Dwarf woolly-heads, specific populations, COSEWIC assessment status report: chapter 6
Biology
General
Very little has been written about the life history, demography or physiology of Psilocarphus brevissimus or other species within the genus. Cronquist’s 1950 monograph on the genus remains one of the best sources of information, although some aspects of life history can be inferred from studies of other annual vernal pool species with similar phenological characteristics.
Reproduction and dispersal
Species of Psilocarphus are annual plants that most often occur in vernally moist environments. Pollination likely occurs within the flower head, since there are no structures to aid in wind or animal dispersal. Furthermore, only a small amount of pollen is produced in staminate flowers and it is unlikely to be carried by the wind because the flowers are sheltered by the upper stem leaves and ‘wool’ which encircle each head. Appendages on each receptacular bract guide the long, slender styles of the outer (pistillate) flowers toward pollen-producing flowers in the centre of the lobe (Cronquist 1950).
Movements/dispersal
Many vernal pool plants lack apparent dispersal-enhancing traits, perhaps because the island-like distribution of suitable habitat penalizes a species that tends to disperse widely (Zedler 1990).
The receptacular bracts, which persist around the mature achene, may buoy it up when the site is flooded, thereby dispersing achenes short distances throughout a pond or pool (Cronquist 1950).
Some of the light, fluffy receptacular bracts may catch in the wind, dispersing the attached achenes over slightly longer distances. Generalist mammalian herbivores may also play a role in medium-distance dispersal. In a California study, seeds of Psilocarphus brevissimus germinated from cottontail rabbit droppings collected from the vicinity of vernal pools, including ‘new’ pools which were artificially created. The data suggest that cottontails, though weak dispersers of seeds, may play an important role in the colonization and re-establishment of P. brevissimus in habitat patches (Zedler and Black 1992). Cottontail rabbits occur in the grasslands of southern Alberta and Saskatchewan and may play a role in medium-distance dispersal of P. brevissimus among vernal pools and other suitable habitats. Cottontails have not been observed in the Princeton area of British Columbia but other small mammals may play a similar role.
Some seeds may be carried long distances, in mud on the feet of shorebirds and waterfowl (Cronquist 1950). A number of waterfowl, shorebird and passerine species that frequent vernal pools and moist prairie depressions (Silveira 1998) may serve as vectors. Long-distance dispersal is probably a rare event, given the absence of the species from many suitable habitats within its extent of occurrence in Canada.
There is no evidence regarding the species’ ability to bank seeds in the soil but this is a common adaptation among vernal pool species that allows them to persist over drought years.
Germination and seedling ecology
Many annuals of moist depressions are adapted to limited growth under water during cool periods, followed by rapid growth and maturation as the pools warm up and then dry out (Zedler 1990). It is not certain, however, that Psilocarphus brevissimus actually germinates under water. Psilocarphus elatior, a closely related species of vernal pools in coastal British Columbia, either germinates after standing water disappears or is too small to detect at that time (pers. obs.).
Survival
Psilocarphus brevissimus is not capable of clonal growth. Vegetative development, flowering and fruit production are probably regulated by the rate at which the soil desiccates. Late spring and early summer rains may prolong the growing period, increasing fecundity. Consistent levels of high moisture in the spring and early summer likely favour an increase in the cover of perennial herbs, which can outcompete P. brevissimus. There is no evidence of herbivory on the plants.
Physiology
There is no information on the physiology of Psilocarphus brevissimus that is relevant to assigning at-risk status or developing recovery plans.
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