Contrasting responses of epiphytic and terricolous lichens to variations in forest characteristics in northern boreal ecosystems1

The main environmental factors that drive lichen communities are still poorly known in northern boreal ecosystems. This study compares the effects of forest stand characteristics (height, canopy cover, and age) on fruticose epiphytic and terricolous lichen communities across a large region located at the interface between closed-crown boreal forests and northern open woodlands in the province of Quebec (Canada). The dataset consists of 875 plots spread across a 242 000 km² territory that ranges from the eastern to the western extremities of the province. The biomass of fruticose epiphytic lichens (Alectoria, Bryoria, Evernia, and Usnea) was evaluated at the branch, tree, and plot levels, and terricolous lichen cover (Cladonia spp.) was evaluated at the plot level. The results indicate that epiphytic and terricolous lichens respond significantly but differently to variations in forest characteristics. At the plot level, epiphytic lichen biomass was highest in the oldest stands (>100 years) and lowest in stand with low canopy cover (<25%) or in stands dominated by relatively short trees (<7 m). By contrast, terricolous lichen cover was highest in stands dominated by short (<7 m) or mid-sized (7–12 m) trees and lowest in stands with a relatively high canopy cover (>40%) or stands of intermediate age (60 to 100 years old). Species composition of epiphytic communities was also examined, and some species or genera exhibit a strong association with older stands (Alectoria sarmentosa (Ach.) Ach., Bryoria spp.) or with specific regions along the ca. 1500 km east–west gradient (Evernia mesomorpha Nyl. in the western part, Bryoria spp. in the central part, and A.sarmentosa in the eastern part). In terms of conservation, these results indicate that epiphytic lichens communities are potentially sensitive to the preferential logging of older stands. Both epiphytic and terricolous lichen communities are also potentially sensitive to expected climate change effects such as increased fire frequencies or increased forest growth.