Coastal Sage Scrub 6,550 acres
Coastal sage scrub is comprised of low, soft-woody subshrubs, generally no higher than three feet (one meter). This vegetation community is typically located on dry sites, such as steep, south-facing slopes or clay-rich soils that are slow to release stored water. Dominants may include California(Artemisia californica), flat-top buckwheat, laurel sumac, white sage, broom baccharis, and San Diego sunflower. Other, less frequent, constituents include spiny redberry, deerweed, and yellow bush-penstemon. Native understory species include foothill stipa, ashy spike-moss, chalk live-forever, and coast barrel cactus (Ferocactus viridescens).
When the Refuge was mapped in 2007, coastal sage scrub vegetation covered approximately 70% (about 6,550 acres) of the lands included within the Otay-Sweetwater Unit.
Chaparral 2,110 acres
This shrubland vegetation is widely distributed throughout California on dry slopes and ridges at low and medium elevations where it occupies thin, rocky, or heavy soils. It is typically composed of hard-stemmed, leathery leaved shrubs, with a species composition that varies considerably with location. The plants of this community have adapted to wildfire by either re-sprouting from underground roots following a burn, and/or producing seeds that require a fire-related cue to stimulate germination. If fires occur too frequently, the chaparral vegetation may be replaced with weedy, nonnative vegetation.
Four distinct chaparral associations on the refuge include southern maritime chaparral, southern mixed chaparral, chamise chaparral, and scrub oak chaparral.
Southern maritime chaparral is generally comprised of low, relatively open vegetation characterized by such species as wart-stemmed ceanothus, Del Mar manzanita, and summer-holly. Geographically, southern maritime chaparral is restricted primarily to the coastal fog belt.
For southern mixed chaparral, broad-leaved sclerophyllous (i.e., hard-leaved) plants that occupy protected north-facing slopes dominate. For example, chamise, toyon, and mission manzanita. These dominant shrubs in this community are generally about five feet high on slopes, and 6-8 feet high in ravines. The understory of dense stands of mixed chaparral is fairly sparse, however, mariposa-lilies can be found here.
Chamise chaparral is characterized by large stands of chamise, three to ten feet in height. Additional shrub species, such as mission manzanita and our Lord’s candle (Yucca whipplei), may be present, but contribute little to the overall cover. Chamise chaparral occurs on xeric slopes and ridges, and is found on shallower, drier soils or at somewhat lower elevations than southern mixed chaparral.
Scrub oak chaparral is a dense, evergreen chaparral association that approaches 20 feet in height and is dominated by scrub oak. This habitat occurs on more mesic sites than other chaparral associations and often at slightly higher elevations, allowing it to recover from fire more quickly than other chaparral types. Some of the understory species that may be present include poison oak and bedstraw.
Rock outcrops are common within areas supporting chaparral vegetation. Where present, these rocky outcrops often provide distinct microhabitats that support plant species generally absent or uncommon throughout the surrounding chaparral. Such species include melic grass (Melica frutescens), California bee-plant (Scrophularia californica var. floribunda), cotton fern (Cheilanthes newberryi), California brickelbush (Brickellia californica), and caterpillar phacelia (Phacelia cicutaria).
The vegetation mapping conducted in 2007 indicates that approximately 23% of the lands included within the Otay-Sweetwater Unit support chaparral vegetation. Within the Del Mar Mesa Vernal Pool Unit, approximately 99 percent of the lands support chaparral vegetation.
Riparian Forest 250 acres
Southern Cottonwood-Willow Riparian Forest
This vegetation community occurs along streams and rivers, occupying relatively broad drainages and floodplains. It consists of trees that are generally greater than 20 feet high. Dominated by mature winter deciduous trees, including Fremont’s cottonwood and several species of tree willows, this community often has a dense understory of shrubby willows, mulefat, and mugwort. The dominant species require moist, bare mineral soil for germination and establishment, an environment that is provided after flood waters recede.
Southern Coast Live Oak Riparian Forest
Denseforests of coast live oak located in bottomlands and outer floodplains along larger streams. This vegetation tends to be richer in herbs and poorer in understory shrubs than other riparian communities. In addition to coast live oak, these areas are characterized by mugwort, toyon, California wild rose, desert elderberry, and poison oak.
