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Drought limits the long-distance transport of water in the xylem due to the reduced leaf-to-soil water potential difference and possible embolism-related losses of conductance and of sugars in the phloem due to the higher viscosity of the dehydrated sugary solution. This condition can have cascading effects in water and carbon (C) fluxes that may ultimately cause tree death. We hypothesize that the maintenance of xylem and phloem conductances is fundamental for survival also under reduced resource availability, when trees may produce effective and low C cost anatomical adjustments in the xylem and phloem close to the treetop where most of the hydraulic resistance is concentrated. We analyzed the treetop xylem and phloem anatomical characteristics in coexisting Scots pine trees, symptomatic and non-symptomatic of drought-induced dieback. We selected the topmost 55 cm of the main stem and selected several sampling positions at different distances from the stem apex to test for differences in the axial patterns between the two groups of trees. We measured the annual ring area, the tracheid hydraulic diameter (Dh) and cell wall thickness (CWT), the conductive phloem area and the average lumen diameter of the 20 largest phloem sieve cells (Dph). Declining trees grew less than the non-declining ones, and despite the similar axial scaling of anatomical traits, had larger Dh and lower CWT. Moreover, declining trees had wider Dph. Our results demonstrate that even under drought stress, maintenance of xylem and phloem efficiencies is of primary importance for survival, even if producing fewer larger tracheids may lead to a xylem more vulnerable to embolism formation.

In 2018, Central Europe experienced one of the most severe and long-lasting summer drought and heat wave ever recorded. Before 2018, the 2003 millennial drought was often invoked as the example of a “hotter drought”, and was classified as the most severe event in Europe for the last 500 years. First insights now confirm that the 2018 drought event was climatically more extreme and had a greater impact on forest ecosystems of Austria, Germany and Switzerland than the 2003 drought. Across this region, mean growing season air temperature from April to October was more than 3.3°C above the long-term average, and 1.2 °C warmer than in 2003. Here, we present a first impact assessment of the severe 2018 summer drought and heatwave on Central European forests. In response to the 2018 event, most ecologically and economically important tree species in temperate forests of Austria, Germany and Switzerland showed severe signs of drought stress. These symptoms included exceptionally low foliar water potentials crossing the threshold for xylem hydraulic failure in many species and observations of widespread leaf discoloration and premature leaf shedding. As a result of the extreme drought stress, the 2018 event caused unprecedented drought-induced tree mortality in many species throughout the region. Moreover, unexpectedly strong drought-legacy effects were detected in 2019. This implies that the physiological recovery of trees was impaired after the 2018 drought event, leaving them highly vulnerable to secondary drought impacts such as insect or fungal pathogen attacks. As a consequence, mortality of trees triggered by the 2018 events is likely to continue for several years. Our assessment indicates that many common temperate European forest tree species are more vulnerable to extreme summer drought and heat waves than previously thought. As drought and heat events are likely to occur more frequently with the progression of climate change, temperate European forests might approach the point for a substantial ecological and economic transition. Our assessment also highlights the urgent need for a pan-European ground-based monitoring network suited to track individual tree mortality, supported by remote sensing products with high spatial and temporal resolution to track, analyse and forecast these transitions. ISSN:1439-1791 ISSN:1618-0089

International audience; Climate change threatens food security, and plant science researchers have investigated methods of sustaining crop yield under drought. One approach has been to overproduce abscisic acid (ABA) to enhance water use efficiency. However, the concomitant effects of ABA overproduction on plant vascular system functioning are critical as it influences vulnerability to xylem hydraulic failure. We investigated these effects by comparing physiological and hydraulic responses to water deficit between a tomato (Solanum lycopersicum) wild type control (WT) and a transgenic line overproducing ABA (sp12). Under well-watered conditions, the sp12 line displayed similar growth rate and greater water use efficiency by operating at lower maximum stomatal conductance. X-ray microtomography revealed that sp12 was significantly more vulnerable to xylem embolism, resulting in a reduced hydraulic safety margin. We also observed a significant ontogenic effect on vulnerability to xylem embolism for both WT and sp12. This study demonstrates that the greater water use efficiency in the tomato ABA overproducing line is associated with higher vulnerability of the vascular system to embolism and a higher risk of hydraulic failure. Integrating hydraulic traits into breeding programmes represents a critical step for effectively managing a crop's ability to maintain hydraulic conductivity and productivity under water deficit.

