Hawaiian tropical high shrublands

Hawaiian tropical high shrublands are a unique ecoregion found on the main Hawaiian Islands, primarily at elevations between approximately 1,300 and 3,000 meters. This zone, often situated above the cloud layer, is characterized by a cooler, drier climate that supports a distinctive community of hardy, often endemic shrubs and grasses. It represents a critical transitional habitat between lower-elevation forests and the alpine deserts of the highest peaks like Mauna Kea and Mauna Loa.

Geography and Climate

This ecoregion occurs on the islands of Hawaiʻi, Maui, Molokai, Lanai, and Kauai, typically on the leeward slopes of the tallest volcanoes. The geography is often rugged, comprising ancient lava flows, cinder cones, and volcanic ash plains. The climate is markedly different from the wetter zones below; it is cooler due to altitude and experiences less precipitation because it lies in the rain shadow of the trade winds, above the persistent cloud layer. Frequent inversion layers trap moisture below, leaving the high shrublands in a zone of intense solar radiation, strong winds, and significant diurnal temperature fluctuations. This creates an environment akin to a tropical montane desert or alpine climate.

Flora

The vegetation is adapted to the harsh, dry, and windy conditions, forming open, low-stature communities. Dominant plants include endemic shrubs such as ʻaʻaliʻi (Dodonaea viscosa), pūkiawe (Styphelia tameiameiae), and kūkaenēnē (Leptecophylla tameiameiae). Native bunchgrasses, including species of Deschampsia and Trisetum, are common. The ecoregion also features remarkable silversword alliances, most famously the Haleakalā silversword (Argyroxiphium sandwicense subsp. macrocephalum) on Haleakalā and the Mauna Kea silversword (Argyroxiphium sandwicense subsp. sandwicense) on Mauna Kea. These iconic plants, along with other endemics like the Hawaiian geraniums (Geranium spp.), demonstrate spectacular adaptations to the extreme environment.

Fauna

The faunal community is sparse but includes highly specialized and often endangered species. The most notable inhabitants are native forest birds that forage in these areas, such as the ʻamakihi (Chlorodrepanis virens) and the critically endangered ʻakiapōlāʻau (Hemignathus wilsoni). The nēnē (Branta sandvicensis), the state bird of Hawaii, nests in these shrublands. Invertebrate fauna includes endemic insects like the Mauna Kea wēkiu bug (Nysius wekiuicola), a predator adapted to the freezing summit climate, and various species of Hawaiian happy-face spiders (Theridion grallator). Historically, the now-extinct ʻalalā (Corvus hawaiiensis) also utilized this habitat.

Conservation Status and Threats

The conservation status is considered critical, with the ecoregion listed as endangered. Primary threats are pervasive and devastating. Invasive ungulates, particularly mouflon sheep, axis deer, and feral goats, degrade habitat through browsing and trampling. Invasive plant species like firetree (Morella faya), fountain grass (Pennisetum setaceum), and gorse (Ulex europaeus) outcompete native flora and alter fire regimes. Climate change poses a long-term threat by potentially shifting cloud layers and precipitation patterns. The small, isolated populations of endemic species are extremely vulnerable to these combined pressures, with many, like the Mauna Kea silversword, listed under the Endangered Species Act.

Human Impact and Management

Human impact began with Polynesian arrival, which introduced rats and likely initiated some local changes, but accelerated dramatically after European contact in the 18th century with the introduction of livestock and aggressive weeds. Today, active management is conducted by agencies including the Hawaii Department of Land and Natural Resources, the National Park Service (within Hawaiʻi Volcanoes National Park and Haleakalā National Park), and the U.S. Fish and Wildlife Service. Key management strategies include extensive fencing to exclude ungulates, rigorous invasive species control programs, and captive breeding and outplanting efforts for species like the nēnē and silverswords. Research by institutions such as the University of Hawaii at Manoa and the Pacific Cooperative Studies Unit informs these conservation actions to preserve this unique and fragile high-elevation ecosystem.

Category:Ecoregions of Hawaii Category:Tropical and subtropical grasslands, savannas, and shrublands Category:Environment of Hawaii