High‐diversity seed additions promote herb‐layer recovery during restoration of degraded oak woodland

Abstract

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.

Common, showy, and perennial species dominate a restoration species pool

“…There is an enormous opportunity to improve seed‐based restoration capacity, especially for woodlands in the Ozark ecoregion. Seeds for woodland species in this region are generally hand harvested in wildlands, which is the only option when species are not included in SPAs (Pedrini et al 2020; Reid et al 2020; Kaul et al 2023). Understory species grow slower, need shadier conditions, and may be more difficult to harvest using mechanical methods, making it hard to scale up profitably for vendors.…”

“…The dominant ecosystems are fire‐maintained oak or mixed oak‐pine forests, woodlands, and savannas, while bottomland forests, prairies, and glades (rocky grasslands) are also present (Nelson 2010). Restoration of the herbaceous flora in these systems is a priority for land management, especially in the woodlands and forests, which have lost herbaceous species due to densification and mesofication (McCarty 1998; Reid et al 2020; Kaul et al 2023). Thus, we focus our analysis on herbaceous angiosperms in upland habitats, given their priority for restoration.…”

Restoration age affects microbial‐herbaceous plant interactions in an oak woodland

“…Restoration of oak ecosystems usually involves prescribed burning or a combination of burning and canopy disturbance to control woody vegetation and encourage passive recovery of a diverse herbaceous flora (Hanberry et al., 2020). Although native plant species richness and abundance often increase after fire or thinning (Lettow et al., 2014; Maginel et al., 2019; Peterson et al., 2007; Vander Yacht et al., 2020), restored oak woodlands often lack conservative species in the understory (Kaul et al., 2023; Reid et al., 2020), leading to questions about whether SMCs can, in part, explain the lack of conservative plant species in restored sites. To date, few studies have explored the relationship between restoration age, the soil microbial communities, and reintroduced plants in woodlands.…”