Bog bird’s-foot trefoil (Lotus pinnatus) COSEWIC assessment and status report: chapter 9

• Previous page
• Table of contents
• Next page

Population Sizes and Trends

• General
• Extirpated Populations

General

Lotus pinnatus has been reported from nine sites in British Columbia (Table 1), all in the Nanaimo area on Vancouver Island and on Gabriola Island, nearby. Of the nine reported records, seven populations were verified during this study. The remaining two sites have been extensively disturbed and it is unlikely that these populations currently exist. Between 1500 and 2000 individual plants were counted in Canada in 2003 over an area of 650 m².

Relatively little is known of demographic characteristics and population trends of L. pinnatus due to a lack of long-term monitoring, though the number of individual plants appears to have increased at one site, on Gabriola Island. 

Estimates of population trends in L. pinnatus are currently unreliable as the clumped, sprawling habit makes it difficult to identify separate individuals and counting methods have varied with different investigators. Until counting methods are standardized, population numbers should be considered rough estimates. On average, each individual (genet) includes from 3 to 5 shoots.

The inventory for the purposes of this report involved a count of all stems at each site. Each site was surveyed by walking a series of transects and counting individual stems. It was highly impractical to count each individual stem at the Harewood Plains site due to the large number of stems involved, and because they were spread over a large area. Therefore, a grouping method was used to simplify counting at this site. A group of ten, or in some cases fifty, stems were clustered together to gain a visual picture of the area occupied by the stems. The site was then surveyed by walking a series of transects and stems were counted based on the grouping method. 

The disappearance of the populations at Departure Bay in Nanaimo has resulted in a decline of 15% in the extent of occurrence for this species, from an area of 118 km² in 1939 (assuming that all currently extant populations also existed at that time) to 100 km² in 2003. As the British Columbia population is geographically isolated from the nearest reported Washington site (for which the current status is unknown) by approximately 240 kilometres, the chances of a healthy population returning to Canada if local populations become extirpated are highly unlikely.

Harewood Plains, S of Nanaimo

The population of L. pinnatus at Harewood Plains occurs in three discrete areas. They are best treated as separate populations because they are well separated and the intervening terrain is not conducive to pollen and seed dispersal. The majority of plants (approx. 22 subpopulations) are located at the north end of the site; two small populations occur in open meadows further south. In 2003, a total of about 1500 individual plants were observed at these three sites. This estimate includes three previously undocumented patches. If each individual plant includes an average of four shoots per clump, any change in the number of plants from estimates reported in 1998 is likely to be smaller than the errors in counting, associated with the method of enumeration. 

The bedrock at this location, which belongs to the “Millstream Member” of the Extension Formation (Bickford and Kenyon, 1988), consists of thick-bedded pebble-conglomerate, with interbeds of coarse-grained, gritty sandstone (Bickford 1993). 

Lotus pinnatus grows in dense clusters in vernal seepages at this site, often at the margins of moist thickets populated by Rosanutkana, Physocarpus capitatus, Holodiscus discolor and Salix spp. The two most northern subpopulations, overlooking the parkway, occur in scattered openings within a Pseudotsuga menziesii/Arbutus menziesii (Douglas-fir/arbutus) plant community. Plants were also observed growing along the sides of tire ruts where moisture is retained for prolonged periods. The most common associated species at this location include Mimulus guttatus, Triteleia hyacinthina, Plectritis congestus, Plagiobothrys scouleri, Veronica beccabunga ssp. americana and Montia parvifolia. Overall, the vigour of most sub-populations appears to be good, although two populations have declined in areas where the soil has been eroded from off-road vehicle use and Anthoxanthum odoratum dominates adjacent grassy meadows. Morphological differences such as the number of flowers per inflorescence are evident among subpopulations at this location indicating the existence of genetic variation and outcrossing. 

Three other plant species known from this locality are rare in British Columbia (Douglas et al. 2002). Epilobium densiflorum and Carex feta appear on the provincial Red list maintained by the British Columbia Conservation Data Centre. Allium amplectens is on the provincial Blue list.

