Causes fonctionnelles et structurales du déclin de productivité des fôrets avec l'âge : analyse expérimentale d'une chronoséquence de peuplement de Pin maritime.

Ce travail de thèse avait pour objectif la détermination des processus physiologiques et structuraux à l'origine du déclin de la productivité primaire nette des forêts avec l'âge. Il concerne plus particulièrement les processus régulant les flux et les états hydriques dans la plante au cours de sa croissance. Nous nous sommes attachés à développer une approche expérimentale dans le cadre d'une chronoséquence de Pin maritime dans le Sud-Ouest de la France. La démarche envisagée a consisté en l'étude de plusieurs paramètres écophysiologiques sur une chronoséquence constituée de quatre peuplements équiennes et monospécifiques de Pin maritime âgés de 10 à 91 ans. Nous avons montré que la production primaire nette aérienne diminue de 60 % entre 10 (673 gC m-2 an-1) et 91 ans (275 gC m-2 an-1). Même si les effets d'un accroissement de la respiration autotrophe et d'une dérive avec l'âge de l'allocation de carbone dans la plante n'ont pas fait l'objet d'une approche quantitative, nous avons pu attribuer essentiellement ce déclin à une diminution de la production primaire brute (GPP). Nous avons démontré q'il est expliqué à la fois par une réduction de l'indice foliaire du peuplement et une diminution de la productivité par unité de surface foliaire. Nous avons décrit le fonctionnement hydrique de l'arbre, en phase liquide et en phase vapeur, à l'aide de paramètres intégrés résumant les limitations au transfert liées à la structure hydraulique et au fonctionnement stomatique. La conductivité hydraulique spécifique foliaire (KL) diminue avec l'accroissement en hauteur de l'arbre. Nous avons montré que la plante maintient son potentiel hydrique au-dessus d'une limite critique (- 2 MPa), quelles que soient les conditions de disponibilité en eau du sol et indépendamment de son âge et de sa hauteur. Ce maintien est assuré par deux types d'ajustement. A moyen terme, le rapport surface foliaire / surface d'aubier diminue, mais cette diminution ne compense pas en totalité l'effet de la hauteur sur KL. L'homéostasie hydrique est complétée à court terme par la régulation stomatique. Ces deux mécanismes de compensation participent au déclin de productivité aérienne. Nous montrons par une étude comparative des capacités de photosynthèse suivant l'âge des arbres, qu'une diminution de la vitesse maximale de carboxylation foliaire pourrait s'ajouter aux processus de régulation hydraulique après l'âge de 20 ans. Dans la chronoséquence étudiée, les variations inter-peuplements de la photosynthèse sont bien expliquées par la teneur foliaire en phosphore. Ce résultat complète des observations empiriques qui étayent l'hypothèse que la séquestration des nutriments limitants comme le phosphore dans le cas du Pin maritime pourrait aussi contribuer à l'affaiblissement de la production primaire brute avec l'âge. En plus de leur impact sur l'assimilation du carbone de l'arbre et du couvert, le déclin de l'indice foliaire et de la conductance stomatique diminuent la transpiration du peuplement de 70 % entre les peuplements jeunes et les peuplements âgés. Cette réduction de transpiration des arbres est compensée par une augmentation de l'évapotranspiration du sous-étage : sol et strate herbacée, ce qui ne permet pas de réduire le déficit hydrique annuel du sol. Nous concluons que les facteurs trophiques au sens large expliquent la plus grande part de l'évolution de la productivité d'un peuplement de Pin maritime entre la régénération et la fin de la révolution forestièr

Mechanism of water-stress induced cavitation in conifers: bordered pit structure and function support the hypothesis of seal capillary-seeding

Resistance to water-stress induced cavitation is an important indicator of drought tolerance in woody species and is known to be intimately linked to the anatomy of the xylem. However, the actual mechanical properties of the pit membrane are not well known and the exact mode of air-seeding by which cavitation occurs is still uncertain. We examined the relationship between cavitation resistance and bordered pit structure and function in 40 coniferous species. Xylem pressure inducing 50% loss of hydraulic conductance (P50, a proxy for cavitation resistance) varied widely among species, from −2.9 to −11.3 MPa. The valve effect of the pit membrane, measured as a function of margo flexibility and torus overlap, explained more variation in cavitation-resistance than simple anatomical traits such as pit membrane, pit aperture or torus size. Highly cavitation resistant species exhibited both a high flexibility of the margo and a large overlap between the torus and the pit aperture, allowing the torus to tightly seal the pit aperture. Our results support the hypothesis of seal capillary-seeding as the most likely mode of air-seeding, and suggest that the adhesion of the torus to the pit border may be the main determinant of cavitation resistance in conifers

Adaptation of forest trees to climate change

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Invasive Acer negundo outperforms native species in non-limiting resource environments due to its higher phenotypic plasticity

