Allelopathic Control of Herb Growth in the Fire Cycle of California Chaparral

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Abstract
Chaparral is a mixture of shrub dominants generally lacking an understory of herbaceous plants. Recurrent fires destroy the shrub cover at intervals of 10 to 40 years. In the 1st growing season following fire, a luxuriant growth of annual herbs and bulb-forming perennials appears. Most shrub species, furthermore, produce seedlings. Shrub regeneration largely depends upon sprouting of surviving underground parts. With regeneration of shrub cover, seed germination ceases about 5 or 6 years following fire and herbs are again eliminated. Open shrub stands with as much as 50 [degree]/o bare ground, full sun, and soils annually saturated throughout the season of growth initiation fail to exhibit seed germination. Mechanical removal of shrub crowns without soil disturbance results in germination and seedling growth of some 30 species of herbs and shrubs identical in timing and aspect to that following fire. Adenostoma fasciculatum, Arctostaphylos glandulosa, and A. glauca release water soluble toxins from their crowns and leaf litter. Salvia mellifera and Lepechinia calycina release volatile terpenes. All are effective in inhibiting seed germination and herb growth. As the shrub stand regenerates following fire, toxic suppression of herbs increases until the next fire reinitiates the cycle by destroying both the toxins and their shrub source.

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