COS 9-1 - Woodland-grassland transitions are strongly associated with soil hydrology but not fuel loads or fire in Serengeti savannas

Monday, August 12, 2019: 1:30 PM
L013, Kentucky International Convention Center
Ricardo M. Holdo, Odum School of Ecology, University of Georgia, Athens, GA and T. Michael Anderson, Biology, Wake Forest University, Winston-Salem, NC
Background/Question/Methods

At the continental scale, shifts from grass- to tree-dominated biomes are associated with mean annual precipitation and fire frequency. At small spatial scales, such transitions have been attributed either to soil-mediated hydrological factors or to tree-mediated feedbacks between grass biomass and fire. Few studies, however, have compared support for these two mechanisms or explicitly explored the relationship between tree cover and soil moisture dynamics. We obtained volumetric water content (θ) time series and quantified fractional tree cover using Google Earth across eight sites comprising 1-km transects straddling woodland-grassland transitions in Serengeti National Park, Tanzania. We generated water retention curves in grassland and woodland habitats to compare soil hydrological properties between habitats, and to translate θ to plant-available water over the course of an annual cycle. We also quantified fuel biomass at 100 locations per site to test the hypothesis that variation in tree cover is related to fuel loads.

Results/Conclusions

We found a strong positive relationship between tree cover and the rate of soil moisture drainage, found that grasslands had more negative water potentials than woodlands for a given value of θ, and experienced periods of water stress for plants that are twice as long as those of woodland sites over the course of the year. In contrast, we found no evidence for a relationship between fuel loads or fire frequency and tree cover. This suggests potentially important water extraction constraints in grasslands despite abundant soil moisture, and suggests that fine scale variation in tree cover is strongly constrained by hydrological (edaphic) factors unrelated to mean annual precipitation or fire. The strong modulation of soils on the length of stressful periods has implications for the effects of rainfall timing on the ability of trees to colonize grassland sites. We show that changing rainfall patterns in Serengeti have implications for the relative cover of grassland and woodland in the system.