A delay in leaf form transition as a shade avoidance response in two populations of Hawaiian Acacia koa from contrasting environments

by Kyle Earnshaw and Doug Jacobs

HTIRC, Department of Forestry and Natural Resources, Purdue University

hacko-lfpetiole04302Acacia koa , like many acacias, produces two different kinds of leaves: bipinately-compound true leaves and modified petioles, called phyllodes. The true leaves are found on juvenile plants and usually transition to phyllodes during the first year of growth. The structure and physiology of true leaves are thought to provide an advantage for fast early growth and light capture in shaded conditions, whereas phyllodes are structured to provide greater drought resistance.

          Koa occupies a dominant canopy position in forests with rainfall ranging from 800 to 6000 mm per year. In each forest type, regenerating koa seedlings face distinct challenges, including both shade and drought. However, most previous studies on the differences between true leaves and phyllodes have examined only single populations. In our study, we are exploring how the timing of the transition from true leaves to phyllodes differs among koa populations from contrasting forest types under various levels of shade and water stress. We also wanted to know whether a delay in the transition to phyllodes was correlated with other shade avoidance responses, such as decreased branching and longer internode lengths. These traits are desirable for timber production, especially for koa which is notorious for its proclivity to produce low branches when grown in open conditions. 

          Decreased branching in lower light, even if accompanied by a small decrease in growth rates, would suggest that it might be beneficial to plant koa seedlings under nurse trees or in mixed species plantations. These planting strategies have been applied in the production of timber-quality blackwood (Acacia melanoxylon), another phyllodinous acacia species. 

          The current study is being conducted in the Horticulture and Landscape Architecture Greenhouses at Purdue University. We are using two populations of koa: one from a wet forest and another from a dry forest on Hawaii Island. Seedlings from these forests are grown under three light levels: full sun, 70% full sun, and 30% full sun. These treatments include decreasing red to far red ratios to mimic canopy effects. The lowest light level is coupled with a shade cloth treatment that has low light intensity, but a red to far red ratio indistinct from full sun. Within each light treatment, plants were either watered daily or when they reached a set soil water content level. After four months, the plants are beginning to transition from true leaves to phyllodes. As expected, plants from the dry forest population and those grown in full sun are transitioning faster. We await further results that may shine light on the relative adaptability of these two populations of koa to environmental changes. The results of this experiment will have implications for improved management of koa in plantations and restoration.

Subirrigation as an innovative new technology to reduce water consumption for native plant nursery production

by Douglass F. Jacobs

HTIRC, Department of Forestry and Natural Resources, Purdue University












The purpose of this demonstration project was to test the use of a scientifically-proven irrigation technology known as sub-irrigation for native plant nursery production for forest restoration programs in Hawaii. This technology greatly reduces irrigation water consumption because water is recycled in a closed-system instead of being released as runoff. It also reduces the amount of fertilizer entering the environment as runoff from nursery operations.

We established subirrigation demonstration at three nurseries on the Island of Hawaii: Waimea State Tree Nursery, Hakalau Forest National Wildlife Refuge Native Plant Nursery, and the Kulani Prison Native Plant Nursery. At these nurseries, we successfully grew three native forest tree species: koa (Acacia koa), ohia (Metosiderous polymorpha), and mamane (Sophora chrysophylla). After growing in the nursery to plantable size, seedlings produced in our subirrigation trials were outplanted into various demonstration field plots on the windward slope of Mauna Kea on the Island of Hawaii. We monitored development (growth and survival) of seedlings in demonstration plots over time. These data confirmed that, in all cases, subirrigated seedlings performed as well as or better than those produced under overhead irrigation.  This indicates that subirrigation can be used to produce high quality plants for forest restoration in Hawaii, while simultaneously allowing for potential reductions in water use and wastewater discharge during nursery culture.

Workshops and field tours

To bring this technology to a wider audience, we held workshops for nursery managers and technicians at each of the three participating nurseries during various stages of the project.  Additionally, we organized a 2-day workshop in March, 2010 at the USDA Forest Service Institute of Pacific Islands Forestry, Pacific Southwest Research Station that was attended by a capacity crowd of more than 50 nursery and land managers. The first day of this workshop included several presentations of our subirrigation demonstration trials. On the second day, we visited nurseries and conducted field tours to directly demonstrate the methods and results of our subirrigation trials. We presented results from these trials at the Nāhelehele Dryland Forest Symposium in February 2011 in Kailua-Kona.

Adoption by Native Plant Nurseries in Hawaii

Based at least partly on our current efforts, subirrigation systems have recently been adopted at several nurseries in Hawaii including Hakalau Forest National Wildlife Refuge Native Plant Nursery, the Kulani Prison Native Plant Nursery (recently closed), Ulupalakua Ranch (Maui), and Native Nursery (Maui). Several additional nurseries and management organizations have requested assistance from us in development of subirrigation systems for their operations. 

More information

Our group has written  two extension publications that are available online at Native Plants Journal http://www.nativeplantnetwork.org/journal

Davis A.S., Pinto J.R., Jacobs D.F. 2011. Early field performance of Acacia koa seedlings grown under subirrigation and overhead irrigation. Native Plants Journal 12:94-99.

Schmal, J.L., Dumroese, R.K., Davis, A.S., Pinto, J.R., and Jacobs. D.F. 2011. Subirrigation for production of native plants in nurseries – concepts, current knowledge, and implementation. Native Plants Journal 12:81-93.

