Factors determining the distribution of Opuntia fragilis in the boreal forest of southeastern Manitoba

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Abstract
Opuntia fragilis (Nutt.) Haw. is restricted to south-facing rock outcrops on the shores of certain rivers and lakes in the boreal forest of southeastern Manitoba. Limitations in propagule dispersal, establishment, and maintenance were studied at Bird River (50°25′ N, 95°41′ W) between October 1979 and October 1982. Propagules studied were stem units (pads) because this cactus propagates entirely by vegetative means in this region. Mature terminal pads broke off most easily in the late spring while turgid and rolled downhill (geochory), thus establishing new plants locally. Some pads fell into the river and could float long distances (hydrochory) before being washed ashore. Perhaps less frequently, pads were dispersed over long distances on animals (epizoochory). Marked pads placed in potential habitats showed significantly greater establishment (α = 0.05) on south-facing outcrops than on north-facing or forest sites. Censuses (1979–1982) of an established colony indicated that pad number was increasing by 51% per year. Approximately 6% of the pads were dispersed from the outcrop. The present distribution of O. fragilis appears to be both habitat and dispersal limited.

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