Widespread breakdown in masting in European beech due to rising summer temperatures.

Climate change effects on tree reproduction are poorly understood, even though the resilience of populations relies on sufficient regeneration to balance increasing rates of mortality. Forest-forming tree species often mast, i.e. reproduce through synchronised year-to-year variation in seed production, which improves pollination and reduces seed predation. Recent observations in European beech show, however, that current climate change can dampen interannual variation and synchrony of seed production and that this masting breakdown drastically reduces the viability of seed crops. Importantly, it is unclear under which conditions masting breakdown occurs and how widespread breakdown is in this pan-European species. Here, we analysed 50 long-term datasets of population-level seed production, sampled across the distribution of European beech, and identified increasing summer temperatures as the general driver of masting breakdown. Specifically, increases in site-specific mean maximum temperatures during June and July were observed across most of the species range, while the interannual variability of population-level seed production (CVp) decreased. The declines in CVp were greatest, where temperatures increased most rapidly. Additionally, the occurrence of crop failures and low seed years has decreased during the last four decades, signalling altered starvation effects of masting on seed predators. Notably, CVp did not vary among sites according to site mean summer temperature. Instead, masting breakdown occurs in response to warming local temperatures (i.e. increasing relative temperatures), such that the risk is not restricted to populations growing in warm average conditions. As lowered CVp can reduce viable seed production despite the overall increase in seed count, our results warn that a covert mechanism is underway that may hinder the regeneration potential of European beech under climate change, with great potential to alter forest functioning and community dynamics.

Changes in carbon stocks according to stand development stages in oriental beech forests in the Marmara Region of Türkiye.

This study was conducted to determine the changes in carbon stocks of oriental beech (Fagus orientalis) according to stand development stage in the Marmara Region of Türkiye. For this purpose, sample plots were taken from a total of 32 areas encompassing four stand development stages (young, middle age, mature and overmature stand). The diameter at breast height and height of all trees in the sample plots were measured, and only three dominant trees's ages per plot were determined. Aboveground carbon stock was calculated using equations developed for beech forests, while the coefficients in the Agriculture, Forestry and Other Land Use guide were used to determine belowground carbon stocks. A soil pit was dug in each plot and soil samples were taken at different depths (0-10, 10-30, 30-60, 60-100 cm). In addition, litters were sampled from four different 25 × 25 cm sections in each plot, and then the physical and chemical properties of the soil and litters were analysed. The variations in carbon stocks in above- and below-ground tree mass, litter and soil, and in ecosystem carbon stocks according to development stage were examined by analysis of variance and Duncan test, and the relationships between the carbon stocks were investigated by correlation analysis. Aboveground (AG) and belowground (BG) tree, soil and ecosystem carbon stocks showed significant differences between the four stand development stages (P < 0.05), but not the litter carbon stocks (P > 0.05). AG and BG tree and ecosystem carbon stocks increased with progressive stand development stages, while the soil carbon stock was the highest at the young stage. These findings will contribute to the preparation of forest management plans and the national greenhouse gas inventory.

Localization and Dynamics of the Methionine Sulfoxide Reductases MsrB1 and MsrB2 in Beech Seeds.

Beech seeds are produced irregularly, and there is a need for long-term storage of these seeds for forest management practices. Accumulated reactive oxygen species broadly oxidize molecules, including amino acids, such as methionine, thereby contributing to decreased seed viability. Methionine oxidation can be reversed by the activity of methionine sulfoxide reductases (Msrs), which are enzymes involved in the regulation of many developmental processes and stress responses. Two types of Msrs, MsrB1 and MsrB2, were investigated in beech seeds to determine their abundance and localization. MsrB1 and MsrB2 were detected in the cortical cells and the outer area of the vascular cylinder of the embryonic axes as well as in the epidermis and parenchyma cells of cotyledons. The abundances of MsrB1 and MsrB2 decreased during long-term storage. Ultrastructural analyses have demonstrated the accumulation of these proteins in protein storage vacuoles and in the cytoplasm, especially in close proximity to the cell membrane. In silico predictions of possible Msr interactions supported our findings. In this study, we investigate the contribution of MsrB1 and MsrB2 locations in the regulation of seed viability and suggest that MsrB2 is linked with the longevity of beech seeds via association with proper utilization of storage material.

