Regeneration and coexistence of two subalpine conifer species in relation to dwarf bamboo in the understorey

Microhabitats for seedling establishment and gap regeneration in subalpine forests of northern Japan were studied for two conifers, Abies sachalinensis and Picea glehnii. The abundance of understorey dwarf bamboo (Sasa spp.) was different for the four plots examined. Two types of microhabitats were recognized for the two conifers: ground and elevated woody substrates (fallen logs and buttresses). Picea regenerated mostly on elevated sites, while Abies regenerated on both ground and elevated sites. The densities of Picea were independent of those of Sasa, but Abies densities decreased with increasing abundance of Sasa because Sasa reduced regeneration on the ground. Density of Abies on elevated sites was higher than that of Picea, irrespective of Sasa and of the density of adult trees. There was no significant difference in growth in sapling trunk height between the two conifers, but Picea grew more slowly under the canopy than Abies and was aggregated into gaps. Thus, in forests with less Sasa, the recruitment capacity of Abies was greater than that of Picea. The long life span of Picea compensated for its low density on elevated sites. Examination of a dynamic system model showed that Picea was excluded by Abies in forests without Sasa because regeneration on the ground is more advantageous than on elevated sites, but the two conifers could coexist in forests with Sasa because of the increased relative success of regeneration on elevated sites by Picea saplings.