Freeze tolerance and genetic variability of Acacia koa A. Gray, a key Hawaiian tree species

Koa (Acacia koa A. Gray) is a charismatic tree species endemic to Hawaii that plays an important role, socioeconomically, culturally and ecologically in the Hawaiian Islands. Koa forests in Hawaii have been heavily depleted and degraded due to harvesting and land use changes. To contribute to the success of koa restoration initiatives, two research projects were conducted as part of this thesis. The first project consisted of analyzing the freeze tolerance of seedlings from koa populations distributed across an elevational gradient, while the second project involved determining heritability estimates and the genetic gain of traits within and among high-elevation koa families in progeny trials. In the first project (Chapter 2), the objective was to determine if different ecotypes of koa show variation in freeze hardiness as a mechanism to tolerate cold and if exposure to hardening conditions prior to frost exposure can modify such cold tolerance adaptation. Thirteen populations of koa (Acacia koa A. Gray) were grown from seeds collected across an elevational range from 603 m to 2050 m on the Island of Hawai’i. Four-month-old greenhouse-grown seedlings obtained from these seeds were then divided into control seedlings (maintained at 25 °C day, 22 °C night) and chilling-acclimated seedlings (held at 8 °C day, 4 °C night). After six weeks, ten acclimated and ten non-acclimated seedlings per population were tested for freeze tolerance by electrolyte leakage at temperatures from 5 °C to –20 °C. Results showed a higher index of cold damage in the non-acclimated seedlings for most of the populations at the two lowest test temperatures. There were some differences in the index of damage among the population elevations, depending on the test temperature. In the second project (Chapter 3), the objective was to determine if there is genetic variability among 20 koa families for height, basal diameter, and other parameters of interest, measured at 4 years of age, at a high-elevation progeny trial site on the windward slopes of Mauna Kea on the Island of Hawai’i. Height, basal diameter, height to living crown, and projected crown area showed significant differences among families. The estimates of heritability of family means were higher than the estimates of individual-tree heritability for height, projected crown area, and gross crown volume, yet showed the same value of the estimates of individual-tree heritability for basal diameter and live crown ratio. Height, basal diameter, and height to live crown had relatively high heritability estimates > 0.4. There were significant differences among the tested families in height, basal diameter, height to living crown and projected crown area. The relative performance rankings among families produced from this progeny test will allow forestry managers to make selections for relatively high performing koa families to supply regionally-adapted, improved koa seeds for reforestation in the vicinity of this test site.