The removal of woody encroachment restores biophysical gradients in Midwestern oak savannas.
Abstract
Savannas throughout the world are characterized by spatial gradients of resources created by scattered overstorey trees. These gradients are important for maintenance of understorey biodiversity; however, they may be disrupted by woody encroachment, resulting in alterations to understorey vegetation. Little is known about the status of understorey gradients in encroached savannas, or whether they redevelop during restoration and if so, by what mechanism. We used a large-scale restoration experiment with Midwestern oak savannas (USA) to address these issues. We established understorey transects radiating from overstorey Quercus alba L. trees to inter-canopy gaps in four control and four restoration treatment sites. Along each transect, we sampled understorey vegetation over three field seasons and we recorded physical factors in the final year of study. Restoration produced a strong relationship between visible sky (e.g. light penetrating to the understorey) and distance from overstorey trees, while none existed in control sites. Restored sites had greater variability in soil moisture due to higher levels immediately after rain at all distances from trees, coupled with greater drying rates, particularly at farther distances from overstorey trees. With restoration, a positive relationship between cover by understorey vegetation and distance from overstorey trees developed and strengthened over time, whereas no relationship existed in control sites. Cover by each of the major functional groups, forbs, graminoids, and woody species, contributed to this pattern. Furthermore, after restoration, species richness increased with distance from overstorey trees in the final year of study. Nonmetric multidimensional scaling (NMS) showed that common understorey species were correlated with gradients of canopy cover and soil moisture, which were associated with restoration plots, and gradients of soil texture and N, which were associated with both restoration and control plots. Furthermore, restoration strengthened correlations between NMS plot scores and distance from overstorey trees. Synthesis and applications. Re-establishing overstorey structure was an important step during restoration of the oak savannas in this study. While encroached savannas contained a homogenized understorey, restored sites contained understorey patterning typical of intact savannas. The timeframe for re-establishment - within years of restoration, after decades of degradation - demonstrates high resiliency and suggests restorability of other highly degraded sites; however, we recognize the importance of prescribed fire for maintaining open savanna structure and, probably, promoting further understorey pattern development.