Stand-level competition and local climate influence tree responses to increased drought at the regional scale. To evaluate stand density and elevation effects on tree carbon and water balances, we monitored seasonal changes in sap-flow density (SFD), gas exchange, xylem water potential, secondary growth, and non-structural carbohydrates (NSCs) in Abies pinsapo. Trees were subjected to experimental thinning within a low-elevation stand (1200 m), and carbon and water balances were compared to control plots at low and high elevation (1700 m). The hydraulic conductivity and the resistance to cavitation were also characterized, showing relatively high values and no significant differences among treatments. Trees growing at higher elevations presented the highest SFD, photosynthetic rates, and secondary growth, mainly because their growing season was extended until summer. Trees growing at low elevation reduced SFD during late spring and summer while SFD and secondary growth were significantly higher in the thinned stands. Declining NSC concentrations in needles, branches, and sapwood suggest drought-induced control of the carbon supply status. Our results might indicate potential altitudinal shifts, as better performance occurs at higher elevations, while thinning may be suitable as adaptive management to mitigate drought effects in endangered Mediterranean trees.

The availability of water and nutrients are two main factors limiting tree growth and forest productivity. Despite the strong legacy of research on forest nutrition, the role of nutrients in modulating tree drought responses remains largely unknown. This thesis aimed to evaluate the role of nutrient availability on key traits related to water and carbon relations such as growth and its determinants, water-use efficiency (WUE), vulnerability to cavitation, or non-structural carbohydrate (NSC) contents, in poplars under contrasting drought intensities. Research experiments relied on two complementary experimental designs, one in a greenhouse and the other in an outdoor common garden, and considered a maximum of four genotypes primarily selected for their contrasting growth and stomatal regulation. The main findings are as follows: 1) a higher nutrient availability increases growth and WUE under moderate drought, 2) a higher nutrient availability does not necessarily increase the risk of xylem hydraulic failure, 3) NSC depletion, in particular starch, co-occurs with massive xylem embolism under lethal drought, and 4) a higher nutrient availability reduces NSC storage, especially under moderate drought. This study shows for the first time the temporal covariance between the dynamics of xylem hydraulic failure and carbon starvation in a fast-growing but highly vulnerable tree species, and gives an example of how these relationships are modulated by nutrient availability.; La disponibilité en eau et en nutriments sont deux facteurs limitant la croissance et la productivité des forêts. Malgré un historique important de travaux sur le rôle physiologique des principaux éléments minéraux, l’impact du statut nutritif des arbres sur leur capacité de réponse à la sécheresse demeure très largement inconnu. Cette thèse visait à évaluer le rôle de la disponibilité en nutriments sur des caractères clés indicateurs du fonctionnement hydrique et carboné comme la croissance et ses déterminants architecturaux, l’efficience d’utilisation de l’eau (WUE), la vulnérabilité à la cavitation ou encore les teneurs en glucides non structuraux, en réponse à des sécheresses d’intensités variées chez le peuplier. Deux dispositifs expérimentaux complémentaires ont été support des expériences, l’un en serre et l’autre en pépinière, et les travaux se sont concentrés sur un maximum de quatre génotypes sélectionnés pour leur comportement contrasté en termes de croissance et de régulation stomatique. Les résultats majeurs sont les suivants : 1) une disponibilité en nutriments plus importante augmente la croissance et WUE en conditions hydriques limitantes, 2) une disponibilité en nutriments plus importante n’augmente pas nécessairement les risques de dysfonctionnements hydrauliques, 3) l’épuisement des réserves carbonées, notamment de l’amidon, coïncide avec l’embolie massive du xylème à mortalité en cas de sécheresse sévère, et 4) une plus grande disponibilité en azote diminue la mise en réserve glucidique, notamment en condition de sécheresse modérée. Cette étude illustre pour la première fois de façon explicite la covariance au cours du temps entre les dysfonctionnements hydrauliques et l’épuisement des réserves chez une essence à croissance rapide très vulnérable à la sécheresse, et donne un exemple de la façon dont ces relations sont modulées par la disponibilité en nutriments.

