Vegetation zones in Hawaii are related principally to rainfall and altitude. The wet and dry sides of islands have plant associations that differ, depending upon the rainfall. To a considerable extent there is also a correlation between the vegetation and the soils of the islands. A lower limit of forest growth determined by low rainfall is found only on the leeward side; the upper limit at about 10,000 feet is related to temperature. Because trees suitable for higher elevations were never introduced, boreal-type plants are few.
The number and the nature of the original plant species were greatly influenced by the wide separation of Hawaii from large bodies of land, and the broad ocean prevented the migration of many species. Of 1729 described indigenous species of seed plants about 85 per cent are endemic. Fosberg 1 believes that the native Hawaiian flora was evolved by differentiation from the introduction of comparatively few parent plants.
The wild vegetation of the present day contains many introduced plants, and the number of such exotic species is estimated at over 2000, which have been brought in during the last 175 years. Examples of introduced plants that have gone wild are guava, algaroba, lantana, and cactus. Introduced plants have become dominant over the native flora on large areas.
Five generalized vegetation zones in Hawaii have been recognized by Ripperton and Hosaka. Zone A, on the leeward side of the islands, consists of coastal flats and of slopes from sea level to elevations of 500 feet, except on the dry side of the island of Hawaii where it extends to 2000 feet. It also occupies the low ends of islands and portions of the windward coasts where the rainfall happens to be light. If the land is irrigated, sugar cane can be raised; otherwise it is used mainly for pasture. The introduced algaroba (mesquite), thorny shrubs like koa haole and cactus, and drought-resistant grasses are now predominant. Native trees include the wiliwili (Erythrina) and the hola (Pandanus).
Zone B, or the lower forest belt, lies above zone A, where zone A is present; and probably the most common tree, along with the koa haole bushes, is the kukui. Shrubs like the ilima form thickets as does also the introduced cactus and lantana. Pili grass, other grasses, and annuals are available for grazing.
The middle forest zone, zone C, is located where the rainfall is about 40 to 60 inches annually, and in elevation the zone rises to 4000 feet and on windward coasts may descend to sea level. The ohia lehua and koa are important trees, and there is luxuriant undergrowth, including small tree ferns. In the lower and less rainy sections, guava forms thickets. Bermuda grass, staghorn fern, and lantana often occupy openings in the drier forests. The soil is somewhat leached but generally fertile, and much of this forest land has been cleared and planted in sugar cane and pineapples especially on the central plateau of Oahu. This type of forest originally covered much of Oahu and is common on other islands.
Zone D develops in areas of very heavy rainfall, and originally the forests in this belt were of very dense growth. There are some differences in the composition of the forest, depending on altitude. The upper phase begins at about 4000 feet and extends to about 8000 feet and is found only on Hawaii and Maui, which alone among the islands have this upper elevation. On the windward side of the islands the lower zone begins at sea level and occupies ground on which the rainfall is 60 to 300 inches annually. Dense thickets of guava, shrubs, staghorn, and other ferns, ohia lehua, and koa occur. The upper levels are mostly in forest reserves. On Hawaii and to a limited extent on other islands the lower levels have been cleared and planted in sugar cane.
Zone E occurs only on Maui and Hawaii and extends from about 4000 to 10,000 feet, which is the tree line. Forest reserves and national parks include much of this area, little of which is for agriculture although there is some grazing. Some sandalwood grows in this zone along with wild strawberries and akala berries. The high tablelands and mountain tops like Mount Kaala on Oahu and Waialele on Kauai have bogs with a peculiar flora related to plants found in New Zealand, the southern Andes Mountains, and the Falkland Islands. Possibly some of these plants were introduced by aquatic wild fowl.
The forests of Hawaii are more useful today for the conservation of ground water, flood control, and prevention of soil erosion than as a source of commercial timber. Originally there were many sandalwood trees, and the export of the logs was among the first industries developed in the islands, but this resource was exhausted in a generation. At present some koa (Hawaiian mahogany), and ohia are used for furniture, interior decoration, and curios. The introduced algaroba (mesquite) covers 100,000 acres of dry lowlands and furnishes blossoms for bee pasture, wood for fuel, and seeds (beans) for stock feed. Some of the wooded land is grazed. Nearly 700,000 acres of Territorial government land and 357,000 acres of privately owned land are included in forest reserves that are located in the mountains of the five largest islands. They amount to 25.6 per cent of the area
of the Territory. The city of Honolulu guards the forests on the watersheds used for municipal water supply, and many sugar plantations likewise maintain forest reserves from which comes water for irrigation.