A power function for forest structure and regeneration pattern of pioneer and climax species in patch mosaic forests.

Abstract

The DBH-class distribution in natural deciduous broad-leaved forests was elucidated with a power function. A power function (y=ax b, y: stem density, x: represents DBH class, a and b: constants) fits the distribution better than an exponential function (y = a exp bx). The parameter b in the power function is approximately −2. This means that the natural forests studied have a patch-mosaic structure and that tree cohorts regenerate from gaps. Parameter a implies the number of juveniles, and b means size-dependent mortality. The value of −2 for parameter b means that when trees in a given DBH class double their DBH, the density of the size class should decrease by one-fourth. This phenomenon results from self-thinning and is caused by horizontal space competition among trees, called the `tile model'. The parameter describing DBH-class distribution for a forest with self-thinning patches should be approximately −2. I call this the `−2 power law' for DBH-class distribution. In a typical natural forest dominated by deciduous broadleaf tree species, trees are recognized as pioneer or climax species by the parameters describing their regeneration patterns. When I applied the power functional model to the DBH-class distribution of each dominant species, in pioneer species parameter a was high and b was less than −2 (markedly less than zero), suggesting that there are many juveniles, but mortality is high. On the other hand, in climax species parameter a was low value and the value of b was larger (negative, but closer to zero), suggesting that there are not many juveniles, but mortality is low. A power-function analysis of DBH-class distribution can be used to clarify the patch mosaic structure of a forest, and to clarify the regeneration pattern of pioneer and climax species by applying the function for each species.