How the mortality and growth of tree species vary with the iso-anisohydric continuum and xylem vulnerability is still being debated. We conducted a precipitation reduction experiment to create a mild drought condition in a forest in the Baotianman Mountains, China, a sub-humid region. Three main sub-canopy tree species in this region were examined. After rainfall reduction, Lindera obtusiloba showed severe dieback, but two other co-occurring species did not show dieback. The water potential at stomatal closure of Dendrobenthamia japonica, L. obtusiloba and Sorbus alnifolia was −1.70, −2.54 and −3.41 MPa, respectively, whereas the water potential at 88 % loss in hydraulic conductivity of the three species was −2.31, −2.11 and −7.01 MPa, respectively. Taken together, near-anisohydric L. obtusiloba with vulnerable xylem was highly susceptible to drought dieback. Anisohydric S. alnifolia had the most negative minimum water potential, and its xylem was the most resistant to cavitation. Isohydric D. japonica conserved water by rapidly closing its stomata. Ultimately, the hydraulic safety margin (HSM) of L. obtusiloba was the smallest among the three species, especially in precipitation-reduced plots. In terms of the stomatal safety margin (SSM), L. obtusiloba was negative, while S. alnifolia and D. japonica were positive. Of the two species without dieback, rainfall reduction decreased growth of D. japonica, but did not influence growth of S. Alnifolia; meanwhile, rainfall reduction led to a decrease of non-structural carbohydrates (NSCs) in D. japonica, but an increase in S. alnifolia. It is concluded that HSM as well as SSM allow interpreting the sensitivity of the three sub-canopy species to drought. The drought-induced dieback of L. obtusiloba is determined by the interaction of stomatal behaviour and xylem vulnerability, and the species could be sensitive to climate change-caused drought although still in sub-humid areas. The isohydric/anisohydric degree is associated with NSCs status and growth of plants., Climate change-induced drought has caused increasing plant mortality around the globe. In this study, we artificially reduced annual rainfall in the field of temperate continental mountains, and investigated different responses of three understory tree species to drought and their response strategies. This study aimed to explore how future climate change-caused droughts might affect vegetation succession in the region. Plants have two important attributes that regulate water balance: closable pores in leaves (stomata) and long-distance water transport channels (xylem conduits). It was found that the synergistic effect of stomatal adjustment and xylem anti-embolism ability was the key to drought resistance.

Catastrophic failure of the water transport pathway in trees is a principal mechanism of mortality during extreme drought. To be able to predict the probability of mortality at an individual and landscape scale we need knowledge of the time for plants to reach critical levels of hydraulic failure. We grew plants of eight species of Eucalyptus originating from contrasting climates before allowing a subset to dehydrate. We tested whether a trait-based model of time to plant desiccation t(crit), from stomatal closure g(s90) to a critical level of hydraulic dysfunction psi(crit) is consistent with observed dry-down times. Plant desiccation time varied among species, ranging from 96.2 to 332 h at a vapour-pressure deficit of 1 kPa, and was highly predictable using the t(crit) model in conjunction with a leaf shedding function. Plant desiccation time was longest in species with high cavitation resistance, strong vulnerability segmentation, wide stomatal-hydraulic safety, and a high ratio of total plant water content to leaf area. Knowledge of t(crit) in combination with water-use traits that influence stomatal closure could significantly increase our ability to predict the timing of drought-induced mortality at tree and forest scales.

Understanding which structural and functional traits are linked to species’ vulnerability to embolism formation (P50) may provide fundamental knowledge on plant strategies to maintain an efficient water transport. We measured P50, wood density (WD), mean conduit area, conduit density, percentage areas occupied by vessels, parenchyma cells (PATOT) and fibers (FA) on branches of angiosperm and gymnosperm species. Moreover, we compiled a dataset of published hydraulic and anatomical data to be compared with our results. Species more vulnerable to embolism had lower WD. In angiosperms, the variability in WD was better explained by PATOT and FA, which were highly correlated. Angiosperms with a higher P50 (less negative) had a higher amount of PATOT and total amount of nonstructural carbohydrates. Instead, in gymnosperms, P50 vs PATOT was not significant. The correlation between PATOT and P50 might have a biological meaning and also suggests that the causality of the commonly observed relationship of WD vs P50 is indirect and dependent on the parenchyma fraction. Our study suggests that angiosperms have a potential active embolism reversal capacity in which parenchyma has an important role, while in gymnosperms this might not be the case.

