Habitat Types

hopper cyn photo

The vegetation at Hopper Mountain NWR is composed of approximately 679 acres of coastal sage scrub, 673 acres of California black walnut woodland, 627 acres of annual and perennial grassland, 278 acres of chamise chaparral, 47 acres of coast live oak woodland, and 29 acres of riparian vegetation. In addition, the refuge has a small, 1-acre fresh-water marsh, dominated by bulrushes and other emergent vegetation and surrounded by approximately 5 acres of willow-dominated wetlands.

Coastal Sage Scrub

Coastal sage scrub covers approximately 28% of the refuge, or approximately 679 acres, and is typically dominated by purple sage (Salvia leucophylla). Common species within this community on the refuge include coffeeberry (Rhamnus californica), golden yarrow (Eriophyllum confertiflorum), giant wildrye (Leymus condensatus), grape soda lupine (Lupinus excubitus), sugar sumac (Rhus ovata), and California sagebrush (Artemisia californica). Coastal sage scrub is found primarily on the Calleguas shaly loam and Castaic-Balcom complex soil types at the refuge, often on very steep slopes. Coastal sage scrub is a threatened vegetation community in the southwest region of California, and the type that occurs on Hopper Mountain NWR is the least protected type of coastal sage scrub (Davis et al. 1994; Davis et al. 1995). A large proportion (approximately 87%) of the coastal sage scrub landscapes dominated by purple sage are on private lands within the western Transverse Ranges. Hopper Mountain NWR is located where the South Coast Range meets the western Transverse Ranges—publically owned lands that are geographically near the 87% of the community that is under private ownership. The greatest threats to coastal sage scrub are habitat loss, fragmentation, and degradation, which typically are associated with increasing fire frequencies, invasion of non-native plant species, and unregulated livestock grazing (CalPIF 2004).

California Walnut Woodland

California walnut groves (or woodlands) (classified in the Manual of California Vegetation as the Juglans californica alliance), are endemic to southern California and are primarily dominated by California black walnut. This vegetation type covers approximately 673 acres or approximately 27% of the refuge and is the most common woodland plant community present. In more open-canopy areas, California black walnuts associate almost exclusively with blue elderberry (Sambucus nigra ssp. cerulea). In canyons and the northeastern portion of Hopper Mountain NWR, black walnut mixes with other species, including coast live oak (Quercus agrifolia) and canyon oak (Quercus chrysolepis). Other species associated with
California walnut groves at Hopper Mountain NWR include hollyleaf redberry (Rhamnus ilicifolia) and California flowering ash (Fraxinus dipetala). California walnut woodland is a plant community that is endemic to southern California. Southern California black walnut (Juglans californica var. californica) is the dominant tree species in this community and it is state-listed as vulnerable. The California black walnut stands located on Hopper Mountain NWR exist as some of the last and largest remaining stands in southern California. See also Special Status and Culturally Important Vegetation in this chapter.

California Annual and Perennial Grassland

California annual and perennial grassland covers approximately 627 acres or approximately 25% of the refuge. The grasslands at the refuge are predominately composed of non-native annual grasses such as wild oats (Avena spp.), brome grasses (Bromus spp.), and foxtail barley (Hordeum murinum), but native species such as perennial needle grasses (Nassella spp.) and small fescue (Vulpia microstachys) are also present. Grasslands on the refuge contain numerous forb speciess, including native wildflowers like butterfly Mariposa lily (Calochortus venustus), checkerbloom (Sidalcea sp.), and golden stars (Bloomeria crocea). Some areas of the refuge also have a high percentage of non-native herbaceous species such as mustards (Brassica nigra and Hirschfeldia incana) and Italian thistle (Carduus pycnocephalus). Mustard species are especially prevalent in the grassland area between the wetland and the house. Grasslands cover only 3% of the vegetation in the southwest region of California (Davis et al. 1995). Almost all of those grasslands are nonnative grasslands. According to Davis and others (1995), non-native grasslands [not only native grasslands] are at risk in this region because only 6% are found in areas managed for maintenance of biodiversity, and 21% are on public lands managed for multiple uses. A large proportion (73%) is found on private lands not managed primarily for maintenance of biodiversity. The greatest threats to California grasslands are habitat loss and habitat fragmentation (CalPIF 2000). Most grasslands in California have been converted to agricultural or urban lands. In addition to loss of habitat, the patch size of remaining grasslands has decreased. Research from other regions in North America has demonstrated that grassland bird species (including some that breed in California) can be sensitive to patch size (CalPIF 2000).

