Rare Plant Communities

A plant community is an assemblage of plant species that interact with one another and their environment in a given area. Environmental factors such as topography, soil, climate, hydrology and geology are important in determining where plant communities occur. Rare plant communities are those that are of highly limited distribution. These communities may or may not include special status plant (species that are officially "listed" by federal and state agencies) or others that, based on available data, meet the definition of rare, threatened, or endangered. Northern California hosts a number of these rare plant communities, such as vernal pools, serpentine outcrops, Great Valley riparian meadow and alkaline grasslands.

Because of their limited, localized distribution, rare plant communities are more susceptible than other habitats to development and land conversion.

Northern Hardpan Vernal Pools

This type of vernal pool occurs on old alluvial fans along the eastern margins of California's Central Valley, where acidic, iron-silica cemented soils form a hardpan where water pools seasonally. Evaporation, and not runoff, empties the pools in the spring. When rising spring temperatures evaporate the seasonal pools, concentric bands of colorful vegetation and blooms circle the pools, sometimes called ‘rim blooms'. Conditions lending themselves to this type of habitat often occur over continuous areas, rather than in isolated spots, so vernal pools in the Central Valley tend to occur in clusters called "complexes". Within these complexes, pools may be fed or connected by low drainage pathways called "swales".

The dominant plants within this habitat are generally native, facultative wetland to obligate wetland plants typical of vernal pools that have relatively short seasonal inundation. Plant species commonly found in northern hardpan vernal pools include navarettia, Fremont's goldfields, Toad rush, coyote thistle, and Downingia spp.

Vernal pools are considered a threatened ecosystem in California, with a significant proportion of their distribution lost to cultivation or urbanization. In California, they are among the few ecosystems still dominated by native flora and are critical habitat for numerous endemic and rare species.

Fresh Emergent Marsh Wetland

Emergent marsh wetlands characterize many different, specific wetland habitats, and are most common on gently rolling topography. These wetland types are flooded frequently, so that the habitats are dominated by rooted herbaceous hydrophytes. The vegetation is typically dominated by perennial monocots, and ranges in size from small clumps to areas covering several kilometers. The vegetation zones characteristically follow basin contours and reflect the relative depth and duration of flooding. Soils are predominantly silt and clay, although coarser sediments and organic material may be intermixed. In some areas organic soils may constitute the primary growth medium.

The acreage of emergent marsh wetlands in California has decreased dramatically since the turn of the century due to drainage and conversion to other uses, primarily agriculture. Fresh emergent wetlands are among the most productive wildlife habitats in California. They provide food, cover, and water for many different species of birds and numerous mammals, reptiles, and amphibians.

Vernal Swales

Vernal swales differ from vernal pools in that they function distinctly as shallow, seasonal conveyance channels. They typically connect vernal pools or convey shallow seasonal flows down gradual inclines often collecting water in a vernal pool or seasonal wetland. Vernal swales and pools may share plant species, successive ‘rim bloom' plant assemblages and soil types.

Waters of the United States

Protected by the federal Clean Water Act, "Waters of the United States" describes a diversity of relatively permanent, standing or continuously flowing bodies of waters. This includes lakes, rivers, and streams, but also features such as intermittent streams, lakes, sloughs, wetlands and ponds. Man-made water bodies can be protected waters as well, if they have positive indicators such as wetland vegetation, anaerobic soils, or wetland hydrology.

Waters of the U.S. provide critical ecosystem benefits. Approximately 35% of all federally-listed rare and endangered animal species either live in or depend on these waters, and they serve as important habitat for migratory birds. Waters of the U.S. also provide important social benefits, providing recreation, flood protection, drinking water and navigation. While thousands of construction projects threaten the presence and ongoing health of these resources each year, ongoing federal efforts are focusing on a net gain in wetlands across the country.

California Annual Grassland

California annual grassland is an upland plant community typically dominated by non-native annual grasses, but containing a diverse assemblage of native and nonnative forbs. Common wildlife species that use annual grassland habitats include the garter snake, western rattlesnake, California ground squirrel, western burrowing owl, short-eared owl, horned lark, western meadowlark, northern harrier, and American kestrel.

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