This area is dominated throughout the year by tropical maritime air. In the summer equatorial air masses invade the region, bringing increased temperatures and higher humidity. In the winter polar continental air masses occasionally move out over the islands from Asia. These are much modified by the time they reach the Bonins, and they arrive in the southern Marianas as weak cold fronts bringing greater precipitation and cloudiness. In the Bonins, however, they result in a wider range of temperature.
Because of the area's attenuation, the climate can best be generalized by a consideration of the meteorological records of the northernmost and southernmost stations in the chain. 6 Owing to the warm Kuroshio and the mitigating influence of the ocean waters, temperature variability over so extended an area is not so great as might be expected. The average annual temperature at Sumay, Guam, is 80.9° F, and the annual range is 3.3° F; at Omura, Chichi-shima, the average annual temperature is 72.9° F and the annual range 18.2°F. Rainfall decreases from south to north, ranging from 89.4 inches at Sumay to 64.4 inches at Omura. Prevailing winds throughout the entire area are easterly. In Guam and Saipan wind direction is predominately northeast; in the Bonins east winds prevail, but only by a slight percentage over winds from other quadrants.
In general summary, the climate of the southern islands is tropical maritime, and that of the Bonins is subtropical. In the northern islands temperatures decrease, annual and diurnal ranges increase, and rainfall decreases but is more effective because of lower temperatures and more even distribution.
On a generic basis, three categories of soil may be distinguished: volcanic, coralline, and coralline-volcanic. On the geologically recent volcanic islands much of the lava surface is bare or covered with volcanic detritus. On gentle slopes and in the valleys fine clay soils of fair fertility have formed from a few inches to several feet in depth. The coralline soils, occurring on the raised terraces, are often shallow and with occasional outcrops of limestone. When fully developed, these soils are reddish brown, fine-textured, and claylike, and they usually contain pieces of undecomposed coral throughout the profile. The water-holding capacity of these soils is slight, and during the dry season vegetation growth is retarded. Volcanic-coralline soils occur in valleys and over other areas where alluvial action has resulted in a mixture of the other two soil types. These soils are heavy clay, finegrained, but sometimes contain coral chunks. All these soils are lateritic, having evolved under high temperatures and heavy rainfall.
The native vegetation is tropical and shows a generic relationship with the plant life of Malaysia. Only in the Bonins is there any indication of northern elements, and these are few in number. According to the early explorers the original cover was forest and grass. The entire area lies within the zone of coconut palms, but in the Bonins the tree is believed to have been introduced by man. Elevations are not of sufficient height to result in zoning, and, dependent upon soil conditions, strand plants may be found on the highest points. Several ecological associations occur, resulting largely from edaphic factors:
Mangrove. Found in Guam, and to a very limited and weak extent in Saipan.
Beach. A complex of vines and low shrubs, with breadfruit, coconut palms, Formosan koa (Acacia confusa), and Australian pine or Polynesian ironwood (Casuarina equisetifolia) along the beach ridge.
Upper strand. Vines, short, coarse, grass, and low shrubs; a treeless association occurring on thin, rocky soils at the top of seaside cliffs and within range of salt spray. Most highly developed along exposed eastern coasts.
Secondary forest. No part of the primary forest remains undisturbed. Cliff sides and deep ravines, covered with a tangle of vines, ferns, and trees, give some impression of the former native jungle. Nearly all the timber trees have been removed, and the second growth is small. Japanese reforestation projects have covered some areas with fast-growing Australian pine and Formosan koa.
Marsh. Limited in extent and consisting of grasses, ferns, and sedges.
Grassland. In the southern Marianas this is frequently made up of wide stretches of sword grass with cutting scabrous edges. This grass is chiefly associated with volcanic soils, but its area has been extended as the result of native practices of burning.
Because of the isolation of the islands and their recent geological formation, the number of indigenous genera and species is small. Plants with adaptations for migration over so wide a water area as separates these islands from the mainland are few. Accidental or intentional introduction by man has resulted in modifying the greater part of the plant life in the islands; on Guam, for instance, 58 per cent of the vegetation has been introduced.
Repatriation of the Japanese has left much land unused and has resulted in vigorous competition among the plants for capture of the formerly cultivated areas. Sugar cane has strongly resisted invasion, and it still dominates the vegetation landscape on Tinian, on Saipan, and to a lesser extent on Rota. The other fields are invaded first by hardy and tolerant weeds, two or three species of which tend to become dominant before trees begin to encroach on the area.