Ph.D. - Tropical Plant and Soil Sciences
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ItemPredicting Sediment Export into Tropical Coastal Ecosystems to Support Ridge to Reef Management([Honolulu] : [University of Hawaii at Manoa], [May 2016], 2016-05)Sediment, a principal land-based source pollutant, can negatively affect coral reef ecosystems and impact the services they provide to society, such as recreation and fishing. Watershed managers interested in evaluating options and trade-offs require decision support tools that predict sediment export and retention. However, sediment load has previously been difficult to model in Hawaii because 1) observed annual sediment load data is not widely available and 2) Hawaii’s volcanic hydrology makes it difficult to use models designed on the continent. One method to estimate annual sediment loads is by measuring deposition rates in reservoirs. As of 2013, Hawaii had 140 regulated water storage reservoirs that perform as retention basins, yet the extent of their functionality had not been analyzed. I conducted a state-wide survey of reservoir capacity. Results demonstrated that only 15% of the total water storage capacity of Hawaii’s reservoirs is currently used, and 40% of all reservoirs are affected by sedimentation, representing a significant maintenance cost and potential risk. I also investigated spatial patterns of a broader array of ecosystem services including sediment retention, nitrogen retention, water yield, carbon sequestration, and agricultural production in West Maui. Between 1778 and 1920, I determined that sediment export increased by 18 times, while nitrogen export increased by 11 times over the same period. I demonstrated that past impacts from agriculture were more severe than predicted future development and climate change. Lastly, to calculate the first state-wide estimates of annual sediment export, I used a RUSLE-based model coupled with an estimate of the sediment delivery ratio (SDR), and adapted input datasets, including the erodibility and cover factors. To calibrate the model, I analyzed 60 watersheds with observed data for annual sediment load. I calculated that only 42% of the total land area contributes to sediment export, and that Hawaii and Maui export 57% of the total sediment load. Annual specific sediment yields ranged from 26 to 273 tons km-2 yr-1 across all islands, with Kahoolawe and Maui having the highest rates. These projects combined provide a base for watershed managers to identify management strategies to mitigate sediment export within an ecosystem context.
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ItemCharacterization of Dormancy, Establishment and Seed Production of Waltheria indica and Panicum torridum([Honolulu] : [University of Hawaii at Manoa], [August 2015], 2015-08)
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ItemBiochar as an Amendment to Acid Soils([Honolulu] : [University of Hawaii at Manoa], [May 2015], 2015-05)The capacity of biochar to improve acid soil productivity and enhance nutrient retention was the main focus of this study. The specific objectives were to characterize six wood-derived biochars, to assess biochars’ liming effects on Hawaiian and Indonesian acid soils, and to study nutrient retention of biochars. Six and another two biochars were collected, characterized, and then were used to evaluate their liming effect on a Hawaiian and two Indonesian acid soils with Desmodium intortum and soybean (Glycine max) as test plants, respectively. Two biochars in combination with two composts (both at 2%) as nutrient sources were used to investigate their nutrient retention with pak choi (Brassica rapa) as the test plant. The results showed that six wood-derived biochars were different in their properties, including ash content, pH, cation exchange capacity (CEC), CaCO3 equivalent, basic cations and surface functional groups. Based on their CaCO3 equivalent, leucaena (Leucaena leucocephala) and lac tree (Schleichera oleosa), Hilo mixed wood and she oak (Casuarina junghuhniana), and mahogany (Sweitenia mahagoni) and mountain gum (Eucalyptus urophylla), were grouped into the highest, moderate, and lowest liming potential biochars, respectively. Additions of six biochars at 2% and 4% with or without 2 cmolc/kg of lime to a Hawaiian acid soil increased soil pH and CEC, reduced exchangeable Al, enriched plant nutrients and enhanced Desmodium growth with lac tree and leucaena being most effective, followed by she oak and Hilo mixed wood biochars. Similar results were obtained from lac tree wood and rice husk biochars (4 and 8%) applied to two Indonesian acid soils. Addition of lac tree wood and Hilo mixed wood biochars in combination with vermicompost or thermocompost to a Ultisol and a Oxisol of Hawaii showed a positive interaction effect on EC, P and K, cabbage fresh and dry matters. Biochars increased soil pH, plant tissue Ca, retention of K, Ca and Mg, and reduced exchangeable Al in both soils. Overall, the liming capacity and nutrient retention potential of selected biochars have been positive.
