Long-term changes in dispersal and persistence traits of the oak savanna understory

Sam Ahler, Department of Botany, University of Wisconsin - Madison, Madison, WI, Laura M. Ladwig, Biology, University of Wisconsin - Oshkosh, Oshkosh, WI, Katherine Charton and Ellen I. Damschen, Department of Integrative Biology, University of Wisconsin-Madison, Madison, WI

Background/Question/Methods
Savanna communities have historically relied on fire and grazing to ensure an open canopy structure. Largely due to fire suppression, climate change, and grazer removal, oak savannas have experienced canopy closure and mesophication over the past century. In addition to changes in the canopy structure and composition, understory communities are changing as well. Functional traits are morpho-physio-phenological features that impact the fitness of a species. Traits affecting species’ ability to disperse (e.g. seed mass, vegetative height, flowering height and dispersal-mode) and persist could provide insight into the directionality and causes of community change in savanna understories. Here, we use rare long-term frequency data and functional trait measurements to ask how functional composition of Wisconsin savanna understories changed over the past half century. We predict site homogenization as the diversity of microsite conditions decrease due to canopy closure. In addition, we expect trait diversity to depict a transition away from a diverse grass and forb system towards a shrub dominated system.
Results/Conclusions
Savanna understories shifted in their functional composition over the past 60 years. The understory has become significantly more homogenous with regards to taxonomy and function. Understories are now composed of taller plant species while short-statured, shade-intolerant forbs and grasses have declined or been lost entirely. There are more large-seeded, tall species in 2014, and these species are also more abundant. Seed dispersal also changed, with unassisted species disappearing and being replaced by animal-dispersed species. Finally, short-persisting species have increased in abundance. These changes suggest that current oak savannas have become heavily encroached by species with woody habits, animal-dispersed seeds, and taller structures while herbaceous forms and grasses have been lost. The remaining highly persistent perennials suggest an extinction debt, owing their continued presence in the community to their belowground investment. Without the germination of new individuals, this group will likely vanish. These systems need fire and active management to reinstate an open understory and restore savanna canopy structure. Fire is needed to prevent the extirpation of highly persistent species by promoting germination and may help lost savanna species reestablish in the system.