Examination of the coexistence mechanism of two major conifers in Hokkaido, northern Japan, based on differences in suitable germination conditions and shade tolerance

The coexistence mechanism of Abies sachalinensis and Picea jezoensis, which usually regenerate on fallen logs, was examined with respect to differences in suitable conditions for germination and shade tolerance in Hokkaido, northern Japan. Germination conditions were evaluated through the effect of log hardness, moss height, light condition, and shading by taller seedlings on fallen logs, and shade tolerance was evaluated by physiological and morphological traits and survival dynamics in a natural coniferous forest. In contrast to P. jezoensis, the number of germinated seedlings of A. sachalinensis increased with moss height. The number of germinated seedlings decreased with shading only in P. jezoensis. Consequently, it is suggested that germination of P. jezoensis is limited to fresh, recently fallen logs. The survival rate of A. sachalinensis under low-light conditions (< 5% of relative photosynthetic photon flux density) was higher than that of P. jezoensis. Abies sachalinensis had a higher leaf mass ratio (total leaf mass/total mass) and leaf area ratio (total leaf area/total mass) than P. jezoensis. In conclusion, earlier establishment of P. jezoensis on fallen logs and higher shade tolerance of A. sachalinensis are suggested to contribute to the coexistence of these 2 species.