Howell’s triteleia (Triteleia howellii) COSEWIC assessment and status report: chapter 5


Habitat requirements

Triteleia howellii sites in British Columbia are found in the Quercus garryana (Garry oakFootnote1) ecosystem in the Dry Coastal Douglas-fir zone (Nuszdorfer et al. [1991]) of southeastern Vancouver Island. This area is in a rainshadow belt created by the Olympic mountains to the south, resulting in a relatively warm and dry Mediterranean climate with warm, dry summers and mild, wet winters.

Triteleia howellii occurs on rock outcrops, in Quercus garryana woodlands and in highly disturbed sites dominated by weeds in private yards and on roadsides. In the highly disturbed sites, dominants include Dactylis glomerata (orchard grass), Vicia sativa (common vetch), Bromus rigidus (rip-gut brome), B. hordeaceus (soft brome), Lolium perenne (perennial ryegrass) and Sanicula crassicaulis var. crassicaulis (Pacific sanicle), all introduced except for the latter.

The Quercus garryana woodland in the Cowichan Garry Oaks Preserve is classified as a Quercus garryana/Dactylis glomerata plant community (Douglas et al. 2001) and is characterized by deep, dark soils up to a metre in depth. It is likely that prior to understory dominance by D. glomerata in this, and other Quercus stands of the region, this plant community would have fallen within the Q. garryana/Bromus carinatus (California brome) community type (Roemer 1972). An extremely rich low shrub and herb stratum is present during the spring. The most prominent species in the Cowichan Garry Oaks Preserve Garry oak stand are Sanicula crassicaulis var. crassicaulis and D. glomerata (Douglas et al. 2001). Other species with moderate to high constancies associated with T. howellii includeCamassia spp. (common camas), Bromus spp., Dodecatheon hendersonii ssp. hendersonii (broad-leaved shooting-star), Galium aparine (cleavers), and Symphoricarpos albus (common snowberry). A marked change in composition takes place by mid-summer. Many of the conspicuous native plants (e.g., Camassia leichtlinii (great camas), C. quamash, Dodecatheon hendersonii, and Viola praemorsa ssp. praemorsa (yellow montane violet)) have completed their yearly life cycle and have essentially disappeared. Perennial grasses that were not recognizable or had not initiated growth in the spring and numerous introduced annuals, well adapted to the drier soils, dominate the understory. At this time, Dactylis glomerata and Vicia species are the most prominent species with greatly increased mean covers. Other prominent species include the native grasses, Bromus carinatus and Melica subulata (Alaska oniongrass) and the introduced grasses, Bromus sterilis (barren brome) and Poa pratensis (Kentucky bluegrass).

Triteleia howellii also occurs in a Quercus garryana-Arbutus menziesii (Arbutus) stand at the base of rock outcrops on Horth Hill. The shrub layer is more prominent at this site and is dominated by Mahonia aquifolium (tall Oregon-grape) andHolodiscus discolor (oceanspray). Associates include Lonicera hispidula (hairy honeysuckle), Bromus rigidus, Galium aparine, Nemophila parviflora (small-flowered nemophila) and Cynosurus echinatus (hedgehog dogtail).


The habitats in which Triteleia howellii occurs within the Quercus garryana ecosystems have been converted to urbanized centers and agricultural land over most of their range.  There is little undisturbed Q. garryana ecosystem left and most of these stands contain an abundance of introduced species.  In urban areas the only remnants of oak stands are more often just a few veteran trees with an understory of lawn grass or pavement.  The woodlands are also threatened by conifer encroachment as a result of the exclusion of fire throughout most of their range (Thilenius 1968).


The nine extant populations of Triteleia howellii occur in regional and municipal parks, as well as on private properties.

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