Abstract
Pinus contorta-dominated montane forests of western Canada with relatively dense tree canopies have ground layers with abundant bryophytes, especially the feather mosses (Pleurozium schreberi and Hylocomium splendens), while those with more open canopies are dominated by species of reindeer lichens, especially Cladonia arbuscula s.l. and C. rangiferina s.l. Woodland caribou (Rangifer tarandus caribou), which are a threatened species in Alberta, prefer open, Cladonia-dominated forests for their winter food supply. This study investigated if opening the canopy by thinning mature montane forests of the Canadian Rocky Mountain foothills would change the abundance of lichens and bryophytes. In 1997, forests were thinned by removing 20%, 40%, and 60% by volume. In 2016, 19 years after treatment, we re-surveyed a subset of these plots (n = 97) for lichen and bryophyte abundance and species richness by utilizing the amount of canopy opening at the plot level as our prime gradient. We then used ordination to determine the relationship of control plots to treatment plots. In uncut forest, the control plots were highly variable, but were mostly dominated by feather mosses, with little or no bare ground. Feather moss abundance was lower in treatment plots when compared to control plots, while cover of bare ground was greater. Overall, 19 years after treatment, we found that, in treatment plots, lichen abundance remained stable or slightly increased, feather mosses decreased markedly, and unoccupied space was double that of the control plots. We conclude that the canopy opening had little effect on understory and ground layer diversity, but considering species abundance (1) bryophytes have not recovered after canopy opening, (2) populations of reindeer lichens increased marginally, but have not colonized areas left bare from bryophyte dieback, and (3), after 19 years there, remains unoccupied areas of bare ground in plots with a reduced canopy cover. Our study demonstrated that, with canopy cover reduction resulting from forest thinning operations, the ground layer diversity is maintained, but recovery of ground layers in old-growth pine-dominated forests is not promoted. Therefore, timber harvest that partially opens the tree canopy is unlikely to benefit caribou by augmenting or accelerating winter food availability and habitat suitability for caribou.
Highlights
Upland coniferous forests in boreal western Canada often have a ground layer composed of a mosaic of mosses and lichens
More open stands have ground layers dominated by lichens, especially Cladonia arbuscula (s.l.), while stands open stands have ground layers dominated by lichens, especially Cladonia arbuscula (s.l.), while stands with more closed canopies have ground layers dominated by bryophytes, especially the feather moss, with more closed canopies have ground layers dominated by bryophytes, especially the feather moss, Pleurozium schreberi
The small increases in lichen abundances in the treatment plots (8.2%) suggest that these small increases are due to some expansion of existing populations and not the establishment of new populations in areas of bare ground. These findings demonstrate that lichens respond minimally to opening the canopy, and after 19 years, lichen abundances have not increased substantially and reindeer lichens have not yet expanded into areas left bare by feather mosses, which leaves large areas of the bare ground
Summary
Upland coniferous forests in boreal western Canada often have a ground layer composed of a mosaic of mosses and lichens. Ground layers in many stands are a mosaic of patches of lichens and mosses [1]. Shade-adapted bryophytes have limited ability to recover from prolonged periods of drying [7], fruticose lichens of the ground layer are adapted for surviving long periods of drought and high light environments [8]. These attributes of lichens and bryophytes contribute to sensitivity to disturbance, including sensitivity to mechanical damage from logging operations and changing environmental conditions post-harvest. Other characteristics of the substrate play a limiting role for ground layer species, including the amount of organic matter, moisture availability, and soil chemistry and texture [13]
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