northwest, USA, that were dying from recent seawater exposure.The simulations matched well with observations of photosynthesis, transpiration, nonstructural carbohydrates concentrations, leaf water potential, the percentage loss of xylem conductivity, and stand‐level mortality rates. The simulations suggest that seawater‐induced mortality could decrease by c. 16.7% with increasing atmospheric CO2 levels due to reduced risk of carbon starvation. Conversely, rising VPD could increase mortality by c. 5.6% because of increasing risk of hydraulic failure.Across all scenarios, seawater‐induced mortality was driven by hydraulic failure in the first 2 yr after seawater exposure began, with carbon starvation becoming more important in subsequent years. Changing CO2 and climate appear unlikely to have a significant impact on coastal tree mortality under rising sea levels. [ABSTRACT FROM AUTHOR]

To study how gap-graded soil reinforced with fibers of different types restrains suffusion in embankment dams, the hydraulic response of fiber-reinforced soil (FRS) containing polypropylene fibers (PPF), coir fibers (CF) or basalt-tape fibers (BTF) was investigated in one-dimensional seepage tests. The critical hydraulic gradient and fine-particle loss of the FRS were analysed quantitatively by changing the fiber type, fiber content, and reinforcement-layer thickness. The test results show that the three types of FRS all had restraining effect on the development of suffusion. The netting reinforcement effect of PPF was the best, followed by the water absorption and expansion of CF to reduce pore volume, and finally the effect of BTF was to block migration channels under the same reinforcement conditions. With increasing fiber content and the reinforcement-layer thickness, the critical onset hydraulic gradient (icr,onset), and critical failure hydraulic gradient (icr,failure) increased, improving the internal erosion resistance of gap-graded soils against suffusion. The fine-particles loss decreased and the restraint rate of fine-particle loss (RF) increased meant that the permeability stability of soil was enhanced. Linear regression analysis showed that the reinforcement-layer thickness has greater influence on restraining effect parameters than fiber content.

Pathogenic diseases frequently occur in drought‐stressed trees. However, their contribution to the process of drought‐induced mortality is poorly understood. We combined drought and stem inoculation treatments to study the physiological processes leading to drought‐induced mortality in Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) saplings infected with Heterobasidion annosum s.s. We analysed the saplings' water status, gas exchange, nonstructural carbohydrates (NSCs) and defence responses, and how they related to mortality. Saplings were followed for two growing seasons, including an artificially induced 3‐month dormancy period. The combined drought and pathogen treatment significantly increased spruce mortality; however, no interaction between these stressors was observed in pine, although individually each stressor caused mortality. Our results suggest that pathogen infection decreased carbon reserves in spruce, reducing the capacity of saplings to cope with drought, resulting in increased mortality rates. Defoliation, relative water content and the starch concentration of needles were predictors of mortality in both species under drought and pathogen infection. Infection and drought stress create conflicting needs for carbon to compartmentalize the pathogen and to avoid turgor loss, respectively. Heterobasidion annosum reduces the functional sapwood area and shifts NSC allocation patterns, reducing the capacity of trees to cope with drought. Summary Statement: Interrelated decreases in carbon and water pools drive drought‐ and pathogen‐induced mortality mechanisms in Picea abies infected by the pathogen Heterobasidion annosum. The pathogen reduces the functional sapwood area and induces a defence response, reducing available carbon to face drought impacts. [ABSTRACT FROM AUTHOR]

Acoustic emission analysis is promising to investigate the physiological events leading to drought-induced injury and mortality. However, their nature and source are not fully understood, making this technique difficult to use as a direct measure of the loss of xylem hydraulic conductance. Acoustic emissions were recorded during severe dehydration in lavender plants (Lavandula angustifolia) and compared with the dynamics of embolism development and cell damage. The timing and characteristics of acoustic signals from two independent recording systems were compared by principal component analysis (PCA). Changes in water potential, branch diameter, loss of hydraulic conductance, and cellular damage were also measured to quantify drought-induced damages. Two distinct phases of acoustic emissions were observed during dehydration: the first one associated with a rapid loss of diameter and a significant increase in loss of xylem conductance (90%), and the second with slower changes in diameter and a significant increase in cellular damage. Based on PCA, a developed algorithm discriminated hydraulic-related acoustic signals from other sources, proposing a reconstruction of hydraulic vulnerability curves. Cellular damage preceded by hydraulic failure seems to lead to a lack of recovery. The second acoustic phase would allow detection of plant mortality. [ABSTRACT FROM AUTHOR]

