Nothofagus dieback on Mt. Giluwe, Papua New Guinea!

The phenomenon of patch dieback in stands of Nothofagus on Mt. Giluwe, in the southern highlands of Papua New Guinea, is described. Ecological studies that have been carried out are reviewed. Based upon the results of surveys on Mt. Giluwe, the role of Phytophthora cinnamomi in contributing to the dieback is examined. A comparison is made of Nothofagus dieback on Mt. Giluwe with Metrosideros dieback in Hawaii. AN IMPORTANT COMPONENT of the midmontane forest zone ofNew Guinea is Nothofagus Blume (Johns 1982), and about 13 species are represented (Ash 1982). On Mt. Giluwe, an extinct shield-shaped volcano in the southern highlands of Papua New Guinea (Figure 1), three species of Nothofagus, N. grandis Steen, N. rubra Steen, and N. pullei Steen, are found, with N. pullei extending to a higher altitude toan the6tlier fwospecies (Asli1982fExfeii= sive stands of single species may be formed, and there is a mosaic of small areas of trees, each dominated by a single size class that is apparently even-aged (Ash 1975). A feature of these stands, and of stands of Nothofagus in other parts of Papua New Guinea, is the occurrence of patches of dead and dying mature N. pullei and N. grandis of large diameter (Ash 1982, Cartledge, Shaw, and Stamps 1975, Paijmans 1976). Death of trees is preceded by defoliation of the crown and development of epicormic shoots on the bole and lower crown. Initially, only a few trees die, but the patch may enlarge laterally at a rate of up to 5 mfyr (Ash 1975). Good regeneration ofNothofagus has been recorded in old dead patches, and there have been no symptoms of widespread mortality among the regrowth. Figures 2 and 3 show typical dieback patches. Several hypotheses have been proposed regarding the cause of this dieback phenomenon (Ash 1982, Cartledge, Shaw, and Stamps 1 Manuscript accepted 5 October 1983. 2Forest Research Station, P.O. Box 134, Bulolo, Papua New Guinea. 1975, Paijmans 1976, Robbins and Pullen 1965), and these are examined in this paper in conjunction with results obtained from a soil survey for root pathogens carried out at Mt. Giluwe. _DIEBACK :_ANATURALI'HENOMENON1. The hypothesis that dieback is a natural phenomenon necessary for the regeneration of Nothofagus is one of two hypotheses, the other being that the dieback is a result of attack by a root pathogen newly introduced into the area (Cartledge, Shaw, and Stamps 1975). It has been observed that regeneration of Nothofagus spp. is generally poor under closed-canopy forests, with other species being favored (Ash 1975). Although seed crops may be heavy, very few seedlings develop and their growth is very slow. Upon opening of the canopy, seedlings grow rapidly, suggesting that they are light-demanding. In addition to seedlings, much regeneration originates from suckers which may arise from fallen tree trunks (Ash 1982). Thus, the regeneration of Nothofagus spp. appears to depend on the creation of gaps in the canopy, either through deaths of trees or by other means. Mortality is restricted to trees of large diameters. Ash (1975) has shown that canopy closure occurs at about age 75 yr, maximum growth occurs at approximately age 150-200 yr, and senescence commences after this age. Groups of trees appear to be even-aged, representing a single cohort of individuals.