Guinea Grass Establishment in South Texas Is Driven by Disturbance History and Savanna Structure

Abstract

Non-native grass invasion is a leading driver of biodiversity loss and degradation of rangeland ecosystem services. The reproductive mode and output of alien species mediate invasion, and their differential propagule pressure has implications for management outcomes. Propagule success, however, is context dependent. Resource availability promotes invasion and is heavily influenced by disturbance regimes. This study measured the reproductive phenology and establishment of Guinea grass's vegetative and seed-based reproduction in the field and the greenhouse. We asked how differences in vegetation type (woodland vs. grassland), disturbance history (brush management), and their interaction mediate the invasion process of Guinea grass. We documented differences in reproductive phenology as a function of vegetation type and disturbance history in the field. We measured seedling emergence and establishment in the greenhouse by fully crossing environmental variables with soils from two different disturbance levels. Finally, we estimated propagule pressure across vegetation types and disturbance history using data collected from the field and greenhouse trials.We found that areas associated with higher soil nutrients (under nitrogen-fixing mesquite trees and soil disturbance) had higher seed and stolon propagule pressure. Conversely, low disturbance and grassland areas had lower propagule pressure and were associated with lower nutrients. In the greenhouse, seedling establishment interacted with vegetation and disturbance. Seedlings growing in soils from mottes in a high-disturbance pasture had higher biomass and shoot length. However, the low disturbance pasture's specific root length was highest in mottes, indicating a higher allocation to roots, investing toward resource capture. Subsequently, we posit a hypothesis for Guinea grass spread in South Texas. Our findings suggest potential management steps, including treating Guinea grass under wooded cover to reduce overall propagule pressure and minimize soil disturbance during brush management of partially invaded woodlands.