Except in the more sheltered coastal lowlands such as the north shore of New Britain, the protected valleys of the northeastern part of that island, and along the west coast of New Ireland, the climate of the archipelago is not very favorable for agriculture and human settlement, although clusters of natives are found at intervals along all coasts and small numbers of natives manage to eke a living, if a poor one, off the mountainous, forested interior. The southern slopes of New Britain are exceedingly wet, the annual rainfall at Gasmata in the south-central region averaging about 250 inches a year. This coast is wet principally because it is fully exposed to the southeast monsoon for half the year and receives a considerable carry-over of rainfall from the northern side of the island during the northwest monsoon the other half of the year. The eastern shores of New Ireland are less wet because the winds during both monsoons blow more or less parallel to the coastline a good part of the time, and the lower interior produces less orographic lifting of the air masses that do penetrate into the interior. The more exposed coasts of the islands are normally breezy places, especially during the height of the monsoonal air flow. This is a definite advantage from the standpoint of human comfort in these low latitudes where a still atmosphere is a hot atmosphere; but from the agricultural point of view this is a disadvantage because of higher rainfall.
The soils of the two main islands are neither uniformly poor nor exceptionally fertile. Some of the volcanic soils, notably those derived from certain basalts, are highly productive when properly handled. But it is by no means true that all volcanic soils are good soils. Some of the poorest soils on the islands are of this type, for they are little more than accumulations of recent ash showers from the volcanoes. Except for the local influence of parent materials, we may generalize perhaps to the extent of saying that the higher the rainfall in this region the poorer the soils, for high rainfall means rapid leaching of the upper soil profile. Again, the north coast of New Britain and the west coast of New Ireland are the favored localities for agriculture.
The natural vegetation of the Bismarck Archipelago is somewhat similar to that of New Guinea, except that rainforest extends to the summits of most of the higher mountain peaks, and there are fewer indications of Australian elements in the plant associations. The rainforest, as in New Guinea, consists of a multi-storied assemblage of tall, buttressed, forest trees of varied species; medium-high growth of lesser trees; and lower growth of various shrubs, ferns, and other plants; all levels being densely interlaced by climbing vines and embellished by a wide range of parasitic and epiphytic forms. Some of the giant liana vines, indeed, grow to the very tops of the forest in their struggle for sunlight, throttle the host tree, and assume the nature of trees themselves.