The role of seed limitation and microsite trait in seedling establishment: Harvester ants and fire alter recruitment of perennial forbs in a semi-arid savanna

The comparative role that biotic and abiotic agents play is important for seedling recruitment and our understanding of plant population dynamics and community restoration.  Fire, a major abiotic driver of population dynamics in savannas, provides better microsites for perennial forbs recruitment, such as increasing light and available nitrogen.  Alternatively, seed limitation occurs by post-dispersal seed predators resulting in lowered plant populations.  We hypothesized that seed removal by the granivore Pogonomyrmex barbatus (red harvester ant) alters perennial forb seedling establishment more than post-fire microsite properties.  To assess seedling recruitment by P. barbatus foragers, an ant-protected exclosure study was established in three colony densities, none (control), low, and high.  Seed addition of five forb species were sown in burned and unburned locations after a spring prescribed burn. To quantify probable seedling establishment for restoration planning, these species were germinated in laboratory conditions and field-tested for granivory rates.  Seedlings were significantly higher in exclosures compared to open arenas for all species combined, Simsia calva, and Engelmannia pinnatifida.  Slight increases were observed in burned locations.  Factors such as soil depth may be more important than burn properties in forb recruitment.  No differences were observed between low and high colony density likely because P. barbatus colonies protect their foraging areas.  Probability of a species' seed surviving to a seedling ranged from 2.6% to 38.1% indicating variable species preference by foragers and germination rates.  Seed limitation can be important in forb seedling recruitment even with microsite improvement after fire.