Grazing and defoliation timing effects in Great Plains ponderosa pine woodland following a large summer wildfire

Authors: WILLIAMS, AMANDA - Montana State Extension Service; Vermeire, Lance; Waterman, Richard; MARLOW, CLAYTON - Montana State University

Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2022
Publication Date: 6/30/2022
Citation: Williams, A.R., Vermeire, L.T., Waterman, R.C., Marlow, C.B. 2022. Grazing and defoliation timing effects in Great Plains ponderosa pine woodland following a large summer wildfire. Forest Ecology and Management. 520. Article 120398. https://doi.org/10.1016/j.foreco.2022.120398.
DOI: https://doi.org/10.1016/j.foreco.2022.120398

Interpretive Summary: Grazing management decisions following wildfire are often thought to require a choice between the economic cost of not grazing and the ecological cost of slowing recovery by grazing. The Lodgepole Complex fire was the largest U.S. wildfire during 2017, burning 109,346 ha in east-central Montana and reburning an area previously burned by the Bureau of Land Management to reduce fuel loads. We tested the effects of moderate grazing and timing of defoliation the first growing season after fire on herbaceous productivity and species composition in ponderosa pine woodlands. Eight exclosures (25 x 15 m) were built, 4 on reburned area and 4 on wildfire only sites. To determine grazing effects, a 15x10 m sampling area in each exclosure was paired with a grazed section outside the exclosure. To determine seasonal defoliation effects, four 5x10-m plots were established within each exclosure and were mowed to 10 cm in June, July, August, or not mowed. Differences between grazed and non-grazed sites in 2019 were limited. There was a trend for less old dead biomass in grazed than non-grazed treatments (P = 0.0600). Species richness was greater on grazed sites than nongrazed or reburn sites (P = 0.0335). Native species richness was greater on grazed than nongrazed sites (P = 0.0183) and reburn sites had an intermediate level of native species richness. Standing crop was reduced by defoliation (P < 0.0001), with current-year and perennial grass biomass being unaffected. The reduction in standing crop was driven by less old dead biomass on defoliated plots (P < 0.0001). June defoliation reduced cool-season grasses (P = 0.0148) and increased warm-season grasses (P = 0.0263), and June and July defoliation each reduced annual grasses (P =0.0004). Total diversity (P = 0.0011) and native diversity (P < 0.0001) were greater on defoliated plots. Results indicate ponderosa pine woodland is resilient to post-fire grazing.

Technical Abstract: There remains a high level of ambiguity around post-fire grazing management. The Lodgepole Complex fire burned 109,346 ha in east-central Montana in July 2017, including areas previously burned in 2003 by the Bureau of Land Management for fuels mitigation. We hypothesized that herbaceous productivity and species composition in ponderosa pine woodlands are resistant to moderate grazing and timing of defoliation the first growing season after fire. Eight exclosures (25 x 15 m) were built, 4 on reburned area and 4 on wildfire area. To determine grazing effects, a 15 x 10 m sampling area in each exclosure was paired with a grazed section outside the exclosure. Additionally, four 5x10-m plots were established within each exclosure and were mowed to 10 cm in June, July, August, or not mowed to determine seasonal defoliation effects. Differences between grazed and nongrazed sites in 2019 were limited. There was a trend for less old dead biomass in grazed than nongrazed treatments (P = 0.0600). Species richness was greater on grazed sites than nongrazed or reburn sites (P = 0.0335). Native species richness being greater on grazed sites than nongrazed sites (P = 0.0183) and reburn sites had an intermediate level of native species richness. Standing crop was reduced by defoliation (P < 0.0001), with current year and perennial grass biomass being unaffected. The reduction in standing crop was driven by less old dead biomass on defoliated plots (P < 0.0001). June defoliation reduced C3 grasses (P = 0.0148) and increased C4 grasses (P = 0.0263), and June and July defoliation each reduced annual grasses (P =0.0004). Simpson’s (P = 0.0011) and native Simpson’s (P < 0.0001) diversity were greater on defoliated plots. Results indicate ponderosa pine woodland is resilient to moderate post-fire grazing.