Dominance of California Annual Grasslands by Brassica nigra

Abstract

Brassica nigra (L.) Koch. is a dicotyledonous member of the annual grassland vegetation of southern California. Pasture grassland areas often show pure stands of this species surrounded by areas inhabited by various mixtures of the grassland associates. Yearly regeneration of these pure stands occurs at the first rainfall of late autumn and initiates the growing season of all annual species of this vegetative type. At this time the weedy grass species which characterize these annual grasslands are notably absent in the areas of Brassica dominance. The exclusion of the grasses, such as Avena fatua, Bromus rigidus and Bromus mollis occurs primarily as a result of an allelopathic mechanism, involving the leaching of water soluble toxins from standing dead stalks and dead leaf material remaining in the soil. The pattern of geographic isolation of the species was shown not to be a result of spatial differences in soil texture, pH or temperature. Competitive mechanisms involving light or soil factors failed to be effective in this case of interference. Animal foraging was shown to be of minimal consequence. A potentially highly allelopathic, volatile toxin from living vegetative parts of Brassica was shown to lack an ecologically effective mechanism. Exudates from germinating seeds exhibited no effect upon grass seed in the laboratory and the possibility that this mechanism could be operative under field conditions is negligible. Water soluble phytotoxins, however, provide an ecologically effective mechanism which is operative in this field-inhibition phenomenon. Water soluble toxins inhibit the annual grassland species and permit the invasion and control of extensive grassland areas by Brassica nigra. INTRODUCTION Brassica nigra (L.) Koch. is a dicotyledonous member of the annual grassland vegetation of southern California. This vegetation is composed of a mixture of many species of grasses and broad-leaved herbs, though the grasses usually predominate (Heady, 1956; McNaughton, 1968). Perennial species vary in abundance depending on location, but generally comprise less than 1% of the plant cover (Talbot and Biswell, 1942). Like most of the dominant grasses, B. nigra is an introduced species, native to Europe and the Mediterranean. Some of the most common associates include Avena fatua, Bromus rigidus and B. mollis (McNaughton, 1968). All the aforementioned species can be characterized as weedy opportunists which quickly take advantage of any form of disturbance. This vegetational type has now generally re1 From a dissertation for the doctorate submitted by the first author to the Graduate Division of the University of California, Santa Barbara. 2 Financial support from National Science Foundation Grants GB4058 and GB6814 to C. H. Muller. 3 Present address: Department of Forestry, University of Illinois, Urbana 61801.