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

Abstract

Riparian zones and the streams they border provide vital habitat for organisms, water quality protection, and other important ecosystem services. These areas are under pressure 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, 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 3 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 reseeded 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 2 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.

Impacts of Woody Encroachment on the Fate of Soil Co2 in Grassland Watersheds

“…Findings by Keen et al, (2022) et al 2006). In other environments, as well as at Konza Prairie, where woody vegetation was removed streamflow did not rebound (Dugas, Hicks, and Wright 1998;Wilcox 2002;Huang et al 2006;Dodds et al 2023). At Konza Prairie woody vegetation was removed outside the riparian area with the expectation that stream flow would rebound.…”

“…Woody vegetation uses more water via transpiration and can access deeper sources of water than grasses, so by removing woody vegetation more water would be available for stream flow. Dodds et al, (2023), did not observe a change in stream flow and attributed this to woody encroachments' ability to permanently change soil structure and preferential flow paths. Consistent with observations made in Edwards Plateau, that as woody plants die the leave behind decaying roots that increase preferential flow (Leite et al 2023).…”

“…It is co- Konza Prairie is historically a tallgrass prairie with continuous gallery forests around riparian areas (Abrams 1986). The dominant vegetation is a diverse group of C4 grasses, the most abundant of which are Big Bluestem (Andropogon gerardii), Little Bluestem (Schizachyrium scoparium), switchgrass (Panicum virgatum), and indiangrass (Sorghastrum nutans) (Keen et al 2022;Dodds et al 2023). Land management practices, suppressing wildfires allowed woody vegetation to become more abundant across the prairie (Knight, Briggs, and Nellis 1994;Knapp, Conard, and Blair 1998;Briggs et al 2005).…”

“…Land management practices, suppressing wildfires allowed woody vegetation to become more abundant across the prairie (Knight, Briggs, and Nellis 1994;Knapp, Conard, and Blair 1998;Briggs et al 2005). Common woody vegetation in Konza Prairie is bur oak (Quercus macrocarpa), chinquapin oak (Quercus muehlenbergii), and rough-leaf dogwood (Cornus drummondii) (Keen et al 2022;Dodds et al 2023). The expansion of rough-leaf dogwood in Konza Prairie has been well documented and is the major player in woody expansion at this site.…”

“…Encroachment was first documented via aerial photography at Konza Prairie in 1939 (Knight, Briggs, and Nellis 1994) and continues to be studied and measured. Once woody encroachment is established it is difficult to reverse (Dodds et al 2023). Fire frequency patterns are linked to woody encroachment expanse across Konza Prairie.…”