Accurately predicting global drought-induced tree mortality remains a major challenge facing plant science and ecology. Stem hydraulic safety margin (HSM, the difference between water potential at the minimum value and the value that causes xylem vulnerability to embolism) performs as one of the best hydraulic traits in predicting global drought-induced tree mortality, however, HSM is time-consuming and very difficult to measure. We proposed to use leaf turgor loss point (TLP, the water potential at which leaves start to wilt) as a proxy for HSM because HSM may be highly correlated to TLP, as both of them are tightly linked with water potential changes after stomatal closure. Since TLP is more easy and rapid to measure than HSM, if we find strong HSM-TLP relationships at the global scale, TLP can be used in predicting global drought-induced tree mortality. However, no study has quantified the relationships between HSM and TLP at the global scale. Here we draw together published data on HSM and TLP for 1,773 species from 370 sites worldwide to check whether HSM and TLP are highly associated. We found that HSMs and TLPs are merely highly related in tropical forests, thus TLP can be a reliable surrogate of HSM to predict drought-induced tree mortality in tropical forest. Here we are certainly not advocating for the use of TLP instead of HSM to predict drought-induced tree mortality in tropical forests, but simply for predicting drought-induced tree mortality in tropical forests in supplementary of HSM in the future.

Unprecedented tree dieback across Central Europe caused by recent global change‐type drought events highlights the need for a better mechanistic understanding of drought‐induced tree mortality. Although numerous physiological risk factors have been identified, the importance of two principal mechanisms, hydraulic failure and carbon starvation, is still debated. It further remains largely unresolved how the local neighborhood composition affects individual mortality risk. We studied 9435 young trees of 12 temperate species planted in a diversity experiment in 2013 to assess how hydraulic traits, carbon dynamics, pest infestation, tree height and neighborhood competition influence individual mortality risk. Following the most extreme global change‐type drought since record in 2018, one third of these trees died. Across species, hydraulic safety margins (HSMs) were negatively and a shift towards a higher sugar fraction in the non‐structural carbohydrate (NSC) pool positively associated with mortality risk. Moreover, trees infested by bark beetles had a higher mortality risk, and taller trees a lower mortality risk. Most neighborhood interactions were beneficial, although neighborhood effects were highly species‐specific. Species that suffered more from drought, especially Larix spp. and Betula spp., tended to increase the survival probability of their neighbors and vice versa. While severe tissue dehydration marks the final stage of drought‐induced tree mortality, we show that hydraulic failure is interrelated with a series of other, mutually inclusive processes. These include shifts in NSC pools driven by osmotic adjustment and/or starch depletion as well as pest infestation and are modulated by the size and species identity of a tree and its neighbors. A more holistic view that accounts for multiple causes of drought‐induced tree mortality is required to improve predictions of trends in global forest dynamics and to identify mutually beneficial species combinations. [ABSTRACT FROM AUTHOR]

Between late 2015 and early 2016, more than 7,000 ha of mangrove forest died along the coastline of the Gulf of Carpentaria, in northern Australia. This massive die-off was preceded by a strong 2015/2016 El Niño event, resulting in lower precipitation, a drop in sea level and higher than average temperatures in northern Australia. In this study, we investigated the role of hydraulic failure in the mortality and recovery of the dominant species, Avicennia marina , 2 years after the mortality event. We measured predawn water potential (Ψpd) and percent loss of stem hydraulic conductivity (PLC) in surviving individuals across a gradient of impact. We also assessed the vulnerability to drought-induced embolism (Ψ50) for the species. Areas with severe canopy dieback had higher native PLC (39%) than minimally impacted areas (6%), suggesting that hydraulic recovery was ongoing. The high resistance of A. marina to water-stress-induced embolism (Ψ50 = −9.6 MPa), indicates that severe water stress (Ψpd < −10 MPa) would have been required to cause mortality in this species. Our data indicate that the natural gradient of water-stress enhanced the impact of El Niño, leading to hydraulic failure and mortality in A. marina growing on severely impacted (SI) zones. It is likely that lowered sea levels and less frequent inundation by seawater, combined with lower inputs of fresh water, high evaporative demand and high temperatures, led to the development of hyper-salinity and extreme water stress during the 2015/16 summer. [ABSTRACT FROM AUTHOR]

Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models. [ABSTRACT FROM AUTHOR]

The sand boiling phenomenon occurs downstream of hydraulic structures, and the continuation of this phenomenon could lead to a structural failure. In this paper, by developing a device to simulate sand boiling, experimental tests have been performed to determine the critical hydraulic gradient on various sandy soil. The effect of different parameters was evaluated such as four relative densities (0, 20, 50 and 80%), uniformity coefficient, woven and non-woven geotextiles in both single and double-layer states and the particle size distribution. Based on the results, with increasing the relative density and uniformity coefficient, the sand resistance against the hydraulic failure improved and also in soils with Cu = 1, coarser sands have indicated more resistance to the boiling. In improvement by geotextile, reinforced sands beard more water load than natural sands, and the type and layer number of reinforcements have affected the test results. In addition, a statistical comparison has been performed between the experimental results and the proposed equation by Terzaghi to calculate the critical hydraulic gradient. Finally, by using experimental results, a linear and nonlinear regression model is presented to predict the required seepage force for the hydraulic failure of the specimens. [ABSTRACT FROM AUTHOR]

Drought‐induced tree mortality may increase with ongoing climate change. Unraveling the links between stem hydraulics and mortality thresholds, and the effects of intraspecific variation, remain important unresolved issues. We conducted a water manipulation experiment in a rain‐out shelter, using four provenances of Schima superba originating from a gradient of annual precipitation (1124–1796 mm) and temperature (16.4–22.4°C). Seedlings were droughted to three levels of percentage loss of hydraulic conductivity (i.e., P50, P88 and P99) and subsequently rewatered to field capacity for 30 days; traits related to water and carbon relations were measured. The lethal water potential associated with incipient mortality was between P50 and P88. Seedlings exhibited similar drought responses in xylem water potential, hydraulic conductivity and gas exchange. Upon rehydration, patterns of gas exchange differed among provenances but were not related to the climate at the origin. The four provenances exhibited a similar degree of stem hydraulic recovery, which was correlated with the magnitude of antecedent drought and stem soluble sugar at the end of the drought. Results suggest that there were intraspecific differences in the capacity of S. superba seedlings for carbon assimilation during recovery, indicating a decoupling between gas exchange recovery and stem hydraulics across provenances. Summary Statement: Schima superba may start to die between P50 and P88, with mortality being more likely than survival after P88. There were intraspecific differences in the capacity for carbon assimilation during recovery, indicating a decoupling between gas exchange recovery and stem hydraulics across provenances. [ABSTRACT FROM AUTHOR]

Between late 2015 and early 2016, more than 7,000 ha of mangrove forest died along the coastline of the Gulf of Carpentaria, in northern Australia. This massive die-off was preceded by a strong 2015/2016 El Niño event, resulting in lower precipitation, a drop in sea level and higher than average temperatures in northern Australia. In this study, we investigated the role of hydraulic failure in the mortality and recovery of the dominant species, Avicennia marina, 2 years after the mortality event. We measured predawn water potential (Ψpd) and percent loss of stem hydraulic conductivity (PLC) in surviving individuals across a gradient of impact. We also assessed the vulnerability to drought-induced embolism (Ψ50) for the species. Areas with severe canopy dieback had higher native PLC (39%) than minimally impacted areas (6%), suggesting that hydraulic recovery was ongoing. The high resistance of A. marina to water-stress-induced embolism (Ψ50 = −9.6 MPa), indicates that severe water stress (Ψpd < −10 MPa) would have been required to cause mortality in this species. Our data indicate that the natural gradient of water-stress enhanced the impact of El Niño, leading to hydraulic failure and mortality in A. marina growing on severely impacted (SI) zones. It is likely that lowered sea levels and less frequent inundation by seawater, combined with lower inputs of fresh water, high evaporative demand and high temperatures, led to the development of hyper-salinity and extreme water stress during the 2015/16 summer.

Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models.

