Thinning and removal of woody vegetation is the first step in restoring ecosystem structure to systems altered by woody encroachment. However, pile burning—a common method of eliminating woody residue at restoration sites—can promote the establishment of exotic species and adversely impact soils and native vegetation via extreme soil heating. Despite its widespread use, pile burning effects remain poorly understood in oak woodlands compared to coniferous forests. We examined how pile burning influenced soil properties and herb‐layer recovery in a Midwestern, U.S. oak woodland undergoing restoration via exotic shrub removal, tree thinning, and prescribed burning. We quantified soil properties and passive vegetation recovery inside and adjacent to pile burn scars after 3 years, and tested whether native seed additions in year 1 increased native cover and reduced exotic cover in year 3. Bare ground cover, soil pH, and concentrations of P and Ca remained elevated 3 years after pile burning. In contrast, native cover, native richness, and floristic quality recovered to levels similar to adjacent control locations. Pile burning did not promote exotic invasions, increase inorganic forms of N, or alter patterns of the native cover of most growth forms. However, Carex species failed to reestablish in burn scars. Compared to passive recovery, seed additions increased native cover in burn scars and reduced exotic cover in adjacent control locations. Our results indicate that burn scars can naturally recover in oak woodland, but native seed additions may accelerate this process and improve restoration outcomes.

Seed limitation represents a fundamental constraint to the restoration of native plant communities, and practitioners often apply seed additions to overcome this barrier. However, surprisingly few studies have experimentally tested whether seed additions can increase diversity in herbaceous communities of oak woodlands, which have undergone large‐scale transformation due to logging, altered fire regimes and invasion by non‐native species. Previous studies suggest that structural (thinning of woody biomass) and process‐based (prescribed fire) restoration treatments alone are unlikely to restore the full breadth of taxonomic and functional diversity in the herb layer, which accounts for most species in woodland ecosystems. To explore whether seed additions can improve restoration outcomes in an oak woodland, we sowed high‐diversity seed mixes in paired transects (seeded vs. controls) along a topographic gradient in a degraded site undergoing restoration with non‐native shrub removal, selective tree thinning and prescribed fire. Seed mixes contained native forbs, grasses and sedges from locally sourced material (n = 169 total species) in the regional species pool, and were designed to match species' habitat affinity to appropriate locations along the topographic gradient. The herb flora was sampled pre‐seeding, and for two consecutive years after additions. Seed additions significantly altered community and functional composition, and increased native species richness by 29% (43.0 vs 55.4), and floristic quality by 30% relative to controls. However, fewer than half of the sown species were established 2 years after planting, suggesting that dispersal and establishment limitation are both important barriers to the recovery of the herb flora in oak woodlands. We also tested if species' sown abundance, conservatism or functional group predicted establishment success. Species sown at high abundances and less conservative species recruited the most reliably. Grass and forb establishment rates were more dependent on seeding rate than sedges or legumes, and the mechanisms behind this trend merit further investigation. We found that adding high‐diversity seed mixes in conjunction with non‐native shrub removal, canopy thinning and burning, can accelerate recovery of herbaceous communities in a highly degraded woodland.

No abstract

No abstract