Diese Arbeit beschäftigt sich mit dem Thema des hydraulischen Versagens der Baugruben im Grundwasser. Dabei werden die unterschiedlichen Versagensarten hinsichtlich der hydraulischen Standsicherheit der Baugruben näher betrachtet. Dabei wird der Schwerpunkt auf das Aufschwimmen, hydraulischen Grundbruch sowie Erosion gelegt. Der Nachweis des Aufschwimmens kann aus der Gleichgewichtsbedingung zwischen treibenden und widerstehenden Kräften abgeleitet werden. Im Gegensatz dazu erscheinen die beiden anderen Versagensarten etwas komplexer. Hydraulischer Grundbruch und innere Erosion treten häufig an den Schwachstellen des Bauwerkes bzw. Baugrundes auf. Hierzu werden die Nachweisführungen auf Grundlage der der gültigen Richtlinien beschrieben und die einzelnen Ansätze exemplarisch anhand von Berechnungsbeispielen erläutert. Darüber hinaus werden in der gegenständlichen Arbeit die unterschiedlichen Arten von Baugrubensicherungen unter Grundwasserbedingungen diskutiert. This thesis deals with the topic pf hydraulic failure of excavation pits in groundwater. The different types of failure about the hydraulic stability of the excavation pits are examined in more detail. the focus is on floating, hydraulic failure and erosion. The proof of floating can be derived from the condition of stability between driving and resisting forces. In contrast, the other two types of failure are more complex. Hydraulic failure and internal erosion often occur at the weak points of the structure or subsoil. For this purpose, the verification procedures are described on the basis of the valid guidelines and the individual approaches are explained exemplarily using calculation examples. In addition, the different types of excavation pit protection under groundwater conditions are discussed in this paper.

Embolism repair ability has been documented in numerous species. Although the actual mechanism driving this phenomenon is still debated, experimental findings suggest that non-structural carbohydrates (NSC) stored in wood parenchyma would provide the osmotic forces to drive the refilling of embolized conduits. We selected 12 broadleaved species differing in vulnerability to xylem embolism (P-50) and amount of wood parenchyma in order to check direct evidence about the possible link(s) between parenchyma cells abundance, NSC availability and species-specific capacity to reverse xylem embolism. Branches were dehydrated until similar to 50% loss of hydraulic conductivity was recorded (PLC similar to 50%). Hydraulic recovery (Delta PLC) and NSC content was, then, assessed after 111 of rehydration. Species showed a different ability to recover their hydraulic conductivity from PLC similar to 50%. Removing the bark in the species showing hydraulic recovery inhibited the embolism reversal. Strong correlations between the Delta PLC and: a) the amount of parenchyma cells (mainly driven by the pith area), b) the consumption of soluble NSC have been recorded. Our results support the hypothesis that refilling of embolized vessels is mediated by the mobilization of soluble NSC and it is mainly recorded in species with a higher percentage of parenchyma cells that may be important in the hydraulic recovery mechanism as a source of carbohydrates and/or as a source of water.

The maintenance of hydraulic function during and after a drought event is crucial for tree survival, but the importance of non-structural carbohydrates (NSCs) in the recovery phase is still debated. We tested whether higher NSC availability facilitates post-drought hydraulic recovery, by applying a short-term drought (Sdr) and a long-term drought combined with shading (Ldr+sh) in Fraxinus ornus and Ostrya carpinifolia. Plants were then re-irrigated and recovery was checked 24 h later, by measuring water potential, stem percentage loss of hydraulic conductance (PLC) and NSC content. The relative magnitude of hydraulic and carbon constraints was also assessed in desiccated plants. During drought, PLC increased only in F. ornus, while it was maintained almost constant in O. carpinifolia due to tighter stomatal control of xylem pressure (i.e. more isohydric). In F. ornus, only Sdr plants maintained high NSC contents at the end of drought and, when re-irrigated, recovered PLC to control values. Whereas hydraulic failure was ubiquitous, only F. ornus depleted NSC reserves at mortality. Our results suggest that preserving higher NSC content at the end of a drought can be important for the hydraulic resilience of trees.

No claim to original US government works New Phytologist © 2018 New Phytologist Trust 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.