<p>Abstract</p> <p>Background</p> <p>To identify the determinants of invasiveness, comparisons of traits of invasive and native species are commonly performed. Invasiveness is generally linked to higher values of reproductive, physiological and growth-related traits of the invasives relative to the natives in the introduced range. Phenotypic plasticity of these traits has also been cited to increase the success of invasive species but has been little studied in invasive tree species. In a greenhouse experiment, we compared ecophysiological traits between an invasive species to Europe, <it>Acer negundo</it>, and early- and late-successional co-occurring native species, under different light, nutrient availability and disturbance regimes. We also compared species of the same species groups <it>in </it><it>situ</it>, in riparian forests.</p> <p>Results</p> <p>Under non-limiting resources, <it>A. negundo </it>seedlings showed higher growth rates than the native species. However, <it>A. negundo </it>displayed equivalent or lower photosynthetic capacities and nitrogen content per unit leaf area compared to the native species; these findings were observed both on the seedlings in the greenhouse experiment and on adult trees <it>in situ</it>. These physiological traits were mostly conservative along the different light, nutrient and disturbance environments. Overall, under non-limiting light and nutrient conditions, specific leaf area and total leaf area of <it>A. negundo </it>were substantially larger. The invasive species presented a higher plasticity in allocation to foliage and therefore in growth with increasing nutrient and light availability relative to the native species.</p> <p>Conclusions</p> <p>The higher level of plasticity of the invasive species in foliage allocation in response to light and nutrient availability induced a better growth in non-limiting resource environments. These results give us more elements on the invasiveness of <it>A. negundo </it>and suggest that such behaviour could explain the ability of <it>A. negundo </it>to outperform native tree species, contributes to its spread in European resource-rich riparian forests and impedes its establishment under closed-canopy hardwood forests.</p

Where is the optimum? Predicting the variation of selection along climatic gradients and the adaptive value of plasticity. A case study on tree phenology

International audienceMany theoretical models predict when genetic evolution and phenotypic plasticity allow adaptation to changing environmental conditions. These models generally assume stabilizing selection around some optimal phenotype. We however often ignore how optimal phenotypes change with the environment, which limit our understanding of the adaptive value of phenotypic plasticity. Here, we propose an approach based on our knowledge of the causal relationships between climate, adaptive traits, and fitness to further these questions. This approach relies on a sensitivity analysis of the process-based model Phenofit, which mathematically formalizes these causal relationships, to predict fitness landscapes and optimal budburst dates along elevation gradients in three major European tree species. Variation in the overall shape of the fitness landscape and resulting directional selection gradients were found to be mainly driven by temperature variation. The optimal budburst date was delayed with elevation, while the range of dates allowing high fitness narrowed and the maximal fitness at the optimum decreased. We also found that the plasticity of the budburst date should allow tracking the spatial variation in the optimal date, but with variable mismatch depending on the species, ranging from negligible mismatch in fir, moderate in beech, to large in oak. Phenotypic plasticity would therefore be more adaptive in fir and beech than in oak. In all species, we predicted stronger directional selection for earlier budburst date at higher elevation. The weak selection on budburst date in fir should result in the evolution of negligible genetic divergence, while beech and oak would evolve counter-gradient variation, where genetic and environmental effects are in opposite directions. Our study suggests that theoretical models should consider how whole fitness landscapes change with the environment. The approach introduced here has the potential to be developed for other traits and species to explore how populations will adapt to climate change

Maternal effects shape the seed mycobiome in Quercus petraea

The tree seed mycobiome has received little attention despite its potential role in forest regeneration and health. The aim of the present study was to analyze the processes shaping the composition of seed fungal communities in natural forests as seeds transition from the mother plant to the ground for establishment. We used metabarcoding approaches and confocal microscopy to analyze the fungal communities of seeds collected in the canopy and on the ground in four natural populations of sessile oak (Quercus petraea). Ecological processes shaping the seed mycobiome were inferred using joint species distribution models. Fungi were present in seed internal tissues, including the embryo. The seed mycobiome differed among oak populations and trees within the same population. Its composition was largely influenced by the mother, with weak significant environmental influences. The models also revealed several probable interactions among fungal pathogens and mycoparasites. Our results demonstrate that maternal effects, environmental filtering and biotic interactions all shape the seed mycobiome of sessile oak. They provide a starting point for future research aimed at understanding how maternal genes and environments interact to control the vertical transmission of fungal species that could then influence seed dispersal and germination, and seedling recruitment.Peer reviewe

Desiccation and Mortality Dynamics in Seedlings of Different European Beech (Fagus sylvatica L.) Populations under Extreme Drought Conditions

European beech (Fagus sylvatica L., hereafter beech), one of the major native tree species in Europe, is known to be drought sensitive. Thus, the identification of critical thresholds of drought impact intensity and duration are of high interest for assessing the adaptive potential of European beech to climate change in its native range. In a common garden experiment with one-year-old seedlings originating from central and marginal origins in six European countries (Denmark, Germany, France, Romania, Bosnia-Herzegovina, and Spain), we applied extreme drought stress and observed desiccation and mortality processes among the different populations and related them to plant water status (predawn water potential, ΨPD) and soil hydraulic traits. For the lethal drought assessment, we used a critical threshold of soil water availability that is reached when 50% mortality in seedling populations occurs (LD50SWA). We found significant population differences in LD50SWA (10.5–17.8%), and mortality dynamics that suggest a genetic difference in drought resistance between populations. The LD50SWA values correlate significantly with the mean growing season precipitation at population origins, but not with the geographic margins of beech range. Thus, beech range marginality may be more due to climatic conditions than to geographic range. The outcome of this study suggests the genetic variation has a major influence on the varying adaptive potential of the investigated populations