Cessation of Native American burning and introduction of non-native slug are both implicated in decline of rare clover

Showy Indian clover (Trifolium amoenum) was historically distributed in grasslands surrounding San Francisco Bay. Presumed extinct by 1984, a single plant was discovered in 1993. Factors thought by others to have been responsible for the near extinction of the species are habitat loss, livestock grazing, and competition from non-native plants. I believe that another factor may be the cessation of indigenous burning aimed at increasing food resources in grasslands. Clovers were eaten as fresh greens by many California Tribes and T. amoenum was one of several species eaten by the Coast Miwok and probably the Southern Pomo. Clover fields were traditionally managed by small-scale, controlled burns conducted seasonally. In autumn 2003, I began a 2-year field study by planting T. amoenum seeds at two sites in Sonoma County to identify habitat requirements and threats, and to quantify the effects of burning on establishment and survival. Both sites experienced high seedling mortality due to herbivory by the European gray field slug, Deroceras reticulatum. Slug exclusion measures in the second year’s experimental design, however, were not completely effective in controlling seedling loss due to slugs. Plant survival and growth were significantly greater in burn treatments than in nonburn treatments. The results of this study, along with preliminary slug surveys at other possible reintroduction sites, indicate: 1) that the introduction of D. reticulatum into California in the 1880s is another factor in T. amoenum decline; and, 2) that reintroducing indigenous burning practices can be a useful tool to enhance restored populations.