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THE VEGETATION AND FLORA OF THE MARIN ISLANDS, CALIFORNIA

Robert Ornduff and Michael C. Vasey
Madroño
Vol. 42, No. 3 (JULY-SEPTEMBER 1995), pp. 358-365
Stable URL: http://www.jstor.org/stable/41425083
Page Count: 8
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THE VEGETATION AND FLORA OF THE MARIN ISLANDS, CALIFORNIA
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Abstract

The Marin Islands in San Francisco Bay are two very small islands a short distance off the shore of San Rafael, Marin County, California. The vegetation and floras of the two islands are strikingly different. The herbaceous flora of the plateau on West Marin Island is dominated by a dense sward of the introduced annual grasses Bromus diandrus and Avena fatua, whereas the flora of the plateau on East Marin Island is dominated by a mixture of smaller, introduced grass species growing with several perennial and annual native forbs and grasses. Snowy Egrets, Great Egrets, Blackcrowned Night Herons, Great Blue Herons, Western Gulls, and Canada Geese nest on West Marin Island; of these, only Canada Geese nest sparingly on East Marin Island. We attribute the floristic and vegetational differences between these islands to alterations of soil chemistry by bird guano and physical disturbances of the vegetation caused by the birds on West Marin Island, factors that are largely missing from East Marin Island, which was, until very recently, inhabited. The vegetation of both islands includes mixed evergreen forest, coastal prairie, coastal salt marsh, and northern coastal scrub. Several species of introduced ornamental trees, shrubs, and herbs grow on East Marin Island but not on West Marin Island. Both islands support a partially fringing woodland of Quercus agrifolia, Aesculus californica, and Umbellularia californica. The native Sanícula crassicaulis, Stachys ajugoides var. rígida, Dryopteris arguta, Lonicera hispidula var. vacillans, and Symphoricarpus albus var. laevigatus were frequent on East Marin Island but were not observed on West Marin Island; Claytonia perfoliata and Polypodium californicum were abundant on the former island but were sparingly represented on the latter one. The known native vascular flora of the two islands consists of 65 species; 26 of these occur on both islands, 37 are known only from East Marin Island, and two were collected only on West Marin Island.

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