Koa trees often have low forks and inherently poor stem form, which may limit the potential economic and cultural returns from koa forestry. Gap silvicultural techniques may be used to help maintain apical dominance and thereby improve stem form, allowing the possibility to sustainably manage for high value koa (e.g., canoe logs). Gap silviculture has promise in part because koa has been hypothesized to have adapted a strategy to regenerate in natural forests within canopy gaps. Koa regeneration performs well within canopy gaps and its morphology can respond favorably to light conditions in gaps. However, the relationship between gap structure and koa survival and performance is complex and the resultant stem form improvement from managing koa in canopy gaps has not been verified.
To examine the effects of nutrition and canopy structure on stem form development, five-year-old koa interplanted within a sugi pine (Cryptomeria japonica) plantation on Department of Hawaiian Home Lands near Humuula was remeasured during June 2019. Measurements included height, diameter at breast height (DBH), branch diameter below 8 ft, straightness, and the number of stems. Although fertilized trees in more shaded conditions far outperformed non-fertilized trees in more open conditions after two years, preliminary results suggest that this effect was beginning to diminish. Increased shade still resulted in greater height after five years and increased fertilization promoted larger DBH, but the interaction was no longer significant. Notably, fertilized trees were more likely to have multiple stems, a lower height to the first fork, and a less straight stem in shaded conditions, relative to more open conditions. Moreover, these trends were reversed for trees that received either no fertilizer or a low rate of fertilizer at planting.
These preliminary results demonstrate the need for further research into the effects of nutrition and canopy structure on koa stem form. Site selection is now underway to identify native forest areas to study soil fertility and canopy gap structure effects on koa recruitment, performance, and