Browsing and fire decreases dominance of a resprouting shrub in woody encroached grassland.

Abstract

North American grasslands have experienced increased relative abundance of shrubs and trees over the last 150\xa0years. Alterations in herbivore composition, abundance and grazing pressure along with changes in fire frequency are drivers that can regulate the transition from grassland to shrubland or woodland (a process known as woody encroachment). Historically, North American grasslands had a suite of large herbivores that grazed and/or browsed (i.e. bison, elk, pronghorn, deer), as well as frequent and intense fires. In the tallgrass prairie, many large native ungulates were extirpated by the 1860 s corresponding with increased homesteading (which led to decreased fire frequencies and intensities). Changes in the frequency and intensity of these two drivers (browsing and fire) has coincided with woody encroachment in tallgrass prairie. Within tallgrass prairie, woody encroachment can be categorized in to two groups: non-resprouting species that can be killed with fire, and resprouting species that cannot be killed with fire. Resprouting species require additional active management strategies to decrease abundance and eventually be removed from the ecosystem. In this study we investigated plant cover, ramet density and physiological effects of continuous simulated browsing and prescribed fire on Cornus drummondii C.A. Mey, a resprouting clonal native shrub species. Browsing reduced C. drummondii canopy cover and increased grass cover. We also observed decreased ramet density that allowed for more infilling of grasses. Photosynthetic rates between browsed and unbrowsed control shrubs did not increase in 2015 or 2016. In 2017, photosynthetic rates for browsed shrubs were higher in the unburned site than the unbrowsed control shrubs at the end of the growing season. Additionally, after the prescribed fire, browsed shrubs had\xa0~\xa090% decreased cover, ~50% reduced ramet density, and grass cover increased by\xa0~\xa080%. In the roots of browsed shrubs after the prescribed fire, non-structural carbohydrates (NSC) experienced a 2-fold reduction in glucose and a 3-fold reduction in both sucrose and starch. The combined effects of browsing and fire show strong potential as a successful management tool to decrease the abundance of clonal-resprouting woody plants in mesic grasslands and illustrate the potential significance of browsers as a key driver in this ecosystem.