Substrate-associated seedling recruitment and establishment of major conifer species in an old-growth subalpine forest in central Japan

Abstract

In this study, we investigated substrate-associated seedling recruitment and establishment of major conifer species, Abies mariesii, Abies veitchii, Picea jezoensis var. hondoensis and Tsuga diversifolia, in a subalpine forest in central Japan. For all species, the abundance of currently germinated seedlings significantly reflected seed rain abundance and did not differ between substrate types, suggesting that seedlings emerge where seeds fall and that there is little difference in germination rate between substrates. This is probably because moisture-retention capacity, which is important for seedling germination, is usually adequate at the time of germination irrespective of substrate type. However, for all species, the survival rate of current seedlings was higher on woody debris (WD) than soil, and as a result older seedlings (≥1-year-old) were more abundant on WD. It is probable that several agents such as summer drought and pathogenic fungi reduced the survivorship of current seedlings on soil. Although mortality agents influence the survival of not only current seedlings but also older ones, results suggest that a relatively drastic change occurs from the current to 1-year-old seedling stage. Consequently, WD functions as safe sites, especially for first-year establishment, and thereby older seedlings are preferentially established on WD. Additionally, results suggest that the properties of WD have a significant effect on seedling survival and establishment, since older seedling densities seemed to differ among WD decay classes. Furthermore, compared to Picea and Tsuga, a relatively large proportion of 1-year-old Abies seedlings were maintained on soil due to their higher survival rate on soil substrates. This results from the large seed/seedling size of Abies, which probably provides several advantages at the time of first-year establishment. Thus, the events determining variations in safe sites for the seedlings of each conifer species in this subalpine forest specifically occur at the time of first-year establishment following germination.