Riparian vegetation communities change rapidly following passive restoration at a northern Utah stream

Riparian vegetation may recover quickly from disturbance when the disturbance vector is removed or reduced. Grazing is a disturbance that removes plant biomass through herbivory, while overgrazing is a more severe disturbance that can deplete plant propagule pools and inhibit plant community recovery. We tested the hypothesis that riparian vegetation communities can shift quickly from ruderal grasslands to hydrophytic shrubs and graminoids when grazing is largely eliminated from riparian areas. We used a before-after-control study design to collect vegetation community data at six restored reaches and two grazed control reaches prior to and immediately following the construction of a cattle exclosure. We identified trends in Carex and Salix species abundance and quantified shifts in riparian vegetation community composition across time at each reach using PERMANOVA, multi-level pattern analysis and non-metric multidimensional scaling. Vegetation composition changed rapidly in the four years following removal of grazing disturbance. Indicator species for all impact reaches shifted away from grazing tolerant graminoids and forbs, and toward hydrophytic graminoid and shrub species. Over the same timespan control reach indicator species remained grazing-tolerant graminoids and forbs. There was little change in Salix abundance over time at control or impact reaches but Carex abundance increased at restored reaches. We conclude that herbaceous plant communities may recover rapidly following the removal of grazing disturbance, but that woody species may lag in recovery without active vegetation manipulation. We postulate that low woody-species recruitment may affect the potential of the riparian zone to quickly shade stream channels and facilitate undercut bank formation, common riparian restoration objectives. To prevent halted riparian succession, designers should proactively identify potential limitations to woody vegetation colonization. We close discussing active approaches to overcome stalled riparian ecosystem development and suggest metrics for assessing woody species recovery. Published by Elsevier B.V.