Past, present and future distributions of Oriental beech (Fagus orientalis) under climate change projections.

Species distribution models can help predicting range shifts under climate change. The aim of this study is to investigate the late Quaternary distribution of Oriental beech (Fagus orientalis) and to project future distribution ranges under different climate change scenarios using a combined palaeobotanical, phylogeographic, and modelling approach. Five species distribution modelling algorithms under the R-package `biomod2`were applied to occurrence data of Fagus orientalis to predict distributions under present, past (Last Glacial Maximum, 21 ka, Mid-Holocene, 6 ka), and future climatic conditions with different scenarios obtained from MIROC-ESM and CCSM4 global climate models. Distribution models were compared to palaeobotanical and phylogeographic evidence. Pollen data indicate northern Turkey and the western Caucasus as refugia for Oriental beech during the Last Glacial Maximum. Although pollen records are missing, molecular data point to Last Glacial Maximum refugia in northern Iran. For the mid-Holocene, pollen data support the presence of beech in the study region. Species distribution models predicted present and Last Glacial Maximum distribution of Fagus orientalis moderately well yet underestimated mid-Holocene ranges. Future projections under various climate scenarios indicate northern Iran and the Caucasus region as major refugia for Oriental beech. Combining palaeobotanical, phylogeographic and modelling approaches is useful when making projections about distributions of plants. Palaeobotanical and molecular evidence reject some of the model projections. Nevertheless, the projected range reduction in the Caucasus region and northern Iran highlights their importance as long-term refugia, possibly related to higher humidity, stronger environmental and climatic heterogeneity and strong vertical zonation of the forest vegetation.

Electrophysiological Responses of the Beech Leaf-Mining Weevil, Orchestes fagi, to Seasonally-Variant Volatile Organic Compounds Emitted by American Beech, Fagus grandifolia.

The beech leaf-mining weevil, Orchestes fagi, is a common pest of European beech, Fagus sylvatica, and has recently become established in Nova Scotia, Canada where it similarly infests American beech, F. grandifolia. We collected volatile organic compounds (VOCs) emitted by F. grandifolia leaves at five developmental stages over one growing season and simultaneously analyzed them for volatile emissions and O. fagi antennal response using gas chromatography-electroantennographic detection (GC-EAD). Volatile profiles changed significantly throughout the growing season, shifting from primarily ß-caryophyllene, methyl jasmonate, and simple monoterpene emissions to dominance of the bicyclic monoterpene sabinene during maturity. Two VOCs dominant during bud burst, (R)-(+)-limonene and geranyl-p-cymene, may be of biological relevance due to the highly specific oviposition period of O. fagi at this stage though antennal responses were inconclusive. Senescence showed a decrease in blend complexity with an increase in (Z)-3-hexenyl acetate and (Z)-3-hexen-1-ol as well as a resurgence of α-terpinene and geranyl-p-cymene. We present a novel electroantennal preparation for O. fagi. Antennae of both male and female O. fagi responded to the majority of detectable peaks for host volatiles presented via GC-EAD. Females displayed greater overall sensitivities and less specificity to host volatiles and it is hypothesized that this translates to more generalist olfaction than males. It is clear that olfactory cues are important physiologically though their implications on behaviour are still unknown. The results presented in this study provide a baseline and tools on which to connect the complex and highly time-specific phenology of both F. grandifolia and the destructive pest O. fagi through which olfactory-based lures can be investigated for monitoring systems.

Climate Change Strengthens Selection for Mast Seeding in European Beech.

