Canopy Closure, a Factor in Rata (Metrosideros)-Kamahi (Weinmannia) Forest Dieback in Westland, New Zealand

Increased dieback in Westland rata (Metrosideros umbellata)-kamahi (Weinmannia racemosa) forests has been linked to the build-up of populations of the Australian brush-tailed possum (Trichosurus vulpecula). Within these forests young even-aged stands are observed to be more resilient to dieback than older stands. The effect of possum browsing on individual rata trees was related to the level of defoliation. Trees which had been not or only lightly browsed maintained intact canopies. Where browsing resulted in canopy opening exposed leaf bunches continued to deteriorate, even in the absence of further browsing. In even-aged rata-kamahi stands which were still undergoing natural thinning crown size was small, and the loss of individual trees (whether by competition or possum browsing), did not affect the ability of the stand to maintain an intact canopy cover. Once thinning was complete, the loss of large crowned mature trees led to the opening up of the stand and to continuing damage by wind, fungi, and insects. Dieback is thus a positive feedback process in mature stands, but a buffered one in immature, self-thinning stands.