Hawaii has rainforests, which are a unique and significant part of the islands’ natural environment. They contribute to the islands’ exceptional biodiversity and play a role in maintaining ecological balance.
The Hawaiian Climate and Topography
Hawaii’s rainforests owe their existence to a unique combination of climatic and topographical factors. The islands are situated in the tropics, where warm temperatures persist year-round. Temperatures remain warm year-round, generally ranging from 79-90°F.
A primary influence on Hawaii’s climate is the consistent northeasterly trade winds. These winds carry humid air from the North Pacific towards the islands. As this moist air encounters the tall volcanic mountain ranges, it is forced upward.
This upward movement causes the air to cool and condense, leading to significant rainfall on the windward (northeastern) sides of the islands. This phenomenon is known as orographic lift. Consequently, areas like the Hamakua Coast on the Big Island can receive up to 300 inches of rain annually, while Mount Wai’ale’ale on Kauai averages 373 inches of rainfall per year, making it one of the wettest places on Earth. The leeward (southwestern) sides of the islands, shielded by the mountains, experience much drier conditions, creating distinct microclimates within short distances.
Defining Characteristics of Hawaiian Rainforests
Hawaiian rainforests are characterized by dense vegetation, high humidity, and consistent rainfall. These forests vary in elevation, occurring in windward lowlands, montane areas, and even on mountain tops of smaller islands. They are found in areas receiving over 80 inches (200 cm) of rain per year, with some parts receiving over 390 inches (1000 cm) annually.
A continuous canopy layer covers lower subcanopy trees, understory shrubs, and a ground layer of ferns and herbs. Two of the most common native tree species forming the canopy are ‘ōhi’a lehua (Metrosideros polymorpha) and koa (Acacia koa). ‘Ōhi’a lehua is especially resilient, often being one of the first plants to colonize new lava flows.
Hawaiian rainforests are categorized into types based on elevation. These include lowland wet forests, mixed mesic forests (found at 750–1,250 meters elevation), and montane wet forests, which can extend up to 1,700 meters. Cloud forests, a type of montane rainforest, are found on wind-exposed upper slopes. They are characterized by a low-statured tree canopy and abundant epiphytic mosses and ferns due to near-daily ground-level clouds and fog drip.
Ecological Significance and Unique Biodiversity
Hawaiian rainforests are important habitats, supporting rich biodiversity with a high degree of endemism. Over 90% of Hawaii’s native species are endemic, meaning they are found nowhere else in the world. These forests are home to numerous endemic plants and animals, including unique birds, insects, and land snails.
The ‘ōhi’a lehua tree, for instance, is the most common native Hawaiian tree, comprising 80% of Hawaii’s native forests. It provides habitat and food for native birds like the ‘apapane and ‘i’iwi. Koa trees also serve as a habitat for many endemic insects. The isolation of the Hawaiian Islands over millions of years has led to the evolution of these distinct species.
Beyond their role as biological habitats, Hawaiian rainforests provide important ecosystem services. They are important for water capture and watershed protection, absorbing rainfall and replenishing underground aquifers, the primary source of fresh water for the islands. The multi-layered structure of these forests, with dense canopies and understories, helps soak up precipitation and reduce runoff. This also aids in soil stabilization, preventing erosion and the flow of sediment into coastal waters, which can harm coral reefs.