This ecoregion represents the moist forests on Halmahera, Morotai, Obi, Bacan, and the other nearby Maluku Islands in the northeastern Indonesian Archipelago. Based on the Köppen climate zone system, this ecoregion falls in the tropical wet climate zone (National Geographic Society 1999). The geologic history of these islands is a very complex mixture of inner volcanic island arcs, outer volcanic island arcs, raised coral reefs, and fragments of continental crust. Halmahera is a product of a collision between two islands approximately 1-2 million years ago. The eastern half of the island was part of an outer arc on the Philippines tectonic plate and consists of sedimentary and intrusive igneous rocks. The western half of Halmahera and Morotai was part of an inner arc consisting of volcanic materials. Bacan is a mixture of volcanic inner island arc and some crustal materials (Monk et al. 1997).
The natural vegetation of these islands was tropical lowland evergreen and semievergreen forest (Stattersfield et al. 1998). Most of the remaining habitat in this ecoregion is semi-evergreen rain forest and includes eight characteristic dipterocarp species: Anisoptera thurifera, Hopea gregaria, H. iriana, H. novoguineensis, Shorea assamica, S. montigena, S. selanica, and Vatica rassak. Volcanic soils and good aspect combine to produce almost optimal growth conditions. Most of the trees reach 30 m or more and carry thick-stemmed lianas and woody and herbaceous epiphytes. Rattans that grow to 130 m and other epiphytes are common in old-growth forests. The most luxuriant rain forests occur in northwest Morotai and north Halmahera, as opposed to the south arm of Halmahera, which is in the rain shadow of north Halmahera and Bacan. Low, shrubby vegetation is found in poor soil conditions on patches of ultrabasic rocks (Monk et al. 1997).