The variation that exists in both time and space in riparian plant communities was explored in five streams within the Salmon National Forest and surrounding Bureau of Land Management (BLM) lands of central-eastern Idaho that are managed for cattle grazing. In this study, we evaluated the ability of commonly used vegetation metrics (live vegetation cover, species richness, % forb, litter, bare ground, wetland indicator rating, % graminoid, bank alteration, soil compaction, and % native) and different plot types (static or dynamic) to assess changes in plant communities over time and in areas grazed by cattle. We were particularly interested in evaluating the stability of metrics over time and the responsiveness of metrics to cattle grazing. We found that the metrics wetland indicator rating, % graminoid, and % native were stable across the season, while live vegetation cover, species richness, % forb, litter, bare ground, bank alteration, and soil compaction were affected by seasonal variation. The metrics that responded to grazing similarly at all streams and sites were live vegetation cover, litter, bare ground, and bank alteration, while species richness, % forb, wetland indicator rating, % graminoid, soil compaction, and % native responded differently to cattle grazing at individual streams. The metrics that were most sensitive to cattle grazing within the season were live vegetation cover, species richness, % forb, litter, and bare ground. Plot type did not have an effect on the majority of the metrics at the majority of the streams. This information can be used by land managers to determine which metrics are suitable for short- and long-term monitoring, and which ones are appropriate for monitoring the effects of cattle grazing.

L'urbanisation représente une menace majeure pour la biodiversité. Ce mémoire de maîtrise vise à comprendre ses effets sur la composition fonctionnelle et l'homogénéisation biotique dans les forêts riveraines. Des inventaires floristiques ont été réalisés dans 57 forêts riveraines de la région de Montréal. Afin d'étudier la variation de la composition fonctionnelle avec l'urbanisation, des moyennes pondérées de traits par communauté ont été calculées pour les arbres, arbustes et herbacées. Chaque forêt a été caractérisée par des variables relatives au paysage urbain environnant, aux conditions locales des forêts et aux processus spatiaux. Les conditions locales, notamment les inondations, exerçaient une pression de sélection dominante sur les traits. L'effet du paysage était indirect, agissant via l'altération des régimes hydrologiques. La dispersion le long des rivières était aussi un processus important dans la structuration des forêts riveraines. Les changements dans la diversité β taxonomique et fonctionnelle des herbacées ont été étudiés entre trois niveaux d'urbanisation et d'inondation. Alors que l'urbanisation a favorisé une différenciation taxonomique, les inondations ont favorisé une homogénéisation taxonomique, sans influencer la diversité β fonctionnelle. L'urbanisation était l'élément déclencheur des changements de la diversité β, directement, en causant un gain en espèces exotiques et une diminution de la richesse totale dans les forêts très urbanisées, et, indirectement, en entraînant un important turnover d'espèces par l'altération des régimes hydrologiques. Globalement, ces résultats suggèrent que la modification des processus naturels par les activités anthropiques est le principal moteur de changements dans les communautés riveraines urbaines. / Urbanization is a major threat to biodiversity worldwide. This thesis aims at understanding its effects on plant functional composition and biotic homogenization in riparian forests. Floristic inventories were carried out in 57 riparian forests of the Montreal area. To investigate changes in functional composition with urbanization, community weighted means were computed for trees, shrubs and herbs using eight functional traits. Each forest was characterized by variables related to the surrounding urban landscape, local forest conditions and spatial processes. The relative importance of these three subsets on the functional composition was quantified by variation partitioning using redundancy analyses. Local conditions, especially flood intensity, exerted an overriding selection pressure on riparian functional communities. The effect of the landscape was suspected to be indirect, acting on trait patterns likely through alteration of hydrological disturbances in riparian forests. In addition to environmental filtering, dispersal along rivers was also an important process structuring riparian forests. Changes in taxonomic and functional β-diversity for herb species were studied between three urbanization and flood levels. While urbanization led to taxonomic differentiation, flood intensity fostered taxonomic homogenization, both without influence on functional β-diversity. Urbanization was the main trigger for all β-diversity changes, as it directly caused a gain in exotic species and a net species loss in highly urban forests, and indirectly fostered an important species turnover through alteration of the hydrological regime. Overall, these results suggest that the alteration and interruption of natural landscape-scale processes by human activities are major mechanisms underlying changes in urban riparian communities.