Growth response of Acacia koa trees to thinning, grass control, and phosphorus fertilization in a secondary forest in Hawai‘i

Abstract

Koa ( Acacia koa A. Gray) is an endemic Hawaiian hardwood tree of high ecological, cultural and economic value. Despite its multiple values, research on the silviculture of koa has been minimal until recently because the preferred land-use was pasture for livestock, and logging was done mainly to facilitate and reduce the costs of conversion. This study examined growth response of selected potential crop trees to three treatments: (1) thinning of surrounding non-crop koa trees; (2) herbicide control of exotic grasses; and (3) herbicide grass control plus phosphorus (P) fertilization. These treatments were applied in a split-plot design to a 24-year-old stand of koa located at 1700-m elevation on the island of Hawai‘i. Treatments had variable effects on stem diameter increment of crop trees, measured at breast height during the second and third years after treatments were imposed. Neither thinning alone nor grass control alone significantly increased stem diameter increment or leaf nutrient concentrations of crop trees, or soil nitrogen (N) or P availability. Grass control in combination with P fertilization in the unthinned plots increased stem increment by 50% compared with unthinned control subplots, but again not significantly so. In these unthinned plots, grass control plus P fertilization greatly increased soil P availability and foliar P, but not those of other nutrients. Thinning in combination with grass control and P fertilization significantly increased annual diameter increment at breast height by 118%. Crown vigor and live crown to total tree height ratio were correlated with crop tree growth rate, which emphasizes the need to select crop trees that have healthy, full crowns and maintain a high live crown ratio, in addition to straight, defect-free stems. Overall, our results suggest that the benefits of release thinning of intermediate age koa crop trees on similar sites can be enhanced when combined with weed control and fertilization. Although even greater benefits might be realized if treatments are imposed before crown vigor and live crown ratio decline, the timing will need to be balanced against higher cost of thinning denser stands and the ability of managers to identify potential crop trees.