Dynamic Structure of Wave-Regenerated Abies Balsamea Forests in the North- Eastern United States

Abstract

Many high-altitude forests of balsam fir (A. balsamea (L.) Mill.) in the northeastern USA are characterized by wave regeneration, a peculiar type of disturbance pattern in which trees continually die off at the front edge of a wave and are replaced by vigorous regeneration. This apparently takes place because when an opening occurs in the forest, the canopies of the trees at the leeward edge of the opening, exposed to the prevailing wind, die from a combination of loss of branches and needles in winter due to heavy rime accumulations, death of needles due to winter desiccation and decreased primary productivity due to cooling of needles in summer. This exposes the trees behind them, which die in turn, and so a wave of dying trees moves through the forest. The direction of motion of the waves appears to be controlled by the prevailing wind, while the speed which varies from 1-3 m yr-1 is determined by the degree of exposure to the wind. Wave-regenerated forests are fairly common in the northeast, having been observed in the mountains of New York, New Hampshire and Maine [USA]. Very similar forests have been described in Japan. The growth pattern of this forest is basically cyclical. Regeneration waves tend to follow each other at approximately 60 yr intervals, so the average stand developing after a wave has passed lasts about 60 yr before another wave comes through and begins the regeneration process again. Because of the regularity of this process, all stages of degeneration and regeneration are present in the forest at all times; moreover, degenerative changes in one part of the forest are typically balanced by regenerative changes elsewhere, so the composition of the forest as a whole remains relatively constant with time. The ecosystem is in what might be termed a steady-state.