Responses and resilience of tallgrass prairie streams to patch-burn grazing.
Abstract
Patch-burn grazing (PBG) can promote terrestrial heterogeneity and biodiversity, but can temporarily increase stream nutrients and ecosystem metabolism, and alter macroinvertebrate assemblages. The impacts of grazing on stream channel morphology and post-PBG recovery patterns are unclear. We assessed the influence of grazing in PBG managed grassland streams in Missouri, USA, and subsequent recovery when grazing ceased for 2 years. We hypothesized that grazing would degrade water quality, stream biotic integrity and channel morphology, but that riparian fencing would mitigate these effects. We predicted that biological and chemical variables in unfenced streams would return to pre-PBG levels within 2 years after grazing ceased, but channel morphology would not. Six small headwater streams (two in ungrazed control watersheds, two in PBG watersheds with 10 m fenced riparian zones and two in PBG unfenced) were sampled over 7 years; 2 years before PBG, 3 years during PBG and 2 years post-PBG. We sampled macroinvertebrates and water chemistry monthly when water was present and surveyed channel morphology at least once each study period. During grazing, unfenced watersheds showed the greatest changes in channel width, depth and area. During the post-PBG period, one of the two unfenced watersheds showed partial recovery of channel morphology. Although grazing increased concentrations of nutrients and chlorophyll-a, concentrations returned to pre-PBG conditions after grazing ceased, indicating recovery. Very fine organic sediments increased in the unfenced watersheds compared to the control during grazing but recovered afterwards. Contributions of Chironomidae to total invertebrate abundance increased in the unfenced watersheds during grazing, and then decreased during the post-PBG period. Riparian fencing mostly mitigated effects of grazing on the streams. Unfenced streams were resilient to effects of grazing in a PBG managed grassland, with most metrics recovering within 2 years after grazing ceased, except for channel morphology. Synthesis and applications. Grazing in a patch-burn grazed managed grassland coupled with riparian fencing could be an effective conservation tool for land managers of prairies, with relatively modest influences on stream water quality and biotic integrity. Persistent changes in stream channel morphology and effects of longer periods of grazing deserve further research.