Distribution and Succession of Epiphytic Lichens on Black Spruce near Cochrane, Ontario

The distribution and succession of corticolous lichens on black spruce in an unburned area near Cochrane, Ontario, were investigated. The frequencies of the epiphytes in point samples are statistically dependent on the following microclimatic indices: aspect, height, and distance to the nearest tree in the sampled quadrant. Differences in the lichen flora between whole trees of different absolute age are attributable to their different heights, but there were distinct differences in the flora of bark surfaces of different ages, indicating an apparent succession. Regressions of the frequencies of Alectoria nadvornikiana and Parmelia physodes on the microclimatic indices were per- formed separately for surfaces in a number of age classes. The resulting regres- sion coefficients were plotted against time, revealing several sharp changes in the rate of corticolous succession. It was found that these changes could be ex- plained by tree morphology, i.e. changes from crown to bole to base, and by presumed microclimatic gradients correlated with tree morphology. Because the observed changes in epiphytic vegetation are caused by growth of the host tree and associated with environmental variation, and not by the activities of the plants themselves, it was concluded that corticolous succession is not a true suc- cession as defined by Clements.