Context: It has been proposed that the distribution of resprouter and seeder Erica in the Cape Floristic Region (CFR) of South Africa is determined by drought rather than by fire frequency. Seedlings of Erica seeders are predicted to withstand the mild droughts of the southwest CFR better than those of Erica resprouters, which would account for the abundance of seeders in this region. Aims: This study aimed to test this hypothesis using Erica coccinea (L.), which contains both a resprouter and a seeder form. Methods: One-year-old seedlings of both forms were subjected to a progressively severe 50-day drought in the greenhouse. Key results: Contrary to expectations, seeder seedlings had 67% higher mortality than resprouters during the initial phase of drought (30% vs 18% after 32 days). However, both forms were unable to survive as the drought progressed, resulting in 95% mortality by Day 50. There were limited differences between the forms in gas exchange and water potential, and no difference in root:shoot ratios. However, resprouters had significantly higher soluble carbohydrates than did seeders, which may have aided in delaying mortality. Conclusions: Our results showed no evidence of resprouter seedlings being more vulnerable to mild drought than seeders in E. coccinea. Implications: Our findings challenge the hypothesis that the distribution of this species can be explained by the vulnerability of resprouter seedlings to mild drought. We tested drought sensitivity in reseeder and resprouter seedlings of E. coccinea , a species that contains both reseeder and resprouter morphs. Contrary to expectations, seeder seedlings were more drought sensitive than were resprouter seedlings. Our results challenge the drought-distribution hypothesis for Erica in the Cape Floristic region. [ABSTRACT FROM AUTHOR]

European beech is one of the most important deciduous tree species in natural forest ecosystems in Central Europe. Its dominance is now being questioned by the emerging drought damages due to the increased incidence of severe summer droughts. In Switzerland, Fagus sylvatica have been observed in the Intercantonal Forest Observation Program since 1984. The dataset presented here includes 179176 annual observations of beech trees on 102 plots during 37 years. The plots cover gradients in drought, nitrogen deposition, ozone, age, altitude, and soil chemistry. In dry regions of Switzerland, the dry and hot summer of 2018 caused a serious branch dieback, increased mortality in Fagus sylvatica and increased yellowing of leaves. Beech trees recovered less after 2018 than after the dry summer 2003 which had been similar in drought intensity except that the drought in 2018 started earlier in spring. Our data analyses suggest the importance of drought in subsequent years for crown transparency and mortality in beech. The drought in 2018 followed previous dry years of 2015 and 2017 which pre-weakened the trees. Our long-term data indicate that the drought from up to three previous years were significant predictors for both tree mortality and for the proportion of trees with serious (>60%) crown transparency. The delay in mortality after the weakening event suggests also the importance of weakness parasites. The staining of active vessels with safranine revealed that the cavitation caused by the low tree water potentials in 2018 persisted at least partially in 2019. Thus, the ability of the branches to conduct water was reduced and the branches dried out. Furthermore, photooxidation in light-exposed leaves has increased strongly since 2011. This phenomenon was related to low concentrations of foliar phosphorus (P) and hot temperatures before leaf harvest. The observed drought effects can be categorized as (i) hydraulic failure (branch dieback), (ii) energy starvation as a consequence of closed stomata and P deficiency (photooxidation) and (iii) infestation with weakness parasites (beech bark disease and root rots).

Hydraulic failure caused by severe drought contributes to aboveground dieback and whole‐plant death. The extent to which dieback or whole‐plant death can be predicted by plant hydraulic traits has rarely been tested among species with different leaf habits and/or growth forms. We investigated 19 hydraulic traits in 40 woody species in a tropical savanna and their potential correlations with drought response during an extreme drought event during the El Niño–Southern Oscillation in 2015. Plant hydraulic trait variation was partitioned substantially by leaf habit but not growth form along a trade‐off axis between traits that support drought tolerance versus avoidance. Semi‐deciduous species and shrubs had the highest branch dieback and top‐kill (complete aboveground death) among the leaf habits or growth forms. Dieback and top‐kill were well explained by combining hydraulic traits with leaf habit and growth form, suggesting integrating life history traits with hydraulic traits will yield better predictions. [ABSTRACT FROM AUTHOR]

