Recoupling cross-scale interactions in tall fescue-invaded tallgrass prairie

Vegetation quantity and quality influence the degree to which large grazers shape grassland structural heterogeneity. Invasive plants threaten the function of cross-scale interactions that exist when multi-scale effects such as fire and grazing interplay to form patterns of grassland structural heterogeneity. We investigated how grazing pressure and time since fire at the patch scale influenced patch utilization and production as well as forage quality in experimental grassland pastures dominated by an invasive grass, tall fescue (Schedonorus arundinaceous). We also assessed the response of tall fescue utilization and production to interactive fire-and-grazing under moderate and heavy grazing pressure. We collected data on vegetation quantity and quality over two grazing seasons to evaluate the role of fire and grazing across time in shaping structural heterogeneity among patches in invaded tallgrass prairie in Iowa, USA. We anticipated greater initial patch-scale utilization in patches burned for the first time in two years than patches not burned in two years in our experimental pastures. We expected that greater utilization in recently-burned patches would reduce tall fescue production, mostly where grazing pressure was highest. The contrast in the availability of live herbage between patches was half the level typical for native-dominated tallgrass prairie. Under increased grazing pressure, the interplay between fire and grazing did not result in greater broad-scale heterogeneity (among-patch heterogeneity) or an invasive grass reduction in experimental pastures. Yet, increased grazing early in the season did promote native-grass production in this invaded grassland landscape. Our results suggest that the dominance of tall fescue mediates the lack of structural heterogeneity induced by patch-level prescribed fire and grazing. Unlike native perennial grasses, tall fescue provides access to forage in unburned patches through its low-stature growth form. Diminished cross-scale interactions through a weak coupling of fire and focal grazing in invaded tallgrass prairie may facilitate structural homogenization of fire-dependent grassland.