Déterminisme environnemental de la fécondité du chêne vert

Climate change is placing increasing pressure on forests around the world, and its potential impacts on the reproductive capacity of individuals are yet poorly understood. Many forest tree species reproduce following so-called masting patterns, in which seed production is intermittent and synchronized among individuals, whose drivers need to be identified in order to better understand the impact of climate on the fecundity of forest trees. The general objective of this thesis is to deepen our knowledge of the environmental drivers of the fecundity of forest trees and their interactions, by focusing on the holm oak, a widespread species in the Mediterranean basin. It also aimed to determine how fecundity could be impacted by climate change, and to develop a process-based model capable of predicting this impact. This thesis work combines analyses of long-term monitoring in a holm oak forest exposed to a rainfall exclusion experiment, experimental manipulations of source-sink interactions involved in the resource allocation to reproduction, and modelling. I have shown that reproduction was strongly regulated by weather conditions during the reproductive cycle and in particular by the water availability in summer-autumn, and to a lesser extent by the availability of carbon and nutrients, as well as by competition between fruits and new buds. My experimental results show that the allocation of resources to reproduction could be modified in case of resource limitation due to defoliation; and that developing fruits have a negative impact on the dynamics of nitrogen and zinc reserves in branches, as well as on the production of female flowers the following year. Long-term observations show that multi-year rainfall reduction induce a decrease in the number of fruits produced which is additional to the interannual variability and that tree reproduction does not acclimate to drought. Finally, within the PHENOFIT model, I have developed a fecundity model based on the assumptions and results from my work and from the literature.Le changement climatique exerce une pression croissante sur les forêts du monde entier, et ses impacts potentiels sur la fécondité des arbres sont encore mal compris. De nombreuses espèces d’arbres forestiers se reproduisent suivant des patrons de fructification intermittente et synchronisée entre les individus, dits de masting, dont les déterminants complexes doivent être identifiés pour comprendre l’effet du climat sur la fécondité des arbres forestiers. Cette thèse a comme objectif général d’approfondir notre connaissance des déterminants environnementaux de la fécondité des arbres forestiers et de leurs interactions, en se focalisant sur le chêne vert, une espèce très répandue en milieu méditerranéen. Elle a aussi pour objectif de déterminer comment la fécondité du chêne vert pourrait être affectée par le changement climatique, et de développer un modèle basé sur les processus capable de prédire cet impact. Ce travail de thèse a donc combiné des analyses de suivis à long terme dans une forêt de chêne vert soumise à une manipulation des précipitations, des expérimentations de manipulation des relations sources-puits pour l’allocation des ressources à la reproduction à l’échelle individuelle, et de la modélisation. J’ai montré que la reproduction était fortement régulée par les conditions météorologiques durant le cycle reproducteur, et en particulier par la disponibilité en eau en été-automne, et dans une moindre mesure par la disponibilité en carbone et nutriments ainsi que par la compétition entre fruits et nouveaux bourgeons. Mes résultats expérimentaux montrent que l’allocation de ressources à la reproduction peut être modifiée en cas de limitation des ressources par une défoliation ; et que les fruits en développement ont un impact négatif sur les dynamiques des réserves en azote et en zinc dans les branches ainsi que sur la production des fleurs femelles l’année suivante. Les observations en forêt indiquent qu’une réduction continue des précipitations sur plusieurs années induit une diminution du nombre de fruits produits, indépendamment des effets de la variabilité interannuelle et sans que les arbres semblent être capables de s’y acclimater. Enfin, j’ai développé, au sein du modèle de fonctionnement de l’arbre PHENOFIT, un modèle de fécondité se basant sur les hypothèses et résultats issus de mes travaux et de la littérature

S’inscrire dans un domaine d’activité à la gouvernance en construction: Les bases d’une stratégie territoriale dans le champ des mesures compensatoires environnementales à l’Office National des Forêts

Pour répondre à des enjeux de gouvernance, financiers et de biodiversité, l’ONF souhaite être davantage identifié comme opérateur de la compensation en région méditerranéenne, disposant pour cela d’une forte légitimité technique et territoriale. Afin de pouvoir s’affirmer dans ce domaine, l’ONF devra clarifier sa gouvernance interne du sujet et développer les outils juridiques, financiers et fonciers qui lui font encore défaut. Enfin, il pourra construire une stratégie territoriale sur laquelle communiquer pour regagner en visibilité et en lisibilité auprès des autres acteurs locaux de la compensation

Divergent selection on flowering phenology but not on floral morphology between two closely related orchids

