The ecology of two northern marginal disjunct populations of Celtis tenuifolia Nutt. in Ontario, Canada

Celtis tenuifolia (Nutt.), the Dwarf Hackberry, reaches its northern range margin in Ontario, where it occurs in disjunct populations. Two Ontario populations were examined to determine whether a temperate woody plant species which apparently reaches its range margins in southern Canada, exhibits the same characteristics found in marginal arctic and alpine tree-line species.;No significant loss in germinative ability was noted between seed lots collected in 1988, 1989, and 1990. The mean germination success rate for both populations was 73%. Seed weight was not correlated with germination date or with germination success. Abiotic factors causing seedling mortality at Point Pelee were seed and root desiccation, sun and frost damage, and lack of nutrients. Frost damage was the most serious threat to seedling survival at Northville. The most important biotic factor threatening seedling survival was snail predation; this was only seen at Point Pelee.;Dwarf Hackberry does not have a reduced competitive ability at its northern range limits. It was found in a variety of plant communities sampled at both Point Pelee and Northville. Fecundity was greatest in the more northern Northville population, which had about three times more seedlings than at Point Pelee. Both populations showed clumped distribution patterns, indicating short dispersal distances and clustering at favourable habitats, usually beneath the shade of a parent tree.;It is concluded that these populations of Dwarf Hackberry are not at the climatic limits for the species. Dwarf Hackberry appears to not occupy all of its potential northern range because of both a lack of seed dispersal vectors, and by a shortage of disturbance events creating safe sites for establishment.;Several statistically significant differences were found between the two populations. Point Pelee trees are older and less vigorous than those at Northville, which were typically producing more new growth, and exhibiting far less structural damage. Tree mortality at Point Pelee increased through 1989 and 1990, while no mortality was noted at Northville. Some species of bark beetle new to Canada were discovered to be major insect pests contributing to the mortality of Dwarf Hackberry adults at Point Pelee, but not at Northville.