Habitat fragmentation and the reproductive success of Trillium grandiflorum (Liliaceae) in southern Ontario

Habitat fragmentation in the Peterborough region of southern Ontario was assessed for its effect on the reproductive success of Trillium grandiflorum (Liliaceae), a bee-pollinated woodland perennial. Populations were examined in 1995 and 1996, and estimates of population size, as well as habitat characteristics, floral associates and site nutritional features were measured. Controlled hand-pollinations and a marker gene analysis of outcrossing rates demonstrated that the species is self-incompatible and largely outcrossing. Comparison of seed set from open- versus supplemental-pollinations in 18 populations from large woodlands, small woodlands and disturbed sites were undertaken to evaluate the intensity of pollen limitation. On average, open-pollinated plants set 25% less seed than supplemental-pollinated plants. However, significant variation in the intensity of pollen limitation was detected among populations. This variation was not related to habitat type or various demographic and environmental factors. The lack of an association between ecological factors and pollen limitation was interpreted as the result of the generalized pollination system of T. grandiflorum. These results illustrate that knowledge of a plant's reproductive biology is necessary to determine the extent to which habitat fragmentation is likely to adversely effect plant populations.