Parent tree selection and wood quality of a 9-year-old Acacia koa stand

by Oriana Rueda-Krauss and Charles Michler

HTIRC, Department of Forestry and Natural Resources, Purdue University


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.



Caterpillar outbreak hits Hawaii Island forests

by Robert Hauff

Hawaii Department of Forestry and Wildlife

Koa moth Puuwaawaa

HawaiiIsland KoaDefoliation 1 01242013-DLNR

defoliated koa

caterpillars on koa WH

Scotorythra paludicola adult-e1360977995358

The native koa looper moth (Scotorythra paludicola) has given foresters a scare on the island of Hawaii this year, defoliating over 50,000 acres of koa forest. The outbreak of these small, leaf-devouring caterpillars was first noticed in January 2013, although it likely began a month or two earlier. The insect is normally present at low levels but, for unknown reasons, populations occaissionally explode, leading to wide-scale koa defoliation. This is the largest recorded outbreak and the first on the Hawaii island since the 1950’s, although there have been several outbreaks on Maui more recently.

The island of Hawaii has the largest tracts of intact koa forest left in the state. Fortunately, infested trees have already been observed re-sprouting leaves in the original infestation area in the Hamakua and Laupahoehoe areas. In past outbreaks, most trees have recovered. It’s hoped that the same will occur this time. However, in one documented case on Maui during the 1970’s, 30% of trees included in a forestry study died following a series of repreated outbreaks. Researchers speculated the high mortality was due to the dense and stressed conditions in the stand (Stein and Scowcroft 1984). After a more recent outbreak in Haleakala National Park on Maui, researchers saw near complete recovery of the forest canopy.

Foresters are currently monitoring whether the refoliated trees will get hit a second time by the caterpillars or whether the population will crash as in past outbreaks. It is unknown what causes the moth outbreaks to end, but parasitism or disease likely play a role. The koa psyllid, which arrived in Hawaii in the 1960’s, prefers young foliage of terminal branches and could flourish in a refoliating forest. This raises concerns over interactions with invasive species not present during earlier outbreaks. Invasive plants in the understory such as strawberry guava and clidemia could quickly respond to increased light levels and the nutrient pulse from insect frass, increasing their foothold in native forests.

For young plantations that are unlucky enough to get defoliated, growth rates will likely decrease for a year or two, but vigorous trees are expected to recover. It is not known whether the defoliation will harm tree form, but a secondary infestation by the koa psyllid would be very damaging for commercial plantations as this insect destroys the terminal shoot of young trees. Nothing is known about the interaction of the two species, but we are witnessing an ongoing experiment and are documenting this aspect as well as other characteristics of the outbreak.

Entomologists at UH are investigating the biology of koa moth outbreaks, including the role pheromones play in adult moth behavior. Females have been observed congregating in large clouds outside defoliated areas, perhaps leaving the males behind after mating. The role of pheromones could help explain dispersal patterns as well as offer tools for managing future outbreaks if the moths were determined to be deleterious to koa forests (ecologists believe the role of moth outbreaks might be an important disturbance promoting healthy stand development, as well as a valuable food source for remaining forest birds and the Hawaiian hoary bat, Hawaii’s only native mammal).  Mating disruption with pheromones could offer managers a way to slow moth outbreaks and protect commercially valuable koa stands in the future.

For more information and updates on the current koa moth outbreak on the big island, please visit Hawaii Division of Forestry and Wildlife website http://dlnr.hawaii.gov/blog/2013/05/30/nr13-060/

Citation: Stein, J.D. and Scowcroft, P. G. 1984 . Growth and refoliation of koa trees infested by the koa moth, Scotorythra paludicola (Lepidoptera: Geometridae). Pacific Science. 38:4 pp. 333-339.

Photos by William P. Haines

KoaMoth Flight MKea KoaForest 06 2013

Acacia koa pollination biology and genetic markers for koa wilt resistance

by Susan C. Miyasaka

Department of Tropical Plant and Soil Sciences, University of Hawaii, Manoa




Very little is known about the flowering biology of koa.  In a preliminary trial to determine whether deliberate cross-pollination of koa could be carried out successfully, we made 648 hand-crosses of nine koa trees planted at the Hamakua Experiment Station (2400 ft. elevation).  Our success rate was less than 5% and 29 flowering structures produced 55 pods, with some producing more than one pod per flowering structure. Unfortunately, in part due to damage by koa moth caterpillars (Scotorhythra paludicola), we lost 43 pods.  We   are germinating the remaining 12 pods, and will plant the koa seedlings in the greenhouse.  In the future, we plan to take cuttings from these koa seedlings and challenge them with the koa wilt pathogen and to determine whether genetic differences in resistance to koa wilt exist.
Our overall goal is to improve the sustainability of koa in Hawaii through increased resistance to koa wilt. Specifically, we are working to develop genetic markers associated with koa wilt resistant families. Previous research shows that different maternal lines (families) of koa produce seedlings that differ in resistance to the virulent strains of this fungus. Genetic markers for koa resistance would give us the ability to conduct early tests of seedlings to   determine whether they carry genes responsible for disease resistance: seedlings lacking the genetic markers associated with wilt resistance could be discarded prior to outplanting.


Janice Uchida, UH-M Dept. of Plant and Environmental Protection Sciences

J.B. Friday, UH-M Dept. of Natural and Environmental Management

Heather Kimball, UH-M Dept. of Tropical Plant & Soil Sciences

Photos by Kiley Umeda & Robert Dundas. 




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