Distribution and regeneration strategies of major canopy dominants in species-rich subtropical warm temperate rainforests in south-western Japan

The distribution and regeneration strategy of the major canopy dominants in species-rich subtropical/warm temperate rainforests in south-western Japan was studied in a hilly zone below 1100 m a.s.l. Using the patch sampling method, four dominance-community types were numerically identified and they corresponded to four habitats which represented a combination of topography and altitude (i.e. ridges us slopes, and below vs above 500 m a.s.l.) Seven major canopy dominants had their respective distributional core in one of these topo-altitudinal habitats. The seven canopy dominants could be classified into three species groups according to their size structure and growth habits. Group A (Tsuga sieboldii and Quercus gilva) and group B (Quercus acuta, Quercus salicina, Machilus thunbergii) were restricted to the emergent and canopy layer, respectively. These two groups had only a few subcanopy trees and saplings. Saplings of group B showed a rapid growth rate in canopy gaps. Group C (Distylium racemosum) was characterized by many subcanopy trees and saplings that grew steadily under the closed canopy. Castanopsis sieboldii showed intermediate characteristics between group B and C in the size structure and growth habit of its saplings. The density of D. racemosum canopy trees was markedly reduced on ridges and slopes above 500 m and on slopes below 500 m. In these marginal habitats, the three species groups coexisted by sharing different strata within a community. This situation was possible due to the differences in regeneration strategies among the canopy species.