The Social Status of Agathis in a Rain Forest in Melanesia

Abstract

Of the small group of conifers which occur in the lowland tropics species of Agathis, Dacrydium and Podocarpus grow mixed with angiosperms in tropical rain forest. Dacrydium and Podocarpus are predominantly montane genera with a few species descending into the lowlands, Agathis has rather more lowland species. In its present distribution (Florin 1963) and habitat Agathis is the most euthermic confer. Mixed conifer-angiosperm forests occur scattered through Malesiat and the southwest Pacific region. Their ecology has scarcely been investigated: this is important because of its bearing on the evolutionary history of the relationships between conifers and angiosperms. Further, tropical conifers, like temperate ones, are sometimes associated with podsolic soils and a study of them might be expected to throw light on the phenomenon of podsolization in the tropics. The temperate podocarp-angiosperm forests in New Zealand, and the forests of Queensland and New Guinea where species of Araucaria grow as emergents from an under-layer of Tropical Lowland and Lower Montane Rain Forest, have received some attention from ecologists. The native vegetation of much of the lowlands of New Zealand is a series of forests which range from pure angiosperm through angiosperm-podocarp mixture to pure podocarp. The main podocarp is Dacrydium cupressinum Soland. There are various hypotheses to explain this series, which Robbins (1962) has summarized. Cockayne considered that it represents an ecological succession, leading from conifer forest to angiosperm forest climax. Poole and Holloway later both suggested alternation of the forest types on any one site. Holloway later abandoned this explanation and suggested that a climatic change about 800 years ago had favoured the angiosperms and was leading to replacement of the conifers, still incomplete. Robbins himself postulated that there are two forest types, an ancient conifer forest and a geologically more recent angiosperm forest; both are found in pure form but in many places they have fused and the angiosperms are exercising a progressive supremacy over the conifers (Robbins 1962). Agathis australis Salisb. occurs in the north of New Zealand around Auckland. Apparently it does not regenerate in its own shade and in the climax vegetation is replaced by other species (Cockayne 1928, p. 127; Mirams 1957). According to Mirams (1957) it invades the secondary vegetation which springs up after clearing and at one stage in the secondary succession is well represented by all size classes; the big trees in