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.

Decaying logs are important seedbeds in boreal and subalpine forests. However, biotic and abiotic factors and their interactions related to seedling colonization patterns on logs remain unclear. In the present study, we evaluated the influence of bryophyte communities, wood decay type (white‐, brown‐, and soft‐rot) owing to decomposer fungal activities, and environmental abiotic factors on seedling establishment in an old‐growth subalpine coniferous forest in Japan. Among the tree species recorded on the conifer logs, Picea jezoensis var. hondoensis was the most dominant. Log surfaces were covered with distinctive patches of liverwort Scapania bolanderi and moss Pleurozium schreberi (approximately 33% cover for each). Redundancy analysis showed that brown‐rot in sapwood significantly affects the bryophyte and seedling community on the logs. Generalized linear models suggested that the total bryophyte cover, Scapania cover, and white‐rot in heartwood positively associate with Picea seedling density, whereas Pleurozium cover and basal area of adjacent Picea adults negatively associate with Picea seedling density. Results of structural equation modeling suggested that the brown‐rot of sapwood positively associates with Scapania cover that has a positive effect on Picea seedling density. Furthermore, brown‐rot of sapwood inhibited the Pleurozium cover, thus contributing to the Scapania dominance on the logs. These results suggest that fungal wood decomposer activities affect colonization of Picea seedlings in an indirect way via structuring bryophyte community on the logs.

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Hondo spruce (Picea jezoensis var. hondoensis (Mayr) Rehder)) is separately distributed among several mountainous regions in central Japan as remnant populations of the last glacial period. To identify factors that affect Hondo spruce seedling regeneration on decaying logs, we investigated the relationships between climatic conditions, log properties, including decay type by fungi, and Hondo spruce seedling density on logs using data from seven subalpine Hondo spruce forests in central Japan. The results showed that the presence of soft rot was associated with higher seedling density, and the effect of brown rot in sapwood and white rot in heartwood on the predicted number of spruce seedlings on logs switched from positive to negative with increasing temperature and precipitation. Because soft rot occurs under humid conditions, the use of forest management techniques that increase the number of logs with soft rot in sapwood (e.g., by keeping the forest floor moist) are recommended for the sustainable regeneration of Hondo spruce. However, the relationships between wood decay type and seedling regeneration can also be affected by climate condition and thus are more complex than previously thought.