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Global environmental change represents one of the greatest challenges facing forest resource managers today. The uncertainty and variability of potential future impacts related to shifting climatic and disturbance regimes on forest systems has led resource managers to seek out alternative management approaches to sustain the long-term delivery of forest ecosystem services. To this end, forest managers have begun incorporating adaptation strategies into resource planning and are increasingly utilizing the outcomes of forest landscape simulation and climate envelope models to guide decisions regarding potential strategies to employ. These tools can be used alongside traditional methods to assist managers in understanding the potential long-term effects of management and climate on future forest composition and productivity. This study used a spatially explicit forest landscape simulation model, Landis-II, to examine and evaluate a range of long-term effects of current and alternative forest management under three projected climate scenarios within a 50,000-hectare forested landscape in southeastern Vermont, USA. Multiple scenarios were examined within this mixed ownership landscape, allowing for an evaluation of the influence of management and climate on future forest conditions in the region. These simulations indicate that land-use legacies and the inertia associated with long-term forest successional trajectories are projected to be an important driver of future forest composition and biomass conditions for the next 100 years. Nevertheless, climate is projected to have a greater influence on species composition and aboveground biomass over the next two centuries, with forests containing a greater abundance of species from more southerly regions and lower levels of aboveground biomass, resulting in shifts in the future provisioning of ecosystem services. Key words: Vermont, USA; climate change; forests; LANDIS-II; forest adaptation; forest management; above ground biomass; landscape inertia; land use recovery; forest composition

The impacts of possum herbivory were assessed on 713 individual plants of the nationally endangered Pittosporum patulum and 115 individual plants of the threatened ('Gradual Decline') mistletoes (Alepis flavida & Peraxilla tetrapetala) over three years in the Temple and Huxley river valleys in the Ohau catchment, South Island New Zealand. Statistical models were used to test the influence of several explanatory variables on the probability of survival and growth rates of these rare palatable plants. Increasing defoliation levels were the most significant predictors of mortality for both P. patulum and mistletoe, and reduced growth rates in P. patulum. Mortality of P. patulum increased with plant size although smaller plants were also affected by mortality. P. patulum mortality was greater for non-forest habitats, and growth rates were greatest on warm aspects. Mistletoe mortality increased with warm aspect, steeper slopes and at lower altitudes. Plant survival and growth increased in the Temple valley following possum control. Although indicative of a positive treatment effect, other factors may have also influenced this result. Possums are the major herbivore responsible for the widespread decline of P. patulum and beech forest mistletoes throughout New Zealand, although the autecology of P. patulum predisposes it toward extinction more so than mistletoe. Monitoring in conjunction with possum control operations provide good opportunities for understanding the impacts of possums on palatable plants and ecosystems. Assessing defoliation levels on indicator species like P. patulum and mistletoes can serve as a guide for managers to assess ecosystem stress from herbivory.

Contexte. Après des siècles de diminution, la surface forestière de la plupart des pays développés augmente, un phénomène appelé « transition forestière ». Le stock de bois sur pied présente une augmentation plus rapide mais son évolution, associée à des problématiques actuelles majeures, reste négligée. Il est indispensable de caractériser et comprendre ces évolutions, d’en connaître les causes sous-jacentes, et de les reconstituer sur le long terme afin de pouvoir situer les ressources forestières dans une trajectoire d’ensemble et anticiper leur dynamique future. Cette thèse est consacrée aux forêts françaises métropolitaines et repose principalement sur les données de l’IFN. Objectifs. (1) Analyser les changements de surfaces, de stock et de densité de stock et leurs hétérogénéités spatiales et temporelles, sur 40 ans (1975 – 2015) en fonction de facteurs présumés structurer ces évolutions (géographie, propriété et composition). L’existence de changements de vitesse d’expansion a été recherchée. Le lien entre les changements de stock et des propriétés de la forêt (stock et densité de stock initiaux, augmentation récente de surface) ont été analysés. (2) Analyser les mécanismes des changements de stock et décomposer l’expansion selon des ensembles forestiers homogènes du point de vue de leur dynamique. L’analyse se fonde sur une estimation des flux de stock : croissance, recrutement, mortalité et prélèvement. (3) Situer l’expansion actuelle du stock dans une dynamique séculaire. Les stocks de 1892, 1908 et 1929 (associés aux surfaces de statistiques anciennes) ont été estimés par une approche d’imputation conditionnelle de la densité. Un modèle synthétique de densification du stock des forêts a été testé afin d’étudier à quelles conditions sur cette densification il est possible de retracer la chronologie présumée du stock. Résultats. (1) Sur 40 ans, l’expansion en stock a été trois fois plus rapide que celle des surfaces, soulignant