Chamise Chaparral

Chaparral on the refuge is typically dominated by chamise (Adenostoma fasciculatum) and is generally found on steep slopes with rocky, poorly developed soils in the southern portion of the refuge. The far southern and much of the far eastern portions of the refuge have no roads or only very narrow ATV trails, which are difficult and dangerous to traverse. In addition, the terrain in these areas is extremely steep, with much exposed and loose rock. Because of these conditions, this plant community was not well sampled in field surveys. It is estimated to cover approximately 278 acres or approximately 11% of the refuge. Chamise chaparral includes a diverse mix of plants, with common species such as manzanita (Arctostaphylos spp.), white sage (Salvia apiana), black sage (Salvia mellifera), yerba santa (Eriodictyon crassifolium), coyote brush (Baccharis pilularis), hoary ceanothus (Ceanothus oliganthus), poison oak (Toxicodendron diversilobum), penstemon (Penstemon sp.), deerweed (Acmispon glaber var. glaber [Lotus scoparius var. scoparius]), buckwheats (Eriogonum spp.), and spineflower (Chorizanthe sp.).
There are 17 types of chaparral in the southwest region of California covering approximately 36% of the current land cover (Davis et al. 1995). Chamise, the most widespread chaparral plant species in the region, is found in many types of chaparral and occurs as a dominant or co-dominant species on approximately 67% of the chaparral in the region. The chamise chaparral vegetation type specifically constitutes 10% of the chaparral in the region. Chamise chaparral is fairly well represented in areas managed for maintenance of biodiversity (11%) and public lands managed for multiple uses (47%). Only 42% is found on private lands not managed primarily for maintenance of biodiversity. The greatest threats to chamise chaparral are the same as those of coastal sage scrub. These threats are habitat loss, fragmentation, and degradation such as sometimes occurs with increasing fire frequencies, invasion of non-native plant species, and unregulated livestock grazing (CalPIF 2004).

Coast Live Oak Woodland and Riparian Forest

Coast live oaks are found scattered throughout the refuge in areas with more developed soils and more moisture. In canyon bottoms and in areas with flowing water, they form riparian forest, whereas in other parts of the refuge, they are described as coast live oak woodlands. Both the riparian and non-riparian woodlands are part of the Quercus agrifolia alliance, but are separated here because of the important nature of riparian forests on the refuge and due to differences in management actions for those areas. The coast live oak woodlands cover approximately 47 acres, or nearly 2% of the refuge, while the coast live oak riparian forests cover approximately 29 acres, or a little more than 1% of the refuge. While there is much less than 1% cover of southern coast live oak riparian woodlands throughout the southwest region of California, this vegetation type is fairly well represented (16%) in areas managed primarily or maintenance of biodiversity in this region (Davis et al. 1995). In the northeastern part of the refuge, at some of the highest elevations of Hopper Mountain NWR, coast live oak are associated with other large tree species such as bigleaf maple (Acer macrophyllum) and bigcone Douglas-fir (Pseudostuga macrocarpa). On north-facing slopes, this alliance may contain species associated with coastal sage scrub, whereas on hotter, drier slopes, the understory vegetation is composed of grassland species. Several of the oak trees in the central part of the refuge (south of the southernmost end of the refuge road) burned in the 2003 Piru Fire. Some of those oaks were killed, but many have resprouted and are showing vigorous growth. Coast live oaks in the riparian areas of the refuge are associated with other riparian species such as willow (Salix lasiolepis and S. exigua), as well as a small number of California sycamore (Platanus racemosa) and Fremont cottonwoods (Populus fremontii subsp. fremontii). Riparian ecosystems in arid western landscapes are ecologically important because they have the most diverse bird communities and are critical in maintaining the quality of in-stream habitat (Riparian Habitat Joint Venture 2004). Riparian areas in California provide important breeding and over-wintering grounds, migration stopover areas, and corridors for dispersal of birds. In fact, the loss of riparian habitats may be the most important cause of population decline among landbird species in western North America (DeSante and George 1994). Riparian habitats have been lost and degraded over the past 100 years through water management practices, agricultural conversions, channelization, recreation development, and the introduction of nonnative species (Knopf et al. 1988).

Arroyo Willow Thickets

Arroyo willow thickets (Salix lasiolepis alliance) are found in the area surrounding the man-made marsh in the central part of the refuge. They cover roughly 5 acres, or less than 1% of the refuge land. A natural spring to the northwest of the house complex creates the wet conditions that these willows require. Within the arroyo willow thickets, Salix lasiolepis may form pure to nearly-pure stands. However, arroyo willows are also found in the riparian forests as an understory species.
Freshwater Marsh
In 1989, a stone wall, made from large chunks of mineral deposits from the riparian areas, was constructed around a 330-foot by 225-foot marsh at the refuge, in close proximity to the house. This wall was built to keep cattle from grazing in this area and trampling the vegetation. Cattle were removed completely from refuge property in 1991, and the marsh began to grow and expand. The
stone wall remains and continues to impound water that supports the marsh. Since 1991, the marsh and surrounding wetland area has expanded to over 5 acres, and it continues to expand. The freshwater marsh portion of this wetland area covers approximately 1 acre and is dominated primarily by bulrushes (Scirpus americanus and S. microcarpus), but other wetland plants are found here, including nettles (Urtica spp.), broad-leaved cattail (Typha latifolia), hemp dogbane (Apocynum cannabinum), and poison oak (Toxicodendron diversilobum). Together, the vegetation of the freshwater marsh and arroyo willow thicket provide important wetland habitat for many avian, amphibian, reptilian, invertebrate, and mammalian species.