MULTIPLE CONTROLS OF CLIFF-EDGE DISTRIBUTION PATTERNS OF THUJA-OCCIDENTALIS AND ACER-SACCHARUM AT THE STAGE OF SEEDLING RECRUITMENT

(1) Along the Niagara Escarpment in Ontario, Canada, a stunted forest of old-growth Thuja occidentalis occurs on cliff faces, but is replaced by a deciduous forest dominated by Acer saccharum within 5 m of the cliff edge on the upper plateau. (2) The stem frequencies of all seedling and sapling size-classes of Thuja decreased with distance from the cliff, but for Acer the smallest size classes displayed the greatest increase in stem frequency away from the cliff edge. (3) Seed input did not change significantly across the cliff-edge-deciduous-forest continuum for either species. (4) Recruitment patterns of the two species differed significantly across the gradient. Seed emergence of Thuja was limited by litter in the deciduous forest but not at the cliff edge. Herbivory restricted recruitment of Acer more at the cliff edge than in the deciduous forest. (5) The maintenance of the spatial pattern appears to be due to an array of interacting factors which change across the environmental gradient and over time.