The two types of coastal dunes in San Diego are active coastal dunes and foredunes. Active coastal dunes occur on barren, mobile sand accumulations . The size and shapes of these formations are determined by abiotic site factors such as, wind direction and speed, site topography, sand source, and grain size, rather than by stabilizing vegetation. Coastal dunes are unvegetated and will often overrun adjacent foredunes. Foredunes are similar to active coastal dunes, but with less wind and/or a smaller supply of sand and/or more available groundwater. Foredunes are drier, slightly warmer, and exposed to weaker and less persistent onshore winds. A coastal bluff is a naturally formed precipitous landform that generally has a gradient of at least 200 percent (1:2 slope) with a vertical elevation of at least 10 feet . The southern coastal scrub vegetation community exists on coastal bluffs . Southern California bluff scrub is a low growing scrub up to 2 meters tall. The scrub forms continuous mats or may be more scattered. The vegetation consists of dwarf shrubs, herbaceous perennials, and annuals.
The coastal dunes alliance, Ambrosia chamissonis-Abronia maritima, occurs widely up and down the Pacific coastal strand of California and is restricted to the coastal dunes and beaches . The most extensive strands occur from Silver Strand State Beach to the Tijuana River mouth. In this alliance, Abronia latifolia and/ or Ambrosia chamissonis mix with other perennial herbs, grasses, and low shrubs to form a low herbaceous canopy. Species characteristic of the foredunes include native plants such as: red sand verbena (Abronia maritima), beach sand verbena (A. umbellata), beach bur (Ambrosia chamissonis), beach salt bush (Atriplex leucophylla), beach morning glory (Calystegia soldanella), shruby beach primrose (Camissoniopsis cheiranthifolia suffruticosa), inland saltgrass (Distichlis spicata), and Menzie’s goldenbush (Isocoma menziesii), as well as nonnative plants such as European searocket (Cakile maritima) and iceplant (Carpobrotus edulis) . The foredune vegetation community also includes the Ambrosia chamissonis-Abronia maritima-Cakile maritima Association. This association includes Ambrosia chamissonis with Abronia maritima or A. umbellata, and/or the nonnative Cakile maritima as dominants in a low herbaceous canopy. This vegetation type is limited to coastal dune habitats throughout San Diego County. Species characteristic of the coastal bluff vegetation community include natives such as: saltbush (Atriplex spp.), coast morning glory (Calystegia macrostegia ssp. cyclostegia), island morning glory (C. macrostegia), Indian paintbrush (Castilleja affinis), Orcutt’s spineflower (Chorizanthe orcuttiana), giant coreopsis (Leptosyne gigantean), sea dahlia (L. maritima), dudleya (Dudleya spp.), California encelia (Encelia californica), seaside golden yarrow (Eriophyllum staechadifolium), goldenbush (Ericameria spp.), cliff aster (Malacothrix saxatilis), chilicothe (Marah macrocarpa), coast prickly pear (Opuntia littoralis), and lemonade berry (Rhus integrifolia), as well as nonnatives such as sea fig (Carpobrotus aequilateralus) and crystalline iceplant (Mesembryanthemum crystallinum) .
Active coastal dunes are found along the Pacific Coast where sandy beaches are present and coastal headlands are absent .
Dunes and coastal bluff vegetation in the MSPA are included in the “other” vegetation community for mapping and acreage calculation, so the exact acreage of dunes and coastal bluffs in unknown.
The plants found on coastal sand dunes are primarily prostrate herbs with creeping stems and long taproots . The leaves are typically small, somewhat succulent, and often hairy and grayish in color. These features aid the plants in tolerating drought, salt stress, and intense sunlight. The vegetation of foredunes is dominated by succulents, perennial herbs, and subshrubs . The vegetation lacks perennial grasses, but has a higher proportion of subshrubs that grow up to 30 cm tall. Coverage varies from complete to scattered. Growth and flowering occur in early to mid-spring. Coastal bluff vegetation is exposed to varying, moisture-laden winds with high salt content. The soils on coastal bluffs are usually rocky and poorly developed.
Coastal dunes are dynamic habitats affected by waves, tides, wind, and trampling . Dunes develop when there is a substantial amount of blown, dry sand. Coastal bluffs are escarpments or steep faces of rock, decomposed rock, or soil resulting from erosion, faulting, or folding of the land mass and are located in the coastal zone .
Coastal dunes in California have been significantly threatened and reduced by development, off road vehicle use, and exotic species invasion . Human disturbance and competition for the use of beaches by beach-goers can endanger birds that rely on beaches for nesting habitat, such as the least tern (Sternula antillarum) and snowy plover (Charadrius nivosus) . Coastal bluffs are threatened by the increasing pressure to develop shorelines, as well as a number of natural processes. High tides and large waves can contribute to cliff failure and can threaten the coastal scrub bluff vegetation community. Large waves facilitate coastal bluff erosion by removing protective beach sediment and allowing waves to directly attack the toe cliff . Moderate-to high-intensity ENSO events can elevate sea levels and lead to high wave energy, which can cause sea-cliff erosion and shoreline recession. Problems from coastal bluff failures have emerged from Torrey Pines to San Onofre . In March 2016 a large sea cave opened up beneath a Carlsbad bluff, and in August 2018 large chunks of the Del Mar cliffs collapsed, impacting train traffic and narrowly missing sunbathers below .