Growth responses of Lobaria retigera to forest edge and canopy structure in the inland temperate rainforest, British Columbia

The foliose cyanolichen Lobaria retigera is a sensitive old-growth forest indicator in British Columbia's inland rainforest. These forests are increasingly being fragmented by harvesting, raising concerns about edge effects and the maintenance of canopy lichen communities in remaining forest patches. We have examined the response of L. retigera to edge effects, using transplant experiments to compare growth responses between abrupt (hard) edges created by clearcut harvesting and more buffered (soft) edges created by variable-retention harvesting. L. retigera thalli placed along hard edges showed high mortality rates, with most thalli (from 64 to 100%) losing biomass at the end of each of the 3-year's measurement periods. Biomass loss along soft edges was much lower, occurring in fewer than 7% of thalli in the best year, up to 42% in the worst year. When growth rates were expressed against light availability at each branch location, thalli along variable-retention transects showed a positive response to increasing light availability. In contrast, thalli along the hard edge transects showed an abrupt decline in growth rates at equivalent levels of high light availability (canopy openness values >30%), where presumably the benefits of greater light availability were offset by heat or dessication stress. These results show a much greater sensitivity to edge effects in L. retigera, compared to earlier measurements at this site with the more common lichen Lobaria pulmonaria, a species that has a dominant green-algal photobiont partner. We suggest that increased use of variable-retention harvesting systems in which a substantial number of residual trees are left along cutblock edges has the potential to reduce the impacts of forest harvesting on canopy lichen biodiversity.