North American prairies are acknowledged to have evolved with grazing following fire. Given this evolutionary fire-grazing interaction, our objective was to determine whether seasonal timing of defoliation following fire alters subsequent productivity and species composition. Following the April 2013 Pautre wildfire in the Grand River National Grasslands of South Dakota, we installed exclosures in three locations along the border of the fire. Grazing exclosures were paired across the fire line to create a burned and nonburned exclosure at each location. Four plots were demarcated in each exclosure. Three plots were defoliated via mowing to 6 cm either 2, 4, or 6 mo following fire, with the fourth maintained as a control. Productivity and species composition data were collected in November 2013, June 2014, August 2014, and July 2015. Fire increased productivity 56% during the 2013 growing season following fire.During the 2014 growing season, there was a tendency for burned sites to maintain greater production. June defoliation resulted in the greatest current-year productivity in 2014 regardless of fire treatment, whereas all other treatments resulted in similar productivity. Fire and defoliation effects on productivity were undetectable in 2015. Community composition was not affected by fire in 2013. Melilotus officinalis was increased by spring defoliation in 2014 and by fire in 2015. Litter was initially reduced by fire but was similar to nonburned levels by 2015. Results indicate that positive fire effects on productivity are limited to the first two growing seasons following fire, whereas defoliation effects manifest the second growing season following fire (first growing season after defoliation). With the exception of old dead material, fire and timing of defoliation affected biomass and community composition independently. Postfire rest from grazing appears unnecessary for the maintenance of plant productivity and species composition in northern mixed-grass prairie.
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Vol. 70 • No. 4