Cyanolichen Distribution in Young Unmanaged Forests: A Dripzone Effect?

In young forests of humid south-central British Columbia, Lobaria pulmonaria and other epiphytic “cyanolichens” attain optimum development over the lower branches of conifers growing within the dripzone of Populus. To account for this, we propose the existence of a “dripzone effect”, in which nutrient-rich leachates from the upper branches of Populus enhance the pH of nearby conifers to the benefit of cyanolichen colonization. The dripzone effect is presumably most pronounced in stands located over nutrient-rich soils, in which Populus plays a pivotal role in the early establishment of nitrogen-fixing epiphytic cyanolichens. This phenomenon is assumed to be widespread, but it apparently leads to colonization of conifers by cyanolichens only in humid regions not subject to acid rain. Though the presence of cyanolichens on trees belonging to the Pinaceae is probably indicative of allochthonous nutrient enrichment wherever it occurs, we stress that the dripzone effect is only one of many mechanisms promoting the enrichment of conifer bark.