Differing effects of savanna trees and forest trees on savanna understory plant diversity

Background/Question/Methods

Emerging theory in savanna ecology recognizes that savanna trees and forest trees represent distinct functional groups that support different suits of traits related to fire tolerance and resources acquisition. The functional differences between forest trees and savanna trees are thought to influence savanna-forest boundaries, and thus affect savanna understory plant communities, which comprise the majority of plant diversity in most savannas. Using data from 162 pine savannas in the Southeastern USA, we developed a structural equation model to test the effects of broadleaf savanna trees, savanna pine trees, and broadleaf forest trees on understory species richness. To account for underlying environmental gradients, our model incorporated fire frequency and soil moisture, two important factors that influence both overstory tree density and understory plant diversity.

Results/Conclusions

Our results indicate that among broadleaf species, the distinction between savanna and forest trees is relevant to understanding understory plant diversity in savannas. We also provide evidence that among savanna species, broadleaf savanna trees and savanna pines represent distinct functional groups that have different effects on understory species richness and respond differently to fire. In the structural equation model, densities of forest trees and pines were negatively related to understory richness, but there was no correlation between broadleaf savanna trees and understory richness. Fire promoted richness directly, and indirectly by limiting forest tree density. Fire did not affect pine densities but did limit broadleaf trees; amongst broadleaf tree species, forest tree densities were more strongly limited by fire than savanna trees. From a theoretical perspective, our results demonstrate that overstory trees belonging to different function groups can have different effects on savanna understory plant diversity.  From applied perspective, our results suggest that to promote plant diversity in savannas, ecosystem managers should control encroaching forest trees (through prescribed fire) and control pine densities (through cutting), but preserve broadleaf savanna tree species.