Closely related species often differ in traits that influence reproductive success, suggesting that divergent selection on such traits contribute to the maintenance of species boundaries. Gymnadenia conopsea ss. and Gymnadenia densiflora are two closely related, perennial orchid species that differ in (a) floral traits important for pollination, including flowering phenology, floral display, and spur length, and (b) dominant pollinators. If plant–pollinator interactions contribute to the maintenance of trait differences between these two taxa, we expect current divergent selection on flowering phenology and floral morphology between the two species. We quantified phenotypic selection via female fitness in one year on flowering start, three floral display traits (plant height, number of flowers, and corolla size) and spur length, in six populations of G. conopsea s.s. and in four populations of G. densiflora. There was indication of divergent selection on flowering start in the expected direction, with selection for earlier flowering in two populations of the early‐flowering G. conopsea s.s. and for later flowering in one population of the late‐flowering G. densiflora. No divergent selection on floral morphology was detected, and there was no significant stabilizing selection on any trait in the two species. The results suggest ongoing adaptive differentiation of flowering phenology, strengthening this premating reproductive barrier between the two species. Synthesis: This study is among the first to test whether divergent selection on floral traits contribute to the maintenance of species differences between closely related plants. Phenological isolation confers a substantial potential for reproductive isolation, and divergent selection on flowering time can thus greatly influence reproductive isolation and adaptive differentiation

Quelle interministérialité pour le Conseil National de l'Alimentation ?: Vers une gestion plus efficace et ciblée de l’interministérialité pour améliorer l’impact et la reconnaissance du CNA

Cette analyse s'inscrit dans un contexte de prise de poste récente de la secrétaire interministérielle du CNA et du démarrage d'une nouvelle mandature (2016-2019). Ce projet doit permettre au commanditaire de mieux appréhender les enjeux de la dimension interministérielle du CNA et de son fonctionnement. Nous proposons des préconisations et des outils pour améliorer la coordination interministérielle dans un futur proche

Retour d'expérience sur 15 ans de suivi de la diversité spécifique forestière en France

International audienceTrends in species diversity represent an important challenge to be addressed by forest monitoring in response to environmental changes, of which climate is critical. However assessing such changes faces methodological issues. Useful lessons can be drawn from practical experience. In France, forest ecosystems have been intensively monitored since 1992 on 102 permanent plots (RENECOFOR network), in the framework of the ICP-Forests Pan-European monitoring program. Among ecosystem parameters, plant composition has been recorded every five years since 1995, inside and outside the fenced central area of each plot. Fungi species (basidiomycota) were inventoried on 51 plots. Special attention has been paid to assessment quality: inventories by expert taxonomists, assessments on permanently delimited areas, use of harmonized taxonomy, observer effects quantified with intercalibration exercises and control assessments. For both plant and fungi surveys, detecting and identifying species are affected by important observer effects. In average per census, 19% of plant species are undetected by each observer team compared to consensus inventory. Also the species richness continuously increases with the number of inventories over time. Therefore species richness hardly appears as a relevant indicator to be monitored. After 15 years, temporal changes in plant composition and indicator values were of weak magnitude, not unidirectional and inconsistent with significant changes measured in soil chemistry (pH, C/N). The most significant temporal change observed was the diverging trajectories of plant communities between fenced and unfenced areas, indicating the prominent role of large wild mammals. Fungi inventories revealed significant spatial patterns of communities, responding to environmental factors. Results were used to evaluate the sampling effort needed for detecting further significant response to each factor. What trends can be actually detected in biodiversity? This should be carefully considered by stakeholders, regarding time scales, magnitude of changes and relevant response indicators

Resource manipulation through experimental defoliation has legacy effects on allocation to reproductive and vegetative organs in Quercus ilex

International audienceBackground and Aims In plants, high costs of reproduction during some years can induce trade-offs in resourceallocation with other functions such as growth, survival and resistance against herbivores or extreme abioticconditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortageat particular moments of the reproductive cycle. Because plants are modular organisms, strategies for resource allocationto reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was totest how allocation to reproduction was impacted by resource limitation.• Methods We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercusilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal onfruit development (survival, growth and germination potential) and on the production of vegetative and reproductiveorgans the year following defoliation.• Key Results We found that defoliation had little impact on fruit development. Fruit survival was not affected bythe intense defoliation treatment, but was reduced by moderate defoliation, and this result could not be explainedby an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germinationsuccess. However, in the following spring defoliated branches produced fewer shoots and compensated for leafloss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocationto reproduction the following season but did not affect sex ratio.• Conclusions Our results support the idea of a regulation of resource allocation to reproduction beyond theshoot scale. Defoliation had larger legacy effects than immediate effects

Reproduction alternation in trees: testing the resource depletion hypothesis using experimental fruit removal in Quercus ilex

