Growth and Physiology of Thuja occidentalis L. from Cliffs and Swamps: Is Variation Habitat or Site Specific?

To determine if patterns of growth and physiology of Thuja occidentalis differed between dry and wet habitats or were site specific within habitats, three cliff and three swamp populations in southern Ontario, Canada, were intensively investigated. The average annual increases in height, basal diameter, and estimated oven dry mass were determined for all trees in four 16 m2 quadrats per site. The net photosynthetic response to photosynthetically active radiation was measured in situ at each site and on potted cliff and swamp saplings kept in a mesic common garden. Possible environmental factors controlling growth, such as tissue nutrient levels and canopy shading, were also investigated. Significant differences were found among the six sites in all three measures of productivity, in the maximum net photosynthetic rate at light saturation (Pmax), in the foliar levels of N, P, Ca, and the N:P ratio, and in the degree of canopy shading. Variations in all these characteristics were site specific and independent of habitat type. Significant differences between cliffs and swamps were found only in the foliar levels of Mg (cliff > swamp). All foliar nutrient levels (with the possible exception of P at one cliff site) were above values considered limiting. Plants in the common garden had the same photosynthetic light response as their in situ counterparts. There was no correspondence among the site-specific patterns of productivity, Pmax, foliar nutrients, and canopy shading. It is concluded that the factors controlling the growth of T. occidentalis are site but not habitat specific. No evidence of ecotypic differentiation was found.