Alternative field fertilization techniques to promote restoration of leguminous Acacia koa on contrasting tropical sites

Field fertilization can promote early growth and survival of planted trees on degraded pastures and agricultural lands where low soil fertility and high herbaceous competition inhibit regeneration success. Controlled-release fertilizers (CRF) may improve the effectiveness of fertilization relative to that of immediately available fertilizers (IAF) because CRF gradually release nutrients directly to the root zone, thereby limiting nutrient losses. Despite past research in boreal and temperate landscapes, few studies have tested the efficacy of similar applications in tropical systems where year-round high temperatures can increase release rates of CRF and intensity of competing vegetation. On two contrasting sites on the Island of Hawaii, USA, we evaluated early growth and survival responses of koa (Acacia koa Gray), a fast-growing legume, using ten treatments: a control, four IAF formulations, and five rates of polymer-coated CRF (15N-9P-12K; 15-75 g). At Pahala, a productive site, we detected no significant growth, survival, or foliar nitrogen (N) or phosphorous (P) responses to the fertilizer treatments. At Volcano, a rockier and cooler site on younger soil, height increased by 36-49% for the highest performing CRF and IAF relative to the control; diameter likewise increased by 55-92%. Growth responses appeared to be a result of P fertilization rather than N. The highest performing IAF had a reduced survival rate relative to the lowest CRF (46% vs. 83%). Although total nutrient application rates were much lower for CRF, our results suggest that on tropical restoration sites, CRF may promote seedling performance at least equally to that of IAF. There is a need to more carefully evaluate the effects of site-specific interactions that may determine field fertilizer responses, across a range of genera and functional groups.