Assessing grassland restoration success: Relative roles of keystone, dominant, and early emerging species

Tallgrass prairies are highly diverse at both alpha and beta scales, and high diversity is difficult to restore. Seeding species with traits that favor species establishment could enable higher species diversity in restorations. In a series of experiments, species that differed in their trait values were added or deleted from plantings to determine if they affect alpha and beta diversity. Species differed in potentially important plant traits such as seed size, longevity, leaf width and C:N ratio, biomass and litter production, and growth form. Early emerging species identity caused divergent species compositions but no changes in alpha diversity over the first three years of the study, and plant traits most important in predicting community divergence were related to fire intensity (temperature and proportion of plot burned). In a second experiment, dominant grass species differed in their growth rates and lateral spread during establishment, ranging from tall, narrowly-based species (e.g. big bluestem) to short, widely-based species (e.g. side-oats grama). Rare forb species recruited more successfully into plots dominated by shorter species than they did into plots dominated by taller species. A companion study found that exclusion of bison grazing led to increased dominance by tall species, and that these grasses greatly suppressed forb seedling recruitment and species diversity. Taken together, these results suggest that restoration success rate will be highest when natural disturbance agents are returned to the system to reduce dominance by tall grasses, or when tall grasses are added after shorter species have established.