Ongoing global warming increases the frequency and severity of tropical typhoons and prolonged drought, leading to forest degradation. Simultaneous and/or successive masting events and climatic extremes may thus occur frequently in the near future. If these climatic extremes occur immediately after mass seed reproduction, their effects on individual trees are expected to be very severe because mass reproduction decreases carbohydrate reserves. While the effects of either a single climate extreme or masting alone on tree resilience/growth have received past research attention, understanding the cumulative effects of such multiple events remains challenging and is crucial for predicting future forest changes. Here, we report tree hazards compound by two successive climate extremes, a tropical typhoon and prolonged drought, after mass reproduction in an endemic tree species (Distylium lepidotum Nakai) on oceanic islands. Across individual trees, the starch stored within the sapwood of branchlets significantly decreased with reproductive efforts (fruit mass/shoot mass ratio). Typhoon damage significantly decreased not only the total leaf area of apical shoots but also the maximum photosynthetic rates. During the 5‐month period after the typhoon, the mortality of large branchlets (8–10‐mm diameter) increased with decreasing stored starch when the typhoon hit. During the prolonged summer drought in the next year, the recovery of total leaf area, stored starch, and hydraulic conductivity was negatively correlated with the stored starch at the typhoon. These data indicate that the level of stored starch within branchlets is the driving factor determining tree regrowth or dieback, and the restoration of carbohydrates after mass reproduction is synergistically delayed by such climate extremes. Stored carbohydrates are the major cumulative factor affecting individual tree resilience, resulting in their historical effects. Because of highly variable carbohydrate levels among individual trees, the resultant impacts of such successive events on forest dieback will be fundamentally different among trees. [ABSTRACT FROM AUTHOR]

Predicted increases in forest drought mortality highlight the need for predictors of incipient drought‐induced mortality (DIM) risk that enable proactive large‐scale management. Such predictors should be consistent across plants with varying morphology and physiology. Because of their integrative nature, indicators of water status are promising candidates for real‐time monitoring of DIM, particularly if they standardize morphological differences among plants. We assessed the extent to which differences in morphology and physiology between Pinus ponderosa populations influence time to mortality and the predictive power of key indicators of DIM risk. Time to incipient mortality differed between populations but occurred at the same relative water content (RWC) and water potential (WP). RWC and WP were accurate predictors of drought mortality risk. These results highlight that variables related to water status capture critical thresholds during DIM and the associated dehydration processes. Both WP and RWC are promising candidates for large‐scale assessments of DIM risk. RWC is of special interest because it allows comparisons across different morphologies and can be remotely sensed. Our results offer promise for real‐time landscape‐level monitoring of DIM and its global impacts in the near term. Relative water content (RWC) predicts drought‐induced mortality risk in morphologically and physiologically distinct Pinus ponderosa seedling populations. Mortality curves based on RWC show incipient mortality thresholds close to turgor loss point that are consistent across organs and populations. [ABSTRACT FROM AUTHOR]

Sand boiling is a type of hydraulic failure occurring the downstream of dams with huge damages. In this study, the reinforcement effect of Firuzkuh sand with bulk continuous filament (BCF) fibers and geotextile (woven and nonwoven) on the resistance to boiling is examined on a laboratory scale. For this purpose, an upward seepage apparatus was designed and fabricated. In reinforcement with BCF fibers, with increasing the weight percentage of fibers, soil strength increased and the permeability coefficient was changed. In soil reinforcement with geotextile, two types of woven and nonwoven geotextiles were used in two modes of one and two layers. Based on the results of the performed tests on these samples, the reinforced samples have shown more resistance to boiling than the natural samples. Also, the increase in critical hydraulic gradient and seepage force (to sand boiling) was found to be a function of the type and number of geotextile layers. Moreover, in this research, the effect of the relative compaction of the sample on permeability and critical hydraulic gradient of samples (reinforced and unreinforced) was investigated. According to the results, in denser samples, the boiling is delayed and soil improvement (a reinforcement with fibers and geotextile) is significantly effective. [ABSTRACT FROM AUTHOR]

