Super‐abundant C₄ grasses are a mixed blessing in restored prairies

Abstract

Forbs comprise most of the plant diversity in North American tallgrass prairie and provide vital ecosystem services, but their abundance in prairie restorations is highly variable. Restoration practitioners typically sow C4 grasses in high abundances because they are inexpensive, provide fuel for prescribed fires, can dominate reference sites, and suppress weeds that suppress sown forbs. However, C4 grasses can also suppress sown forbs, calling this practice into question. We evaluated how C4 grasses influence the abundance and diversity of sown forbs in 78 restored prairies across Illinois, Indiana, and Michigan. We found that the direct negative effects of C4 grasses on sown forbs outweighed indirect positive effects that occurred as C4 grasses suppressed nonsown species, which in turn suppressed sown forbs. This pattern was especially strong for the C4 grass big bluestem (Andropogon gerardii). Therefore, strategies to promote big bluestem and other C4 grasses would not promote sown forbs. Although C4 grass cover was not strongly related to two hypothesized drivers (time since fire or site age), seeding density of C4 grasses increased their cover. Sown forb cover also increased with forb seeding density, increased indirectly with fire (through its negative effect on nonsown species), and decreased indirectly with soil water‐holding capacity (through its positive effect on nonsown species). These results highlight the complex interplay of species groups during grassland restoration and show how managers can promote sown forbs in restored prairies: increasing forb seeding density and reducing time since fire and the abundance of C4 grasses and weeds.

Planting time, first‐year mowing, and seed mix design influence ecological outcomes in agroecosystem revegetation projects

“…Grasses are typically seeded at low rates in pollinator habitat conservation projects to reduce competition for native forbs (USDA 2021). Some C 4 perennial grasses, such as big bluestem ( A. gerardii ) are known to outcompete forbs (Grman et al 2021), suggesting that planting in the dormant‐season would improve forb persistence. Third, stands planted in the dormant‐season had more spring forbs and fall forbs than stands planted in the spring.…”

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

“…Our study demonstrates that seed additions can rapidly shift plant community composition to native species with higher conservation value during the early-stages of oak woodland restoration. ( Grman et al, 2020). To quickly establish native cover on bare ground after non-native shrub removal and to promote flammability in the ground-layer, we intentionally sowed high densities of perennial C 3 grasses, which averaged 41% of the total mass of seeds among our three mixes and accounted for 39% of cover in sown transects after 2 years.…”

“…Matrix and conservative grass species, especially Bromus pubescens ( C = 5), Cinna arundinacea ( C = 7) and Chasmanthium latifolium ( C = 4), increased substantially in cover within 2 years after sowing and disproportionately contributed to the greater floristic quality in sown transects, owing to their apparent displacement of ruderal species. Native grasses are often sown at high densities in grassland restorations because seeds are easily collected, inexpensive, establish reliably, and they provide ecosystem services in degraded sites including improved soil stabilization, productivity and biotic resistance to non‐native invasion (Grman et al, 2020). To quickly establish native cover on bare ground after non‐native shrub removal and to promote flammability in the ground‐layer, we intentionally sowed high densities of perennial C 3 grasses, which averaged 41% of the total mass of seeds among our three mixes and accounted for 39% of cover in sown transects after 2 years.…”

Community assembly history alters relationships between biodiversity and ecosystem functions during restoration

“…Finally, variation in biodiversity and ecosystem functioning depends on the effects of dominant species, in our case big bluestem. Big bluestem populations can quickly dominate, thereby reducing native local diversity (Grman et al, 2020 ) and homogenizing restorations (Catano et al, 2022 ). As we show here, these influences extend to aspects of ecosystem functioning and are modified by successional age and establishment year.…”