Our work in various subzones of the Interior Cedar–Hemlock biogeoclimatic zone suggests that nutrient transfer from the soil into the canopy is mediated at least in part by the roots of cottonwood trees and trembling aspen. We hypothesize that such trees can effectively function as “nutrient pumps.” The nutrients transported into the canopy by them are later released into the surrounding forest in what may be called a “drip zone effect”. Though cottonwood and aspen do not themselves support diverse assemblages of epiphytic cyanolichens, they do permit the development of impressive nodes of cyanolichen diversity on conifers growing adjacent to them. By their presence, they indirectly provide habitat for many of British Columbia’s rarer epiphytic species. If it is true that nitrogen-fixing cyanolichens play a significant role in the nitrogen budgets of some forest ecosystems (Pike 1978), then aspen and cottonwood should be considered “keystone” organisms.
Arsenault, André, Goward, Trevor. 1999. The Drip Zone Effect: New Insights Into the Distribution of Rare Lichens (in Proc. Conference Biology & Management of Species and Habitats at Risk). Ministry of Environment, Lands and Parks; University College of the Cariboo. Proceedings - Conference Biology & Management. Vol. 2