National audience

Plant hydraulic conductance (k s) is a critical control on whole-plant water use and carbon uptake and, during drought, influences whether plants survive or die. To assess long-term physiological and hydraulic responses of mature trees to water availability, we manipulated ecosystem-scale water availability from 2007 to 2013 in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between k s and subsequent mortality using more than 5 years of physiological observations, and the subsequent impact of reduced hydraulic function and mortality on total woody canopy transpiration (E C) and conductance (G C). For both species, we observed significant reductions in plant transpiration (E) and k s under experimentally imposed drought. Conversely, supplemental water additions increased E and k s in both species. Interestingly, both species exhibited similar declines in k s under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole-plant k s also reduced carbon assimilation in both species, as leaf-level stomatal conductance (g s) and net photosynthesis (A n) declined strongly with decreasing k s. Finally, we observed that chronically low whole-plant k s was associated with greater canopy dieback and mortality for both piñon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy E C and G C. Our data indicate that significant reductions in k s precede drought-related tree mortality events in this system, and the consequence is a significant reduction in canopy gas exchange and carbon fixation. Our results suggest that reductions in productivity and woody plant cover in piñon-juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both piñon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.

Wood density, considered a heritable trait, is often associated with drought tolerance and can be a useful trait for tree breeders to select drought-tolerant provenances. Provenance trials are a useful tool to disentangle genetic variation from plasticity of adaptive traits among populations within the same species. If wood density is a strongly heritable trait selected by climatic conditions, we hypothesize that its variation in provenance trials should be shaped by the provenance climatic variables. To test our hypothesis we collected wood density data of provenance trials from the literature. We used thirty publications published between 1966 and 2015 including 25 species (13 angiosperms and 12 gymnosperms). At the intraspecific level, eight species showed significant positive correlations between wood density and drought conditions of the provenance, two species showed the opposite trend and the remaining 15 species showed no correlation with the climatic conditions of the provenances. For the 10 species with wood density strongly linked to the climatic conditions of the provenance, such relationship reflects a strong genetic determinism and a theoretical fitness optimum. For the other 15 species, wood density does not seem to be the main selection target of drought. This information is critical in the frame of breeding programs for selecting provenances more adaptable to different climate change scenarios.

The genus Quercus comprises important species in forestry not only for their productive value but also for their ability to withstand drought. Hence an evaluation of inter- and intraspecific variation in drought tolerance is important for selecting the best adapted species and provenances for future afforestation. However, the presence of long vessels makes it difficult to assess xylem vulnerability to embolism in these species. Thanks to the development of a flow centrifuge equipped with a large rotor, we quantified (i) the between species variability of embolism resistance in four native and two exotic species of oaks in Europe and (ii) the within species variability in Quercus petraea. Embolism resistance varied significantly between species, with the pressure inducing 50% loss of hydraulic conductivity (P-50) ranging between -7.0 and -4.2 MPa. Species native to the Mediterranean region were more resistant than pan-European species. In contrast, intraspecific variability in embolism resistance in Q. petraea was low within provenances and null between provenances. A positive correlation between P-50 and vessel diameter among the six oak species indicates that the more embolism resistant species had narrower xylem vessels and a higher amount of hydraulic bridges between vessels. However, this tradeoff between hydraulic efficiency and safety was not observed between Q. petraea provenances.

© 2018 The Authors. Functional Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society. Insular woodiness refers to the evolutionary transition from herbaceousness towards derived woodiness on (sub)tropical islands and leads to island floras that have a higher proportion of woody species compared to floras of nearby continents. Several hypotheses have tried to explain insular woodiness since Darwin's original observations, but experimental evidence why plants became woody on islands is scarce at best. Here, we combine experimental measurements of hydraulic failure in stems (as a proxy for drought stress resistance) with stem anatomical observations in the daisy lineage (Asteraceae), including insular woody Argyranthemum species from the Canary Islands and their herbaceous continental relatives. Our results show that stems of insular woody daisies are more resistant to drought-induced hydraulic failure than the stems of their herbaceous counterparts. The anatomical character that best predicts variation in embolism resistance is intervessel pit membrane thickness (T PM ), which can be functionally linked with air bubble dynamics throughout the 3D vessel network. There is also a strong link between T PM vs. degree of woodiness and thickness of the xylem fibre wall vs. embolism resistance, resulting in an indirect link between lignification and resistance to embolism formation. Thicker intervessel pit membranes in Argyranthemum functionally explain why this insular woody genus is more embolism resistant to drought-induced failure compared to the herbaceous relatives from which it has evolved, but additional data are needed to confirm that palaeoclimatic drought conditions have triggered wood formation in this daisy lineage. A plain language summary is available for this article. ispartof: FUNCTIONAL ECOLOGY vol:32 issue:6 pages:1467-1478 status: published

