Growth rate responses of Lobaria pulmonaria to canopy structure in even-aged and old-growth cedar–hemlock forests of central-interior British Columbia, Canada

Abstract

The ability of cyanolichens to sustain growth in even-aged forests that develop after clear-cut harvesting is a major conservation biology concern in British Columbia's inland rainforest. This and other conservation issues have led to proposals advocating partial-cut rather than clear-cut harvesting to better maintain the structures that support biodiversity in old-growth forest stands. However, evaluating the potential impacts of alternative harvesting practices cannot occur without first understanding lichen growth responses to the existing range of natural variability in canopy structure. Using transplant experiments with thalli of Lobaria pulmonaria (a tripartite foliose cyanolichen), we evaluate growth responses to canopy structure in both naturally occurring 120 year old small-patch even-aged stands and in adjacent old-growth uneven-aged forest stands. Canopy microclimate was also measured in old-growth reference trees, allowing calculation of energy conversion efficiency for old-growth L. pulmonaria thalli. Growth rates of L. pulmonaria transplants showed a strong correlation with canopy light transmission (CLT). Mean annual lichen dry matter gain over the 2-year measurement period ranged from less than 5% on branches in closed canopy even-aged stands (<1 mol m −2 day −1 CLT) to near 20% on branches under canopy gaps in old-growth stands (>18 mol m −2 day −1 CLT). Intercepted irradiance of hydrated L. pulmonaria thalli ( I wet) under old-growth stand conditions was 24.7 and 45.5 MJ m −2 annually in years 2 and 3 of the study, resulting in an energy conversion efficiency of 1.33 and 0.61%, respectively. Open sky (gap fraction) above lichen transplants in old-growth stands was much greater than that found in even-aged stands, allowing for longer duration sunfleck events in the lower canopy of the old-growth forest stands. These findings suggest that the creation of small openings by partial-cut harvesting in the inland rainforest may result in conditions favourable to continued growth of the dominant canopy cyanolichen, L. pulmonaria. They also support existing concerns about future cyanolichen growth in even-aged forests regenerating after clear-cut harvesting.