Fire and simulated herbivory have antagonistic effects on resistance of savanna grasslands to alien shrub invasion

Abstract

Question: Resistance of the native community has been identified as an important factor limiting invasion success and invader impact. However, to what extent resistance interacts with disturbance to control invasion success remains unclear. We studied the interaction between biotic resistance, fire and small-scale disturbances mimicking those of large mammalian herbivores (hoof action and grazing) on invasion success of the alien shrub Chromolaena odorata. Location: Hluhluwe-iMfolozi Park, South Africa. Methods: We performed a seedling transplant experiment in a savanna grassland. We manipulated the grass layer by clipping (grazing), created small-scale soil disturbances (hoof action) and transplanted seedlings under native tree canopies to assess potential nursing effects. Subsequently, we burned half of our plots. We assessed the effect of fire and small-scale disturbances on seedling growth, survival and biomass allocation. Results: Seedling survival of C. odorata was 26% in undisturbed savanna grassland, reducing to 5% post-fire. Small-scale disturbances increased seedling biomass and survival and modified biomass allocation, whereas fire greatly reduced seedling survival. Root allocation increased in response to grass clipping, while stem allocation decreased. Tree shading increased seedling survival in the absence of fire, but greatly reduced post-fire survival. Conclusions: Grass communities in savannas exert a high level of biotic resistance to C. odorata invasion. However, small-scale disturbances, mimicking those of large mammalian herbivores, can facilitate the invasion of C. odorata in savanna grassland both directly, by creating micro-sites for establishment, and indirectly, by reducing the negative effect of fire. Thus, small-scale disturbances may aid the long-term persistence of woody species invading grass-dominated systems. Biotic resistance limits invasion success, yet to what extent resistance interacts with disturbance remains unclear. We transplanted seedlings of the alien shrub Chromolaena odorata in savanna grasslands and show that fire reduced seedling survival, whereas small-scale disturbances increased both seedling survival and biomass. We conclude that small-scale disturbances may aid the long-term persistence of woody species invading grasslands.