Projected climate change scenarios such as frequently occurring dry summer spells are an enormous threat to the health of boreal conifer forests. We identified visible features indicating wood with tracheids predisposed for hydraulic and mechanical dysfunction in Norway spruce, suggest why this is formed during severe summer drought and hypothesised on mechanism that would cause tracheid collapse and stem cracks. Trees from southern Sweden that showed signs of severe reaction to drought, i.e. stem cracks along the trunk, were compared to healthy, undamaged trees. Rings investigated included those formed in 2006, a year with an extremely dry summer season in the study region. In southern Norway, we investigated trees with and without drought-induced top dieback symptoms. We analysed anatomical features such as tracheid lumen diameter, thickness of cell wall and its various layers (S1, S2 and S3), applied Raman imaging in order to get information on the lignin distribution in the cell wall and the compound middle lamellae and performed hydraulic flow and shrinkage experiments. Although tracheids in annual rings with signs of collapse had higher tangential lumen diameters than those in “normal” annual rings, we conclude that collapse of tracheid walls depends mainly on wall thickness, which is genetically determined to a large extent. Spruce trees that produce earlywood with extremely thin cell walls can develop wall collapse and internal cracks under the impact of dry spells. We also present a new diagnostic tool for detecting individuals that are prone to cell wall collapse and stem cracks: Lucid bands, i.e. bands in the fresh sapwood with very thin cell walls and inhomogeneous lignin distribution in the S-layers and the compound middle lamellae that lost their hydraulic function due to periods of severe summer drought. The detection of genotypes with lucid bands could be useful for an early selection against individuals that are prone to stem cracks under the impact of severe summer drought, and also for early downgrading of logs prone to cracking during industrial kiln drying.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; International audience; Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere–atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function

Different tree species have various strategies in response to different drought types. Black locust (a deciduous tree) and arborvitae (an evergreen tree) present different physiological death processes when exposed to fast− (FD) or slow−drought (SD) stress. In this study, black locust shed its leaves when subjected to drought, underwent hydraulic failure and carbon depletion regardless of FD or SD. However, arborvitae retained its needles when exposed to drought, only suffered hydraulic failure under FD, while it experienced both hydraulic failure and carbon depletion under SD., Drought-induced tree mortality has been observed worldwide. Nevertheless, the physiological mechanisms underlying this phenomenon are still being debated. Potted Robinia pseudoacacia and Platycladus orientalis saplings were subjected to drought and their hydraulic failure and carbon starvation responses were studied. They underwent simulated fast drought (FD) and slow drought (SD) until death. The dynamics of their growth, photosynthesis, water relations and carbohydrate concentration were measured. The results showed that during drought, growth and photosynthesis of all saplings were significantly reduced in both species. The predawn water potential in both species was ~ −8 MPa at mortality. The percentage loss of conductivity (PLC) was at a maximum at mortality under both FD and SD. For R. pseudoacacia and P. orientalis, they were >95 and ~45 %, respectively. At complete defoliation, the PLC of R. pseudoacacia was ~90 % but the trees continued to survive for around 46 days. The non-structural carbohydrate (NSC) concentrations in the stems and roots of both FD and SD R. pseudoacacia declined to a very low level near death. In contrast, the NSC concentrations in the needles, stems and roots of P. orientalis at mortality under FD did not significantly differ from those of the control, whereas the NSC concentrations in SD P. orientalis stems and roots at death were significantly lower than those of the control. These results suggest that the duration of the drought affected NSC at mortality in P. orientalis. In addition, the differences in NSC between FD and SD P. orientalis did not alter mortality thresholds associated with hydraulic failure. The drought-induced death of R. pseudoacacia occurred at 95 % PLC for both FD and SD, indicating that hydraulic failure played an important role in mortality. Nevertheless, the consistent decline in NSC in R. pseudoacacia saplings following drought-induced defoliation may have also contributed to its mortality.

