Data from: Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs

1. Although there is a well-known association between tree cover and soil
texture in savannahs, the hydrological drivers of tree cover variation
have not been systematically explored, particularly in parallel with
factors such as fire, herbivory, and tree-grass interactions. The
relationship between hydrological factors and tree cover is important for
resolving the relative contribution of bottom-up vs. top-down factors in
structuring savannah vegetation. 2. We quantified soil moisture dynamics
across eight 1-km transects spanning tree cover gradients from open to
woody savannah in Serengeti National Park in Tanzania using soil moisture
sensors coupled with dataloggers. We mapped tree cover at two spatial
scales through supervised classification of high-resolution satellite
imagery. We simultaneously produced water retention curves in open and
woody habitats within each transect to compare soil hydrological
properties and to convert volumetric water content (θ) from dataloggers to
plant-available water over the course of an annual cycle. We also
quantified grass biomass at 100 locations per transect, estimated fire
frequency from MODIS satellite data, and quantified herbivore occupancy
with paired camera traps situated in open and woody habitats within each
transect. 3. We found a positive relationship between tree cover and soil
moisture drainage rate, and found that open habitats had more negative
water potentials than woody habitats for a given value of θ. In contrast,
we found no evidence for a consistent relationship between grass biomass
or fire frequency and tree cover. We found evidence for higher browser
occupancy in woody than open habitats, but no habitat effects on
herbivores as a group (browsers plus grazers), suggesting that herbivory
is unlikely to be the dominant factor explaining variation in tree cover.
4. Our results suggest that variation in tree cover is partly driven by
hydrological (edaphic) factors unrelated to fire, herbivory, tree-grass
interactions or mean annual precipitation at these spatial scales in
Serengeti. We contrast our findings with previous work attributing tree
cover shifts in Serengeti to precipitation gradients.