Although the importance of bryophyte colonies on tree seedling establishment on downed logs is widely known, the mechanisms of how various bryophyte species affect seedlings are poorly understood. In this study, we analyzed the relationships between bryophyte community (colony coverage and thickness of each species) and tree seedlings growing on logs along the decay process. In an old-growth subalpine coniferous forest dominated by spruce, juvenile seedling density of spruce on downed logs was positively associated with the coverage of a liverwort Scapania bolanderi and the thickness of the bryophyte colonies, but was negatively associated with the coverage of a moss Hylocomium splendens. Coverage of S. bolanderi was also associated with shoot length of spruce seedlings, but this effect was negative. Coverage of S. bolanderi increased with log decay and became the most dominant species in the penultimate stage of log decomposition, but was replaced by thicker H. splendens in the final decay stage. These results suggested that S. bolanderi colonies, dominant only in the mid-stage of log decomposition, may indicate a ‘window of time’ for spruce seedlings to colonize on logs in this old-growth subalpine coniferous forest. Thus, when trying to understand sustainable regeneration mechanisms in subalpine forests, this interaction should be considered.
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