Southern Arroyo Willow Riparian Forest
This vegetation type consists of winter-deciduous riparian forests with closed or nearly-closed canopies that are dominated by moderately tall broad-leafed trees, primarily arroyo willow. Typically occurring on frequently flooded areas along rivers and streams, southern arroyo willow riparian forest and the understory usually consists of mulefat and shrubby willows, including sandbar willow and occasional Goodding's (black) willow. Understory plants can include western ragweed, mugwort, and spiny rush.
Grassland (native and non-native) 140 acres
Native Perennial Grassland
Perennial grassland is grassland dominated by native bunchgrass. Within the Refuge, it is typically dominated by dense, irregular tussocks of native purple needlegrass interspersed with several other herbs and grasses including shooting-star, blue-eyed grass (Sisyrinchium bellum), morning glory, splendid mariposa lily, and several non-native grasses, including fescue and soft chess. This plant association, which typically alternates with coastal sage scrub on some clay soils, often occurs on more mesic exposures and at the base of slopes. Approximately 15 acres within the McGinty Mountain area have been mapped as native grassland vegetation. Native grassland is also present in other smaller areas within the Otay-Sweetwater Unit.
Non-native grassland is composed of annual grasses often associated with numerous species of showy-flowered native annual forbs. Characteristic species include wild oats, foxtail chess, ripgut grass, ryegrass, and mustard. Most of these species originated from the Mediterranean region, similar to California, making it easy for them to thrive. Plant germination in these grasslands occurs with the onset of the late fall rains, well before many native forbs have sprouted; giving the non-natives a competitive head start in growth. The plants are usually dead through the dry season, persisting as seeds. Over 100 acres on the Otay-Sweetwater Unit were mapped as non-native grasslands in 2007. Also, some non-native grassland areas within the San Miguel Mountain and Sweetwater River burned in the 2007 Harris Fire. Although dominated by non-native grasses and annuals at the time of mapping, these areas may ultimately recover from the effects of the fire and eventually support coastal sage scrub or native grassland habitat.
Oak Woodland 90 acres
Coast Live Oak Woodland. This woodland is dominated by evergreen coast live oaks (Quercus agrifolia) reaching 32 to 82 feet (ten to 25 meters) in height. This vegetation typically occurs on north-facing slopes or in shaded ravines, and intergrades with coastal sage scrub or mixed chaparral on drier sites (Holland 1986). The shrub layer is typically poorly-developed but may include toyon (Heteromeles arbutifolia), currant (Ribes spp.), laurel sumac (Malosma laurina), and desert elderberry (Sambucus mexicana). The herbaceous component is continuous and often dominated by weedy species.
Vernal pools are a unique, specialized form of seasonal wetlands that occur in a geographical area extending from southern Oregon through California into northern Baja California, Mexico. In southern California, these pools form in areas where downward percolation of water is prevented by an impervious subsurface layer consisting of claypan, hardpan, or volcanic stratum. Under these conditions, the pools appear as shallow depressions filled with rainwater during fall and winter months, and as dry depressions in the summer after the water in the pools has evaporated. As a result, vernal pools support a distinctive living community adapted to extreme variability in hydrologic conditions.
The Del Mar Mesa Vernal Pool Unit of the Refuge, exhibits a herbaceous community dominated by annual herbs and grasses. Sensitive plant species include San Diego button celery (Eryngium aristulatum var. parishii), little mousetail (Myosurus minimus ssp. apus), spreading navarretia, Orcutt’s brodiaea (Brodiaea orcuttii), California adder’s tongue-fern (Ophioglossum lusitanicum ssp. californicum), and San Diego mesa mint. The surrounding vegetation is often chamise chaparral.
In the Otay-Sweetwater Unit of the Refuge, vernal pools are generally characterized by lower, overall vegetative cover than hardpan pools found at the Del Mar Mesa Vernal Pool Unit. Typical sensitive plant species in these pools include San Diego button celery; little mousetail; spreading navarretia; California Orcutt’s grass (Orcuttia californica), and Otay mesa mint (Pogogyne nudiuscula). These pools are generally surrounded by grassland or sparse coastal sage scrub rather than chaparral.
Historically, vernal pool habitat, which was scattered throughout San Diego County in locations with appropriate soil and hydrological conditions, covered approximately 200 square miles, or about 6% of the County. Only a fraction of this habitat remains intact today. Current estimates indicate the between 95 – 97% of the vernal pool habitat in the San Diego County has been lost to urbanization and agriculture. In recent years, efforts have been made to restore and/or recreate vernal pool habitat on preserved lands, including within the Otay-Sweetwater Unit of the San Diego NWR, where vernal pools were historically present.