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Implications of pyric-herbivory on central North American grassland ecology, management and production

Scasta, John Derek

The interaction of fire and grazing is an ecological process regulating fire-dependent ecosystems. Prior to European settlement, grasslands were regulated by fires and focal grazing by herbivores (pyric-herbivory), a disturbance pattern largely removed from the landscape. Pyric-herbivory, applied as patch-burn grazing, can sustain cattle production by reducing input costs and maintaining herbaceous plant communities for forage.

Management for heterogeneity with interactive fire and grazing has been effective in many ecosystems, but its efficacy has been variable in fragmented and invaded grasslands. We assessed factors constraining the fire-grazing interaction in Iowa, USA from 2007 - 2013. The most informative model included stocking rate, burn completion and precipitation. The lightest stocking rate did not establish low vegetative structure in the burn patch and the heaviest stocking rate did not maintain low vegetative structure in the burn patch. The intermediate stocking rate resulted in the lowest vegetative structure in the burn patch and the greatest heterogeneity.

We compared the influence of patch-burn grazing to traditional range management on the most economically injurious fly parasites of cattle in Oklahoma and Iowa in 2012 and 2013. Horn flies and face flies were below economic thresholds with patch-burn grazing but at or above economic thresholds in unburned pastures in Iowa. Pastures that burned completely had fewer horn flies but did not have fewer face flies when compared to no burning in Iowa. Stable flies on both treatments were below economic thresholds suggesting that regular fire can help maintain low levels of infestation.

We assessed how Symphoricarpos orbiculatus (buckbrush), the dominant shrub of tallgrass prairie, was affected by patch-burn grazing, complete pasture fires and grazing or complete fire and grazing exclusion in Iowa from 2011 - 2013. Height in burned plots was lower than unburned plots but S. orbiculatus reached ~ 84% of pre-burn height 120 days after fire. Stems per ramet were 2x greater in the most recently burned plots. Burned pastures had marginally lower densities than plots excluded from fire (P = 0.07). Fire triggered new layering stems. Dormant season fires did not result in mortality, but reduced structural dominance, and maintained lower densities.