l’intensité de la densification des forêts, et ne présente aucun signe de saturation. Les forêts privées, et principalement les forêts feuillues, présentent les expansions de stock et de densité de stock les plus marquées, suggérant le rôle important de l’expansion naturelle et de l’abandon de terres agricoles. Les modèles statistiques révèlent l’effet positif du stock initial et des variations passées de surface sur l’expansion. (2) L’analyse des mécanismes d’expansion a mis en évidence le moindre niveau des prélèvements relativement à la croissance des forêts, et la contribution des forêts jeunes au développement des ressources. Quatre ensembles forestiers synthétiques de dynamiques distinctes et principalement composés de stocks en forêt privée expliquant l’expansion du stock sont identifiés. (3) La reconstitution du stock depuis 1850 suggère une faible densité de stock au début de la période (25 m3/ha) et une augmentation de stock de presque +300% entre 1892 et 2010, soulignant l’importance de cette expansion. Un modèle convexe a été nécessaire pour représenter la densification des forêts, attestant d’une inertie importante à la reconstitution des ressources, interprétée relativement à la baisse progressive des prélèvements ou à une reconstitution progressive de fertilité. Les analyses suggèrent enfin une évolution différenciée dans le temps du modèle de densification pour les forêts provenant de plantations. Conclusion. Ces travaux ont permis de montrer l’importance de l’expansion en stock et la nécessité de contextualiser cette expansion. Cette expansion ancienne ne montre pour l’instant aucune saturation et constitue un stock de carbone croissant qui ne devrait pas diminuer dans les prochaines décennies à conditions contextuelles identiques. L’analyse causale révèle qu’une part importante de l’expansion du stock ne constitue pas une ressource immédiatement disponible. Les futures politiques d’intensification des prélèvements doivent donc être circonstanciées et échelonnées dans le temps / Context. After centuries of decrease, the forest area of most developed countries increase, a phenomenon termed “forest transition”. While current increase in growing stock (GS) is greater than that in area, it remains far less studied. These changes are linked to major current issues. It is essential to assess these changes, to decipher their underlying causes, and to quantify them over the longer term in order to locate current forest resources on a broad trajectory and to anticipate their future dynamics. This thesis is dedicated to French metropolitan forests, which exhibit the most intensive changes in the growing stock in Europe, and relies on data from the French NFI program. Objectives. (1) Analyse forest areal, GS and GS density (GSD) changes and their spatio-temporal variations over 40 years (1975-2015). They were related to factors hypothesized to feature forest changes (geographical contexts, ownership and species composition). We screened for changes in the rate of expansion. The relationships between GS changes and some forest attributes (initial GS and GSD, recent forest area increase) were investigated. (2) Uncover the processes of GS changes and to split the GS expansion magnitude across dynamically-homogeneous forest ensembles. The study was based on GS flux estimation (growth, ingrowth, mortality and harvest). (3) Locate the actual GS expansion in a secular perspective. This analysis consisted in reconstructing the GS chronology since 1850. Levels of GS in 1892, 1908 and 1929 (associated to area of ancient statistics) were estimated using a conditional imputation approach for GSD estimation. Then, a holistic growing stock densification model was implemented to inquire the conditions required on densification patterns and magnitude to simulate the reconstituted GS chronology. Results. (1) Over 40 years, GS increases were three times faster than the areal ones, underlining the intensity of forest densification. No sign of saturation was found. Private forests, and mainly broadleaved ones, presented the greatest GS and GSD increases, suggesting the essential role of natural expansion and agricultural land abandonment. Regression models revealed the positive effect of initial GS and of recent areal increases on GS expansion. (2) The analysis of GS expansion processes evidenced the low level of harvests in comparison to forest growth, and the contribution of recent forests to wood resource development. It led to identify 4 synthetic forest ensembles contributing to the expansion and of distinct dynamics, mainly composed of private forests. (3) GS suggested a very low mean GSD at the beginning of the period (25 m3/ha) and a GS increase by almost +300% between 1892 and 2010, underlying the importance of this expansion. A convex growth model was required to simulate historical forest densification, attesting of a significant inertia in wood resource reconstitution after the forest transition, interpreted based on a gradual decrease in harvest rates for which indices were collected, or to a gradual recovery of site fertility. The analysis also suggested a distinct kinetics for GS densification in plantation forests. Conclusions. These researches reveal the magnitude of GS expansion and the importance of its analysis across forest contexts. This ancient expansion does not present any current sign of saturation and constitute a persistent carbon sink which should not decrease in the next decades assuming similar contextual conditions. According to the process analysis of GS expansion, a significant fraction of the GS increases does not constitute readily available additional wood resources. Thus, future harvest intensification policies must be contextualized and evolving in time