This summer, we selected 9-yr-old trees in an experimental seed orchard and examined wood characteristics of culled trees to further Acacia koa tree improvement research and development. We conducted these experiments at the HARC-A koa stand planted by HARC in 2003 near Mana Rd., located on lands managed by the Department of Hawaiian Home Lands on the eastern slopes of Mauna Kea, Hawaii Island.
Within the HARC-A stand, we graded each tree using using morphological traits: height, height to first fork, crown architecture, straightness, DBH, number of stems, and survival. We culled all but the best potential crop trees, leaving one to four individuals per family. These selected trees are isolated from pollen of other koa trees by a large population of gorse, so they will only cross-pollinate within the stand and only the genetics of the selected trees will be passed onto future progeny. Once the selected trees produce enough seeds, we will perform progeny tests at multiple sites to evaluate the genetic makeup and overall performance of each family. This will represent a substantial advance in the tree improvement program.
To learn more about the wood characteristics of young koa, we examined the wood produced by the trees we harvested from the tree selection experiment. We found that several of the 9-year-old trees we harvested already contained heartwood. Harvested trees averaged around 40% sapwood, though this varied among trees, ranging from 20% to 70% sapwood. The wood density increased from pith to bark, averaging 577 kg/m³ in sapwood and 460 kg/m³ in heartwood. Further, sapwood was stiffer and had lower shrinkage than heartwood. There was no direct relationship between wood density and growth rate. These results provide essential information for future tree improvement.