Climate change is altering patterns of seed production worldwide [1-4], but the potential for evolutionary responses to these changes is poorly understood. Masting (synchronous, annually variable seed production by plant populations) is selectively beneficial through economies of scale that decrease the cost of reproduction per surviving offspring [5-7]. Masting is particularly widespread in temperate trees [8, 9] impacting food webs, macronutrient cycling, carbon storage, and human disease risk [10-12], so understanding its response to climate change is important. Here, we analyze inter-individual variability in plant reproductive patterns and two economies of scale-predator satiation and pollination efficiency-and document how natural selection acting upon them favors masting. Four decades of observations for European beech (Fagus sylvatica) show that predator satiation and pollination efficiency select for individuals with higher inter-annual variability of reproduction and higher reproductive synchrony between individuals. This result confirms the long-standing theory that masting, a population-level phenomenon, is generated by selection on individuals. Furthermore, recent climate-driven increases in mean seed production have increased selection pressure from seed predators but not from pollination efficiency. Natural selection is thus acting to restore the fitness benefits of masting, which have previously decreased under a warming climate [13]. However, selection will likely take far longer (centuries) than climate warming (decades), so in the short-term, tree reproduction will be reduced because masting has become less effective at satiating seed predators. Over the long-term, evolutionary responses to climate change could potentially increase inter-annual variability of seed production of masting species.

Climate warming disrupts mast seeding and its fitness benefits in European beech.

Many plants benefit from synchronous year-to-year variation in seed production, called masting. Masting benefits plants because it increases the efficiency of pollination and satiates predators, which reduces seed loss. Here, using a 39-year-long dataset, we show that climate warming over recent decades has increased seed production of European beech but decreased the year-to-year variability of seed production and the reproductive synchrony among individuals. Consequently, the benefit that the plants gained from masting has declined. While climate warming was associated with increased reproductive effort, we demonstrate that less effective pollination and greater losses of seeds to predators offset any benefits to the plants. This shows that an apparently simple benefit of climate warming unravels because of complex ecological interactions. Our results indicate that in masting systems, the main beneficiaries of climate-driven increases in seed production are seed predators, not plants.

It is not just a 'trade-off': indications for sink- and source-limitation to vegetative and regenerative growth in an old-growth beech forest.

Controls on tree growth are key issues in plant physiology. The hypothesis of our study was that the interannual variability of wood and fruit production are primarily controlled directly by weather conditions (sink limitation), while carbon assimilation (source limitation) plays a secondary role. We analyzed the interannual variability of weather conditions, gross primary productivity (GPP) and net primary productivity (NPP) of wood and fruits of an old-growth, unmanaged Fagus sylvatica forest over 14 yr, including six mast years. In a multiple linear regression model, c. 71% of the annual variation in wood-NPP could be explained by mean air temperature in May, precipitation from April to May (positive influence) and fruit-NPP (negative influence). GPP of June to July solely explained c. 42% of the variation in wood-NPP. Fruit-NPP was positively related to summer precipitation 2 yr before (R2  = 0.85), and negatively to precipitation in May (R2  = 0.83) in the fruit years. GPP had no influence on fruit-NPP. Our results suggest a complex system of sink and source limitations to tree growth driven by weather conditions and going beyond a simple carbon-mediated 'trade-off' between regenerative and vegetative growth.

Phenology and growth of Fagus sylvatica and Quercus robur seedlings in response to temperature variation in the parental versus offspring generation.

Plants are known to respond to warming temperatures. Few studies, however, have included the temperature experienced by the parent plant in the experimental design, in spite of the importance of this factor for population dynamics. We investigated the phenological and growth responses of seedlings of two key temperate tree species (Fagus sylvatica and Quercus robur) to spatiotemporal temperature variation during the reproductive period (parental generation) and experimental warming of the offspring. To this end, we sampled oak and beech seedlings of different ages (1-5 years) from isolated mother trees and planted the seedlings in a common garden. Warming of the seedlings advanced bud burst in both species. In oak seedlings, higher temperatures experienced by mother trees during the reproductive period delayed bud burst in control conditions, but advanced bud burst in heated seedlings. In beech seedlings, bud burst timing advanced both with increasing temperatures during the reproductive period of the parents and with experimental warming of the seedlings. Relative diameter growth was enhanced in control oak seedlings but decreased with warming when the mother plant experienced higher temperatures during the reproductive period. Overall, oak displayed more plastic responses to temperatures than beech. Our results emphasise that temperature during the reproductive period can be a potential determinant of tree responses to climate change.

