Under-saturated distribution of Floerkea proserpinacoides Willd. (Limnanthaceae) at the northern limit of its distribution

Habitat availability and dispersal ability are commonly the major factors c ontrolling the distribution of habitat specialists. However, these species often have an "under-saturated" distribution, i.e., their habitat is more f requent than their populations. Floerkea proserpinacoides is an annual spri ng ephemeral plant of the deciduous forest of eastern North America. It is considered rare in most of its range. In Quebec (Canada), where the species reaches the northern limit of its distribution, F. proserpinacoides is mos tly restricted to rich, low woods on islands of the St. Lawrence River. To elucidate the factors responsible for this peculiar distribution, we determ ined the dispersal ability of F. proserpinacoides nutlets and attempted to establish new populations at apparently suitable, but unoccupied sites with in the species range in Quebec. Primary dispersal is barochorous and nutlet s have no accessory structures to assist in dispersal. Secondary dispersal is also limited, and nutlets typically move distances < 50 cm from their or iginal point of fall. Yet, nutlets are buoyant and hydrochory is a possible means of long-distance dispersal. When nutlets were introduced at apparent ly suitable but unoccupied sites, the seeds could germinate and the individ uals completed their life cycle. However, nutlets from different population s did not have the same success. The under-saturated distribution of F. pro serpinacoides in Quebec is not related to a lack of suitable habitat. Inste ad, long-distance dispersal events may be too infrequent to allow the occup ation of all potential sites, or conditions may be too variable among years on certain sites to allow for population persistence.