Aboveground vs. belowground biomass investment of seedlings might explain floristic composition across a tropical forest-savanna boundary

Abstract

In the seasonal tropics, a dynamic balance can exists between forest advance and retreat. Forest retreat is caused by fire, but forests can advance into savannas in the absence of fire. The present study focuses on limitation of forest expansion in the absence of fire. This is tested by gathering transect data and conduction a nursery experiment. The floristic composition changes gradually from mature forest to open savanna. Forest seedling establishment is critical in understanding the dynamics of forest advance. This establishment is constrained by drought, low soil nutrient availability, competition with herbaceous vegetation, and defoliation by fire and herbivory. This study shows that in the absence of fire, patches of closed savanna exist, but forest species are not dominating. Forest species advance is strongly and negatively correlated with herbaceous cover while is strongly and positively correlated with canopy cover and soil nutrients. Additionally, comparative growth analysis on forest and savanna seedlings shows that the impact of grasses is stronger on forest species than on savanna species. This could be explained by different ecological trade-offs that savanna and forest seedlings make to adapt to their respective environments. Forest species allocate more in above ground biomass than savanna species, which might allow them to compete for light in forest. However, this makes them bad competitors for belowground resources compared to savanna seedlings that invest more in belowground biomass. These results indicate that in the Soutpansberg, South Africa, forest advance into savannas might only be possible if canopy closure by savanna species alters the environment favorable for forest seedling establishment.