Burning reveals cryptic plant diversity and promotes coexistence in a California prairie restoration experiment

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 seed palette. One explanation for the disappearance of certain native species over time is that they are excluded through competition. If so, management that reduces interspecific competition may favor ‘‘subordinate’’ natives and promote greater native species diversity in restored communities. Potential management approaches to accomplish this goal include intraspecific spatial aggregation during seeding and prescribed fire. However, no studies have experimentally evaluated the effects of fire on a controlled (restoration) species pool or the interaction between fire and spatial aggregation. In a previous California prairie restoration experiment, we demonstrated that aggregated plantings protected competitively subordinate species from exclusion and increased community diversity for three years. However, native species richness declined throughout the study, and the benefits of aggregated seeding had begun to disappear by the third year. For the present study, we resurveyed the experimental plots five years after seeding and in the following year carried out controlled burns on half of the plots. The four subordinate species and one of the previously dominant species continued to decline and essentially disappeared aboveground during years four and five. However, burning in year five decreased the cover of dominant natives, triggered the reappearance of the three subordinate species that had disappeared or nearly disappeared in previous years, and increased diversity of the restored community. Seeding treatments (aggregated or interspersed) did not significantly affect community-level responses to the burning treatment. These results confirm that although initial intraspecific aggregation may promote species coexistence in the short term, re-establishing disturbance regimes can allow coexistence over a longer time scale by revealing and potentially renewing seed bank