Metatranscriptomics reveals unsuspected protistan diversity in leaf litter across temperate beech forests, with Amoebozoa the dominating lineage.

Forest litter harbors complex networks of microorganisms whose major components are bacteria, fungi and protists. Protists, being highly selective consumers of bacteria and fungi could influence decomposition processes by shifting competitive microbial interactions. We investigated the eukaryotic diversity from 18 samples of one-year beech (Fagus sylvatica) leaf litter by RNA-based high-throughput sequencing of the small-subunit ribosomal RNA gene. By applying a metatranscriptomics approach, we avoided biases inherent to PCR-based methods, and could therefore focus on elusive protistan groups. We obtained 14 589 eukaryotic assembled sequences (contigs) representing 2223 unique taxa. Fungi dominated the eukaryotic assemblage, followed by an equal proportion of protists and plants. Among protists, the phylum Amoebozoa clearly dominated, representing more than twice the proportion of Alveolata (mostly ciliates) and Rhizaria (mostly Cercozoa), which are often retrieved as the dominant protistan groups in soils, revealing potential primer biases. By assigning functional traits to protists, we could assess that the proportion of free-living and heterotrophs was much higher than that of parasites and autotrophs, opening the way to a better understanding of the role played by the protistan communities and how biodiversity interacts with decomposition processes.

Changes in soil organic matter composition after Scots pine afforestation in a native European beech forest revealed by analytical pyrolysis (Py-GC/MS).

The introduction of coniferous species in former deciduous forests may exert changes in soil organic matter, particularly in its molecular composition. In this work, pyrolysis-gas chromatography-mass spectrometry was used to study changes in SOM quality related to the centennial afforestation of Scots pine in an area formerly covered by European beech forest in the NE-flank of the Moncayo Natural Park (NE-Spain). For each soil profile three organic layers (fresh litter, fragmented litter and humified litter) and mineral soil horizons (Ah, E, Bhs and C) were studied. A total of 128 compounds were identified in the pyrograms, and composition differences were detected among the organic and mineral soil layers as well as between soils under beech and pine, for the main compound classes: nitrogen compounds, aromatics, lignin methoxyphenols, polycyclic aromatic hydrocarbons, lipids and polysaccharide-derived moieties. Such chemical differences were found to be derived from the biomass composition of the predominant vegetation type that was incorporated into the soil and from its progression into the soil profile. The analysis of the distribution of alkanes indicated higher SOM stabilization in the native beech forest soil. The signal of beech biomarkers (long chain n-alkanes C31-C33) found in the pine E horizon indicates the permanence of SOM derived from the natural forest ca. 100 years after the afforestation.

Greater growth stability of trees in marginal habitats suggests a patchy pattern of population loss and retention in response to increased drought at the rear edge.

Species rear range edges are predicted to retract as climate warms, yet evidence of population persistence is accumulating. Accounting for this disparity is essential to enable prediction and planning for species' range retractions. At the Mediterranean edge of European beech-dominated temperate forest, we tested the hypothesis that individual performance should decline at the limit of the species' ecological tolerance in response to increased drought. We sampled 40 populations in a crossed factor design of geographical and ecological marginality and assessed tree growth resilience and decline in response to recent drought. Drought impacts occurred across the rear edge, but tree growth stability was unexpectedly high in geographically isolated marginal habitat and lower than anticipated in the species' continuous range and better-quality habitat. Our findings demonstrate that, at the rear edge, range shifts will be highly uneven and characterised by reduction in population density with local population retention rather than abrupt range retractions.

Regulation of thiol metabolism as a factor that influences the development and storage capacity of beech seeds.

