Trajectories and state changes of a grassland stream and riparian zone after a decade of woody vegetation removal.

Riparian zones and the streams they border provide vital habitat for organisms, water quality protection, and other important ecosystem services. These areas are under pressures from local (land use/ land cover change) to global (climate change) processes. Woody vegetation is expanding in grassland riparian zones worldwide. Here, we report on a decade-long watershed-scale mechanical removal of woody riparian vegetation along 4.5 km of stream channel in a before-after control impact experiment. Prior to this removal, woody plants had expanded into grassy riparian areas, which was associated with a decline in streamflow, loss of grassy plant species, and other ecosystem-scale impacts. We confirmed some expected responses, including rapid increases in stream nutrients and sediments, disappearance of stream mosses, and decreased organic inputs to streams via riparian leaves. We were surprised that nutrient and sediment increases were transient for three years, that there was no recovery of stream discharge, and that areas with woody removal did not shift back to a grassland state, even when re-seeded with grassland species. Rapid expansion of shrubs (Cornus drummondii, Prunus americana) in the areas where trees were removed allowed woody vegetation to remain dominant despite repeating the cutting every two years. Our results suggest woody expansion can fundamentally alter terrestrial and aquatic habitat connections in grasslands resulting in inexorable movement toward a new ecosystem state. Human pressures such as climate change, atmospheric CO 2 increases, and elevated atmospheric nitrogen deposition could continue to push the ecosystem along a trajectory that is difficult to change. Our results suggest that predicting relationships between riparian zones and the streams they border could be difficult in the face of global change in all biomes, even in well-studied sites. This article is protected by copyright. All rights reserved.