Institute for Regional Studies of the Californias

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Map 12. Vegetation and Land Cover

John O'Leary

The bulk of the International Border Planning Area has been developed and also contains substantial amounts of disturbed habitat as seen in Map 12. These altered areas of vegetation and land cover types are confined almost entirely to the western two-thirds of the planning area and represent rapid development that has occurred around Tijuana and Mesa de Otay since the Second World War. Most of the eastern one-third of the planning area is covered largely by relatively undisturbed coastal sage scrub and chaparral vegetation. This area also contains substantial amounts of southern interior cypress forest, especially on or near Otay Mountain, which is within the regional Multiple Species Conservation Program (MSCP). Late in 1999, approximately 18,500 acres (7,493 hectares) of Otay Mountain and surrounding land were set aside by the U.S. federal government as the Otay Mountain Wilderness Area to preserve the habitat (location indicated on Map 3). Coastal sage scrub in this portion of the planning area and elsewhere provides critical habitat to a large number of rare, threatened, and endangered species, most notable the California gnatcatcher (Polioptila californica californica). Further, this region of the planning area serves as a vital dispersion link for the California gnatcatcher and other species between the United States and Mexico around the heavily developed western sector. The Otay Mountain Wilderness Area is also home to approximately 20 sensitive plant species, including the unique Tecate cypress (Cupressus forbesii). Remnant riparian forest and scrub vegetation occur sporadically along the Río Alamar that forms the southeastern border of the International Border Planning Area.

The western two-thirds of the planning area contains other examples of threatened vegetation types that merit serious attention for protection. A substantial area of maritime succulent scrub and coastal sage scrub vegetation remains on the U.S. side of the border immediately north of San Ysidro and west of Interstate 805. In addition, highly valuable wildlife habitat is provided by various riparian and wetland vegetation types (e.g., riparian scrub, freshwater marsh, and southern coastal saltmarsh) that also occur on the U.S. part of the planning area in the Tijuana River Valley.

Located at the mouth of the Tijuana River, the Tijuana River Estuary is one of the few salt marshes remaining in Southern California and northern Baja California. The site of approximately 2,500 acres (1,000 hectares) is an essential breeding, feeding and nesting ground and key stopover point on the Pacific Flyway for over 370 bird species. The estuary supports a wide diversity of plants, including the endangered salt marsh bird's-beak (Cordylanthus maritmus ssp maritimus). California cordgrass (Spartina foliosa) forms robust stands along tidal channels in the northern reaches (lowest elevations) of the estuary. At slightly higher elevations above the spartina-dominated community are found several succulents, including pickleweed (Salicornia spp.) and saltwort (Batis maritima). At higher elevations within the estuary, these succulents grade into a cover of shoregrass (Monanthochlo' littoralis). At the highest elevations, pickleweed codominates with shoregrass.

Another important habitat in the International Border Planning Area is that of the vernal pools. These are depressions on level or gently sloping surfaces that are filled with water each rainy season and contain unique assemblages of animals and plants, including the Otay Mesa mint (Pogogyne nudiuscula) and the San Diego fairy shrimp (Branchinecta sandiegonensis) that are listed as endangered species in the United States. The land occupied by vernal pool habitats is in areas that are highly developable. Most vernal pools have been destroyed or heavily disturbed in the planning area. Remnants of this habitat are found on Otay Mesa and, perhaps, on Mesa de Otay.

Photo

Vegetation restoration experiment in the Tijuana Estuary with Imperial Beach in the background. 1999.

Photo

Highway 1D bridge over the Alamar River at the southern edge of the International Border Planning Area. Note riparian vegetation. 2000.