Evaluating the Interactive Effects of Seasonal Prescribed Fire and Grazing On Invasive Grass Abundance and Woody Brush Encroachment

Abstract

Developing effective restoration techniques for encroached and invaded grasslands requires an understanding of two fundamental ecological processes: fire and grazing. Prescribed fire and grazing are suggested management techniques for preventing shrub encroachment and suppressing non-native species, especially in areas with functionally distinct native and exotic grasses. This study evaluates the effects on two encroached grasslands in South Africa and south Texas. In south Texas, native prairies are undergoing woody brush encroachment and exotic plant invasion concurrently. The exotic C4 grass species (Bothriochloa ischaemum and Dichanthium sericeum) are not physiologically distinct from the native C4 species they displace; therefore, the native and invading species may respond similarly to treatment techniques thus decreasing targeted management options. I established a full-factorial, completely randomized experiment to examine the interactive effects of seasonal prescribed fire and grazing on invasive grass abundance and woody brush density. Thirty-six 10m x 10m plots were assigned six treatments including: i.) summer burned-fenced ii.) summer burned-unfenced iii.) winter burned-fenced iv.) winter burned-unfenced v.) unburnedfenced vi.) unburned-unfenced. Prescribed fires were conducted in August 2013 and February 2014. Each season of burn treatment was followed by a grazing treatment.

Shrub height and cover significantly decreased across all shrub species in response to fire. Fire seasonality had no effect on invasive grass abundance but grazing reduced foliar cover (p=0.02) of the dominant invader (D. sericeum). Both treatments increased total species diversity. These results indicate that both seasonal fire and grazing can be used together to decrease shrub and invasive grass cover and maintain diversity.

This experiment complements another similarly designed study in South Africa, which determines the response of the encroaching shrub, Seriphium plumosum, to single season (winter) prescribed fire and grazing. Results indicate that fire can reduce shrub stature and canopy cover (p<0.01). Both treatments had no effect on herbaceous community composition (p=0.66). Comparing differential plant community responses to prescribed burn season in combination with grazing will provide important insights into the factors influencing woody vegetation dynamics and invasive grass growth and survival. This study provides a baseline of information to assist with the development of management techniques for multiple rangeland objectives.