Wai‘anae Ecological Characterization

Soils

Soils in the Wai‘anae moku originate from its volcanic history. Eruptions of the main shield-building stages of the Wai‘anae volcano began at the ocean floor between 3 and 4 million years ago, depositing lava flows and pyroclastics that built the main mass of the Wai‘anae volcano, which eventually rose to more than 10,000 feet above the ocean surface. Additional volcanic activity occurred between 2.5 and 3 million years ago, with the last activity occurring a half-million years ago, before the volcano’s extinction.

Current surface soils in Wai‘anae exist as a result of millions of years of erosional processes, including rain, stream action, waves, and landslides. Surface soils in the Wai‘anae moku can generally be grouped into three predominant associations (U.S. Department of Agriculture 1971):

1. Lualualei Series, Fill Land, and ‘Ewa Series Association
2. Tropohumults-Dystrandepts Association
3. Rock Land and Stony Steep Land Association

Other soil types and associations exist within Wai‘anae, including the Kemo‘o, Mahana, Mokulē‘ia, and Pulehu series. In addition, rock outcrops are present at various locations throughout the moku. The three predominant soil associations are discussed below.

Lualualei Series, Fill Land, and ‘Ewa Series Association

From the sea level to about 400 feet elevation, the dominating soil type is a combination of Lualualei series, fill land, and ‘Ewa series soils. These soils are deep, nearly level to moderately sloping, well-drained soils that have a fine or moderately textured subsoil. The mean annual soil temperature is between 73 and 75°F.

Lualualei series soils developed in alluvium and colluvium, and have a surface layer of very dark grayish-brown, sticky, and plastic clay that cracks widely upon drying. They are underlain by coral, gravel, sand, or clay at depths below 40 inches. ‘Ewa series soils were developed in alluvium derived from basic igneous rock and have a surface layer and subsoil of dark reddish-brown, friable silty clay loam. The substratum is gravelly alluvium or coral limestone. Fill land consists of various types of fill material and soils.

These soils can be used for agricultural purposes (such as sugarcane and truck crops), livestock and pasture, urban development, and as wildlife habitat. The fill land has primarily been used for commercial, industrial, and residential development. The natural vegetation consists of kiawe, koa haole, bristy foxtail, ‘uhaloa, and fingergrass.

Tropohumults-Dystrandepts Association

This association consists of the mountainous areas and lower slopes of the Wai‘anae Range, at elevations from 1,000 to 4,000 feet. This association is composed of gently sloping to very steep, well-drained soils that are underlain by soft weathered rock, volcanic ash, or colluvium, on narrow ridges and side slopes. The mean annual soil temperature is between 56 and 71°F.

Tropohumults soils occur on the narrow ridges at the upper elevations. They have a surface layer and subsoil of reddish-brown silty clay. The subsoil has subangular blocky structure and is underlain by saprolite (original rock). Dystrandepts occur in concave positions on the steep side slopes. They were derived dominantly from volcanic ash mixed with colluvium. They are dark colored, and in most places the surface is silty clay. The subsoil is generally massive (irregular and lacking structure); however, there are areas where the subsoil is fine textured.

Most of this association is very steep and inaccessible and makes up much of the watershed recharge areas. Some of the minor soils are used for forest and pasture, as well as for agriculture including pineapple and sugarcane. Lantana (tropical shrubs), yellow foxtail, molassesgrass, and Japanese tea can grow at the lower elevations; ‘ōhi‘a, puakeawe, koa, ‘a‘ali‘i, and ferns can grow at the upper elevations. For more information on upper elevation plant communities, please see the Terrestrial and Aquatic Ecosystems: Forests and Streams section.

Rock Land and Stony Steep Land Association

This association consists of stony and rocky, steep to precipitous slopes from sea level to 2,800 feet. The soil material is very shallow. The association is well-drained to excessively drained, rocky and stony land and occurs in gulches and on mountainsides. The mean annual soil temperature is between 67 and 75°F.

Rock land is made up of areas where exposed rock, primarily basalt and andesite, covers 25 to 90 percent of the surface. Rock land outcrops can vary from nearly level to very steep. Rock land can be used for pasture and wild habitat. Natural vegetation at lower elevations may include kiawe, klu, piligrass, Japanese tea, and koa haole; higher elevations can maintain lantana, guava, Natal redtop, and molassesgrass.

Stony steep land consists of boulders and stones deposited by water or gravity in valley bottoms or on side slopes of drainage ways. Stony steep land is primarily used for wildlife habitat including game birds and wild pigs, and recreation including hunting, hiking, and camping. The natural vegetation consists of kiawe, koa haole, and grasses.

Reference Cited

U.S. Department of Agriculture. 1971. Soil Survey of the State of Hawaii. Natural Resource Conservation Service. http://www.ctahr.hawaii.edu/soilsurvey/soils.htm

Back to top