Potential significance of frost, topographic relief, and Acacia koa stands to restoration of mesic Hawaiian forests on abandoned rangeland

Abstract

Beginning in the 1850 s, logging, land clearing, and burning were used to convert high elevation Hawaiian forests to cattle pasture. Recently, declining profits from ranching, the need to expand habitat for endangered species, and diminishing supplies of native saw-timber have prompted interest in restoring native forests. The Forest Service, in cooperation with the Fish and Wildlife Service, is studying climatic, edaphic, and biotic limitations in the reforestation of grassland and savanna in the Hakalau Forest National Wildlife Refuge, island of Hawaii. Frost damage has been implicated in poor survival of Acacia koa, a dominant native nitrogen fixing tree, and other endemic plants. Data showed that for some years freezing temperatures were common above 1800 m elevation, and that A. koa seedling survival was poorest on micro-topographic sites where freezing temperatures were lowest and most prolonged. Artificial frost protection devices enhanced A. koa seedling survival, presumably by reducing radiative cooling. A similar reduction in radiative cooling was observed under a stand of 4 m tall A. koa. Such stands also changed soil chemical properties, making them intermediate between those of grassland and forest. Light levels near the ground under A. koa were similar to those found in forests, but not as low as those found under pasture grasses. Biomass and nutrient pools of the litter-herbaceous layer were affected by micro-topographic position in the landscape and by the presence of stands of A. koa. Using A. koa as a nurse crop may create understory conditions favorable for the establishment of other plant components of a mixed-species forest.