Effects of dwarf bamboo ( Sasa sp.) and forest floor microsites on conifer seedling recruitment in a subalpine forest, Japan

Abstract

To clarify the effects of dwarf bamboo ( Sasa sp.) and forest floor microsites on coniferous ( Abies mariesii, A. veitchii, Picea jezoensis var. hondoensis, and Tsuga diversifolia) seedling recruitment, occurrence and survivorship of current and old (age≥1 year old, height≤15 cm) seedlings on soil and fallen logs were examined in quadrats ( 2 m×2 m) set systematically in a 1 ha permanent plot located in stands with or without Sasa understory of a subalpine forest, central Japan. Occurrence of old seedlings of P. jezoensis var. hondoensis and T. diversifolia was much greater in number on fallen logs than on soil, whereas old seedlings of Abies could occur with similar number on soil as well as fallen logs in the stand without Sasa understory. Whether old Abies seedlings occur on soil or on fallen logs varied largely with degree of Sasa coverage in the stand with Sasa, on the sites with Sasa cover<40% in area, the old seedlings occurred more frequently on soil, but on the site with Sasa cover≥40% they occurred more frequently on fallen logs. The results indicate that the presence of Sasa raises the relative importance of fallen logs for the seedling recruitment of Abies. On the other hand, fallen logs are primarily important for the seedling recruitment of P. jezoensis var. hondoensis and T. diversifolia irrespective of the presence or absence of Sasa. The difference in occurrence of recruited seedlings between these two microsites was formed at some stages in seedling emergence and subsequent mortality process within a current year. The forest floor occupied by fallen logs was small in area, while soil covered large area of the forest floor. However, seedlings on fallen logs may not be affected by the suppression of understory vegetation, such as Sasa as those on soil. From this study, it is suggested that the difference of species life history traits (e.g. seed size), presence of various microsites, and their interactions with Sasa may be one of possible factors to maintain tree species coexistence in a subalpine old-growth coniferous forest.