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Bark is a structure involved in multiple physiological functions, but which has been traditionally associated with protection against fire. Thus, little is known about how the morpho-anatomical variations of this structure are related to different ecological pressures, especially in tropical savanna species, which are commonly subjected to frequent fire and drought events. Here we evaluated how the structural and functional variations of bark are related to the processes of resilience and resistance to fire, as well as transport and storage of water in 31 native species from the Brazilian Cerrado. Because of their thick bark, none of the trees analyzed were top-killed after a severe fire event. The structural and functional variations of the bark were also associated with water storage and transport, functions related to properties of the inner bark. In fact, species with a thicker and less dense inner bark were the ones that had the highest water contents in the wood, bark, and leaves. Lower bark density was also related to higher stem hydraulic conductivity, carbon assimilation, and growth. Overall, we provide strong evidence that in addition to protection from fire, the relative investment in bark also reflects different strategies of water use and conservation among many Cerrado tree species.

Tropical forests store large amounts of biomass despite they generally grow in nutrient-poor soils, suggesting that the role of soil characteristics in the structure and dynamics of tropical forests is complex. We used data for >34 000 trees from several permanent plots in French Guiana to investigate if soil characteristics could predict the structure (tree diameter, density and aboveground biomass), and dynamics (growth, mortality, aboveground wood productivity) of nutrient-poor tropical forests. Most variables did not covary with site-level changes in soil nutrient content, indicating that nutrient-cycling mechanisms other than the direct absorption from soil (e.g. the nutrient uptake from litter, the resorption, or the storage of nutrients in the biomass), may strongly control forest structure and dynamics. Ecosystem-level adaptations to low soil nutrient availability and long-term low levels of disturbance may help to account for the lower productivity and higher accumulation of biomass in nutrient-poor forests compared to nutrient-richer forests.

International audience; Frequency and intensity of heat waves and drought events are expected to increase in Europe due to climate change. European beech (Fagus sylvatica L.) is one of the most important native tree species in Europe. Beech populations originating throughout its native range were selected for common-garden experiments with the aim to determine whether there are functional variations in drought stress responses among different populations. One-year old seedlings from four to seven beech populations were grown and drought treated in a greenhouse, replicating the experiment at two contrasting sites, in Italy (Mediterranean mountains) and Germany (Central Europe). Experimental findings indicated that: (1) drought (water stress) mainly affected gas exchange describing a critical threshold of drought response between 30 and 26% SWA for photosynthetic rate and C-i/C-a, respectively; (2) the C-i to C-a ratio increased substantially with severe water stress suggesting a stable instantaneous water use efficiency and an efficient regulation capacity of water balance achieved by a tight stomatal control; (3) there was a different response to water stress among the considered beech populations, differently combining traits, although there was not a well-defined variability in drought tolerance. A combined analysis of functional and structural traits for detecting stress signals in beech seedlings is suggested to assess plant performance under limiting moisture conditions and, consequently, to estimate evolutionary potential of beech under a changing environmental scenario.

Les écosystèmes forestiers méditerranéens sont un cadre d’étude particulièrement intéressant car ils présentent à la fois des espèces d'arbres vivant en limite sud de leur aire de répartition sensibles aux stress climatiques et des espèces d'arbres vivant au coeur de leur aire de répartition et a priori adaptés à ces mêmes stress. L’existence de tels contrastes dans des populations d’hôtes interroge sur la diversité potentielle des mécanismes régulant la dynamique des populations de leurs parasites, et en retour l’impact des parasites sur la dynamique de leurs hôtes. Les scolytes (Coleoptera : Curculionidae : Scolytinae) constituent un bon modèle pour l'étude de ces interactions hôte-parasite dans le contexte des changements environnementaux car ils alternent des phases endémiques et épidémiques qui dépendent de deux facteurs majeurs : le niveau de disponibilité en arbres affaiblis par un stress biotique et/ou abiotique et le niveau de population de ces insectes. Nous avons développé une approche pluridisciplinaire couplant la caractérisation de traits écophysiologiques et dendrométriques impliqués dans la sensibilité/résistance des populations d’arbres aux attaques de scolytes avec la caractérisation de facteurs écologiques clés pour la démographie des scolytes (traits d'histoire de vie des scolytes et stratégies d'exploitation de l’hôte). Chez le pin d'Alep (Pinus halepensis Mill.), l’Hylésine destructeur T. destruens est présent à un niveau endémique (faibles niveaux de compétition intra- et interspécifique et de prédation) mais des attaques subsistent dans des patchs d'arbres formés par des arbres attaqués et morts suite aux attaques, des arbres attaqués qui ont résisté et des arbres non attaqués. A stress hydrique égal, les arbres résistant aux attaques présentent une plus forte vulnérabilité à la cavitation et une croissance significativement plus faible pour les années sèches précédents l’attaque. Des stratégies de dispersion locale différentes selon les deux grandes phases du cycle de T. destruens semblent favoriser l’exploitation d’une ressource peu abondante. Chez le sapin pectiné (Abies alba Mill.), nous avons montré qu’une communauté abondante de scolytes s’attaque à des arbres déjà affaiblis par des sécheresses répétées et présentant un mauvais état sanitaire et une croissance en baisse, indépendamment de leur taille. L'étude de ces deux écosystèmes forestiers contrastés nous a permis de mieux appréhender les mécanismes impliqués dans les interactions plantes-insectes dans un contexte de changement global, avec l'objectif d'aboutir à plus long terme à des modèles prédisant la réponse conjointe des communautés d'insectes et de la dynamique forestière aux changements climatiques.