Seeds are the basis of propagation for the common beech (Fagus sylvatica L.), but the seed set of the beech is unsteady, with 5-10 years between abundant crops. Beech seeds are very difficult to store and lose their viability quickly even in optimum storage conditions. To date, it has not been possible to determine factors indicative of the aging process and the loss of viability of beech seeds during storage. To address this important economic challenge and interesting scientific problem, we analyzed the adjustment of the redox state during the development and storage of seeds. Many metabolic processes are based on reduction and oxidation reactions. Thiol proteins control and react to the redox state in the cells. The level of thiol proteins increased during seed maturation and decreased during storage. Gel-based redox proteomics identified 17 proteins in beech seeds during development. The proteins could be assigned to processes like metabolism and antioxidant functions. During storage, the number of proteins decreased to only six, i.e., oxidoreductases, peptidases, hydrolases and isomerases. The occurrence of peroxiredoxins (PRX) as thiol peroxidases and redox regulators indicates an important role of cytosolic 1CysPRX and PRXIIC, mitochondrial PRXIIF, and plastidic PRXIIE, 2CysPRX, and PRXQ in beech seeds during development and storage. Particularly, 2CysPRX was present in beech seeds during development and storage and may perform an important function in regulation of the redox state during both seed development and storage. The role of thiol proteins in the regulation of the redox state during the development and storage of beech seeds is discussed.

Evidence of a seasonal trade-off between growth and starch storage in declining beeches: assessment through stem radial increment, non-structural carbohydrates and intra-ring δ13C.

Forest decline is reported in recent decades all over the world. However, developing a clear vision of the associated tree dysfunctioning is still a challenge for plant physiologists. In this study, our aim was to examine the seasonal carbon adjustments of beech trees in the case of a long-term drought-induced decline. We compared healthy and declining trees in terms of stem radial growth, phloem sugar content and δ13C, together with xylem carbohydrates and intra-ring δ13C patterns. The radial growth of declining trees was clearly reduced by lower growth rates and shorter growing season length (44 days compared with healthy trees). The soluble sugar content was higher in the xylem of declining trees compared with the healthy ones, but similar in the phloem except at the end of their growth. Declining trees increased their levels of xylem starch content from budburst until the date of maximal growth rate. These reserve dynamics revealed an early trade-off between radial growth and starch storage that might be the result of an active or passive process. For declining trees, the slight decrease of intra-ring cellulose δ13C pattern during the early growing season was attributed to the synthesis of 13C enriched starch. For healthy trees, δ13C patterns were characterized by a progressive 13C increase along the ring, attributed to increased water-use efficiency (WUE) in response to decreased water availability. Individual variations of the crown area were negatively correlated to the intra-ring δ13C amplitude, which was ascribed to variations in canopy WUE and resource competition for healthy trees and partly to variations in the amount of reserves accumulated during spring for declining ones. Our study highlights the carbon physiological adjustment of declining trees towards reducing spring growth while storing starch, which can be reflected in the individual intra-ring cellulose δ13C patterns.

Short-term nitrogen dynamics are impacted by defoliation and drought in Fagus sylvatica L. branches.

The predicted recurrence of adverse climatic events such as droughts, which disrupt nutrient accessibility for trees, could jeopardize the nitrogen (N) metabolism in forest trees. Internal tree N cycling capacities are crucial to ensuring tree survival but how the N metabolism of forest trees responds to intense, repeated environmental stress is not well known. For 2 years, we submitted 9-year-old beech (Fagus sylvatica L.) trees to either a moderate or a severe prolonged drought or a yearly removal of 75% of the foliage to induce internal N cycling changes. During the second year of stress, in spring and summer, we sprayed 15N-urea on the leaves (one branch per tree). Then, for 14 days, we traced the 15N dynamics through the leaves, into foliar proteins and into the branch compartments (leaves and stems segments), as well as its long-distance transfer from the labeled branches to the tree apical twigs. Defoliation caused a short- and mid-term N increase in the leaves, which remained the main sink for N. Whatever the treatment and the date, most of the leaf 15N stayed in the leaves and was invested in soluble proteins (60-68% of total leaf N). 15N stayed more in the proximal part of the branch in response to drought compared with other treatments. The long-distance transport of N was maintained even under harsh drought, highlighting efficient internal N recycling in beech trees. Under extreme constraints creating an N and water imbalance, compensation mechanisms operated at the branch level in beech trees and allowed them (i) to maintain leaf N metabolism and protein synthesis and (ii) to ensure the seasonal short- and long-distance transfer of recycled leaf N even under drastic water shortage conditions.

