Silviculture of Regenerating Koa Forests

Granting Agency: U.S. Forest Service, Cooperative Agreement
Project Period: May 2003-March 2005
Principal Investigator: Travis Idol
Co-PI: Patrick Baker

Summary

Measuring dbh on Large koa

Koa restoration and reforestation has been successfully achieved across thousands of acres of upland ranches and degraded pasture lands on both public and private lands. However, little professional silvicultural management of these regenerating stands is taking place. Several management priorities have become apparent, such as controlling competition with shade- tolerant grasses, thinning overstocked stands, and potential P limitations to growth. Good silvicultural management may not only improve the growth and quality of the koa itself but also improve the habitat conditions for other native plants and animals. A study has been initiated in one set of stands to investigate the response of koa productivity to thinning, grass control, and P fertilization. Our research will investigate the effects of these experimental treatments on soil nutrient availability and supply, koa nutrition, and nutrient cycling processes.

Dr. Idol and grad student Rodolfo Martinez-Morales

Currently, diameter increment growth of koa is being measured periodically. Results suggest that dbh growth of released trees in thinning plots are significantly improved when combined with grass control and P fertilization. Fertilization has a significant effect on soil P availability, increasing it by more than an order of magnitude. Now, more than 4 years after fertilization, available P remains much higher than background levels. A small portion of this can be traced to increased litterfall and litter P deposition rates. However, a significant portion of this fertilizer P remains in geochemical fractions with presumed "active" and "slow" cycling rates. The ability of koa to maintain access to this P or prevent it from fixation in the reactive volcanic minerals over the long-term is unknown. The soil mineralogy suggests rapid fixation should occur, but the high organic matter content of these soils may be masking the surface charge properties of the soil minerals.

DBH Growth of Selected Trees

DBH Growth of Selected Trees

 	Soil P 												Availability

Soil P Availability

Publications

Meason DF, Idol TW, Friday JB, and Scowcroft PG. 2009. Effects of fertilisation on phosphorus pools in the volcanic soil of a managed tropical forest. Forest Ecology and Management. In press. doi:10.1016/j.foreco.2009.04.001

Meason D and Idol TW. 2008. Comparison of ion exchange resin membranes (PRS-probes) with resin bags in the field: effects of incubation time, soil depth, and P fertilization. Soil Science Society of America Journal 72:1806-1814. doi:10.2136/sssaj2008.0010

Scowcroft PG, Friday JB, Idol TW, Dudley N, Haraguchi J, and Meason D. 2007. Growth response of Acacia koa trees to thinning, grass control, and phosphorus fertilization in a secondary stand in Hawaii. Forest Ecology and Management 239:69-80. doi:10.1016/j.foreco.2006.11.009

Future Directions

We are now expanding this work to younger koa stands in older, deep ash soils along the Hamakua coast. These stands are expected to respond more rapidly to thinning and P fertilization. The youngest stands are also expected to benefit from grass control, which is highly competitive with native plant seedlings. This work is being combined with new grass control techniques developed by Dr. James Leary to improve koa restoration in abandoned pastures.