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Hawaii tropical dry forests

Introduction

Mamani forest, western slope of Mauna Kea, Hawaii, USA. (Photograph by David Olson)

Tropical dry forests of Hawaii typically occurred on the leeward side of the main islands and once covered the summit regions of the smaller islands. Most native lowland forests of Hawaii are either seasonal or sclerophyllous to some degree, and more mesic transition forests occur where conditions are favorable. These transition forests include mixed mesic forests that often contain patches and elements of dry forest communities.

Biological Distinctiveness

Dry forests vary from closed to open canopied forests, can exceed 20 meters (m) in height in montane habitats, and are dominated by the tree genera Acacia, Chamaesyce, Metrosideros, Sapindus, Sophora, Pritchardia, Pandanus, Diospyros, Nestegis, Erythrina, and Santalum. Dry forests harbor a number of specialist species including native hibiscus trees of the genus Hibiscadelphus, Kokia cookei, Caesalpinia kauaiense, and Santalum paniculatum, and several rare endemics such as Gouania, now represented by only a few individuals. Around 22 percent of native Hawaiian plant species occur within this ecoregion, with lower habitat type endemism than tropical moist forests. The palila (Psittirostra bailleui), an endangered finch-like bird, specializes on mamane trees that occur in dry forest habitats. Several shrubland, grassland, and herbaceous formations occur within this ecoregion. Lower Hawaiian dry forest was habitat for several forest birds, such as honeycreepers, fly catchers, flightless rails, other flightless birds (now extinct), and the Hawaiian owl (Asio flammeus sandwicensis).

Conservation Status

Hawaii Volcanoes National Park, Hawaii, USA. (Photograph by David Olson)

Habitat Loss

Tropical dry forests are globally threatened, and Hawaiian dry forests have been reduced by 90 percent. Clearing and burning of lowland dry forests began with arrival of Polynesians and the last remnants are being destroyed today through development, expansion of agriculture and pasture, and burning. Most larger fragments of relatively intact dry forests are in montane areas.

Remaining Blocks of Habitat

A few relictual areas survive such as Pu’u Wa’awa’a on Hawai’i, Pu’u o Kali on Maui, Auwahi on Maui, Kanepu’u on Lana’i, and small stands (a few thousand square meters) in the Wai'anae mountains of O'ahu that is currently surrounded by burned slopes or alien-dominated vegetation. Several other important dry forest conservation sites identified by Sohmer and Gon include the Na Pali Coast of Kaua'i, East Moloka'i Mountains, West Maui Mountains, Leeward East Maui, Lana'ihale-Kanepu'u of Lana'i, and the Kona Subregion of Hawai'i.

Degree of Fragmentation

What little habitat that remains is highly fragmented.

Degree of Protection

Remaining transition forests and dry forests are poorly represented in the existing protected areas system. Strong protection and active management of the remaining remnants of Hawaiian dry forests are needed. Research on effective restoration methods is needed.

Types and Severity of Threats

Introduced plant species are widespread and dense growth and competition for resources prevents the establishment of native plant seedlings. The African fountain grass (Pennisetum setaceum), the shrub Lantana camara, and molasses grass (Melinis minutiflora) are among the major problem species. Introduced rats, plants, and seed-boring insects, grazing by domestic livestock and introduced deer, goats, and pigs, as well as recurring fires inhibit almost any regeneration of native species in most altered habitats.

Conservation Partners

• The Nature Conservancy
• The Nature Conservancy of Hawaii
• Hawaii Natural Heritage Program

Relationship to other classification schemes

The Hawaiin dry forest corresponds to Küchler’s units 1 (Sclerophyllous forest, shrubland, grassland), 5 (Koa forest), and 6 (Koa-mamani parkland). Omernik did not classify Hawaii, and Bailey clumped all of Hawaii into one unit.

Additional information on this ecoregion

• For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
• To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
• World Wildlife Fund Homepage

Further Reading

• This text was originally published in the book Terrestrial Ecoregions of North America: A Conservation Assessment from Island Press ISBN: 1559637226. This assessment offers an in-depth analysis of the biodiversity and conservation status of North America's ecoregions.