Xylella fastidiosa is a xylem-limited bacterium causing the Olive Quick Decline Syndrome, which is currently devastating the agricultural landscape of Southern Italy. The bacterium is injected into the xylem vessels of leaf petioles after the penetration of the insect vector's stylet. From here, it is supposed to colonize the xylem vasculature moving against water flow inside conductive vessels. Widespread vessel clogging following the bacterial infection and causing the failure of water transport seemed not to fully supported by the recent empirical xylem anatomical observations in infected olive trees. We tested the hypothesis that the higher susceptibility to the X. fastidiosa 's infection in Cellina di Nardò compared with Leccino is associated to the higher vulnerability to air embolism of its larger vessels. Such hypothesis is motivated by the recognized ability of X. fastidiosa in degrading pit membranes and also because air embolism would possibly provide microenvironmental conditions more favourable to its more efficient aerobic metabolism. We revised the relevant literature on bacterium growth and xylem physiology, and carried out empirical field, mid-summer measurements of xylem anatomy and native embolism in olive cultivars with high (Cellina di Nardò) and low susceptibility (Leccino) to the infection by X. fastidiosa. Both cultivars had similar shoot mass traits and vessel length (~80 cm), but the highly susceptible one had larger vessels and a lower number of vessels supplying a given leaf mass. Native air embolism reduced mean xylem hydraulic conductance by ~58 % (Cellina di Nardò) and ~38 % (Leccino). The higher air-embolism vulnerability of the larger vessels in Cellina di Nardò possibly facilitates the X. fastidiosa 's infection compared to Leccino. Some important characteristics of the vector–pathogen–plant interactions still require deep investigations acknowledging both the pathogen metabolic pathways and the biophysical principles of xylem hydraulics. [ABSTRACT FROM AUTHOR]

SummaryModel-data comparisons of plant physiological processes provide an understanding of mechanisms underlying vegetation responses to climate. We simulated the physiology of a piñon pine-juniper woodland (Pinus edulis-Juniperus monosperma) that experienced mortality during a 5 yr precipitation-reduction experiment, allowing a framework with which to examine our knowledge of drought-induced tree mortality. We used six models designed for scales ranging from individual plants to a global level, all containing state-of-the-art representations of the internal hydraulic and carbohydrate dynamics of woody plants. Despite the large range of model structures, tuning, and parameterization employed, all simulations predicted hydraulic failure and carbon starvation processes co-occurring in dying trees of both species, with the time spent with severe hydraulic failure and carbon starvation, rather than absolute thresholds per se, being a better predictor of impending mortality. Model and empirical data suggest that limited carbon and water exchanges at stomatal, phloem, and below-ground interfaces were associated with mortality of both species. The model-data comparison suggests that the introduction of a mechanistic process into physiology-based models provides equal or improved predictive power over traditional process-model or empirical thresholds. Both biophysical and empirical modeling approaches are useful in understanding processes, particularly when the models fail, because they reveal mechanisms that are likely to underlie mortality. We suggest that for some ecosystems, integration of mechanistic pathogen models into current vegetation models, and evaluation against observations, could result in a breakthrough capability to simulate vegetation dynamics.

Summary: Eastern Australia was subject to its hottest and driest year on record in 2019. This extreme drought resulted in massive canopy die‐back in eucalypt forests. The role of hydraulic failure and tree size on canopy die‐back in three eucalypt tree species during this drought was examined.We measured pre‐dawn and midday leaf water potential (Ψleaf), per cent loss of stem hydraulic conductivity and quantified hydraulic vulnerability to drought‐induced xylem embolism. Tree size and tree health was also surveyed.Trees with most, or all, of their foliage dead exhibited high rates of native embolism (78–100%). This is in contrast to trees with partial canopy die‐back (30–70% canopy die‐back: 72–78% native embolism), or relatively healthy trees (little evidence of canopy die‐back: 25–31% native embolism). Midday Ψleaf was significantly more negative in trees exhibiting partial canopy die‐back (−2.7 to −6.3 MPa), compared with relatively healthy trees (−2.1 to −4.5 MPa). In two of the species the majority of individuals showing complete canopy die‐back were in the small size classes.Our results indicate that hydraulic failure is strongly associated with canopy die‐back during drought in eucalypt forests. Our study provides valuable field data to help constrain models predicting mortality risk. [ABSTRACT FROM AUTHOR]

