Camissonia benitensis

Description

Camissonia benitensis was discovered by Peter H. Raven on a serpentine stream terrace adajcent to Clear Creek (New Idria serpentine mass; Bureau of Land Management Clear Creek Management Area) in 1960 and scientifically described by him in 1969. The plant's specific epithet, "benitensis", commemorates its discovery in San Benito County. It is a diminutive annual herb that rarely grows larger than 10 centimeters in field conditions, but can become multi-branched and exceed 30 centimeters in diameter within a horticultural setting. Its reddish stems bear plentiful linear leaves less than 1 centimeter long and minutely toothed along the edges. Plants bear four-petaled yellow flowers approximately 6 millimeters in diameter. Flowering occurs in April and May, opening in the morning and closing in early afternoon. The species is fully self-pollinating. The fruit is a cylindrical capsule up to 4 centimeters in length. The fruits remain largely closed at maturity and spring open upon wetting with the first fall rains, dispersing seeds. Seeds are tiny (1 millimeter long, 0.5 millimeter wide) and black to brown in color. The seeds have a smooth seed coat and weigh approximately 0.1 milligram each. Long-distance dispersal likely occurs as seeds in mud stuck to animals or vehicles. Seeds germinate with the first rains in fall or winter.

Camissonia benitensis bears a close resemblance to Camissonia contorta. The two species are virtually indistinguishable with the naked eye when plants are small (under 6 centimeters), as is common in their native habitat, where they occur together. Camissonia strigulosa also grows in this range but is distinguishable by its smaller flowers and prostrate habit. They dichotomous keys in The Jepson Manual first edition are not detailed enough to make certain positive identifications. One key characteristic that can help distinguish the three species is the morphology of the trichomes on the distal inflorescences. C. contorta has trichomes that look like transparent glassy rods. C. strigulosa has white linear or lance-shaped blades. C. benitensis has both kinds of trichomes. This key characteristic was lacking in Jepson Manual first edition, but is now included in Jepson Manual second edition.

A Camissonia which resembles either C. contorta or C. benitensis growing on serpentine soil in upland geologic transition zone habitat or serpentine stream terrace habitat within serpentine masses is virtually always C. benitensis. A Camissonia resembling either of the two species on serpentine stream terraces outside of serpentine masses or mixed alluvium stream terraces could be either C. contorta or C. benitensis. C. strigulosa is the common Camissonia species found growing on nonserpentine alluvial stream and river deposits within the range of C. benitensis.

Range and Habitat

Based upon the original discovery of C. benitensis on a serpentine alluvial stream terrace adjacent to Clear Creek, it was long believed that its only habitat type consisted of serpentine alluvial terraces adjacent to perennial streams and rivers, a very rare type of habitat. Until 2010 few populations of the species had been found in any other habitat. In 2010 numerous additional populations were discovered on other land forms including ancient serpentine alluvium deposits (upland hills), serpentine landslides originating from tectonic masses (upland), and serpentine soils on the periphery of large tectonic serpentine (ultramafic) masses (upland). These recently discovered habitat types are collectively called the "geologic transition zone." This habitat type is now know to constitute the majority of the habitat area and contains most of the known populations of the species. Populations found near Priest Valley in 2010 extended the previously known range 15 kilometers to the southeast.

Common features of the stream terrace are friable serpentine soils that are stable, not eroding, and not prone to frost heave. There is usually little or no clay. The plant communities in the area include chaparral and woodland with gaps in between where there are microclimates. C. benitensis occurs in these gaps. Typical woody vegetation in the area includes Quercus douglasii (blue oak), Juniperus californica (California juniper), Quercus berberidifolia (scrub oak), Quercus durata (leather oak), Arctostaphylos glauca (bigberry manzanita) and Ceanothus cuneatus (buckbrush). Gaps between the woody vegetation have sparse herbaceous plant cover. C. benitensis is generally a poor competitor with other plant species, both native and exotic. Dense woody vegetation (closed shrub gaps) appears to be detrimental to C. benitensis due to shading effects. Some areas of geologic transition zone habitat have been partially invaded by invasive annual grasses such as Bromus madritensis. These infestations are usually sparse due to the extreme physical and chemical characteristics of the serpentine soils and the low level of tolerance of Bromus madritensis to those soil characteristics. Wildfire generally has a positive effect on C. benitensis by removing native and invasive vegetation. Prescribed fire is utilized by the Bureau of Land Management to control Centaurea solstitialis which once threatened to invade stream terrace habitat occupied by San Benito evening primrose near the confluence of Clear Creek and San Benito River. Some areas of geologic transition zone habitat on private land are grazed by cattle which like wildfire, is beneficial to C. benitensis by removing competing vegetation, particularly invasive annual grasses.

The current (2012) known range for Camissonia benitensis is as follows:

Population dynamics

C. benitensis exhibits cyclic population dynamics with one cycle equal to 2 to 3 years. Total plant numbers for the species can fluctuate from a low of a few hundred plants to tens of thousands of plants during a cycle. The reason for the cycling is unknown but it may be related to predation by some type of insect that has a similar cycle. There is no obvious correlation with climate or human factors. Despite the cycling, the overall population remains stable and is slightly increasing.

C. benitensis evening primrose is well-buffered from extinction by enormous soil seed bank 100 - 1000 times that of the number of plants seen with a population in any given year. Seed in the soil seed bank has been documented to be viable for at least 20 years and likely is to be viable for much longer.

Conservation

The town of New Idria was founded in 1848 with the discovery of cinnabar (mercury ore) within the New Idria serpentine mass (BLM Clear Creek Management Area). Numerous European prospectors and cattle ranchers settled throughout the area thereafter. Historic impacts to San Benito evening primrose have included logging and mining (gravel, magnesite, chromite, cinnabar, and chrysotile asbestos). The relatively level stream terraces were historically favored by settlers for home sites and to stage industrial activities. A proposed mineral withdrawal for the New Idria serpentine mass will greatly limit any future mining in the area.

In more recent decades, the BLM Clear Creek Management Area became a popular motorized off-road vehicle (ORV) recreation area. C. benitensis was federally listed as Threatened in 1985. The primary threat cited by the Fish and Wildlife Service was off-road vehicle impacts to habitat. At the time, only nine populations of the species were known with most occurring on serpentine alluvial stream terraces within one of the highest ORV use areas at Clear Creek Management Area, Clear Creek Canyon. Nearly all of the stream terrace habitat in Clear Creek Canyon has since been fenced to exclude human impacts. A formal route designation in 2005 has further reduced ORV impacts.

The Clear Creek Management Area has been under a temporary Emergency Closure since May 1, 2008 when the Environmental Protection Agency reported that natural occurrences of chrysotile asbestos in the soil was above safe limits for people. As of February 2011, BLM lands within the area are still closed. The unmaintained public county road system through the area remains open.

Synopsis

Discovery of the geologic transition zone habitat and numerous additional populations has greatly improved the outlook for C. benitensis. Most of the geologic transition zone habitat has not been subject to high levels of human impacts, unlike most of the stream terrace habitat. As of July 2012, there are more than 380 known populations of C. benitensis. Only 10 populations were known when the species was federally listed in 1985. 60 populations were known by 2009 with most being within alluvial terrace habitat. 320 populations have since been found in 2010, 2011, and 2012 with most of those being within the newly discovered geologic transition zone habitat. Most of the potential geologic transition habitat on both public (BLM and State land) and private land has now been surveyed.