Burning reveals cryptic diversity and promotes coexistence of native species in a restored California prairie

Background/Question/Methods

Grassland and prairie restoration projects in California often result in long-term establishment of only a few native plant species, even when they begin with a diverse palette of species. A likely explanation for the disappearance of certain native species over time is that they are outcompeted by competitively superior species. If this is the case, disturbance may favor these “subordinate” natives and promote greater native species diversity in restored communities. Fire was a historically important disturbance in many California ecosystems, and evidence suggests fire can substantially affect community composition, including releasing a flush of ephemeral early successional species from seed banks (e.g., in chaparral). However, few if any studies have explored the effect of fire in the context of a known post-restoration successional trajectory in California grasslands or prairies. We tested whether burning can reveal species that were initially established in a prairie restoration setting but then disappeared over time. We originally seeded plots with eight native species, all of which initially established. Prior to burning in the sixth year following restoration, two species (Trifolium bifidum and Trifolium wildenovii) had declined to zero cover and one species (Calandrinia ciliata) to near-zero cover.

Results/Conclusions

T. bifidum and T. wildenovii reappeared in almost all burned plots while remaining absent in the unburned control plots, and C. ciliata cover increased significantly and substantially in burned plots relative to controls. These findings suggest that subordinate species, even if only present aboveground for a short time after restoration, can persist “cryptically” in the seed bank through unfavorable periods. Furthermore, burning substantially reduced cover of three of the dominant natives in the restoration palette, suggesting that reappearance of subordinate native species was at least partly facilitated by a reduction in competition from dominants by burning. Taken together, these results imply that periodic burning can be a useful tool for promoting and prolonging the coexistence of dominant and subordinate native species in restored California prairies.