International audienceThe keystones of resource budget models to explain mast seeding are that fruit production depletes tree stored resources, which become subsequently limiting to flower production the following year. These two hypotheses have, however, rarely been tested in forest trees. Using a fruit removal experiment, we tested whether preventing fruit development would increase nutrient and carbohydrates storage and modify allocation to reproduction and vegetative growth the following year. We removed all the fruits from nine adult Quercus ilex L. trees shortly after fruit set and compared, with nine control trees, the concentrations of nitrogen (N), phosphorus (P), zinc (Zn), potassium (K) and starch in leaves, twigs and trunk before, during and after the development of female flowers and fruits. The following year, we measured the production of vegetative and reproductive organs as well as their location on the new spring shoots. Fruit removal prevented the depletion of N and Zn in leaves during fruit growth. It also modified the seasonal dynamics in Zn, K and starch in twigs, but had no effect on reserves stored in the trunk. Fruit removal increased the production of female flowers and leaves the following year, and decreased the production of male flowers. Our results show that resource depletion operates differently for male and female flowering, because the timing of organ formation and the positioning of flowers in shoot architecture differ between male and female flowers. Our results suggest that N and Zn availability constrain flower production in Q. ilex, but also that other regulatory pathways might be involved. They strongly encourage further experiments manipulating fruit development over multiple years to describe the causal relationships between variations in resource storage and/or uptake, and male and female flower production in masting species

Resource manipulation through experimental defoliation has legacy effects on allocation to reproductive and vegetative organs in Quercus ilex

International audienceAbstract Background and Aims In plants, high costs of reproduction during some years can induce trade-offs in resource allocation with other functions such as growth, survival and resistance against herbivores or extreme abiotic conditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortage at particular moments of the reproductive cycle. Because plants are modular organisms, strategies for resource allocation to reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was to test how allocation to reproduction was impacted by resource limitation. Methods We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercus ilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal on fruit development (survival, growth and germination potential) and on the production of vegetative and reproductive organs the year following defoliation. Key Results We found that defoliation had little impact on fruit development. Fruit survival was not affected by the intense defoliation treatment, but was reduced by moderate defoliation, and this result could not be explained by an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germination success. However, in the following spring defoliated branches produced fewer shoots and compensated for leaf loss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocation to reproduction the following season but did not affect sex ratio. Conclusions Our results support the idea of a regulation of resource allocation to reproduction beyond the shoot scale. Defoliation had larger legacy effects than immediate effects

Retour d'expérience sur 15 ans de suivi de la diversité spécifique forestière en France

International audienceTrends in species diversity represent an important challenge to be addressed by forest monitoring in response to environmental changes, of which climate is critical. However assessing such changes faces methodological issues. Useful lessons can be drawn from practical experience. In France, forest ecosystems have been intensively monitored since 1992 on 102 permanent plots (RENECOFOR network), in the framework of the ICP-Forests Pan-European monitoring program. Among ecosystem parameters, plant composition has been recorded every five years since 1995, inside and outside the fenced central area of each plot. Fungi species (basidiomycota) were inventoried on 51 plots. Special attention has been paid to assessment quality: inventories by expert taxonomists, assessments on permanently delimited areas, use of harmonized taxonomy, observer effects quantified with intercalibration exercises and control assessments. For both plant and fungi surveys, detecting and identifying species are affected by important observer effects. In average per census, 19% of plant species are undetected by each observer team compared to consensus inventory. Also the species richness continuously increases with the number of inventories over time. Therefore species richness hardly appears as a relevant indicator to be monitored. After 15 years, temporal changes in plant composition and indicator values were of weak magnitude, not unidirectional and inconsistent with significant changes measured in soil chemistry (pH, C/N). The most significant temporal change observed was the diverging trajectories of plant communities between fenced and unfenced areas, indicating the prominent role of large wild mammals. Fungi inventories revealed significant spatial patterns of communities, responding to environmental factors. Results were used to evaluate the sampling effort needed for detecting further significant response to each factor. What trends can be actually detected in biodiversity? This should be carefully considered by stakeholders, regarding time scales, magnitude of changes and relevant response indicators

Holm oak fecundity does not acclimate to a drier world

International audience(1) Climate change might impact tree fecundity by altering the relative influences of meteorological and physiological drivers, and by modifying resource investment in reproduction. (2) Using a 13-year monitoring of Quercus ilex reproduction in a rainfall exclusion experiment, we analysed the interactive effects of long-term increased aridity and other environmental drivers on the inter-annual variation of fecundity (male flower biomass, number of initiated and mature fruits).(3) Summer-autumn water stress was the main driver of fruit abortion during fruit growth. Rainfall exclusion treatment strongly reduced the number of initiated and mature fruits, even in masting years, and did not increase fruit tolerance to severe drought. Conversely, the relative contribution of the meteorological and physiological drivers, and the inter-annual variability of fruit production were not modified by rainfall exclusion. (4) Rather than inducing an acclimation of tree fecundity to water limitation, increased aridity impacted it negatively through both lower fruit initiation due to changes in resource allocation, and more severe water and resource limitations during fruit growth. Long-term increased aridity affected tree reproduction beyond what is expected from the current response to inter-annual drought variations, suggesting that natural regeneration of holm oak forest could be jeopardized in the future