The effects of tree cover and soil nutrient addition on native herbaceous richness in a neotropical savanna

Abstract

Exotic grasses and high-nutrient availability are common factors that may limit recovery of herbaceous diversity in derived savannas, while tree encroachment is a threat to diversity in old-growth savannas. To understand the impacts of these factors on herbaceous communities, we studied the effect of nutrient addition, and the resulting increase in the exotic grass, Melinis minutiflora, at a savanna-forest boundary in the Brazilian Cerrado. We inventoried richness of reproductive herbs, tree basal area and cover of the exotic grass, Melinis minutiflora, in each plot. Nutrient addition caused a large increase in Melinis and a large decrease in richness of flowering plants. Unexpectedly, structural equation model analysis suggests that the decline in herbaceous richness was a direct result of nutrient addition, rather than an indirect effect caused by the increase in Melinis. Tree density had a strong negative effect on both Melinis cover and herbaceous richness. Our results reveal that high-nutrient availability imposes a barrier to the restoration of a diverse, native herbaceous layer in anthropogenic savannas, while tree encroachment is a threat to diversity in oldgrowth savannas.