Macrolichen colonization on 120–140 year old Tsuga heterophylla in wet temperate rainforests of central-interior British Columbia: a comparison of lichen response to even-aged versus old-growth stand structures

Abstract

Canopy lichen abundance was assessed by size class on regenerating hemlocks, comparing trees of similar size and age (c. 120–140 yrs) growing in the understorey of old-growth cedar-hemlock stands with those growing in adjacent even-aged hemlock stands (natural origin patches c. 1–3 ha in size). Five chlorolichen taxa were associated with old-growth understorey trees: Cavernularia hultenii, Hypogymnia vitatta, Parmelia hygrophila, Platismatia norvegica, and Usnea spp. Lobaria pulmonaria was the most abundant cyanolichen on regenerating hemlock in the old-growth forest stands, particularly in lower canopy (under 12 m) exposures. However, other cyanolichen taxa such as Nephroma helveticum, Sticta fulginosa, and Pseudocyphellaria anomala, reached their greatest abundance at mid-canopy (12–24 m) positions. Smaller cyanolichen thalli (<9 cm²) were abundant on regenerating hemlocks across all canopy positions in the old-growth forest, raising the question as to whether or not cyanolichen thalli in mid- to upper-canopy environments represented long-established individuals facing severe growth constraints, or were simply thalli that had experienced higher rates of fragmentation, and thus did not achieve larger sizes. In comparison, cyanolichens of all taxa were essentially absent from the small-patch even-aged forest stands. Given that dispersal of propagules was not likely a major limiting factor, these 120–140 year old even-age stands may not yet have attained sufficient old-growth characteristics (especially canopy microclimate and canopy throughflow enrichment) to support cyanolichen growth. These findings have major conservation biology implications for wet interior cedar-hemlock forests in British Columbia, where forest harvesting is creating a mosaic of even-aged stands, whose projected age at the time of next harvest (rotation age) will be 120 years or less.