Effects of rain, nitrogen, fire and grazing on tree recruitment and early survival in bush-encroached savanna, South Africa

Moisture, nutrients, fire and herbivory are the principal factors governing tree-grass cover ratios of savannas. We investigated tree (Acacia mellifera) recruitment after fire and under conditions of maximum-recorded rainfall, nitrogen addition and grazing in a completely-crossed field experiment. We employed a similar garden experiment with the exception of the fire treatment. Tree germination in the field was extremely low, probably due to below-average natural rainfall in plots that only received natural rain, and insufficient watering frequency in irrigated plots. Due to low germination in the field experiment, no treatment significantly affected tree recruitment. In the garden experiment, frequent watering, nutrient control (i.e. no nitrogen addition) and grazing enhanced tree recruitment with significant interactions between rain, nitrogen and grazing. We infer that above-average rainfall years with frequent rainfall events are required for mass tree recruitment. Grass defoliation makes space and resources available for tree seedlings. Nitrogen enrichment increases the competitive ability of fast-growing grasses more than that of the N-2-fixing tree component. In contrast to conventional wisdom that grazing alone causes encroachment, we suggest that there are complex interactions between the above-mentioned factors and 'triggering' events such as unusually high rainfall.