Drought-induced decline and mortality of silver fir differ among three sites in Southern France

Abstract : Context : In the Mediterranean area, numerous decline and mortality processes have been reported during recent decades, affecting forest dynamics. They are likely due to increases in summer drought severity and therefore especially affect drought-sensitive species, such as silver fir (Abies alba Mill.). Aims and methods : To understand the relationships between tree growth, crown condition and mortality probability, radial growth trends of healthy, declining (showing crown damages) and dead trees were compared using tree-ring analysis. Factors involved in determining this mortality were also examined at the plot and tree level using altitudinal gradients on three contrasted sites in southeastern France. Results : Individuals with higher inter-annual variability in growth were more prone to dieback. At two sites, dead trees displayed lower growth rates over their entire lifetime, while, on the last site, their juvenile growth rate was higher. Trees with crown damage had higher growth rates than healthy trees on one site, and their radial growth trends over time always differed from those of dead trees. Mortality and crown damage were little related to altitude, but strongly differed between sites and among plots underlining the importance of local edaphic and topographic conditions. Conclusion : These results suggest that the relationships among mortality probability, crown condition and growth can differ among sites, and highlight the impact of soil conditions and the need to assess them in tree mortality studies

Interactions hôtes-parasites en écosystèmes forestiers contrastés : le cas des scolytes (Coleoptera : Curculionidae) en région méditerranéenne

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

Plant-insect interactions in two contrasting forest ecosystems : bark beetles (Coleoptera Curculionidae) in Mediterranean area

Les écosystèmes forestiers méditerranéens présentent des espèces d'arbres vivant en limite sud de leur aire de répartition sensibles aux stress climatiques et des espèces vivant au coeur de leur aire de répartition et a priori adaptées à ces mêmes stress. Ces contrastes interrogent 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 constituent un bon modèle pour l'étude de ces interactions hôte-parasite car ils alternent des phases endémiques et épidémiques. Nous avons développé une approche pluridisciplinaire couplant la caractérisation de traits écophysiologiques et dendrométriques impliqués dans la résistance des populations d'arbres aux attaques de scolytes avec la caractérisation de facteurs écologiques clés pour la démographie des scolytes. Chez le pin d'Alep, T. destruens est présent à un niveau endémique 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. 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, 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. Ces deux écosystèmes forestiers permettent de mieux appréhender les mécanismes impliqués dans les interactions plantes-insectes dans un contexte de changement global.Mediterranean forest ecosystems have both trees living in their southern range and being sensitive to climatic stress and tree species living in the core of their range and being non sensitive to the same stress. These contrasts in tree populations questioned about potential contrasts in their interrelationships with their parasites and on the impact of that parasites have on their host in turn. Bark beetles are a relevant model to study such host-parasite interactions because they alternate endemic and epidemic phases. We developed a multidisciplinary approach combining the characterization of ecophysiological and dendrometrical traits involved in resistance of tree populations to bark beetle attacks with this assessment of key ecological factors involved in bark beetles demography. We focused on the two following contrasted Mediterranean ecosystems in South-Eastern France: a healthy Aleppo pine interacting with a bark beetle population at an endemic state and a declining population of the Silver fir interacting with a bark beetle community at an epidemic state. Our results suggested that this Aleppo pine population, the pine shoot beetle T. destruens showed low abundance levels. All attacks remained in patches formed by dead trees following the attacks. T. destruens dispersal strategies seem to favor the exploitation of a scarce resource. On silver fir, we showed a bark beetle community attacking trees already weakened by repeated droughts and with poor health status and low growth. The study of these two contrasted forest ecosystems allowed us to shed some critical light on the mechanisms involved in plant-insect interactions in the context of global change

Drought-induced decline and mortality of silver fir differ among three sites in Southern France

International audience& Context In the Mediterranean area, numerous decline and mortality processes have been reported during recent decades, affecting forest dynamics. They are likely due to increases in summer drought severity and therefore especially affect drought-sensitive species, such as silver fir (Abies alba Mill.). & Aims and methods To understand the relationships between tree growth, crown condition and mortality probability, radial growth trends of healthy, declining (showing crown damages) and dead trees were compared using tree-ring analysis. Factors involved in determining this mortality were also examined at the plot and tree level using altitudinal gradients on three contrasted sites in southeastern France. & Results Individuals with higher inter-annual variability in growth were more prone to dieback. At two sites, dead trees displayed lower growth rates over their entire lifetime, while, on the last site, their juvenile growth rate was higher. Trees with crown damage had higher growth rates than healthy trees on one site, and their radial growth trends over time always differed from those of dead trees. Mortality and crown damage were little related to altitude, but strongly differed between sites and among plots underlining the importance of local edaphic and topographic conditions. & Conclusion These results suggest that the relationships among mortality probability, crown condition and growth can differ among sites, and highlight the impact of soil con-ditions and the need to assess them in tree mortality studies

Individual vulnerability factors of Silver fir (Abies alba Mill.) to parasitism by two contrasting biotic agents: mistletoe (Viscum album L. ssp. abietis) and bark beetles (Coleoptera: Curculionidae: Scolytinae) during a decline process