Hydraulic failure has been extensively studied during drought-induced plant dieback, but its role in plant-pathogen interactions is under debate. During esca, a grapevine (Vitis vinifera) disease, symptomatic leaves are prone to irreversible hydraulic dysfunctions but little is known about the hydraulic integrity of perennial organs over the short- and long-term. We investigated the effects of esca on stem hydraulic integrity in naturally infected plants within a single season and across season(s). We coupled direct (k s ) and indirect (k th ) hydraulic conductivity measurements, and tylose and vascular pathogen detection with in vivo X-ray microtomography visualizations. Xylem occlusions (tyloses) and subsequent loss of stem hydraulic conductivity (k s ) occurred in all shoots with severe symptoms (apoplexy) and in more than 60% of shoots with moderate symptoms (tiger-stripe), with no tyloses in asymptomatic shoots. In vivo stem observations demonstrated that tyloses occurred only when leaf symptoms appeared, and resulted in more than 50% loss of hydraulic conductance in 40% of symptomatic stems, unrelated to symptom age. The impact of esca on xylem integrity was only seasonal, with no long-term impact of disease history. Our study demonstrated how and to what extent a vascular disease such as esca, affecting xylem integrity, could amplify plant mortality through hydraulic failure. [ABSTRACT FROM AUTHOR]

Understanding the vulnerability of trees to drought-induced mortality is key to predicting the fate of forests in a future climate with more frequent and intense droughts, although the underlying mechanisms are difficult to study in adult trees. Here, we explored the dynamic changes of water relations and limits of hydraulic function in dying adults of Norway spruce (Picea abies L.) during the progression of the record-breaking 2018 Central European drought. In trees on the trajectory to drought-induced mortality, we observed rapid, nonlinear declines of xylem pressure that commenced at the early onset of xylem cavitation and caused a complete loss of xylem hydraulic conductance within a very short time. We also observed severe depletions of nonstructural carbohydrates, though carbon starvation could be ruled out as the cause of the observed tree death, as both dying and surviving trees showed these metabolic limitations. Our observations provide striking field-based evidence for fast dehydration and hydraulic collapse as the cause of drought-induced mortality in adult Norway spruce. The nonlinear decline of tree water relations suggests that considering the temporal dynamics of dehydration is critical for predicting tree death. The collapse of the hydraulic system within a short time demonstrates that trees can rapidly be pushed out of the zone of hydraulic safety during the progression of a severe drought. In summary, our findings point toward a higher mortality risk for Norway spruce than previously assumed, which is in line with current reports of unprecedented levels of droughtinduced mortality in this major European tree species. [ABSTRACT FROM AUTHOR]

A recent data synthesis showed that trees dehydrated to SB lethal sb experienced a > 60% loss of xylem conductivity, suggesting a ubiquitous hydraulic failure in drought-induced tree mortality (Adams I et al i ., 2017). Water potential threshold across species and biomes Across all tree species assessed, the SB lethal sb ranged widely from -1.5 MPa ( I Populus balsamifera i , an angiosperm) to -14.7 MPa ( I Callitris columellaris i , a gymnosperm) (Fig. In sum, our synthesis of data showed that tree SB lethal sb ranged widely from -1.5 to -14.7 MPa across species and biomes, and the SB lethal sb was significantly lower for gymnosperms than angiosperms. Tree SB lethal sb of gymnosperms was close to P50, while for angiosperms, tree SB lethal sb was closer to P88. [Extracted from the article]

Summary: Plants continue to lose water from their leaves even after complete stomatal closure. Although this minimum conductance (gleaf‐res) has substantial impacts on strategies of water use and conservation, little is known about the potential drivers underlying the variability of this trait across species.We thus untangled the relative contribution of water leaks from the cuticle and stomata in order to investigate how the variability in leaf morphological and anatomical traits is related to the variation in gleaf‐res and carbon assimilation capacity across 30 diverse species from the Brazilian Cerrado.In addition to cuticle permeance, water leaks from stomata had a significant impact on gleaf‐res. The differential pattern of stomata distribution in the epidermis was a key factor driving this variation, suggesting the existence of a trade‐off between carbon assimilation and water loss through gleaf‐res. For instance, higher gleaf‐res, observed in fast‐growing species, was associated with the investment in small and numerous stomata, which allowed higher carbon assimilation rates but also increased water leaks, with negative impacts on leaf survival under drought. Variation in cuticle structural properties was not linked to gleaf‐res.Our results therefore suggest the existence of a trade‐off between carbon assimilation efficiency and dehydration tolerance at foliar level. [ABSTRACT FROM AUTHOR]