Modifying Acacia koa Phenotype in Nursery Culture through Container Selection and Nitrogen Hardening to Promote Survival and Growth in the Field

Kaufmann, Bradley
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[Honolulu] : [University of Hawaii at Manoa], [December 2016]
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After centuries of habitat loss, the distribution of Acacia koa (koa) has largely been relegated to high-elevation, fragmented populations. In addition to being one of the most valuable trees in the world, koa provides critical habitat for endangered plant and animal species and is revered in Hawaiian culture. Invasive plant competition, animal browsing, drought, and climate change challenge establishment of koa seedlings. Climate change induced decreases in available soil moisture in conjunction with increases in solar radiation and temperature will greatly stress outplanted seedlings. Ensuring the survival of nursery-grown seedlings on sites that contain limited soil- moisture necessitates the employment of horticultural techniques in the nursery that modify morphological and physiological attributes of field-bound seedlings. Nutrition and container-type influence the survival and growth of outplanted seedlings. The root-to- shoot ratio (R:S) is a standard measure of seedling morphology, which is commonly used to predict drought avoidance potential and establishment success. High-quality seedlings have shoots that are not so large as to have a transpiration requirement that cannot be met by the roots at the time of planting. Nitrogen hardening is a horticultural technique in which the amount of applied nitrogen is reduced in the weeks prior to outplanting to decrease height and shoot growth, while increasing root growth and R:S. Deeper containers train roots to soil depths that can contain increased water, while air- pruning containers create a fibrous root system with an increased quantity of root tips. To test the efficacy of Nitrogen hardening koa for outplanting, seedlings were grown for 13 weeks in DeepotTM (25.4 cm deep) and RootMaker® (10.2 cm deep) containers (both 410 cm3), with and without Nitrogen hardening. Seedlings were outplanted into a field site in the Northwestern Ko‘olau Mountains in January, 2016. At the end of nursery culture, Nitrogen hardened and DeepotTM seedlings exhibited a significantly increased R:S. Nitrogen hardening did not confer survival or growth benefits to seedlings in the field. All seedlings exhibited a high survival rate 8 months after planting (>95%). Container-type was the most influential factor, with DeepotTM containers demonstrating a significantly increased height (+9.4%) and root-collar diameter (+12.5%) after 8 months of field growth compared to RootMaker® containers.
M.S. University of Hawaii at Manoa 2016.
Includes bibliographical references.
koa tree, Acacia koa, nursery production, reforestation, Nitrogen hardening, RootMaker container, Deepot container
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Theses for the degree of Master of Science (University of Hawaii at Manoa). Natural Resources & Environmental Management
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