Evaluating grazing and defoliation effects on ponderosa pine grassland following the lodgepole complex wildfire

Abstract

Previous research indicated rangelands need rest from grazing after fire, while others show grazing the first year following fire has no negative effects on the plant community. This caused uncertainty around post-fire grazing management. In July 2017, the Lodgepole Complex fire burned ponderosa pine grasslands of the northern Great Plains, including areas burned in 2003 for fuels mitigation. We examined effects of post-fire grazing and season of defoliation. For the grazing study we wanted to determine 1) plant community response to grazing or rest the first growing season after fire in ponderosa pine grassland communities, and 2) whether prescribed fire alters plant community response to subsequent wildfire. For the season of defoliation study we wanted to determine 1) timing of defoliation effects on the plant community one growing season after fire and 2) whether defoliation effects are altered by prescribed fire preceding the fire. Eight exclosures (25 x 15 m) were built, 4 reburned sites and 4 wildfire sites. A non-grazed (15 x 10 m) section inside each exclosure was paired with a grazed section outside the exclosure. Plots (5 x 10 m) were mowed in the exclosure to 10 cm in June, July, or August, or not mowed during 2018. In 2019, biomass samples were clipped at peak production, with species composition and diversity measured by point-intercept transects. We observed a trend for reduced cool-season grasses (P =0.0675) and annual grasses (P =0.0071) if defoliated earlier; a trend for reduced forbs (P =0.0699) if defoliated later; and reduced total current-year biomass (P =0.0362) if defoliated. Functional group composition was not changed, but some individual species were shifted due to fire history. The grazing study only showed a trend for greater old dead biomass on non-grazed sites (P = 0.0600), higher composition of forbs on reburn sites (P = 0.0324), and a trend for a higher composition of Psoralidium tenuiflorum (Pursh) Rydb. on reburn sites (P = 0.0559). Results indicate mowing the first year following fire shifts the plant community, but the community is resistant to post-fire grazing. Prescribed fire 14 years before wildfire had small impacts on community composition.