Drought-induced tree decline is a complex event, and recent hypotheses suggest that hydraulic failure and carbon starvation are co-responsible for this process. We tested the possible role of non-structural carbohydrates (NSC) content on post-drought hydraulic recovery, to verify the hypothesis that embolism reversal represents a mechanistic link between carbon starvation and stem hydraulics. Measurements were performed in laurel plants subjected to similar water stress levels either over short or long term, to induce comparable embolism levels. Plants subjected to mild and prolonged water shortage (S) showed reduced growth, adjustment of turgor loss point driven by changes in both osmotic potential at full turgor and bulk modulus of elasticity, a lower content of soluble NSC and a higher content of starch with respect to control (C) plants. Moreover, S plants showed a lower ability to recover from xylem embolism than C plants, even after irrigation. Our data suggest that plant carbon status might indirectly influence plant performance during and after drought via effects on xylem hydraulic functioning, supporting the view of a possible mechanistic link between the two processes.

Durant les darreres dècades s’han documentat arreu del món múltiples casos de mortalitat forestal produïda per sequera i és probable que aquests augmentin amb el canvi climàtic, particularment en aquelles zones on la disponibilitat d’aigua és ja limitada. Aquests episodis de decaïment forestal poden provocar canvis sobtats en l’estructura i funcionament dels ecosistemes. L’objectiu principal d’aquesta tesi doctoral és el d’aprofundir en els mecanismes fisiològics que intervenen en els processos de decaïment induït per sequera en boscos. Els estudis s’han fet en una població de pi roig (Pinus sylvestris L.) a les Muntanyes de Prades, Tarragona. En aquesta població s’han documentat diferents episodis de mortalitat per sequera en el pi roig. L’alzina (Quercus ilex L.) tendeix a ocupar els rodals afectats per mortalitat de pins i és una ferma candidata a substituir el pi com a espècie dominant. En aquesta tesi m’he centrat en estudiar els principals mecanismes fisiològics que s’han proposat com a principals desencadenants de la mortalitat forestal produïda per sequera: l’esgotament de les reserves de carboni i la fallida hidràulica. Tenint en compte que la defoliació és un símptoma del decaïment forestal i que està associada a un major risc de mortalitat, hem caracteritzat les conseqüències fisiològiques de la defoliació en el pi roig, comparant els patrons estacionals de flux de saba, els potencials hídrics foliars i la conductància hidràulica de la planta entre pins defoliats i pins sans (no defoliats) (Capítol 2), per a testar la hipòtesi que pins defoliats serien més sensibles a la sequera. En el Capítol 3, s’ha mesurat la vulnerabilitat a l’embolisme en branques i arrels, i l’estacionalitat en l’embolisme nadiu i els nivells de les reserves de carbohidrats no estructurals en pins defoliats i no defoliats amb l’objectiu de testar si la defoliació s’associa més amb processos de fallida hidràulica o d’inanició per manca de carboni. A més, també s’ha analitzat el paper dels fongs patògens com a possible factor implicat en la defoliació. Posteriorment, he utilitzat dades de microvariacions del radi del xilema per a estudiar la resposta a la sequera de la resistència hidràulica subterrània i de la seva contribució a la resistència total en pins defoliats i no defoliats (Capítol 4). Finalment, s’han estudiat les respostes a la sequera de pins roig i alzines en boscos mixtos i purs (Capítol 5) per tal de determinar possibles diferències fisiològiques que influeixin en la competència entre les dues espècies en un escenari de substitució del pi roig per l’alzina. Els resultats d’aquesta tesi mostren que els pins defoliats són més sensibles a la sequera que els pins no defoliats i presenten una major reducció de la seva transpiració com a conseqüència del tancament estomàtic, associat també a una reducció de la conductància hidràulica al llarg de tot l’arbre (Capítol 2). Els arbres defoliats presenten menys reserves de carbohidrats no estructurals en tots els òrgans, sobretot en els mesos d’estiu (Capítol 3). Al Capítol 4 observem com la resistència hidràulica radicular juga un paper important durant la sequera, però, en aquest cas, els pins no defoliats són els que pateixen majors constriccions en el transport d’aigua en la zona radicular en condicions de sequera. Tot i que les complexes interrelacions entre l’economia de l’aigua i el carboni fan difícil esbrinar quin és el mecanisme precís que porta a la mortalitat produïda per sequera, en el nostre cas la reducció en les reserves de carbohidrats sembla tenir un paper fonamental. Finalment, en el Capítol 5, observem una major resistència a la sequera de l’alzina en comparació amb el pi roig, malgrat una certa tendència en l’alzina a patir un risc més elevat de fallida hidràulica durant sequeres intenses. Aquesta resistència a la sequera de l’alzina, tant en masses pures com mixtes co-habitant amb el pi roig, suggereix que el procés de substitució del pi roig per l’alzina continuarà en el futur., In recent decades, several episodes of