Seed addition and biomass removal key to restoring native forbs in degraded temperate grassland

Questions: Long-term restoration of native forb diversity can only be achieved if native forb species can recruit (colonize and establish) and reproduce. We asked whether native forbs in a temperate grassland were seed-limited, and how the recruitment of native and exotic forbs is affected by grassland structure and resource availability. Location: Australian Capital Territory, south-eastern Australia. Methods: We conducted a field experiment in a temperate grassland dominated by a native tussock grass to assess effects of: (1) addition of native forb seed, (2) thinning of native grass tussocks, (3) leaf litter removal, and (4) exotic plant removal on the recruitment of native and exotic forbs. These four actions can alter grassland structure and the availability of soil nutrients, soil moisture and light. We used GLMM to determine the importance of seed addition, grassland structure and resource availability on the richness and abundance of sown native forbs, and the abundance of exotic forb seedlings and unsown native forbs. Results: Adding seed increased the species richness and abundance of native forbs. Tussock thinning and litter removal increased species richness and abundance of sown native forbs, and the abundance of exotic forb seedlings. Exotic plant removal also increased the abundance of sown native forbs. Abundance of unsown native forb species was unaffected by the experimental treatments. Species richness and abundance of native forbs and abundance of exotic forbs declined with increasing tussock grass cover. Leaf litter restricted the abundance of native forb species more than exotic forb species. Conclusion: Native forb recruitment predominantly relied upon seed addition, suggesting that seed limitation is a major barrier to the recovery of degraded grasslands. Reducing the cover of living grass tussocks facilitated recruitment of native and exotic forbs, and removing litter disproportionally increased recruitment of native forbs compared with exotics. Combining seed addition with the reduction of both living and dead grass biomass should help restore native grassland forbs.