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

Distribution

Global Range

Lotus pinnatus occurs in western North America from Vancouver Island, British Columbia, west of the Cascade Mountains and along the Columbia River Gorge from northwestern Washington to central California and sporadically eastward to Idaho (Isely 1993, Pojar 1999) (Figure 2, Pojar 1999).


Figure 2: Distribution of Lotus pinnatus in North America

Figure 2: Distribution of Lotus pinnatus in North America.

Lotus pinnatus has a disjunct distribution in Washington State, where it is found inland as far as Klickitat County (southern Washington), as well as on the coast (University of Washington Herbarium Database, 2002). Throughout western Oregon, this species occurs in the interior valley west of the Cascade Mountains from Clackamas County in the northern part of the state south to Josephine County adjacent to California (Vrilikas pers. comm. 2003). In California, L. pinnatus is known from the Klamath Range, the northern California Coast Range, the Cascade Range, the northern Sierra Nevada Range and Contra Costa County (Isely, 1981). Originally described from northern California, Lotus pinnatus has also been reported from Santa Barbara County in southern California (CalFlora 2003).


Canadian Range

In Canada, Lotus pinnatus is known from seven extant populations within a small area on the east coast of Vancouver Island near Nanaimo, British Columbia (Pojar 1999). Each of these occurrences, ranging from Harewood Plains, south of Nanaimo to the Woodley Range Ecological Reserve, northeast of Ladysmith, and to Gabriola Island, east of Nanaimo (Figure 3) were verified by the author during surveys in 2003. Despite detailed searches in 2003, two additional occurrences in Nanaimo known from historical records were not relocated.


Figure 3: Distribution of Lotus pinnatus in British Columbia

Figure 3: Distribution of Lotus pinnatus in British Columbia.

The Canadian range of L. pinnatus is restricted due to the limited availability of its preferred habitat (open seepages at the edge of grassy meadows). GIS tools were used to calculate the terrestrial portions of a minimum convex polygon encompassing all sites of occurrences. The extent of occurrence of the species in Canada is approximately 100 km2 (10 000 ha). However, total area of occupancy is less than 1 hectare.

Approximately eight days of fieldwork, conducted in June 2003 during peak flowering, focused on the confirmation of known populations, and the search for new ones. As most known populations in British Columbia are found in vernally moist sites adjacent to grassy meadows, searches for new populations focused on these features. Using aerial photographs and topographic maps, areas of ephemeral drainage at the margins of grassy meadows in the vicinity of known sites at Woodley Range, White Rapids Rd. and Gabriola Island were identified and were accessed wherever possible. An area of approximately 300 hectares was searched. It is possible that additional populations of L. pinnatus remain to be discovered, since some of this potential territory is under private ownership with limited access.

Lotus pinnatus is a conspicuous and highly visible plant that would not be easily overlooked if flowering, even in the course of casual botanical inventory. Despite 50 - 60 hours of careful searching in potential habitat over the past eight years by skilled botanists, Adolf Ceska, George Douglas, Matt Fairbarns and Hans Roemer, L. pinnatus has not been found at any other localities throughout the species’ extent of occurrence. These searches have included: Harmac, Mount Benson and forest openings along White Rapids Road.

In 2003, a GIS analysis using information for the Nanaimo Lowland Ecosection collected by the Sensitive Ecosystems Inventory (SEI)2 and mapped at a scale of 1:20 000 was used to identify potential sites for investigation. The two sensitive ecoysystem categories that correspond most closely to the type of habitat capable of supporting populations of Lotus pinnatus were the Terrestrial Herbaceous (HT) and Wetland (WN) units (Ward et al. 1998).

The HT category includes natural grasslands and bryophyte-dominated ecosystems that occur in small patches, often in a mosaic of several types of herbaceous communities and sometimes within a larger forested site. Typically thin-soiled, most HT sites are dry, open and exposed. However terrestrial herbaceous ecosystems sometimes contain highly specialized microhabitats, including vernal pools and seepage areas that are hotspots for plants of conservation concern (McPhee et al. 2000). Several native plant species typically found in HT ecosystems, such as Mimulus guttatus, Plectritis congesta, Triteleia hyacinthina and Montia parvifolia, are common associates of Lotus pinnatus. Potential microhabitats with suitable seepage were considered most likely to occur in HT sites with rock outcrops as a dominant feature (HT:ro) or within Wetland Meadows (WN:wm). One of the rarest wetland types, herbaceous wetlands are areas of wet soils and moisture-dependent plants (Ward et al. 1998).

Both the HT:ro and the WN:wm units had low representation within the extent of occurrence for L. pinnatus.  Thirty polygons were mapped as HT:ro units, covering 62 ha and eight polygons were mapped as WN:wm ecosystems across 11 ha. Lotus pinnatus was not reported during ground-truthing undertaken at any of the HT:ro and WN:wm ecosystem units. Most of the sites in this area were field-sampled by contracted SEI staff in October. Though seed pods may have been visible at this time of year, the plants are less conspicuous in fruit than when they are flowering. In addition, SEI field surveys were focused on identifying dominant species and percent cover at each site, rather than on rare plant inventory.

Nine HT:ro ecosystem units have been mapped at Woodley Range Ecological Reserve. With the exception of the population observed at the western boundary of the reserve, which was adjacent to SEI polygon # V0165, no further populations of L. pinnatus were observed at this location during field work undertaken in 2003.

The absence of data for L. pinnatus within the areas identified by the SEI may reflect the small size of the specialized habitats where L. pinnatus is found. Although many wetlands smaller than 0.5 ha were mapped by the SEI, the targeted minimum mapping size for most SEI ecosystem types was 0.5 ha (Ward et al. 1998). At 500 m2 (0.05 ha) the area occupied by the largest population of L. pinnatus at Harewood Plains North is considerably smaller than the minimum mapping size of the SEI units. The lack of data for L. pinnatus within the SEI study area may also reflect the rarity of the specialized habitat required by this species. It is likely that less than 100 ha of suitable habitat exists for L. pinnatus in Canada.

The species has not been found to occur on federal lands in the region.




Footnotes

2 The Sensitive Ecosystems Inventory (SEI) is a joint federal/provincial initiative that systematically identified, inventoried, mapped, and evaluated remnant natural ecosystems on the east coast of Vancouver Island and the adjacent Gulf Islands.