In California, Swank and Oechel assessed the individual and combined effects of four main factors (trenching to reduce root competition, caging to reduce herbivory, nutrient addition, and water addition) on herb germination, survival, and growth in a chaparral dominated by chamise, Adenostoma fasciculatum. Results showed strong interactions among these treatments. Therefore, no single factor can be regarded as causing the absence of herbs in chaparral. Herb establishment was limited by herbivory and root competition with shrubs for limited nutrients and/or water. Survival was enhanced by trenching, caging, and water. Growth was enhanced by nutrient addition, but the magnitude of this nutrient effect became appreciable only within trenched, caged plots. Annual species in this old chaparral stand are severely limited by the combined effects of herbivory, competition with the roots of shrubs, and nutrient and water availability. When protected from these effects, herbs flourished in chaparral soil. Although their relative importance may change from year to year and place to place, these mechanisms appear to be sufficient to account for the rarity of annual herbs from Adenostoma chaparral.
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