Frost Tolerance of Acacia Koa Populations Along an Elevational Gradient

Past ProjectsProjects

Frost Tolerance of Acacia Koa Populations Along an Elevational Gradient

Project Collaborators

  • Lilian Ayala Jacobo, Department of Forestry and Natural Resources, Purdue University
  • Aziz Ebrahimi, Department of Forestry and Natural Resources, Purdue University
  • Keith Woeste, Northern Research Station, USDA Forest Service,
  • Douglass F. Jacobs, Department of Forestry and Natural Resources, Purdue University

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Example of non-acclimated and acclimated koa seedlings from low (L=300-750 m), middle (M=750-1,500 m), and high (H=1,500-2100 m) elevation seed sources that were treated with three freezing temperatures (-5, -10, and -20°C). 

Natural populations of koa have severely declined due to intensive harvesting and land clearing for agriculture, and other land uses. The climate is relatively unpredictable at higher elevation sites in Hawai‘i, and exposure of planted seedlings to winter frost may reduce survival and result in failed restoration projects. An improved understanding of potential cold tolerance within and among remaining koa populations may help to restore koa in such high elevation sites.

The objectives of our study are to understand:

  • Whether koa develops cold hardiness as a mechanism to tolerate frost,
  • If populations of koa from higher elevations show greater cold tolerance than populations from lower elevations, and
  • How those populations are genetically related to each other.

Aziz Ebrahimi, formerly a PhD student and now a post-doc, is evaluating the level of koa’s cold tolerance. In addition to the information in the previous report, Aziz’s study has continued to focus on: 1) the level of cold hardiness through whole-plant physiology analysis, 2) genetic diversity across the elevation cline using cold-hardy genes with Wide-seq sequencing methods.

Assessing cold hardiness through a Whole-plant freezing test

Two types of seedlings, non-acclimated (grown in a greenhouse) and cold-acclimated (controlled temperatures), were used for the freezing assessment tests. Entire seedlings were placed in a programmable chest freezer for assessing damage at 4, -5, -10, and -20°C. Seedlings treated at freezing temperatures were kept in the greenhouse for 80 days to evaluate survival and damage rates. Based on the whole-plant freezing results, cold tolerance increased after cold acclimation for all elevations, but the tolerance level increased with elevation. Furthermore, we identified that shoot damage in acclimated seedlings is lower than in non-acclimated seedlings for all the elevations.

Genetic relationship of koa populations using cold-hardy genes

Koa individuals from different elevations were used to assess genetic diversity. We used cold-hardy genes through Wide-seq sequencing methods to evaluate the genetic relationship of koa populations with each other. The level of genetic diversity and detected cold hardy genes in middle elevation seedlings was greater than for low and high elevation seedlings. The number of cold-hardy genes mapped to the genome across the elevation revealed that the middle elevation seedlings contributed more than lower and high elevation seedlings. These samples had higher genetic diversity as well. We conclude that the level of genetic diversity and detected cold hardy genes in seedlings of middle elevation populations is greater than that of low and high elevation populations.


Burr, K.E., 1990. The target seedling concepts: bud dormancy and cold hardiness, in: Rose, R, C.S. (Ed.), Proceedings, Western Forest Nursery Association. U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, pp. 79–90.

Haase, D.L., 2011. Seedling phenology and cold hardiness: moving targets, in: Riley, L.E., Haase, D.L., Pinto, J.R. (Eds.), National Proceedings: Forest and Conservation Nursery Associations - 2010. USDA Forest Service, Rocky Mountain Research Station, Fort Collins, CO, USA, pp. 121–127.

Krauss OR (2014). Parent tree selection and evaluation of frost resistance, wood quality, and seed relatedness of Acacia koa. MS Thesis, Purdue University 116 p.

Scowcroft PG, Jeffrey J (1999) Potential significance of frost, topographic relief, and Acacia koa stands to restoration of mesic Hawaiian forests on abandoned rangeland. Forest Ecology and Management 114: 447-458.

Scowcroft PG, Meinzer FC, Goldstein G, Melcher PJ, Jeffrey J (2000) Moderating night radiative cooling reduces frost damage to Metrosideros polymorpha seedlings used for forest restoration in Hawaii. Restoration Ecology 8: 161-169.