Assembly history alters alpha and beta diversity, exotic-native proportions and functioning of restored prairie plant communities.
Restorations provide a test of community assembly history theory, and practitioners require information on how assembly history might help to restore diverse native species communities. Variation in community assembly history (historical order of species arrival) is hypothesized to generate beta diversity by producing alternate states, but restorations are hindered because there have been few tests using long-term field experiments. We experimentally altered assembly history of native species into formerly exotic-dominated grassland sites while simultaneously removing dispersal limitation to test whether alternate states or a single equilibrium would develop and whether alternate states would generate varying ecosystem-level effects. Assembly history was altered by varying the identity of early-emerging species, timing of seed additions after disturbance (early-emerging species added in spring or summer) and priority effects (common 30-species seed mixture added either at the same time or after an early-emerging species canopy developed). The experiment was conducted at two sites that differed in productivity. Altering timing and priority effects during assembly history had large effects on species composition and diversity. On average, diversity was highest, and the proportion of exotic species was lowest in plots seeded in spring and without priority effects. Identity of early-emerging species did not significantly affect community structure. Differences in species composition affected fuel mass, fire temperatures and peak above-ground primary productivity, key ecosystem processes in tallgrass prairie. Synthesis and applications. Our results indicate that grassland communities can reach alternate exotic- or native-dominated states in uniform environments when perennial exotic species are present. These states were strongly affected by timing of native species arrival and priority effects. Thus, assembly history is a key process that can give rise to beta diversity, and our results suggest that native species should be established early in the restoration process before exotics become fully established.