This two-part session focused on: 1) ecophysiological studies aiming at understanding the adaptive value of xylem and leaf function, including approaches linking xylem traits such as hydraulic capacitance and parameters of hydraulic vulnerability across climatic gradients; 2) genetic studies aimed at estimating the potential for genetic adaptation to climate change in experimental trials as well as in natural populations; 3) investigating how specific functional traits may be fixed or adaptive within a species and related to climatic distribution more broadly; and 4) dendroecological studies aiming at retrospectively investigating tree responses to climate change. Fifteen oral presentations based on reviews and recent studies of novel and applied research were included in the session related to drought responses and adaptation. These were studied within a number of disciplines: ecophysiology, dendroecology, remote sensing and genetics. Some studies were integrated at the genus level (multi-species studies of eucalypts), genetic level (clone and provenance studies of eucalypts, lodgepole pine and radiata pine) or bioclimatic/environmental level (multisite studies of lodgepole pine and eucalypts). Studies employed drought treatments (piñon pine, juniper, radiata pine, and eucalypts) and inter-annual variation of response to drought by direct and retrospective approaches (oak, eucalypts, Siberian pine and fir). In addition there was one multi-generation study on radiata pine, and age effects were indirectly represented by height gradients of Douglas fir. Physiological studies of response to drought were presented at different levels, including: hydraulic vulnerability of leaf and branch xylem with reference also to carbon depletion and starvation. Several studies discussed interspecific variation in functional drought adaptation by comparing seasonal and controlled (water-exclusion or water-deficit) conditions. There was evidence of common plasticity in seasonal adaptation to declining water potential between eucalyptus species from different climatic zones. Other studies suggested within-species variation for drought-resistance strategies (lodgepole pine). In addition, fast natural selection (micro evolution) was apparent in drought resistance in radiata pine. In ecosystems, genus, and species, evidence of variation in drought resistance strategies were related to different levels and components of hydraulic vulnerability from leaves to xylem, with variable relative weights. These levels also varied with environment: between growing season, between sites, and between fertilization treatments. Reliable proxies or methodological improvement for the measurement of some physiological traits including resistance to cavitation (hydraulic vulnerability or hydraulic capacitance) and stomatal regulation of water potential should be investigated to develop large-scale studies involving many species and replicates and for better comparison of results from independent studies.

We studied the growth and photosynthetic capacity of Berberis darwinii shrub growing under different light conditions in their native area of Argentina to test if plant physiology differs from invaded area (using studies carried out in New Zealand). In its native range B. darwinii grows under different light conditions by adjusting shoot and leaf morphology and physiology. Plants of B. darwinii growing under the same light environments show similar physiology in native and invasive ranges. Therefore, intra-specific variations of the functional traits in native area do not condition successful invasiveness., Invasive species' success may depend on ecophysiological attributes present in their native area or those derived from changes that took place in the invaded environment. We studied the growth and photosynthetic capacity of Berberis darwinii shrubs growing under different light conditions (gap, forest edge and below the canopy) in their native area of Patagonia, Argentina. Leaf photosynthesis results determined in the native area were discussed in relation to information provided by studies carried out under the same light conditions in an invaded area in New Zealand. Shoot elongation, leaf production, stem and leaf biomass per shoot, and specific leaf area (SLA, cm2 g−1) were determined in five adult plants, randomly selected in each of the three light conditions at two forest sites. Net photosynthesis as a function of PPFD (photosynthetic photon flux density), stomatal conductance (gs), maximum light-saturated photosynthesis rate (Pmax), Pmass (on mass bases) and water-use efficiency (WUEi) were determined in plants of one site. We predicted that functional traits would differ between populations of native and invasive ranges. In their native area, plants growing under the canopy produced the longest shoots and had the lowest values for shoot emergence and foliar biomass per shoot, while their SLA was higher than gap and forest edge plants. Leaf number and stem biomass per shoot were independent of light differences. Leaves of gap plants showed higher Pmax, Pmass and gs but lower WUEi than plants growing at the forest edge. In its native range B. darwinii grows under different light conditions by adjusting shoot and leaf morphology and physiology. Plants of B. darwinii growing under the same light conditions show similar physiology in native and invasive ranges. This means that for B. darwinii, intra-specific variation of the functional traits studied here does not condition successful spread in new areas.

One challenge of evolutionary ecology is to predict the rate and mechanisms of population adaptation to environmental variations. The variations in most life-history traits are shaped both by individual genotypic and environmental variation. Forest trees exhibit high levels of genetic diversity, large population sizes, and gene flow, and they also show a high level of plasticity for life-history traits. We developed a new Physio-Demo-Genetics model (denoted PDG) coupling (1) a physiological module simulating individual tree responses to the environment; (2) a demographic module simulating tree survival, reproduction and pollen and seed dispersal; and (3) a quantitative genetics module controlling the heritability of key life history traits. We used this model to investigate the plastic and genetic components of the variations of the timing of budburst along an elevational gradient of Fagus sylvatica (the European beech). We used a repeated five years climatic sequence to show that five generations of natural selection were sufficient to develop non-monotonic genetic differentiation in the timing of budburst along the local climatic gradient but also that plastic variation among different elevations and years was higher than genetic variation. PDG complements theoretical models and provides testable predictions to understand the adaptive potential of tree populations

Etude comparée du comportement hydrique de quatre provenances de sapin pectine (Abies alba Mill.)