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

Young, Derek J. N., Porensky, Lauren M., Wolf, Kristina M., Fick, Stephen E., Young, Truman P.
Ecosphere 2015 v.6 no.5 pp. art81
grasslands, indigenous species, interspecific competition, land restoration, prescribed burning, sowing, species diversity, California
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. A likely 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 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, 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. Of the three subordinate species that had become rarer each year in the first three years of the study, all continued to decline and essentially disappeared aboveground over the following two years, as did one of the previously dominant species. However, burning triggered the reappearance of the three subordinate species that had disappeared or nearly disappeared in previous years, decreased the cover of dominant natives, and as a result increased community diversity. However, seeding treatments (aggregated or interspersed) did not 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 diversity.