Muhlenberg's centaury (Centaurium muehlenbergii COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

In Canada, Centaurium muehlenbergii is known from the Coastal Douglas-Fir Biogeoclimatic Zone (Douglas et al. 1999). This zone is limited to a small section of southeastern Vancouver Island, several islands in the Gulf of Georgia, and a narrow strip on the adjacent mainland (Meidinger and Pojar 1991). It is strongly influenced by the rain shadow of the Vancouver Island and Olympic mountains and has a sub-Mediterranean climate characterized by warm dry summers and mild wet winters. The mean annual temperature in Victoria is 10°C, with mean daily temperatures ranging between 4°C in December and 15.6 °C in July. Summer drought is an important feature determining the character of the vegetation in this region. Of the 690 mm of average annual precipitation in Victoria, < 5% falls during July and August. The annual moisture deficit exceeds 350 mm (McMinn et al. 1976, Fuchs 2001).

Within this zone, Centaurium muehlenbergii has been reported from three rather distinct habitat types. The Oak Bay population is in a seasonally flooded meadow (i.e., vernal pool) in a Garry oak (Quercus garryana) woodland, at an elevation of 10 m. Trees are absent due to the wetness of the habitat. The soil beneath the population has not been profiled but is likely a poorly drained Tolmie or Gleysol developed from medium to fine textured marine materials overlying marine clay parent material (Day et al. 1959, H. Roemer, pers. comm. 2003). The associated herb layer consists of a mix of native and non-native species such as Scouler’s popcorn flower (Plagiobothrys scouleri), Kellogg’s rush (Juncus kelloggii), tall woolly-heads (Psilocarphus elatior), water-plantain buttercup (Ranunculus alismifolius var.alismifolius), winged water-starwort (Callitriche longipedunculata), camas (Camassia quamash), toad rush (Juncus bufonius), chaffweed (Anagallis minima), and heterocodon (Heterocodon rariflorum).

Kellogg’s rush, tall woolly-heads, and water-plantain buttercup have all been listed as endangered by COSEWIC. A number of alien, invasive grasses (e.g., Agrostis stolonifera, Bromus hordeaceus, Cynosurus echinatus, Holcus lanatus, and Dactylis glomerata) are also present.

The Gulf Islands population is on a small Gulf Island (E. Chatham Is.) opposite Victoria. Here Centaurium muehlenbergii grows on bare sandy ground at the edge of a saltgrass (Distichlis spicata) marsh, just above the high tide mark. Associated species include purple-leaved willowherb (Epilobium ciliatum), brass buttons (Cotula coronopifolia), hairy cat's-ear (Hypochaeris radicata), and Baltic rush (Juncus balticus).

The Nanaimo population occurs on a sloping, mossy, vernal seep at the edge of a Douglas-fir (Pseudotsuga menziesii) and arbutus (Arbutus menziesii) forest. The substrate is a shallow, poorly developed mineral soil overlying sandstone or conglomerate bedrock of the Nanaimo formation. An invasive shrub, Scotch broom (Cytisus scoparius), has invaded this site and may be in the process of overgrowing it.

Centaurium muehlenbergii is federally classified in the U.S. as a facultative wetland species (USDA-NRCS 2005). Across its U.S. range, described habitats include coastal bluffs, wet openings in woods, and moist places in sagebrush scrub (J. Pringle, pers. comm. 2001); damp places below 500 m in redwood forest, oak woodland and mixed evergreen forest (Munz and Keck 1959); wet prairies (Guard 1995); northern oak woodland, foothill woodland, valley grassland, and northern juniper woodland (Calflora 2000); and serpentine and annual grassland (Callizo et al. 1997).

In summary, Centaurium muehlenbergii displays a fairly broad ecological amplitude, occurring in a variety of community types on soils ranging from sand to poorly drained Chernozem (with a possible tolerance for both saline and serpentine). The species is somewhat shade intolerant and may depend on periodic fire to maintain open habitat and limit competition. In Canada, it appears to prefer moisture-receiving sites that become wetted during the winter and spring but that dry up during summer.

Habitat trends

Little specific information exists as to historical losses or gains in the range and number of Centaurium muehlenbergii habitat patches per se in this region.

However, since the arrival of European settlers on southern Vancouver Island, most of the original Garry oak savanna has been lost as a result of agricultural and urban development. Remaining areas have been heavily impacted by encroachment of woody species due to fire suppression, and by invasions of exotic species. It is currently estimated that < 5% (< 1000 hectares) of the Garry oak ecosystem remains in near-natural condition, and most of this is severely fragmented (Fuchs 2001). Moreover, Prentice and Boyd (1988) examined historical changes to estuarine habitats on the east coast of Vancouver Island, and estimated that estuarine marshes had already declined by >32% by the turn of the century. Although this loss then slowed, they noted that adjacent forests and agricultural lands were being increasingly replaced by other urban land uses.

Southeastern Vancouver Island is currently experiencing one of the most rapid rates of urban expansion in North America. In 1966, the population of metropolitan Victoria numbered around 180,000; by 1996, it had grown to 318,000 and is projected to surpass 400,000 by 2026 (BC Ministry of Management Services 2003). Likewise, the population of Nanaimo is expected to nearly double in size between 2001 and 2026, from 127,016 residents to over 219,000 residents (B.C. Ministry of Management Services 2003). This growth, with its attendant infrastructure, threatens to result in continuing fragmentation and loss of ecosystems through clearing, draining, and conversion to commercial and residential development, industry and agriculture (Ward et al. 1998).

Habitats suitable for Centaurium muehlenbergii were probably never as abundant as Garry oak and associated ecosystems in general. These habitats have certainly experienced at least an equally sharp decline, since price and demand for residential and commercial development are greatest in coastal areas where C. muehlenbergii tends to occur. Specifically, the Oak Bay and Nanaimo populations occur in small green spaces within or immediately adjacent to large residential areas built on similar sites that were formerly capable of supporting C. muehlenbergii.

Similar trends have occurred south of the border, where Centaurium muehlenbergii is a native forb of the remaining wetland prairies of the Willamette Valley (Wilson 2001). These prairies are highly fragmented and considered among the rarest of Oregon’s ecosystems. Their decline can be traced to invasion of both woody plant species and pest organisms, agricultural and urban encroachment, and changes in hydrology (Clark and Wilson 2001).

Habitat protection/ownership

There is no specific legislation for the protection of rare and endangered vascular plants in British Columbia, or for the protection of Garry oak and associated ecosystems. This species can be added to the list of species for protection under the B.C. Wildlife Amendment Act (2004). Less than 1% of the Coastal Douglas-Fir zone is currently protected (Eng 1992).

The Oak Bay population occurs in a large urban municipal park managed by the District of Oak Bay (Table 1). No plans for development at this site exist at this time and are considered unlikely in the future. Nevertheless, this site receives heavy recreational use and does not afford more than a minimal level of protection for species at risk. The Gulf Island population is situated on First Nations land. Future plans for this site are unknown. The Nanaimo population occurs on private land that is slated to have the land title of a portion of the property with the Centaurium muehlenbergii population present transferred to the City Parks Department by the developer (J. Kirby, pers. comm., 2008). Outside of these areas, most of the potential Centaurium muehlenbergii habitat in Canada is held by private landholders, and land use planning falls under the jurisdiction of local governments (GOERT 2002). Other areas are managed by B.C. Parks, Capital Regional District Parks, various non-governmental organizations, and the Department of National Defence. The proportion of potential C. muehlenbergii habitat represented within these different jurisdictions is unknown.

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