tree drought-induced mortality have been reported and they are likely to increase in the future due to climate change, particularly in those areas where water is already limiting. These episodes of forest decline can cause sudden changes in the structure and functioning of ecosystems. The main objective of this thesis is to delve into the physiological mechanisms involved in drought-induced forest decline. The studies were carried out in a population of Scots pine (Pinus sylvestris L.) within PNIN (Paratge Natural d’Interès Nacional) of Poblet (Prades Mountains, Catalonia). Different episodes of Scots pine drought-induced mortality have been documented in this population where, in addition, holm oak (Quercus ilex L.) is a clear candidate to replace pine in the coming years as it is the current dominant understorey species and tends to occupy the gaps opened by pine mortality. In this thesis I focused on the study of the two main physiological mechanisms that have been proposed as major triggers of drought-induced tree mortality: depletion of carbon stores and hydraulic failure. Given that defoliation is a symptom of forest decline and that it is associated with an increased mortality risk, I characterised the physiological consequences of Scots pine defoliation, by comparing the seasonal patterns of sap flow, leaf water potentials and plant hydraulic conductance among defoliated and healthy (non-defoliated) pines (Chapter 2), to test the hypothesis that defoliated pines would be more sensitive to drought. In Chapter 3, I measured vulnerability to embolism in roots and branches and the seasonality of native embolism and non-structural carbohydrates reserves in defoliated and non-defoliated pines, in order to test whether defoliation was more associated with hydraulic failure or carbon starvation processes. In addition, I also analysed the role of pathogenic fungi as a possible factor involved in mortality. Subsequently, I used data on the microvariations of xylem radius to study the drought response of belowground hydraulic resistance and its contribution to total resistance in defoliated and non-defoliated pines (Chapter 4). Finally, I studied the drought responses in Scots pine trees and holm oaks in mixed and pure stands (Chapter 5), in order to determine possible physiological differences influencing competition between both species in a scenario of Scots pine substitution by holm oak. The main results of this thesis showed that defoliated pines were more sensitive to drought than non-defoliated ones and that they showed a transpiration reduction as a result of stomatal closure, which associated with a reduction in tree hydraulic conductance. Thus, defoliation may be more adequately seen as an inevitable consequence of drought than as a strategy to cope with water stress (Chapter 2). Furthermore, roots of defoliated trees showed greater vulnerability to embolism (Chapter 3), besides also having less nonstructural carbohydrate stores in all organs, especially in summer months (Chapter 3). This, combined with the role of pathogenic fungi, makes it very difficult to identify the precise mechanisms triggering drought-induced mortality due to the complex interconnection between them. I highlight, in Chapter 4, how root hydraulic resistance played an important role with increasing drought, but in this case, non-defoliated pines suffered higher constraints on water transport in the root zone under drought conditions. Finally, in Chapter 5, I observed a higher drought resistance in holm oak compared to Scots pine, despite holm oak tending to suffer a higher risk of hydraulic failure under intense droughts. This drought resistance of holm oak, both in pure and mixed stands coexisting with Scots pine, suggests that the process of substitution of Scots pine by holm oak will continue in the future.

Hydraulic failure and carbon starvation are recognized as main causes of drought-induced forest decline. As water transport and carbon dynamics are strictly interdependent, it is necessary to clarify how dehydration-rehydration cycles are affecting the relations between stem embolism and non-structural carbohydrates (NSC). This is particularly needed for conifers whose embolism repair capability is still controversial. Potted Norway spruce saplings underwent two drought-re-irrigation cycles of same intensity, but performed in two consecutive summers. During the second cycle, stem percent loss of hydraulic conductivity (PLC) and NSC content showed no carry-over effects from the previous drought, indicating complete long-term recovery. The second drought treatment induced moderate PLC (20%) and did not affect total NSCs content, while starch was converted to soluble sugars in the bark. After one week of re-irrigation, PLC recovered to pre-stress values (0%) and NSCs were depleted, only in the wood, by about 30%. Our data suggest that spruce can repair xylem embolism and that, when water is newly available, NSCs stored in xylem parenchyma can be mobilized over short term to sustain respiration and/or for processes involved in xylem transport restoration. This, however, might imply dependency on sapwood NSC reserves for survival, especially if frequent drought spells occur.