Woodley Range Ecological Reserve

Within 20 m of the western boundary of Woodley Range Ecological Reserve, Lotus pinnatus was observed growing in shallow water along a small outlet stream connecting two wetlands. In 2003, 120-140 individual plants were observed growing in two sub-populations of 5 m²  and 30 m². Soils at the site were moist, shallow and with frequent sandstone outcrops. Native species associated with L. pinnatusincluded Rosa nutkana, Spirea douglasii, Holodiscus discolor, Plectritis congesta, Mimulus guttatus, Montia parvifolia and Philonotis fontana. Anthoxanthum odoratum and Cytisus scoparius  were also present. Two red-listed plants, Seriocarpus rigidus and Carex feta and two blue-listed plants Allium amplectans and Isoetes nuttallii have also been reported from within the Woodley Range Ecological Reserve. A red-listed Alnus rubra/Carex obnupta [Populus balsamifera ssp. trichocarpa] plant community has been identified at the fen/swamp wetland drained by the creek where L. pinnatusis found. The number of plants at Woodley Range Ecological Reserve (ER) was not reported in 1992, precluding a comparison with plants observed in 2003. The area occupied by the population at Woodley Range ER appears to be unchanged since 1992.

Gabriola Island

Lotus pinnatus was found growing along both sides of an inundated ditch at the edge of a moist thicket on Perry Road. Associated species include Craetagus douglasii (black hawthorn), Holodiscus discolor, Spirea douglasii, Oenanthe sarmentosa, Rosa nutkana and Vicia sativa. Also present at the site were Anthoxanthum odoratum,Plantago lanceolata, Galium aparine and Ranunculus flammula. The soil at this site is a shallow Orthic Dystric Brunisol within the Saturna (ST) Soil Associationon top of sandstone bedrock (frequently exposed throughout the island). This population appears to have increased from a single plant observed in 1996 to about 65-70 in 2003, over an area of 10.5 m². Although the plant has spread to occupy suitable habitat along the ditch, there is no assurance of protection from development for this privately owned property.

Nanaimo, west of Cinnabar Valley

This site is located on private property, south of Nanaimo, west of White Rapids Road. In 2003, about 40 flowering plants were counted over an area of 25 m², in a vernal seepage located in a grassy meadow at the edge of Pseudotsuga menziesii forest with Rosanutkana, Holdiscus discolor, Montia parvifolia, Plectritis congestus, Mimulus guttatus and Camassia quamas. Anthoxanthum odoratum and Holcus lanatus have also become established at this site. The population of L. pinnatus west of Cinnabar Valley appears to have experienced no significant change in numbers since 1998.  As the property is privately owned and frequently used by paintball teams, surveys conducted in 2003 may have been incomplete and it is possible that additional patches ofL. pinnatus may occur in forest openings at this location.

Nanaimo, South of Extension

This site is located on private property, south of Nanaimo. In 2003, ten plants were observed over 5 m², growing in a vernally moist area on the margin of a Physocarpus captitatus thicket with Mimulus guttatus, Allium cernuum, Montia parvifolia and Anthoxanthum odoratum. A second population of 20-35 plants was found in a vernally wet seep in an open meadow at the edge of a Malus fusca (crab apple) thicket with Mimulus guttatus, Allium cernuum and Anthoxanthum odoratum. Four other rare plants have been reported at this location. These include the red-listed speciesEpilobium densiflorum and Carex feta and the blue-listed species Cyperus squarrosus and Botrychium simplex. In 2003, only 50% of the plants reported in 1995 were observed at this site, despite detailed searches of the most suitable habitat. Recreational use of this site by off-road vehicles during the wet season may be responsible for the apparent decline.

Extirpated Populations

Waddington & Departure Bay Roads (Nanaimo)

Lotus pinnatus was collected at this site in 1939 byJ.W. Eastham. Since 1951, the roads in the area have been widened and resurfaced to accommodate the increased development that has taken place. Almost all of the natural vegetation has been eliminated. Given the extent to which this area has been altered and the length of time since L. pinnatus was last observed, it is very unlikely that L. pinnatus is extant at this site.

Departure Bay Rd. and Island Hwy, (Nanaimo)

Lotus pinnatus was collected at this location in 1965 by K. Beamish. Presently, this area is dominated by shopping plazas and highways. Little of the existing vegetation remains in the ditch along Departure Bay Road where almost all of the native vegetation has been replaced by introduced and horticultural species. It is very improbable that L. pinnatus is extant at this locality.

Status Unknown

There is a herbarium record, dated 1918, indicating an observation made in the “foothills” at “Mount Benson”. Since Mount Benson is visible from Harewood Plains, it is possible that this historic record is referring to the sites described for Harewood Plains. According to anecdotal reports from a local naturalist who hikes Mount Benson regularly, Lotus pinnatus has not been observed growing at higher elevations at this location (Thurkill pers. comm., 2003). 

³ The collection by W.R. Carter at Mt. Benson in 1918-06-01 is included with this population as Harewood Plains may be considered the foothills of Mt. Benson.