Tree Competition and Species Coexistence in a Sub-boreal Forest, Northern Japan

Abstract

The growth and mortality patterns and the mode of competition of six tree species forming a sub-boreal climax forest in Hokkaido, northern Japan, were investigated based on the diffusion model at the level of the individual tree ⩾ 2 m height in a 2·3-ha study site. Picea jezoensis, Picea glehnii, Betula ermanii and Abies sachalinensis were dominant species, occupying approx. 94% of the total basal area. Sorbus commixta and Acer ukurunduense were subordinate species occupying approx. 6% of the total basal area. A model for individual growth was developed, considering both intra- and inter-specific competition and the degree of competitive asymmetry. Asymmetry was found in intraspecific competition of Sorbus commixta and Acer ukurunduense. Picea jezoensis, Betula ermanii and Abies sachalinensis showed symmetric intraspecific competition. There was little interspecific competition amongst Picea jezoensis, Picea glehnii and Betula ermanii. Abies sachalinensis competed symmetrically with Picea jezoensis (only very weakly, P < 0·1) and Betula ermanii ( P < 0·01). Picea glehnii gave no indication of inter- or intra-specific competition. The growth of the four dominant species was never affected by the two subordinate species; the growth of the two subordinate species was governed by the abundances of the four dominant species, the sum of which almost amounted to stand crowdedness (i.e. symmetric competitive effect and one-sided competitive direction). On the scale of 2·3 ha of the sub-boreal forest, symmetric competition prevailed over one-sided or asymmetric competition although statistical evidence for any competitive effects was rather weak. This was probably due to the relatively low tree density and stand crowdedness of this climax forest. Little competition between the dominant species suggested by relatively low proportions of r 2-values attributable to competitive effects indicates weak organization amongst the component species (i.e. species were more or less independent of each other) at the level of the individual tree ⩾ 2 m height on the 2·3-ha scale.