Ecological Site Description Ecological Site Characteristics Site Identification
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Plant CommunitiesEcological Dynamics of the Site This ecological site occurs on a soils formed in volcanic ash in warm, moist to wet regions of Kau District of the Island of Hawai`i. Plant communities evolved without the presence of large mammals or the regular occurrence of fires. The original forest plant community is now disturbed and fragmented due to agriculture, urban development, establishment of exotic timber trees, domestic and feral ungulate foraging, and alien species invasion. Foraging by cattle, pigs, and/or goats, or clearing and abandonment accelerate invasion by alien weeds. However, alien weeds appear able to successfully invade native stands regardless of human or ungulate disturbances. Major weeds include strawberry guava, christmasberry, kahili ginger, desmodium, melastomes, night blooming jasmine, and alien grasses. Guineagrass and kikuyugrass pastures become infested with unpalatable grasses and shrubs under conditions of improper pasture and grazing management. State and transition diagram
State 1 – Native Wet Forest Plant Community 1 This state represents the Historic Climax Plant Community. The general aspect is a forest of tall overstory with an open or closed upper canopy of ohia or ohia and koa trees up to 100 feet tall, a secondary canopy of diverse trees species 30 to 60 feet tall, a dense tree fern canopy 10 to 30 feet tall, and a diverse understory of shrubs and ferns. Vines are common, particularly `ie`ie, both on the ground and on trees. All three Big Island tree fern species are present, but hapu`u (Cibotium glaucum) is usually the most common by far. These forests have standing live timber of 300 to 5900 cubic feet per acre, with a representative value of about 5000 cubic feet per acre. Typical low values are about 1500 cubic feet per acre. The lowest values of 300 to 800 cubic feet per acre on steep, continually eroding soils, and in the drier extremes of this ecological site. Overstory tree canopy cover of ohia and koa can vary from about 10% to 80%. However, understory composition is controlled by the cover of the secondary canopy of medium-stature, secondary canopy tree species and especially by the cover of tree ferns, which is usually in the range of 60% to 90%. Koa and ohia do not reproduce successfully in the typically shady understory of intact Native Wet Forest. Tree ferns, medium-stature trees such as olapa, kopiko, kolea lau nui, kawa`u, and olomea, and shrubs such as kanawao and clermontia reproduce well in the understory. The ground layer of small ferns is typically very dense when ungulates are not present. The dominant tree canopy can be ohia trees or a combination of ohia and koa trees. We were unable to discern any consistent correlation between dominant tree canopy composition and soil type, rainfall, elevation, or any other environmental variable. It is probable that long-term disturbance history controls koa occurrence. Koa is a fast growing, opportunistic species that is able to take advantage of temporary openings in the dense forest canopy. Variations on this plant community naturally occur on some soil types within this ecological site. A lower stature forest type without koa and similar to Ecological site F162XY503HI (Shallow Wet Ohia-Olapa Forest, in Puna district) occurs on some delineations of Hilea soil series (some delineations of soil map units 513, 514, 530, and 531) that are particularly shallow and have especially poor drainage that is probably due to the nature of the locally occurring pahoehoe lava. Another low stature forest type occurs on Akihi-Alapai complex, 40-70% slopes (map unit 558) due to constant soil erosion on these cliff slopes. This ecological site grades into ecological site R161AY010HI (Mauna Loa Savannah) in the higher, drier parts of Kahalu`u-Lava flows complex, 2-10% slopes (soil map unit 534) and in Kaholimo medial silt loam, 10-20% slopes (soil map unit 573) where these very shallow to shallow, low available water capacity soils do not receive enough rainfall to support wet forest. The Kahalu`u-Lava flows complex is also on a very young (200-750 years old) lava flow. A drier forest type probably occurred in Fluvents-Kuanene complex, 2-10% slopes (soil map unit 557) at low, dry elevations; however, very little native vegetation remains on this map unit. Pathways from this state/plant community To State 2, Grassland, via “A or E, and B”: A = mechanical land clearing; B = pasture establishment; E = continuous grazing. Native Forest can be converted to Grassland by clearing the forest with heavy machinery; native forest may be cleared gradually by allowing cattle access to the forest. Cattle eventually eat or destroy understory ferns, forbs, shrubs, and saplings, opening up the forest so that pasture grasses will thrive. At higher, cooler elevations kikuyugrass and/or pangolagrass have been planted. At lower elevations where pastures are on old sugarcane plantations, guineagrass (a former weed in the plantations) has volunteered.
H = gradual weed invasion; I = ungulate foraging; J = lack of native plant regeneration. Native Forest can convert to Native Forest with Alien Understory by gradual replacement of the understory by alien shrubs, vines, and small trees that outcompete the native understory species. This process is accelerated by ungulate foraging that disturbs the soil surface and directly destroys native plants and prevents their regeneration.
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