Rosy owl-clover (Orthocarpus bracteosus) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

The single population of Orthocarpus bracteosus known in Canada occupies a very shallow vernal pool (simply a moist depression during dry winters). Soils are 15-30 cm deep and have a pronounced Ah horizon likely built up through the in situ decomposition of forb and grass roots. The soils are lightly churned in the winter, apparently by gulls pecking at the soil surface. The vegetation lacks trees, shrubs or robust herbs. Plant cover peaks in early summer, at about 20%. The dominant species include Grindelia integrifolia, Plantago lanceolata, Hypochaeris radicata, Prunella vulgaris and Orthocarpus bracteosus. Less abundant species include Lotus unifoliatus var. unifoliatus, L. formosissimus, Plantago elongata, Mimulus gutattus, Fragaria chiloensis, Armeria maritima, Festuca rubra, Holcus lanatus, Vulpia bromoides, Geranium molle, Isoetes nuttallii, Dodecatheon pulchellum, Limnanthes macounii and Castilleja ambigua. The vegetation is at a climax stage – encroachment by native trees, shrubs and robust mesophytic herbs is prevented by the shallow nature of the soils, their lack of aeration during the extended period of winter saturation, and the pronounced summer drought. There is a limited degree of invasion by a number of aggressive alien species including Cytisus scoparius, Ulex europaeus, Daphne laureola, Holcus lanatus, Hedera helix, Plantago lanceolata, Hypochaeris radicata, Vulpia bromoides and Geranium molle. The first four species may not represent a significant threat as they appear to be limited by the same factors preventing encroachment by native trees and shrubs. Hedera helix may be capable of advancing across the vernal pool, supported by moisture and nutrients obtained from roots outside the vernal pool. The other four species are now well-established in the vernal pool and it is not clear whether their populations can increase further.

The co-occurrence of Lotus formosissimus, Limnanthes macounii and Castilleja ambigua are of special importance - COSEWIC has designated Lotus formosissimus as Endangered and Limnanthes macounii as a species of Special Concern. Castilleja ambigua, known from fewer than 10 extant sites in Canada, is a likely candidate for future COSEWIC listing.

Older records suggest Orthocarpus bracteosus formerly occurred sporadically across a variety of open, winter wet/summer dry habitats including ditches, prairies and open fields. This is consistent with habitat descriptions for populations in Washington State (Florence Caplow, Botanist, Washington Natural Heritage Program, pers. comm. Nov. 6, 2001).

Vernal pools throughout the historic extent of occurrence tend to be quite isolated from one-another – an extreme case of fragmentation. The same is true of coastal prairies which it favours elsewhere. Ditches and fields likely no longer present suitable habitat as their vegetation has uniformly changed with the ubiquitous invasion of aggressive alien grasses.

Trends

There is no specific information on the trend of vernal pool and coastal prairie habitats in the Victoria area although they almost certainly share the same trends as Garry oak systems with which they are closely associated. Three major trends have influenced Garry oak and associated ecosystems over the past century: (1) cultivation, (2) urbanization and (3) invasion by aggressive alien weeds. Overall, these factors have reduced Garry Oak systems to less than 5% of their original extent in the Victoria area. Garry oak ecosystems persist largely as isolated communities that are heavily fragmented and lack connections that would allow substantial genetic interchange (Lea 2002). Most remaining areas of coastal meadow and vernal pools have very little protection. Many are privately owned and most of the publicly owned areas lack management plans for species at risk or planning safeguards that ensure occurrences of species at risk are protected from management activities and land uses.

Protection/ownership

The sole extant population occurs on Trial Island Ecological Reserve near its boundary with provincial land leased to Seacoast Communications for commercial radio broadcast antennae. The Canada Coast Guard owns the Trial Island Lighthouse station, less than 250 m from the existing population.

Trial Island Ecological Reserve offers nominal protection to all plant species found within its boundaries through the Protected Areas of British Columbia Act, which requires that a valid and subsisting park use permit be acquired before a plant can be destroyed, damaged or disturbed. B.C. Parks does not have any staff stationed on Trial Island and the legal protection afforded ecological reserves is not likely to actually protect the population from major habitat threats.

British Columbia does not have any legislation specifically protecting species at risk.

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