Evaluation de l'integrite ecologique des ecosystemes forestiers du Quebec meridional: Developpement d'une approche basee sur le fonctionnement et la structure du sous-bois

With an increasing proportion of natural forests being replaced by plantations or managed forests, evaluating the impact human-induced transformation of forests is a major challenge for ecologists. This thesis presents a new approach to evaluate forest integrity based on understory seen as an ecosystem driver. More specifically, this thesis synthesizes understory response to the principal human disturbances found in sugar maple-dominated stands of southern Quebec, using a classification of the vegetation based on universal vegetation traits rather than on site-specific vegetation composition. This approach offers the possibility of inter-regional comparisons that could reveal patterns of organization that would be difficult to detect with a taxonomic approach. A global indicator of the ecological integrity of the entire ecosystem is then obtained. This thesis is divided in three parts which each focus on a different aspect of human impact on forest ecosystems:;With the goal of evaluating the potential of pasture to recover an understory flora similar to mature maple forests, changes in trait dominance and forest herb recovery were studied along a chronosequence which extended from active pasture to mature northern hardwood forest of pasture origin. This study indicates that abandoned pastures of the northern hardwood biome have a good potential for natural restoration. However, their complete restoration may take centuries. Sensitive forest herb species with limited dispersal mean were found to comprise a varying recovery capacity. These differences seem in part related to their potential to survive in a non-forested environment.;Using plant traits as well as understory structure and environmental conditions as indicators, this study evaluated the extent to plantations develop understory attributes comparable to natural forests. While the understory functional groups and environmental conditions of deciduous plantations converged toward those of natural forests, conifer plantations show a completely different pathway of understory development. However, richness of high ecological value forest species remains lower in deciduous plantations than in similar unplanted stands. These results suggest that the capacity of natural restoration of plantations may be enhance with modification of our plantation management practices.;Keywords. ecological restoration, forest flora, human disturbance, old growth, functional group, plantation, second growth, sugar maple-dominated stand, vertical structure.;Ecological integrity of sugar maple-dominated stands with a varying history of human disturbance is evaluated by the divergence of their understory species assemblages from those observed in comparable unmanaged forest. Overall, the understory vegetation assemblage was found to be relatively stable among all human disturbances investigated. However, our results suggest some issues of possible long-term conservation concern given a continuation of human disturbances.