Effects of disturbance on old-growth Thuja occidentalis at cliff edges

Thuja occidentalis L. is abundant along cliff edges and cliff faces of the Niagara Escarpment in southern Ontario, Canada, but declines in frequency away from the cliff edge. Hiking and climbing disturbance is intense in such areas, and therefore a study was done to determine the relationship between forest stand structure and disturbance. In eight undisturbed and eight disturbed transects, a total of 441 trees were sampled for height, diameter, age, mass, and average annual productivity. In undisturbed areas most of these characters declined in value away from the cliff edge, whereas in disturbed areas their values increased. Seedlings were almost totally absent from the disturbed areas. It appears that the extensive elimination of ground cover in the disturbed areas results in the release of constraints on the growth of this species, but recruitment of new individuals into the population is stopped. The disturbed parts of the cliff-edge ecosystem are therefore not self-sustaining, and this has considerable significance in view of the great age shown by many of the trees.