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

Beginning in the 1850 s, logging, land clearing, and burning were used to c onvert high elevation Hawaiian forests to cattle pasture. Recently, declini ng profits from ranching, the need to expand habitat for endangered species , and diminishing supplies of native saw-timber have prompted interest in r estoring native forests. The Forest Service, in cooperation with the Fish a nd Wildlife Service, is studying climatic, edaphic, and biotic limitations in the reforestation of grassland and savanna in the Hakalau Forest Nationa l Wildlife Refuge, island of Hawaii. Frost damage has been implicated in po or survival of Acacia koa, a dominant native nitrogen fixing tree, and othe r endemic plants. Data showed that for some years freezing temperatures wer e common above 1800 m elevation, and that A. koa seedling survival was poor est on micro-topographic sites where freezing temperatures were lowest and most prolonged. Artificial frost protection devices enhanced A. koa seedlin g survival, presumably by reducing radiative cooling. A similar reduction i n radiative cooling was observed under a stand of 4 m tall A. koa. Such sta nds also changed soil chemical properties, making them intermediate between those of grassland and forest. Light levels near the ground under A. koa w ere 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 pre sence of stands of A. koa. Using A. koa as a nurse crop may create understo ry conditions favorable for the establishment of other plant components of a mixed-species forest. (C) 1999 Elsevier Science B.V. All rights reserved.