Acacia koa is an important ecological and economic species in Hawaii. Effective silvicultural practices are needed to ensure ecosystem health and economic return. The purpose of this project is to understand the productivity and nutrient cycling responses of regenerating koa forests to thinning and fertilization treatments. The long-term goal is to develop effective silvicultural treatments to ensure ecosystem health, productivity, and economic returns of koa timber.

APPROACH

Regenerating koa forests have been experimentally thinned and thinned + fertilized. We will follow the changes in soil nutrient availability, plant nutrition, and nutrient cycling in response to the treatments. Soil nutrient availability will be measured using ion-exchange membrane stakes placed above and below the surficial root mat. Changes in root length, density, and biomass; litterfall and nutrient content; and canopy nutrient concentration and content will all be assessed directly by collection of samples in the field and laboratory analysis. This data will be combined with complementary studies that will assess stem diameter growth and increases in stand basal area, height, and volume.

This project is focused on determining the productivity and nutrient cycling responses to P fertilization in mid-rotation Acacia koa forest stands. Results over the past year have shown that there are sustained increases in soil P levels in response to fertilization. These increases are reflected in higher live leaf P concentrations, higher leaf litter P concentrations, and increased diameter increment growth of trees in fertilized plots. Increased return of P in litterfall, however, is insufficient to account for the sustained increases in soil P. Chemical fractionation of soil P shows elevated levels in all fractions. This shows that fertilizer P is broadly distributed in the soil but is remaining in actively cycled pools for at least 2 years after application. These results suggest that fertilizer P applications can have long-term effects on available P and thus on forest growth responses. Resin membrane stakes are being used to estimate available soil P. A duration study has investigated the influence of incubation time in the field on estimates of available nutrients. Results from 2-8 weeks of incubation suggest that there is an initial but variable spike in nutrient availability after the first 2 weeks. In the following 6 weeks, nutrient availability either remains steady or declines for most nutrients, but variability is significantly reduced. For nitrate, there is a steady increase in availability. Thus, incubation periods longer than 2-4 weeks do not bias estimates of availability of most nutrients. At the same time, longer incubation periods do not mean continued accumulation above levels measured during the first 2-4 weeks. These results are being compared to the use of more traditional ion exchange resin bags.

Results suggest P fertilization is a viable and effective management option for increasing medium-term soil P availability and koa growth response, even for trees 25 or more years in age. Combined with thinning and grass control, rotation ages can be shortened significantly or, alternatively, larger trees can be harvested over the same rotation age. These results will also affect current efforts to adapt the forest production model 3PG for Acacia koa forests. In particular, the soil fertility factor will need to be modified to reflect P availability and the effects of P fertilization.

PUBLICATIONS (not previously reported): 2005/10 TO 2006/09

1. Meason, D., Idol, T., and Friday, J. B. Infinite sink or dynamic equilibrium? The sorption dynamics of an ion exchange resin membrane (PRS probes) in a tropical Andisol. 2005 Soil Science Society of America meeting.

2. Meason, D., Idol, T., and Friday, J. B., and Scowcroft, P. Phosphorus dynamices in a 25 year old koa forest and the response to management. 2005 Soil Science Society of America meeting.