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COSEWIC assessment and status report on the Bog Bird’s-foot Trefoil in Canada

Habitat

Habitat Requirements

General Physiography and Climate

The British Columbia populations of L. pinnatus occur within the Nanaimo Lowlands Ecosection of the Georgia Depression Ecoprovince (Demarchi, 1996). According to the biogeoclimatic ecosystem classification used in the province (Meidinger and Pojar 1991), all known sites occur within the dry maritime (xm) subzone of the Coastal Douglas Fir (CDF) biogeoclimatic zone. In this region, the Olympic Mountains in Washington State to the south and the Insular Mountains on Vancouver lsland to the west produce a rain shadow effect resulting in a Mediterranean-type of climate with warm, dry summers and mild, wet winters. Most of the rainfall occurs during the winter months and limited precipitation and high temperatures during the summer months result in pronounced summer moisture deficits (Meidinger and Pojar 1991). At Nanaimo, annual precipitation ranges from a maximum of 201 mm in December to a minimum of 24 mm in July. The average maximum temperature is 17.6˚ C in August with an average minimum temperature of 2.3˚ C in January (Environment Canada 2003).

Geology and Soils

The area around Nanaimo is underlain by a succession of alternating conglomerate, sandstone and shale formations that are often exposed along shorelines and ridge tops (Bickford and Kenyon 1988). Populations of L. pinnatus in British Columbia are found in shallow Orthic Dystric Brunisols within the Hiller and Saturna (ST) Soil Associations. These gravelly sandy-loam textured soils have developed on shallow colluvial and morainal deposits overlying an undulating topography of gently sloping sandstone or conglomerate where the bedrock material is frequently exposed near the surface of the soil (Jungen et al. 1985). The soils at all sites are azonal, vary from dark brown to black (organic) and tend to remain moist throughout the late fall to spring, but dry out during the summer. During and shortly after wet periods, water may flow laterally through the saturated subsoil on top of sloping bedrock (Kenney et al. 1989.)

Community Structure and Composition

In British Columbia, Lotus pinnatus grows in open, springy meadows, along the margins of creeks, or in seepages, where underground water comes to the surface and the plants are in close physical contact with cool, flowing water (Roemer, pers. comm., 2003). In all cases, the soils are shallow (< 15 cm), over gently sloping sandstone or conglomerate bedrock with abundant moisture during the growing and blooming period.

Lotus pinnatus is most commonly associated with Mimulus guttatus, Plectritis congesta , Triteleia hyacinthina, Montiaparvifolia, Plagiobothrys scouleri (Scouler's popcornflower ) and Veronica beccabunga ssp. americana (American speedwell). Stands of Pseudotsuga menziesii (Douglas-fir) and dense thickets of Rosa nutkana (Nootka rose), Holodiscus discolor (ocean spray), Physocarpus capitatus (nine-bark) and Salix spp. (willows) exist on the margin of some seepages but Lotus pinnatus does not occur in the shaded understory of these sites, suggesting that the species is shade-intolerant. Prolonged moisture, edge habitat along streams and meadows and shallow soils derived from sedimentary rock are key habitat components. Other factors, such as slope and aspect are variable and do not appear to be critical in defining suitable habitat. In British Columbia, the elevation for this species ranges between 40 m and 150 m.

Populations of L. pinnatusappear to be associated with surface or subsurface seepage throughout the species’ range. West of the Cascades and along the Columbia River Gorge from northwestern Washington to California, L. pinnatus grows from sea level to higher elevations in the mountains where it is found mostly along slow-moving streams that dry up by mid-summer. In contrast to British Columbia, where the species is found primarily in lowland areas, L. pinnatus may be found at elevations of up to 600 m [2000 feet] along the Columbia River from Cape Horn, east to the Klickitat River. Lotus pinnatus is well established in the wetland prairie communities that developed in the Willamette and Umpqua valleys and the southern Puget Trough (Guard 1995).

In California, Lotus pinnatus occurs in wet meadows, bogs, ditches and stream beds from 600 m to 1700 m, throughout the Klamath Ranges, North Coast Ranges, Cascade Range, northern Sierra Nevada foothills and the Central Coast (Isely 1993). In the United States, Lotus pinnatus is federally classified as a facultative wetland species that usually occurs in wetlands (estimated probability of 67% - 99%), but is occasionally found in non-wetlands (USFWS 1988, USDA-NRCS 2002).


Trends

Since there have been no long-term studies of the population dynamics of Lotus pinnatus in British Columbia, little information is available on demographic characteristics and population trends. The sprawling habit of the species makes it difficult to identify separate individuals. Previous population estimates by different investigators have varied considerably. Until counting methods are standardized, population numbers should be considered rough estimates.

Populations of aggressive weedy exotic species such as Anthoxanthum odoratum, Bromus sterilis and Cytisus scoparius have become established at all sites and may threaten the long term ecological integrity of each population.

Although L. pinnatus appears to be relatively secure within Oregon, Washington and California, the status of its critical habitat in Idaho has not been assessed (Mancuso, pers. comm., 2003.)

In Oregon and Washington, Lotus pinnatus occurs within the wetland prairie communities that developed in the Willamette and Umpqua valleys and the southern Puget Trough. Although these moist prairie grasslands once covered extensive areas of the Willamette Valley, only 0.2% of the wetland prairie that existed in 1850 is still present (Guard 1995).


Protection/Ownership

With the exception of the occurrence at the Woodley Range Ecological Reserve, established in 1996, all extant populations of Lotus pinnatus in Canada lack legal habitat protection on privately owned land. Despite efforts to restrict entry to recreational off-road vehicles at Harewood Plains, local residents report that off-road vehicle use has increased in recent years (Thurkill pers. comm. 2003)). In areas with heavy off-road vehicle use, the fragile, shallow soils have been damaged and eroded, exposing the bedrock below.

The population of L. pinnatus at Woodley Range lies near (within 20 m of) the boundary of the Ecological Reserve. Plants within the Ecological Reserve are protected to some extent under the Protected Areas of British Columbia Act, and a valid park use permit is required before a plant can be destroyed, damaged or disturbed. However, the protection afforded by this legislation is unlikely to protect the population from habitat threats because there is little monitoring or enforcement of the Act. Terrestrial herbaceous ecosystems are vulnerable to adjacent land uses and disturbance from logging and other land clearing activities provide an opportunity for the introduction of aggressive alien plant species, which can then spread into the adjacent sensitive ecosystem (Ward et al.1998).