Short photoperiod reduces the temperature sensitivity of leaf-out in saplings of Fagus sylvatica but not in horse chestnut.

Leaf phenology is one of the most reliable bioindicators of ongoing global warming in temperate and boreal zones because it is highly sensitive to temperature variation. A large number of studies have reported advanced spring leaf-out due to global warming, yet the temperature sensitivity of leaf-out has significantly decreased in temperate deciduous tree species over the past three decades. One of the possible mechanisms is that photoperiod is limiting further advance to protect the leaves against potential damaging frosts. However, the "photoperiod limitation" hypothesis remains poorly investigated and experimentally tested. Here, we conducted a photoperiod- and temperature-manipulation experiment in climate chambers on two common deciduous species in Europe: Fagus sylvatica (European beech, a typically late flushing species) and Aesculus hippocastanum (horse chestnut, a typically early flushing species). In agreement with previous studies, we found that the warming significantly advanced the leaf-out dates by 4.3 and 3.7 days/°C for beech and horse chestnut saplings, respectively. However, shorter photoperiod significantly reduced the temperature sensitivity of beech only (3.0 days/°C) by substantially increasing the heat requirement to avoid leafing-out too early. Interestingly, the photoperiod limitation only occurs below a certain daylength (photoperiod threshold) when the warming increased above 4°C for beech trees. In contrast, for chestnut, no photoperiod threshold was found even when the ambient air temperature was warmed by 5°C. Given the species-specific photoperiod effect on leaf phenology, the sequence of the leaf-out timing among forest tree species may change under future climate warming conditions. Nonphotoperiodic species may benefit from warmer springs by starting the growing season earlier than photoperiodic sensitive species, modifying forest ecosystem structure and functions, but this photoperiod limitation needs to be further investigated experimentally in numerous species.

Biomechanical control of beech pole verticality (Fagus sylvatica) before and after thinning: theoretical modelling and ground-truth data using terrestrial LiDAR.

PREMISE OF THE STUDY: Thinning is a frequent disturbance in managed forests, especially to increase radial growth. Due to buckling and bending risk associated with height and mass growth, tree verticality is strongly constrained in slender trees growing in dense forests and poor light conditions. Tree verticality is controlled by uprighting movements implemented from local curvatures induced by wood maturation stresses and/or eccentric radial growth. This study presents the first attempt to compare the real uprighting movements in mature trees using a theoretical model of posture control. METHODS: Stem lean and curvature were measured by Terrestrial LiDAR Scanner (TLS) technology before and 6 years after thinning and compared to unthinned control poles. Measures for several tree and wood traits were pooled together to implement a widely used biomechanical model of tree posture control. Changes in observed stem lean were then compared with the model predictions, and discrepancies were reviewed. KEY RESULTS: Even under a highly constrained environment, most control poles were able to counterbalance gravitational curvature and avoid sagging. Thinning stimulated uprighting movements. The theoretical uprighting curvature rate increased just after thinning, then slowed after 2 years, likely due to the stem diameter increase. The biomechanical model overestimated the magnitude of uprighting. CONCLUSIONS: Most suppressed beech poles maintain a constant lean angle, and uprighting movements occur after thinning, indicating that stem lean is plastic in response to light conditions. Acclimation of posture control to other changes in growth condition should be investigated, and lean angles should be measured in forest inventories as an indicator of future wood quality.

Long-term responses of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) to the contamination of light soils with diesel oil.

