Kohala (mountain) - Ecology

Ecology

The natural habitats in the Kohala district range across a wide rainfall gradient in a very short distance—from less than 5 in (127 mm) a year on the coast near Kawaihae, to more than 150 in (3,810 mm) a year near the summit of Kohala Mountain, a distance of just 11 mi (18 km). At the coast are remnants of dry forests, and near the summit lies a cloud forest, a type of rainforest that obtains much of its moisture from "cloud drip" in addition to precipitation. These large cloud forests dominate its slopes. This biome is rare, and contains a disproportionate percentage of the world's rare and endemic species. The soil at Kohala is nitrogen-rich, facilitating root growth.

The happy combination of small trees, bushes, ferns, vines, and other forms of ground cover keep the soil porous and allow the water to percolate more easily into underground channels. The foliage of the trees breaks the force of rain and prevents the impact of soil by raindrops. A considerable portion of the precipitation is let down to the ground slowly by this three-story cover of trees, bushes, and floor plants and in this manner the rain, falling on a well-forested area, is held back and instead of rushing down to the sea rapidly in the form of destructive floods, is fed gradually to the springs and to the underground artesian basins where it is held for use over a much longer interval...

C S Judd, Superintendent of Forestry in Hawaiʻi, 1924

The forest lies perpendicular to the prevailing trade winds, a characteristic that leads to frequent cloud formation and abundant rainfall. Thus the rain forests carpeting Kohala are vital to the supply of groundwater on the island. The role of Kohala's ecosystem in supporting water flow has long been recognized. As early as 1902, US Forester E. M. Griffith wrote "Forest protection means not only increasing the rainfall, but more important still, conserving the water supply. The future welfare and agricultural prosperity of the Hawaiian Islands depends upon the preservation of the forest."

Prior to the arrival of humans, the wildlife population at Kohala and on the rest of the island was isolated from other species, as it was separated from the nearest major landmass by over 2,000 mi (3,219 km) of open water. The early colonizers of Kohala came by wind, storm or floating on the ocean. Successful colonizations occur on the magnitude of once every 35,000 years. The 2,000 successful ancestors that arrived at Kohala evolved into about 8,500 to 12,500 unique species, resulting in a very high ratio of the species present being found nowhere else on Earth. Only about 2.5% of the world's rainforest is cloud rainforest, making it a unique habitat as well.

The mountain supports approximately 155 native species of vertebrates, crustaceans, mollusks, and plants. A diverse complexion of fungi, liverwort, and mosses further add to the variety. In fact, up to a quarter of the plants in the forest are mosses and ferns. These work to capture the water from clouds, in turn providing microhabitats for invertebrates and amphibians, and their predators. Estimates on the water capacity of the forest range from 792 US gal (2,998 l) to 3,962 US gal (14,998 l) per hectare.

The mountain is also home to several bogs, which exist as breaks in the cloud forests. It is believed that bogs form in low lying areas where clay in the soil prevents proper water drainage, resulting in an accumulation of water that impedes the root systems of woody plants. Kohala's bogs are characterized by sedges, mosses of the genus Sphagnum, and oha wai, native members of the bellflower family. Other habitats include rain forest and mesophytic (wet) forests.

The same isolation that produced Kohala's unique ecosystem also makes it very vulnerable to invasive species. Alien plants and feral animals are among the greatest threats to the local ecology. Plants like the kahili ginger (Hedychium gardnerianum) and the strawberry guava (Psidium cattleinum) displace native species. Prior to human settlement, many major organisms such as conifers and rodents never made it onto the island, so the ecosystem never developed defenses against them, leaving Hawaii vulnerable to damage by hoofed animals, rodents, and predation.

Kohala's native Hawaiian rain forest has a thick layer of ferns and mosses carpeting the floor, which act as sponges, absorbing water from rain and passing much of it through to the groundwater in aquifers below; when feral animals like pigs trample the covering, the forest loses its ability to hold in water effectively, and instead of recharging the aquifer, it results in a severe loss of topsoil, much of which ends up being dumped by streams into the ocean.