Parent tree effects on reestablishment of Acacia koa in abandoned pasture and the influence of initial density on stand development

Abstract

Increasingly private landholders in Hawaii are considering native forest restoration for their lands, and some public agencies have already started such work. Initial efforts have focused on reestablishing Acacia koa to recover alien-grass-dominated sites. This study was done in Hakalau Forest National Wildlife Refuge, Island of Hawaii, to determine the efficacy of disk plowing to stimulate natural regeneration of koa from buried seeds. Sites with four different koa parent tree configurations were treated-single live overhead koa canopy, multiple live canopies, downed snags, and no parent koa tree. Tree growth and survival were assessed periodically over 21 years. Average initial stand den- sities ranged from 100 to 1,500 trees ha -1 of scarified land, although some open areas had as few as 20 trees ha -1 . The distributions of seedlings with increasing distance from plot center were variable within and between parent tree configurations. Initial seedling density was significantly greater for the multiple-live-parent than for the no-parent configuration. Densities for the single-live and dead configurations differed from the no-parent config- uration only when densities were based on the entire scarified area of each plot. Stand densities declined 10-67 % during the next 20 years. Survival was a negative, non-linear function of initial stand density. Initial stand density exerted strong control over stem diameter and crown size at age 21-years, but had little effect on the proportion of trees with single-stems. The relationships between stand basal area and density at 21 years con- formed to the existing koa stocking guidelines. While moderate to high densities of natural regeneration can be expected from scarifying around live and dead koa trees, single trees or low density stands are likely in open areas.