Removal of woody encroachment restores biophysical gradients in Midwestern oak savannas

Savannas throughout the world are characterized by spatial gradients of resources created by scattered trees.  These gradients organize understory biodiversity; however, biophysical gradients may be disrupted by woody encroachment.  We used a large-scale restoration experiment to characterize understory biophysical gradients in woody encroached (n = 4) and restored (encroachment removed; n = 4) Midwestern (USA) oak savannas.  We sampled understory vegetation and physical factors along transects radiating from each of 40 overstory Quercus alba trees to inter-canopy gaps.  In restored sites levels of pH and OM, cover by leaves, down woody material, and species richness decreased with distance from trees, whereas levels of visible sky, cover by understory vegetation, forbs, graminoids, woody species, and species evenness increased.  In control sites levels of N, K, pH, and cover by down woody material decreased with distance from trees, while cover by understory vegetation and woody species increased.  Plots further from trees in restored sites had greater soil moisture levels immediately after rain coupled with greater rates of soil drying.  This resulted in treatment sites displaying greater maximum, but similar to lower minimum soil moisture levels, relative to control sites.  Nonmetric multidimensional scaling (NMS) showed that common understory species were correlated with gradients of canopy cover and soil moisture, which were associated with treatment plots, and gradients of soil texture and N, which were associated with both treatment and control plots.  Furthermore, restoration strengthened correlations between canopy cover NMS plots scores and distance from tree.  This study demonstrates that savanna restoration directly alters the distribution of soil and light resources and suggests that their patterning over fine spatial scales (hundreds m²) is important for structuring understory plant communities.