Fire as a Site Preparation Tool in Grassland Restoration: Seed Size Effects on Recruitment Success

Plants may either produce a large number of small seeds, or a comparatively small number of large seeds. This widely-observed tradeoff in plant reproductive strategies has important implications for ecological restoration by impacting dispersal, establishment, and competitive success of species. Here, we tested how applying fire to decrease environmental stress impacts the establishment success of species with differing seed sizes in a well-replicated, fully randomized planting experiment. Specifically, seedling establishment responses were compared among four pairs of closely-related plant species (one small-seeded vs. one large-seeded species in each pair), and average seed size among species pairs varied by an order of magnitude (from 0.375 to 9.17 mg). Although overall establishment rates were extremely low (< 1%), we found that large-seeded species established more readily than small-seeded species, and that using prescribed fire in site preparation increased establishment success across all species regardless of seed size. Surprisingly, large- and small-seeded species established equally well in unburned plots, which was contrary to our expectations that large-seeded species would establish more readily than small-seeded species in unburned plots. Perhaps a seed size threshold exists, below which seedlings cannot overcome the stress of leaf litter in unburned plots. This study suggests that preparing a restoration site with prescribed fire prior to seeding will substantially increase the establishment success of seeds sown into existing vegetation.