Ecological dynamics

Before European settlement, the natural plant communities for this ecological site range from a California sagebrush and black sage dominated site with very few herbaceous species (also commonly known as coastal scrub) to a native perennial bunchgrasses and perennial and annual forbs site. The reference states for this ecological site are similar to its pre-European state; however the density of the sagebrush and black sage may be different due to fire suppression and the herbaceous species are now heavily dominated by annual grasses and forbs.
The reference state for this ecological site is dominated primarily by California sagebrush (Artemisia californica) and black sage (Salvia mellifera) with California buckwheat (Eriogonum fasciculatum) also commonly present. The herbaceous layer is composed of needlegrasses (Nassella spp.), pine bluegrass (Poa secunda), larkspur (Delphinium spp.), lupine (Lupinus spp.), paintbrush (Castelleja spp.), and wild onions (Allium spp.). Some lesser dominants also include chamise (Adenostoma fasciculatum), foothill pine (Pinus sabiniana), deerweed (Lotus scoparius), monkeyflower (Mimulus spp.), penstemon (Penstemon spp.), phacelias (Phacelia spp.), wooly bluecurls (Trichostema lanatum), and bedstraw (Galium spp.). The non-native annual species that can also be found throughout this site include oats (Avena spp.), bromes (Bromus spp.), annual fescues (Vulpia spp.), and Maltese star-thistle (Centaurea melutinus).

California sagebrush is the key component to the coastal sage scrub plant community, which is composed primarily of unevened-aged stands of low-growing, non-sclerophyllous, shallow-rooted subshrubs (Howard 1993). The coastal sage scrub is found as a plant community mosaic throughout Pinnacles National Monument with the chamise chaparral, blue oak woodlands and annual grasslands. It is found on the steepest portions of the south-facing slopes on shallow, skeletal soils adjacent to the chamise chaparral and across from the blue oak woodlands in the Monument. Coastal sage scrub also commonly consists of black sage, California buckwheat, and deerweed (Lotus scoparius).

California sagebrush is a unique plant that is both a pioneer species and a late seral species. The community boundaries of the coastal sage scrub are considered dynamic, depending on successional patterns influenced by a disturbance; the relative success of different species at establishing seed in the seedbank; and the competitive interactions following a disturbance (Howard 1993). Some research suggests that California sagebrush will invade chamise habitat that has been disturbed, but that most often it’s gradually replaced by chamise in the absence of disturbance. Other research also goes so far as to say that if the disturbances are too frequent, the coastal sage scrub will regress further to annual grassland.

The primary natural disturbance for coastal sage scrub is believed to be fire, although historical documentation of fire in coastal sage scrub is too sparse to allow for estimation of natural fire intervals (Howard 1993). For this ecological site, given it’s proximity to the chamise chaparral plant community, fire intervals would be similar, if not the same at approximately 20 years. The fire intensity is generally higher in the coastal sage scrub than then in the lower seral chamise due to the higher litter accumulations and the higher percentage of terpenes in the scrub vegetation, which would cause them to be more flammable. Light severity fire will top-kill California sagebrush and severe fires will cause plant mortality. However, California sagebrush has adapted itself to fire by sprouting from the root crown in post-fire communities and by establishing seeds that are deeply buried to require charred wood leachate to break dormancy (Howard 1993). Most California sagebrush seed is fire sensitive and their seedbank numbers are moderate, so the seedling densities the first year after fire may be low. Seedling recruitment from sprouts or wind-blown seed disseminated from surviving shrubs is usually good after the first post-fire year, however.

Fire suppression in the coastal sage scrub can create dense stands with little space in between shrubs, which can result in extremely severe, abnormally large fires. Severe fires or fires that occur too frequently would kill the seeds in the seedbank and more than likely diminish the sprouting ability of the current sagebrush on the site, leaving it susceptible to a permanent shift to annual grasslands.