International audience• ContextIn recent decades, there have been increasing reports of forest decline, especially in Mediterranean forest ecosystems. Decline in tree vitality is usually due to complex interactions between abiotic factors and biotic agents that attack weakened trees.• Aims and methodsEstimating dendrometrical characteristics [basal area increment (BAI), age at DBH from tree ring counting, social status, height, and diameter], tree health status, and a competition index, we investigated the individual vulnerability of a French declining silver fir forest to both mistletoe (Viscum album L. ssp. abietis) and bark beetles (Pityophthorus pityographus Ratz., Pityokteines vorontzovi Jac., and Pityokteines spinidens Reitt.).• ResultsBAI was negatively correlated with both mistletoe infection (via mistletoe biomass) and bark beetle attack (number of insects per square meter), but there was evidence of divergence in tree choice between two groups of parasites. Mistletoe preferentially infected isolated and dominant trees that showed higher past growth rates than non-infected ones. Conversely, bark beetles mainly attacked defoliated and preferably declining trees with diameter (DBH) lower than 44.5 cm and slower past growth.• ConclusionWhile successive severe drought periods are thought to greatly weaken southern silver fir populations, mistletoe and bark beetles may contribute actively to their decline processes as inciting and contributing factors, respectively

Distribution of endemic bark beetle attacks and their physiological consequences on Pinus halepensis

One factor that determines a tree’s resilience capacity to drought is its level of interaction with aggravating biotic factors, such as bark beetles, which in turn depends on whether insect populations are at an endemic or epidemic stage. In a Mediterranean natural forest ecosystem, we investigated the ecophysiological characteristics of trees which (i) predispose Pinus halepensis to attacks of endemic populations of bark beetles (Tomicus destruens) and (ii) that arise as a result of these attacks. The annual life cycle of Tomicus destruens features an initial dispersal phase of sexually mature adults towards pine trunks for the purposes of brood production, and a second dispersal phase of callow adults from trunks to pine shoots for sexual maturation. During a three-year survey of endemic bark beetle attacks in a stand of 178 trees, we found that trees targeted for trunk attacks showed a more aggregated distribution pattern than those targeted for shoot attacks. Moreover, shoot-attacked trees were clearly larger and taller, had lower wood density, and were less exposed to competition than non-attacked trees. No differences were found between the characteristics of trunk-attacked trees and non-attacked ones. We found crown defoliation, lower primary growth and alteration of trunk non-structural carbon in phloem in the year following trunk attacks, although this did not affect vulnerability to cavitation of the vascular system, secondary growth, wood density, and xylem starch. Conversely, the health status and physiological variables of the shoot-attacked trees were not altered, which may be explained by their initial vigor. We conclude that the distribution of Tomicus destruens attacks at the plot level was independent of the ecophysiological traits of the host trees. It was mainly determined by the dispersal strategy of the endemic bark beetle population especially during brood production as only trunk attacks significantly weakened the trees.INITIATIVE D'EXCELLENCE AIX MARSEILLE UNIVERSIT

Considering evolutionary processes in adaptive forestry

International audience& Context Managing forests under climate change requires adaptation. The adaptive capacity of forest tree populations is huge but not limitless. Integrating evolutionary consider-ations into adaptive forestry practice will enhance the ca-pacity of managed forests to respond to climate-driven changes. & Aims Focusing on natural regeneration systems, we pro-pose a general framework that can be used in various and complex local situations by forest managers, in combination with their own expertise, to integrate evolutionary consid-erations into decision making for the emergence of an evolution-oriented forestry. & Methods We develop a simple process-based analytical grid, using few processes and parameters, to analyse the impact of forestry practice on the evolution and evolvability of tree populations. & Results We review qualitative and, whenever possible, quantitative expectations on the intensity of evolutionary drivers in forest trees. Then, we review the effects of actual and potential forestry practice on the evolutionary processes. We illustrate the complexity of interactions in two study cases: the evolutionary consequences for forest trees of biotic in-teractions and of highly heterogeneous environment. & Conclusion Evolution-oriented forestry may contribute adapting forests to climate change. It requires combining short-term and long-term objectives. We propose future lines of research and experimentation

Pre- and post-drought conditions drive resilience of Pinus halepensis across its distribution range

International audienceSevere droughts limit tree growth and forest productivity worldwide, a phenomenon which is expected to aggravate over the next decades. However, how drought intensity and climatic conditions before and after drought events modulate tree growth resilience remains unclear, especially when considering the range-wide phenotypic variability of a tree species.We gathered 4632 Aleppo pine (Pinus halepensis Mill.) tree-ring width series from 281 sites located in 11 countries across the Mediterranean basin, representing the entire geographic and bioclimatic range of the species. For each site and year of the period 1950–2020, we quantified tree-growth resilience and its two components, resistance and recovery, to account for the impact of drought and the capacity to recover from it. Relative drought intensity of each year was assessed using SPEI (Standardized Precipitation Evapotranspiration Index), a climatic water deficit index. Generalized additive mixed models were used to explore the non-linear relationships between resilience and its two components and drought intensity, preceding and following years climatic conditions.We found that P. halepensis radial growth was highly dependent on the SPEI from September of the previous year to June of the current year. Trees growing under more arid bioclimates showed higher inter-annual growth variability and were more sensitive to drought, resulting in an increased response magnitude to pre-, during and post-drought conditions. In contrast to our expectations, drought intensity only slightly affected resilience, which was rather negatively affected by favorable preceding conditions and improved by favorable following conditions.Resilience and its components are highly dependent on preceding and following years climatic conditions, which should always be taken into account when studying growth response to drought. With the observed and predicted increase in drought frequency, duration and intensity, favorable conditions following drought episodes may become rare, thus threatening the future acclimation capacity of P. halepensis in its current distribution