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Influences of patch-burn grazing on headwater prairie streams and subsequent recovery

Citation

Fulgoni, Jessica et al. (2020), Influences of patch-burn grazing on headwater prairie streams and subsequent recovery, Dryad, Dataset, https://doi.org/10.5061/dryad.w6m905qmb

Abstract

1. Patch-burn grazing (PBG) can promote terrestrial heterogeneity and biodiversity, but can temporarily increase stream nutrients, ecosystem metabolism, and alter macroinvertebrate assemblages. The impacts of grazing on stream channel morphology and post-PBG recovery patterns are unclear. 2. We assessed the influence of grazing in PBG managed grassland streams in Missouri, USA, and subsequent recovery when grazing ceased for two 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 two years after grazing ceased, but channel morphology would not. 3. Six small headwater streams (two in ungrazed control watersheds, two in PBG watersheds with 10 m fenced riparian zones, and two in unfenced PBG) were sampled over seven 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. 4. 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 ended, 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. 5. 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 two years after grazing ceased, except for channel morphology. 6. Synthesis and applications: Grazing in a PBG managed grassland coupled with riparian fencing could be an effective conservation tool in prairies, with relatively modest influences on stream water quality and biotic integrity. Persistent changes in stream geomorphology and effects of longer periods of grazing deserve further research.

Methods

Channel Morphology

Channel morphology (bankfull width, depth, width:depth, and channel area) was measured within 5 of 6 study streams. The sixth watershed, a control, was not surveyed as it did not have a well-defined channel and bankfull stage could not be confidently determined. All streams were initially surveyed in the spring of 2011 prior to cattle grazing to establish baseline conditions. Streams were re-surveyed in the spring of 2012 and 2013 to determine annual grazing impacts and in the spring of 2016 to assess stream response following recovery.

Ten permanently marked cross sections were established for surveying within each watershed. Cross section locations were spaced at approximately even intervals between the bottom of each treatment and the upstream channel boundary (where the channel turns into hillslope). Average cross section spacing ranged between 24 m for the smallest watershed to 81 m for the largest watershed. Cross section spacing varied between watersheds due to variability in stream length. Within the cattle grazed watersheds, the most upstream cross section was placed at the upstream boundary of the electric cattle enclosure to ensure that all cross sections were contained within the cattle grazed portion of the watershed. We avoided surveying meander bends due to accelerated rates of erosion and deposition. Cross section geometry was measured with a surveyor’s level and leveling rod at 15.24 cm spatial resolution. Bankfull width (m) was defined as the distance from the top of the bank that was lower in elevation to the equivalent elevation on the opposite bank. Cross sectional depth (m) was calculated by averaging all the depth measurements across the bankfull width. Width to depth ratio (w:d) was determined by dividing bankfull channel width by average channel depth and channel area (m2) was determined by multiplying the bankfull channel width by the average channel depth (Harrelson, Rawlins, & Potyondy 1994). Width to depth ratio was included in the analyses as shifts in this parameter may influence habitat for biota by altering water temperature and stream bed stability (Blann et al. 2002, Jansen & Nanson 2010). If bankfull elevation could not be identified, the cross section was excluded from analysis.

Water Chemistry, Benthic Chlorophyll-a, and Macroinvertebrate and Organic Matter Collection and Processing

Water samples for ammonium (NH4+), nitrate (NO3-), soluble reactive phosphorus (SRP), total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), and benthic samples for chlorophyll were collected monthly when flow was present. Samples were analyzed as described in Larson et al. (2016).

Macroinvertebrate and organic matter samples were collected monthly when water was present (study sites were dry July to October). Macroinvertebrate biomass, abundance, relative abundance and biomass, Ephemeroptera Plecoptera and Trichoptera taxa (EPT), functional feeding group and organic matter (benthic organic matter (BOM), CPOM, fine particulate organic matter (FPOM), and VFPOM) were collected and processed as described in Jackson et al. (2015).

Usage Notes

Gray boxes in the Osage Geomorphology Data.xlsx file are cross-sections that were lost for reasons explained in the manuscript.

Funding

Missouri Department of Conservation

Southern Illinois University Carbondale

Kansas State University

Missouri Department of Conservation

Southern Illinois University Carbondale