Natural Resources Conservation Service Ecological site VX160X01X503 Isothermic Forest Accessed: 09/24/2021 General information Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site. MLRA notes Major Land Resource Area (MLRA): 160X–Subhumid and Humid Intermediate and High Mountain Slopes This MLRA occurs in the State of Hawaii on the Big Island of Hawaii and to a small extent on Maui. Elevation ranges from 1000 to 9000 feet (300 to 2725 meters). Slopes are moderate to steep. Topography is sloping to hilly, and cinder cones are common. Lava flows are basaltic or andesitic aa or pahoehoe lava. Aa may form outcrops at higher elevations, but most of the area is covered with material weathered from deep volcanic ash. Areas of volcanic sand and gravel alluvium exist. Average annual precipitation ranges from 20 to 75 inches (500 to 1875 millimeters). Most of the rainfall occurs during kona storms from November through March. In some areas afternoon fog accumulation at higher elevations adds significant moisture to the soil by fog drip and also ameliorates evapotranspiration. Average annual air temperatures range from 50 to 71 degrees F (10 to 20 degrees C), with little seasonal variation. Strong winds are frequent in the saddle between Mauna Kea and Kohala volcanoes. The dominant soil order is Andisols with an isomesic soil temperature regime and ustic or udic soil moisture regime. Native vegetation consists of forests and rangelands that can include medium to tall statured forests, savannas, and grasslands. Classification relationships This ecological site occurs within Major Land Resource Area (MLRA) 160 - Subhumid and Humid Intermediate and High Mountain Slopes. Ecological site concept This ecological site exists on the southwest-facing slopes of Kohala, mauka and makai of the Waimea-Hawi Road (Rte. 250), and on the west-facing slopes of Mauna Kea to the south of Waimea along the upper highway (Rte. 190)through the deep volcanic ash soils to south of Waikoloa Road to the young lava flows. Large parts of it are on private lands managed for livestock grazing. The State of Hawaii and State Division of Hawaiian Homelands own large areas also, and some parts are on the US Army Pohakuloa Training Area. Small, restored examples of the Reference State are found on State and private lands in Kohala, both mauka and makai of the Waimea-Hawi highway. The central concept of the Isothermic Forest is of well drained, shallow to very deep Andisols formed in deposits of volcanic ash ranging from 60,000 to 500,000 years old. Annual air temperatures and rainfall create warm (isothermic), seasonally dry (ustic to torric) soil conditions. Overstory trees are short to medium in stature (16 to 26 feet; 5 to 8 meters). Koaia (Acacia koaia), a relative of koa (Acacia koa) but with shorter stature, smaller leaves (actually phyllodes), and harder wood, is restricted (at least on the Big Island) to this ecological site. Some other common overstory species are lama (Diospyros sandwicensis), wiliwili (Erythrina sandwicensis), mamani (Sophora chrysophylla), naio (Myoporum sandwicense), sandalwood (Santalum ellipticum), and kauila (Colubrina oppositifolia). Many native shrub, vine, forb, fern, grass, and sedge species inhabit this ecological site. Associated sites VX160X01X502 Isomesic-Cool Isothermic Forest F160XY502 Isomesic-Cool Isothermic Forest is a tall stature forest with koa overstory that borders F160XY153 to the east on Mauna Kea in cooler, moister, less windy areas. VX164X01X500 Volcanic Ash Forest F164XY500 Volcanic Ash Forest is a medium stature rain forest bordering F160XY503 at higher elevations on Kohala Mountain. Similar sites VX161B01X500 Ustic Isothermic Forest F161BY500 Ustic Isothermic Forest has similar vegetation to F160XY503 but lacks koaia. It borders F160XY503 to the south, and is on rocky soils with organic and mineral parent materials. Table 1. Dominant plant species Tree (1) Acacia koaia (2) Santalum ellipticum Shrub Not specified Herbaceous Not specified Legacy ID F160XY503HI Physiographic features This ecological site occurs on volcanic ash deposited over lava flows on sloping mountainsides of shield volcanoes. Lava flows are aa (loose, cobbly) or pahoehoe (smooth, relatively unbroken). Table 2. Representative physiographic features Landforms (1) Shield volcano (2) Mountain slope Flooding frequency None Ponding frequency None Elevation 900–4,000 ft Slope 0–60% Water table depth 60 in Aspect SW, W Climatic features Average annual precipitation in this ecological site ranges from 20 to 40 inches (500 to 1000 mm). Extremes of average annual precipitation may range as low as 10 inches (250 mm) and as high as 50 inches (1270 mm). Most of the precipitation occurs from October through April. Average annual temperatures range from 60 to 68 degrees F (15 to 20 degrees C) with extremes as low as 54 degrees F (12 degrees C) and as high as 71 degrees F(22 degrees C). Table 3. Representative climatic features Frost-free period (average) 0 days Freeze-free period (average) 0 days Precipitation total (average) 0 in 0 in 1980 1985 1995 2000 2005 2010 Figure 1. Annual precipitation pattern Influencing water features There are a few small streams flowing at the bottoms of very deep gulches; these streams have no influence on most of the ecological site. Gulches and gullies can carry large amounts of water during heavy rainstorms for short periods of time. Soil features Soils in this ecological site are mineral soils formed from volcanic ash deposited over lava. Kamakoa soils are in flood plains that contain soils weathered from alluvial volcanic ash. Soil temperature regimes are isothermic. Soil moisture regimes are ustic (in most years, dry for more than 90 cumulative days but less than 180 days), or in some cases, torric (dry in all parts for more than half of the cumulative days per year when the soil temperature at a depth of 50 cm from the soil surface is above 5 C; and moist in some or all parts for less than 90 consecutive days when the soil temperature at a depth of 50 cm is above 8 C.). Kamakoa soils have accumulations of calcium carbonate in subsurface horizons, as well as higher salinity and sodium adsorption ratio than other soils in this ecological site. These conditions occur at depths below 34 inches (85 cm), so are unlikely to greatly affect plant growth conditions. The volcanic ash soils of the Island of Hawaii are derived mostly from basaltic ash that varies relatively little in chemical composition (Hazlett and Hyndman 1996; Vitousek 2004)). Most of these volcanic ash soils are classified as Andisols, which have these general management characteristics: ion exchange capacity that varies with pH, but mostly retaining anions such as nitrate; high phosphorus adsorption, which restricts phosphorus availability to plants; excellent physical properties (low bulk density, good friability, weak stickiness, stable soil aggregates) for cultivation, seedling emergence, and plant root growth; resistance to compaction and an ability to recover from compaction following repeated cycles of wetting and drying; and high capacity to hold water that is available to plants. These characteristics are due to the properties of the parent material, the clay-size noncrystalline materials formed by weathering, and the soil organic matter accumulated during soil formation (Shoji et al. 1993). Andisols formed on pahoehoe lava can be very shallow to very deep. Pahoehoe is referred to as a “lithic contact,” which is a boundary between soil and underlying material that is coherent, continuous, difficult to dig with a spade, and contains few cracks that can be penetrated by roots (Soil Survey Staff 1999). Pahoehoe is typically very limiting to root penetration due to the spacing and size of cracks. However, this characteristic of pahoehoe is variable, and there are many instances of stands of large trees growing on very shallow and shallow ash soils over pahoehoe. The lava rock fragments that constitute aa range in size from gravel (2 mm to 76 mm, or up to 3 inches) to stones (250 mm to 600 mm, or 10 to 25 inches), but are primarily gravel and cobbles (76 mm to 250 mm, or 3 to 10 inches). Below the layer of rock fragments is massive lava called “bluerock.” The interstices between rock fragments of Andisols formed in aa are filled with soil from the surface to the blue rock at the bottom of the soil. Some Andisols in aa have few or no rock fragments in the upper horizons, while others may have large amounts of rock fragments in all horizons and on the soil surface. Soils that are moderately deep (20 to 40 inches, or 50 to 100 cm) or deeper over underlying lava appear to present few or no limits on native, pasture, or weedy vegetation, and it seems to make no difference whether the lava rock is pahoehoe or aa. However, these soils may present some tillage difficulties when formed in aa and containing significant amounts of coarse rock fragments near the surface. Very shallow and shallow ash soils over pahoehoe are sometimes ripped to break up the underlying lava and create a deeper rooting zone. Figure 3. Puu Pa soil. Table 4. Representative soil features Parent material (1) Basaltic volcanic ash–basalt Surface texture (1) Medial very fine sandy loam (2) Silt loam Family particle size (1) Loamy Drainage class Well drained to somewhat excessively drained Permeability class Moderate to rapid Soil depth 10–60 in Surface fragment cover <=3" 0–25% Surface fragment cover >3" 0–25% Available water capacity 1.8–10.8 in (0-40in) Calcium carbonate equivalent 0–2% (0-40in) Electrical conductivity 0–2 mmhos/cm (0-40in) Sodium adsorption ratio 0–5 (0-40in) Soil reaction (1:1 water) 5.9–9.4 (0-40in) Subsurface fragment volume <=3" 0–40% (Depth not specified) Subsurface fragment volume >3" 0–90% (Depth not specified) Ecological dynamics The information in this ecological site description (ESD), including the state-and-transition model (STM), was developed using archaeological and historical data, professional experience, and scientific studies.