Rosy owl-clover (Orthocarpus bracteosus) COSEWIC assessment and status report: chapter 6

Biology

Reproduction

Orthocarpus bracteosus is a true annual, incapable of overwintering in British Columbia. Germination at Trial Island begins in March/April and plants die in July. All flowers are chasmogamous and there is no asexual means of reproduction.

It appears that owl-clovers (Orthocarpus, Castilleja in part, Triphysaria) are predominantly outbreeders although there are inbreeding self-compatible taxa such as Triphysaria pusilla. The two contrasting breeding systems may partition the host-environment of these parasitic plants with the showy outbreeders forming compact masses in order to improve the efficiency of cross-pollination while the inbreeders are
more dispersed (Atsatt 1970). It is uncertain whether O. bracteosus is an inbreeder or an outbreeder – the latter seems likely as it does have very showy flowers and at least the Trial Island plants are somewhat clumped. Outbreeding owl-clovers are primarily pollinated by honeybees and native bee species (Atsatt 1970).

Trial Island plants produced an average of 12 mature capsules per plant and there were an average of seven apparently viable seeds per capsule. The seeds have some form of dormancy mechanism – seeds collected and sown in September 2003 did not germinate in a controlled environment (natural light, 25oC, moist potting soil) over a six month period, and seeds grown in a garden environment only began germinating in early March.

Movements/dispersal

Seed dispersal is poorly understood although a number of hypotheses have been advanced. Atsatt (1965) proposed a ‘coordinated dispersal’ hypothesis in which Orthocarpus seeds get caught up in the pappus bristles of Hypochaeris (which often occurs with Orthocarpus) and are wind-dispersed as a result. Chuang and Heckard (1983) were skeptical of the coordinated dispersal hypothesis, believing most seeds are not dispersed beyond the immediate vicinity of the parent. Nevertheless, they did speculate that reticulations on the coats of the small, light-weight seeds may improve aerodynamic properties important to wind-dispersal, or add surface roughness aiding in animal dispersal (ibid.). Alternately, the loosely netted reticulations on the seed coat may trap air and thus add buoyancy to seeds during dispersal by water (Kuijt 1969). On October 16, 2001 – long after plants had withered and died – many capsules on Trial Island plants still contained abundant seeds. They appear to remain on the plant well into winter, rattling out as winter winds shake the capsules like a salt-shaker.

Germination and survival

No specific information is available on germination requirements for Orthocarpus bracteosus. Another owl-clover (Castilleja exerta, formerly known as O. purpurascens) germinates readily after untreated seeds have been scratched into the soil in early fall (http://wildflower.avartech.com/Plants_Online/Native_Plants/Detail.asp?ID=1010, Nov 26 2001).

As an annual, survival is limited to a single year. There is no information on early survivorship but most plants with six or more true leaves (“late juveniles”) survived to produce flowers and fruit in 2002.

Nutrition and interspecific interactions

Species of Orthocarpus have been found to use a C3 photosynthetic pathway (Watson and Dallwitz 1992 and onwards).

Members of the genus Orthocarpus are hemiparasites, containing chlorophyll and fixing carbon through photosynthesis but receiving water and nutrients through parasitic root connections (Kuijt 1969). Despite their photosynthetic abilities, owl-clovers may also obtain photosynthates from their hosts (Atsatt 1970). Many hemiparasites including species of Orthocarpus may obtain secondary compounds from their host plants. These can reduce herbivory without affecting pollinators. It appears that alkaloids may be taken up in the leaves and outer floral tissues of some species but not by their nectar, which has the ultimate effect of increasing seed production and improving fitness (Adler 2000, Adler and Wink 2001, Boros, Marshall, Caterino and Stermitz 1991).

Closely related species of Orthocarpus have been shown to be facultative hemiparasites capable of growing and producing flowers in the absence of a host, but shoot mass tends to be much higher in the presence of a host (Matthies 1997).

Grassland species of Orthocarpus form haustorial connections (root grafts) with a number of grassland associates including annuals and perennials of the legume, grass, composite and many other families. Not all plants are equally good hosts - some appear to reduce the reproductive potential of Orthocarpus. The parasitic habit is unlikely to be a limiting factor given the diversity of potential hosts intermixed within the Trial Island population.

Species of Orthocarpus (sensu lato) may serve as a secondary host for larvae of the Bay Checkerspot (Euphydras editha bayensis) if its primary food source (Plantago spp.) withers before the onset of diapause (USEPA 2000). Coincidently, the closely related Euphydryas editha taylori – a provincially and nationally rare butterfly – has also been recorded from Trial Island. Populations of this butterfly present a potential, but minor, herbivory threat toOrthocarpus bracteosus although they have not been seen on Trial Island for several years. Orthocarpus bracteosus may never have been a significant food source for the butterfly on Trial island because alternative secondary hosts such as Castilleja levisecta are much larger and more abundant.

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