Lichen Colonization and Gap Structure in Wet-temperate Rainforests of Northern Interior British Columbia

Abstract

Abundant canopy lichen communities characterize wet-temperate rainforests on the windward slopes of interior mountain ranges in north-central British Columbia, Canada. Historically, these forests have regenerated through gap-dynamics; however, our knowledge of lichen colonization within gaps is limited. We have now compared lichen biomass on regenerating trees in naturally occurring 1–3 ha gap-disturbances (these gaps presumed to have originated from insect out-breaks in the late 1800's) with those on regenerating trees of similar age growing in the understory of the surrounding old growth forest. Only small differences were seen in total lichen biomass on regenerating trees between the two settings, however, analysis of the individual lichen groups (Alectoria, Bryoria, Foliose, and Cyanolichen) revealed striking differences. The Bryoria group was 35% more abundant on gap trees (632 g/tree) and was distributed vertically through a larger proportion of the tree crown. The Cyanolichen functional group was largely absent from gap trees, despite high levels of biomass loading (1,332 kg/ha) in the surrounding old-growth stand. Alectoria and Foliose functional groups did not differ significantly in biomass or distribution between regenerating trees of the two types. Tree size positively affected lichen loading. Total lichen biomass was 38% greater on the larger size class (31–44 cm dbh) regenerating trees, with the Alectoria functional group alone having 45% greater biomass on larger trees. Presence or absence of leaves on branch substrate had no effect on lichen loading. Stand-level projections indicate that the old growth forest had 19% more arboreal lichen biomass (2,684 kg/ha) and contained greater lichen species diversity than did the “second-growth” regenerating forest patches. The low cyanolichen biomass in naturally occurring gap openings poses concern for the proposed utility of “new-forestry” type harvesting practices to retain canopy biodiversity using current harvest rotation intervals.