Research into trees plays a very important role in evaluations of soil contamination with diesel oil. Trees are ideal for reclaiming contaminated soils because their large biomass renders them more resistant to higher concentrations of pollutants. In the literature, there is a general scarcity of long-term studies performed on trees, in particular European beeches. The aim of this study was to evaluate the responses of Scots pines and European beeches grown for 8 years on soil contaminated with diesel oil. Selected morphological and physiological parameters of trees were analyzed. The biomass yield of Scots pines was not significantly correlated with increasing concentrations of diesel oil, but it was more than 700% higher than in European beeches. Scots pines were taller and had a larger stem diameter than European beeches during the 8-year study. The diameter of trees grown on the most contaminated soil was reduced 1.5-fold in Scots pines and more than twofold in European beeches. The length of Scots pine needles from the most contaminated treatment decreased by 50% relative to control needles. The shortest needles were heaviest. The fluctuating asymmetry (FA) of needle length was highest in Scots pines grown on the most contaminated soil, whereas the reverse was noted in the FA of needle weight. Diesel oil decreased the concentrations of chlorophylls a and b, total chlorophyll, and carotenoids. The Fv/Fm ratio of needles and leaves was influenced by the tested concentrations of diesel oil. The results of the study indicate that the Scots pine better adapts (grows more rapidly and produces higher biomass) to long-term soil contamination with diesel oil than the European beech. In European beeches, growth inhibition and leaf discoloration (a decrease in chlorophyll content) were observed already after the first year of the experiment, which indicates that 1-year-old seedlings of European beech are robust bioindicators of soil contamination with diesel oil.

Close to the edge: effects of repeated severe drought on stem hydraulics and non-structural carbohydrates in European beech saplings.

Severe drought events threaten tree water transport system, productivity and survival. Woody angiosperms generally die when embolism-induced loss of hydraulic conductance (PLC) surpasses 80-90% under intense water shortage. However, the recovery capability and possible long-term carry-over effects of repeated drought events could dictate the fate of species' population under climate change scenarios. Potted saplings of European beech (Fagus sylvatica L.) were subjected to two drought cycles in two consecutive growing seasons, aiming to induce minimum leaf water potentials (Ψmd) of about -4 MPa, corresponding to hydraulic thresholds for survival of this species. In the first cycle, a well-irrigated (C) and a drought-stressed group (S) were formed, and, in the following summer, each group was divided in a well-irrigated and a drought-stressed one (four groups in total). The impact of the multiple drought events was assessed by measuring wood anatomical traits, biomass production, water relations, stem hydraulics and non-structural carbohydrate (NSC) content. We also investigated possible connections between stem hydraulics and carbon dynamics during the second drought event and following re-irrigation. S plants had lower Ψmd and maximum specific hydraulic conductivity (Ks) than C plants in the following growing season. Additionally, aboveground biomass production and leaf number were lower compared to C trees, resulting in lower water consumption. However, PLC was similar between groups, probably due to the production of new functional xylem in spring. The second drought event induced 85% PLC and promoted conversion of starch-to soluble sugars. Nevertheless, 1 week after re-irrigation, no embolism repair was observed and soluble sugars were reconverted to starch. The previous drought cycle did not influence the hydraulic performance during the second drought, and after re-irrigation S plants had 40% higher wood NSC content. Our data suggest that beech cannot recover from high embolism levels but multiple droughts might enhance stem NSC availability.

A method for the quantification of phototropic and gravitropic sensitivities of plants combining an original experimental device with model-assisted phenotyping: Exploratory test of the method on three hardwood tree species.

Perception of inclination in the gravity field and perception of light direction are two important environmental signals implicated in the control of plant shape and habit. However, their quantitative study in light-grown plants remains a challenge. We present a novel method here to determine the sensitivities to gravitropism and phototropism. The method combines: (i) an original experimental device of isotropic light to disentangle gravitropic and phototropic plant responses; and (ii) model-assisted phenotyping using recent models of tropism perception-the AC model for gravitropism alone and the ArC model for gravitropism combined with phototropism. We first assessed the validity of the AC and ArC models on poplar, the classical species model for woody plants. We then tested the method on three woody species contrasted by their habit and tolerance to shade: poplar (Populus tremula*alba), oak (Quercus petraea) and beech (Fagus sylvatica). The method was found to be effective to quantitatively discriminate the tested species by their ratio of tropistic sensitivities. The method thus appears as an interesting tool to quantitatively determine tropistic sensitivities, a prerequisite for assessing the role of tropisms in the control of the variability of the habit and/or tolerance to shade of woody species in the future.