Microsite- and age-specific processes controlling natural populations of Acer saccharum at cliff edges

Abstract

Along cliff edges of the Niagara Escarpment in Ontario, Canada, Acer saccharum Marsh, is rarely found, but within 5 m of the cliff edge it becomes the dominant species of a rich deciduous forest on the plateau. To discover how age-dependent and microsite-dependent processes combine to control the spatial distribution of this species along the environmental gradient, age structures of natural seedlings and saplings between 1 and 25 years old were investigated over a three-year period. Mortality rates over the monitoring period were highest in 1-year-old seedlings and decreased gradually as age approached 10 years. Both spatial and seasonal variation in environmental conditions affected 1-year-old seedlings most, and these effects declined as seedling age increased. The distribution of types of visible injury preceding death in the 1-year-old seedlings suggested that both drought and herbivory influenced survivorship in this species, but to different degrees at different positions on the gradient. Lower seedling heights closer to the cliff edge in 1- to 10-year-old seedlings suggested that environmental conditions constrained productivity and survival. These results clearly demonstrate that variation in age structures along the gradient was determined by both age- and microsite-dependent processes and that these both contribute to the spatial distribution pattern of A. saccharum. Key words: life tables, survivorship, mortality, cliff, distribution, Acer saccharum, Niagara Escarpment.