Savannah trees buffer herbaceous plant biomass against wild and domestic herbivores

Abstract

Questions: Given the growing abundance and dominance of domestic herbivores in savannah ecosystems, can trees maintain plant herbaceous standing biomass under increasing herbivore pressure? Are there differences in the capacity of leguminous and non-leguminous trees in sustaining understory herbaceous biomass? And finally, to what extent does plant community composition underneath trees modulate the effects of herbivore assemblage and abundance on herbaceous biomass? Location: Pasturelands and protected areas along the borders of the Serengeti National Park, Tanzania, East Africa. Method: Monthly herbaceous biomass was monitored non-destructively using a calibrated pasture disc. Measurements were taken within a network of paired exclosures and open plots, underneath large leguminous and non-leguminous tree canopies and outside canopies. Herbivore community assemblage compositions and abundances were characterised using monthly dung counts, and herbaceous plant community composition was surveyed annually every wet season over two years. Results: Overall, we found that trees promote herbaceous standing biomass, particularly in the presence of moderate herbivory rather than under herbivore exclusion. Greater herbivore abundance and livestock dominance reduced herbaceous plant biomass, but trees, particularly leguminous trees, limited these negative effects. This capacity for trees to limit the effect of herbivores was related to herbaceous plant species composition. Understory plant communities that were compositionally typical of protected areas sustained the highest plant biomass when found in pasturelands with high herbivore pressure. Conclusion: Our findings give greater credence to the importance of preserving large trees in savannah landscapes increasingly dominated by high abundances of livestock. Moreover, our results highlight that park managers and pastoralists need to maintain the specialist herbaceous understory community beneath trees in order to benefit from facilitative tree–understory interactions.