Chorro Creek Bog Thistle

Cirsium fontinale var. obispoense

Description

Chorro Creek bog thistle, Cirsium fontinale var. obispoense, is one of two rare varieties of C. fontinale. First-year plants form a rosette of spiny leaves that can reach up to 3 ft (0.9 m) in diameter. In the second year, the plant produces an inflorescence (flowering stalk) up to 7 ft (2.1 m) in height, bearing numerous heads of whitish-to pinkish-lavender-tinged flowers. Chorro Creek bog thistle is distinguished from other thistles in its range by its nodding flower heads and the glandular hairs on its leaves.

Chorro Creek bog thistle is a biennial or short-lived perennial herb that typically lives two or three years. It forms a rosette of leaves the first year and usually flowers its second year. If sufficient reserves remain after flowering, some plants may persist into a third year. The blooming period generally occurs from May through July, occasionally extending into September or October. Seedlings have been observed establishing on small patches of recently disturbed, hummocky, and open soil.

Habitat

Chorro Creek bog thistle is restricted to saturated areas formed by seeps, springs, and slow streams on serpentine soils. Such soils have very low calcium/magnesium ratios compared to other soils, and frequently have high levels of metals, such as chromium and nickel. Chorro Creek bog thistle tolerates these conditions, which are toxic to many plants.

Distribution

Chorro Creek bog thistle is known from multiple colonies at eight to 10 separate sites, depending on how the sites are defined. Since the listing of this species in 1994, an additional occurrence has been identified near Miossi Creek. Seven sites are near San Luis Obispo, and one outlying site is near San Simeon, about 30 mi (48 km) to the north.

Population size of Chorro Creek bog thistle may vary substantially from one year to the next due to its short life span and habitat conditions that may fluctuate with seasonal rainfall. Five of the eight sites support 1,000-2,000 individuals, while the remaining three sites have from 50 to several hundred individuals each.

Threats

Because of its narrow habitat requirements, Chorro Creek bog thistle probably has never been abundant. Extant colonies are threatened by proposed water diversions, development, road right-of-way maintenance, and excessive trampling by cattle. Colonies also may decline, temporarily or permanently, because of drought conditions.

Chorro Creek bog thistle is not usually eaten by cattle, probably because of its spiny leaves. Broken inflorescences and damaged plants have been noted in colonies accessible to cattle, suggesting negative effects on thistle populations. Cattle may also introduce additional non-native plants to the wetland environments through their presence and feces. In other instances, however, thistle seedlings have been observed colonizing patches of saturated soil made bare by cattle hoofprints. If non-native species, such as annual ryegrass, or dense stands of native bulrush and spikerush, occur in a seep area, light or periodic cattle use may be creating germination sites that would otherwise not be available to the thistle. Cattle use occurs at the Miossi Creek, Pennington Creek, and San Simeon sites, on some or all of the colonies. At the Laguna Lake site, two of the three colonies are inside a grazing exclosure constructed to protect the plants. Reopening the thistle's wetland habitat to cattle use at the Chorro Creek site was recently suggested to reduce competition from non-native grasses. On properties adjacent to those supporting colonies at San Simeon Creek, land development proposals include well installation. Because Chorro Creek bog thistle depends on moisture from seeps, it could be negatively affected by diversions of water from above the seeps.

At the Perfumo Canyon site, the population adjacent to the southern edge of Perfumo Canyon Road has been mowed where it extends into the road right-of-way.

At the Froom Ranch site, a residential development project was proposed for the site's relatively flat terraces, but has not proceeded. Additional colonies may occur in the seeps above he terraces, but direct adverse effects from residential development are less likely there because of the steeper slopes.

To control alien species of thistle, the San Luis Obispo County Department of Agriculture introduced the seed-head weevil to several sites in San Luis Obispo County in the early 1980s. The seed-head weevil had been introduced for this purpose throughout California since the early 1970s. Initial reports from field botanists indicated that the seed-head weevils were feeding on Chorro Creek bog thistle, although population-level effects were not known. Biologists have suggested that, because the length of the flowering season of the thistle far exceeds the egg-laying period of the weevil, predation by the weevil may account for only a small reduction in seed availability. Oviposition (egg laying) by the weevil has been recorded at the Camp San Luis Obispo and San Simeon colonies. Inadequate information exists, however, to determine the population-level effects of the weevil and whether control is needed. The weevil has affected other native thistles around the United States.

Conservation and Recovery

The situation for Chorro Creek bog thistle has improved since its listing in 1994. At the end of the twentieth century, multiple colonies at four locations (Laguna Lake, Chorro Creek, Perfumo Canyon, and Pennington Creek) were secured from development. At one site on San Simeon Creek the landowners have entered into a voluntary protection agreement with the Nature Conservancy and participate in a monitoring program for the thistle. California's Department of Fish and Game funded field investigations in 1993 to identify extant colonies, search for additional sites, and describe potential introduction sites at more than 50 specific localities around San Luis Obispo.

Grazing exclosures have been installed around some or all of the colonies at Pennington Creek, Laguna Lake, and Chorro Creek. The California Military Department has fenced, restored, and monitored the Chorro Creek bog thistle at Camp San Luis Obispo since 1994. The Military Department is studying the effects on the Chorro Creek bog thistle of experimental short-term use of the site by cattle.

In 1997 an agreement was reached as part of a development approval process, whereby the City of San Luis Obispo will receive more than 350 acres (140 hectares) in the Perfumo and Froom Creek drainages. These lands support at least three previously recorded colonies of Chorro Creek bog thistle from the Perfumo Canyon location and at least one colony on upper Froom Creek that had not been recorded. This is the southernmost protected site in this taxon's range.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Reference

U. S. Fish and Wildlife Service. 1998. "Recovery Plan for Morro Shoulderband Snail and Four Plants from Western San Luis Obispo County, California." U. S. Fish and Wildlife Service, Portland, Oregon. 75 pp.