Recruitment Processes and Species Coexistence in a Sub-boreal Forest in Northern Japan

Abstract

We investigated the recruitment of saplings (across the 2 m-height threshold) of six species, Picea jezoensis, Abies sachalinensis, Betula ermanii, Picea glehnii, Acer ukurunduenseand Sorbus commixta,in a sub-boreal forest, northern Japan. Data were collected in a 2.48-ha plot over six growing seasons (1989–1994). We used path analysis to analyse the relationships between the recruitment rates of saplings and the stand structural attributes such as mother tree abundance, stand crowdedness, stand stratification, Sasabamboo density on the forest floor, and fallen log abundance. The combination of stand structural attributes affecting recruitment rates of the six sub-boreal forest tree species differed markedly among the species and corresponded to species composition. It is suggested that the size-structure dynamics of adult trees of the sub-boreal forest are regulated largely by different regeneration processes among the species and only slightly by interspecific competition between adult trees because interspecific competition between adult trees was not evident. The dynamics of species coexistence of the sub-boreal forest should be described as a process combining the diversity of recruitment processes of saplings of the component species and the diversity of interspecific competition between adult trees. We propose the boundary condition hypothesis for species coexistence in the sub-boreal forest, that the persistence of each component species is ascribed largely to the different recruitment processes of saplings (boundary conditions for adult tree growth dynamics) and only a little to interspecific adult tree competition.