Forest dynamics and stand-level dieback in New Zealand'sNothofagus forests

Abstract

Emphasis is placed on the role synchronised cohort mortality in the dynamics of beech (Nothofagus) forest in New Zealand. Mature populations are frequently relatively even-aged and have size structures which render them vulnerable to exogenous disturbances from which they cannot recover, except by regeneration. Examples of regeneration systems in pure and mixed stands are described. Mountain beech (N. solandri var.cliffortioides) is a relatively short-lived light demanding species largely dependent upon exogenous canopy destruction for regeneration. Red beech (N. fusca) is a larger longer-lived tree which can maintain its population by autonomous gap-phase regeneration. When these species grow in mixture with silver beech (N. menziesii) in the absence of disturbance, they seem destined to be replaced by it, because silver beech is more shade tolerant and longer-lived. Thus, factors promoting stand synchrony are important for mountain and red beech, because if large scale canopy opening does not occur, sites are more likely to be lost to competitors. Cohort synchrony is associated also with physiological synchrony between individuals. Consequently periodic stresses, such as mast seeding, may affect almost all individuals. Storm damage and drought years are implicated. If only a few individuals (or even parts of individuals) die, this can lead to rapid stand decline, and subsequent large scale regeneration.