THE SIGNIFICANCE OF THE ASYMMETRIC EFFECT OF CROWDING FOR COEXISTENCEIN A MIXED TEMPERATE FOREST

The coexistence of coniferous (mostly Abies homolepis) and broad-leave d tree species (mostly Fagus crenata) in a mixed temperate old-growth forest in Japan was simulated by a size-structure dynamics model incor porating the asymmetrical (one-sided) effect of shading between these two life-form guilds. The model assumes that the crowding effect due t o one-sided competition for light on a tree of a given size regulates the rate of size growth and recruitment. The cumulative basal area of trees larger than a given tree in the forest is employed to express th e intensity of one-sided competition on that tree. Cumulative basal ar eas of both guilds negatively affected the growth rate of any tree. Th e shading effect by conifers on the growth rate of either guild was st ronger than that by broad-leaved species. Two types of model were test ed for recruitment; an additive and a reciprocal model. A reciprocal m odel, where basal area density of conifers and broad-leaved species ha s a negative effect on the recruitment of its own guild but has a posi tive effect on that of the other guild, fit the observed data better t han an additive model where total basal area of the two guilds suppres ses recruitment rates. Simulations using these models showed that, wit hin a particular range of the set of recruitment rates, the two guilds could coexist. The tendency for reciprocal replacement, incorporated in the reciprocal model, substantially widened the range of coexistenc e and shortened the time required for convergence.