Influence of high-contrast and low-contrast forest edges on growth rates of Lobaria pulmonaria in the inland rainforest, British Columbia

Forested landscapes in the upper Fraser River watershed face increasing fragmentation from forest harvesting. The resultant edge effects may have a major impact on canopy lichen communities in the surrounding forest matrix. Growth rate responses of large and small thalli of Lobaria pulmonaria were evaluated in transplant experiments conducted in old-growth cedar–Hemlock forest stands located adjacent to clearcut harvest blocks (high-contrast “hard” edges) and variable-retention harvest blocks (low-contrast “soft” edges). Growth response was also assessed in transplants onto retained trees in the variable-retention harvest blocks. There was little observed difference in growth rate responses of large thalli between “hard” and “soft” edges. In fact, contrary to our initial hypothesis, growth rates of large L. pulmonaria thalli were enhanced near forest edges (25% cumulative growth rate over 2-year study period at 7.5 m from clearcut edges compared to only 17% at forest interior sites). Growth rates of large thalli were somewhat reduced at the immediate edge of clearcuts, but not of variable-retention blocks. Optimal canopy openness for growth of large L. pulmonaria thalli fell broadly between 30 and 40%, while for small L. pulmonaria thalli was closer to 30%. Growth rates of small L. pulmonaria thalli placed on “soft” edge transects were also greatest at or near stand edges and on retained trees in harvest blocks. However, in the “hard” edge transects growth rates of small L. pulmonaria thalli show little or no enhancement near edges. Repeated measures analysis of variance confirms a significant treatment effect (“soft” versus “hard” edges) for the small Lobaria thalli. These findings suggest that edge effects near clearcuts (hard edges) may have little impact on previously established large L. pulmonaria thalli. However, they also suggest that future colonization of remnant forest stands near clearcut edges by small L. pulmonaria thalli may be delayed until edge effects are moderated. The adoption of variable-retention harvesting was demonstrated to be an effective means of reducing the impact of edge effects on both large and small L. pulmonaria thalli.