Although most fire ecology focuses on vegetation responses to fire, invasive species and climate change have prompted the study of fire responses to vegetation changes. Changes that introduce substantially-different fuel types can alter the intensity, frequency, seasonality, or spatial extent of fire, with potential impacts on community succession and biodiversity. While grasses most often accelerate fire regimes when they invade shrub-dominated systems, there are few examples of grass invasions suppressing fire. In sub-Saharan Africa, grasslands are threatened more by climate change than invasive grasses, especially in montane regions, where species distributions can shift as climatically-determined ranges are altered. We studied the impact of encroachment of the temperate C3 grass Festuca costata Nees into C4-dominated grassland at the transition between their subalpine ranges in South Africa’s Drakensberg. We compared empirical data on fuel moisture and fuel load across F. costata-dominated patches in a C4-dominated matrix and used these data in fire spread models to predict the effect of larger, higher-moisture F. costata patches on the spatial extent of fire in these fire-prone grasslands.
Models indicate F. costata reduces fire spread and burn probability in the centre of F. costata patches, and the effect increases as live fuel moisture increases and patches get larger. Greater wind speeds mitigate this effect, although different wind speeds create different burn patterns on the landscape. Results indicate F. costata patches increase variability in the spatial extent of fire in C4-dominated grassland, but the ecological significance of variability on succession and biodiversity remains to be determined. The ecology of F. costata is essentially unknown and these results suggest the continued conservation and management of montane C3/C4 transitions worldwide will be informed by a better understanding of F. costata’s response to fire and climate change.