Canopy Dieback and Successional Processes in Pacific Forests

Abstract

Massive tree dieback has occurred periodically in the Hawaiian montane rain forest. The species mainly involved is Metrosideros polymorpha, which is the prevailing upper canopy tree species throughout most of this forest on all high Hawaiian islands. The canopy dieback occurs in stands over the entire spectrum of sites, from well-drained lava flows over nutritionally rich volcanic ash to permanently wet bogs with toxic soils. A biotic agent could not be found to cause this dieback. Five main dieback patterns have been recognized, and all are site-specific. These patterns suggest certain causal mechanisms, but they explain only a fraction of the dieback syndrome. A number of additional facts were established which have led to a new dieback theory involving a chain-reaction process: (1) cohort senescing as a predisposing factor; (2) a dieback trigger, which can be either internal (a species characteristic) or external (a fluctuating and recurring site-specific perturbation), and (3) a dieback hastening (biotic agents) or stalling mechanism. It is believed that the dieback phenomenon is not restricted to Hawaii but occurs also in other, functionally similar Pacific forests. A corollary to the Hawaiian dieback etiology is a new succession theory, which explains the temporally recurring dieback as a pattern and process sequence in primary succession. The larger dieback patterns are considered a consequence of catastrophic disturbances in the past, such as lava flows, ash blanket deposits, or landslides, which gave rise to large cohorts. Canopy dieback of these large cohorts during their senescing stage then gives rise to new cohorts. However, these become successively smaller and more patchy with each dieback cycle. Concomitantly with the dieback cycles a turnover of successional races or ecotypes appears to occur within the Metrosideros polymorpha species complex. This may allow us to determine pioneer, seral, and near-climax races within this species complex.