Forest Instability and Canopy Tree Mortality in Westland, New Zealand

Abstract

Many researchers in New Zealand have accepted equilibrium models of vegetation change that assume within-stand self-replacement of the dominant tree species as the norm. Consequently, many discontinuous stand structures have been used as evidence of forest instability. For example, the patterns of regeneration and mortality in the rata-kamahi forests of Westland have led many to believe that the present canopy tree mortality is excessive. As a result, there has been considerable research on browsing by the introduced brush-tailed possum as the primary cause of the mortality. We suggest that any interpretation of this forest pattern must include a consideration of the influences on the vegetation of natural disturbances. Abundant evidence suggests that at least some of the mortality is due to senescence of cohorts of trees that originated at approximately the same time after events such as windthrow and mass movements. It may be that browsing by possums hastens the death of trees already susceptible as a result of natural stand development processes.