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ItemSeed dormancy, smoke-stimulated germination and harvest timing of pili grass (heterpogon contortus), a native Hawaiian grass with potential for expanded re-vegetation use([Honolulu] : [University of Hawaii at Manoa], [December 2013], 2013-12)Pili grass (Heteropogon contortus (L.) P. Beauv. ex Roem. & Schult.) is a culturally and ecologically important native species in Hawaiʻi that has been used for re-vegetation of severely degraded land. Currently, it is being evaluated for expanded use on roadside and streambank stabilization, water efficient landscaping and agricultural buffer strip plantings. Despite efforts to increase its use on a variety of re-vegetation applications, planting materials such as seeds are still in limited supply. In order to develop large scale seed production protocols, studies on seed dormancy and seed harvest timing need to be conducted. The three main objectives of this dissertation were to: 1) elucidate the mechanisms involved in pili grass seed dormancy loss through the use of scarification and germination stimulants (i.e., gibberellic acid [GA], fluridone, food grade liquid smoke [FGLS], pili grass smoke infused water, xylose smoke infused water, karrikinolide and cyanide); 2) assess the effect of storage factors (i.e., seed moisture content and temperature) on pili grass seed dormancy loss and viability; and 3) determine an average cumulative growing degree unit (ACGDU) correlated indicator which has potential use for determining the optimum seed harvest timing. Studies on seed scarification, germination with stimulants and dormancy loss under different storage conditions support the placement of pili grass under the non-deep physiological dormancy class of seeds. The observed partial germination with scarification (i.e., exposure of the embryo) suggests the presence of both physical and physiological dormancy controls. Germination assays with the plant growth regulators, fluridone and GA, as well as with FGLS suggests that the abscisic acid (ABA):GA balance plays an important role in pili grass seed dormancy. Seed germination response to chemical stimulants across a gradient in dormancy relief supports the theory of increased effective levels of GA as storage duration increases. Short term seed storage studies (i.e., 1 year or less) show that dormancy and seed viability loss is affected by storage temperature and equilibrium relative humidity (eRH) (i.e., seed moisture content). Storage at 10°C, regardless of eRH, maintained seed dormancy and viability for one year. To optimize dormancy loss while maintaining seed viability, fresh seeds must be stored at either 12% eRH (6% seed moisture content, dry weight basis) at 30°C for 12 months or at 50% eRH (11% seed moisture content, dry weight basis) at 30°C for 9 months. As storage temperature was increased from 20 to 30°C and storage relative humidity was increased from 50 to 75% eRH (11 to 14% seed moisture content, dry weight basis), loss in seed viability became more pronounced over time. The viability of pili grass seeds, relieved of dormancy, can be maintained for at least 6 months with storage at 12% eRH (6% seed moisture content, dry weight basis) and 5°C. FGLS, pili grass smoke infused water and xylose smoke infused water were effective in stimulating germination of dormant pili grass seeds. FGLS and pili grass smoke infused water provided consistent germination stimulation in two batches of seed that represented two levels of dormancy relief. Xylose infused smoke water was less effective since its ability to stimulate germination was not consistent across the two seed batches. Assays which evaluated two bioactive compounds found in smoke (i.e., karrikinolide and cyanide) indicated that only cyanide stimulated germination (20 to 29% germination) of dormant pili grass seeds. Cyanide stimulated germination suggests the role of reactive oxygen species in dormancy loss of pili grass seeds under warm, dry conditions. The presence of cyanide was confirmed in pili grass smoke, but not in FGLS or xylose smoke infused water. Germination observed in these non-cyanide containing smoke sources indicates the presence of other bioactive compounds in smoke. Germination assays conducted throughout this dissertation indicated differences in depth of dormancy with time of year harvest. March seeds exhibited the lowest levels of dormancy compared to seeds harvested in June, July and October. Seasonal dormancy in pili grass seeds can be attributed to differences in growing conditions during seed development. Finally, harvest timing studies identified ACGDU, spike moisture content and the onset of seed head tangling as useful harvest timing indicators for pili grass seed production. Results indicate that maximum seed harvests can be obtained between 768 to 778 ACGDUs (79 to 82 days after cutting) under irrigated conditions. Spike moisture for optimum seed harvest timing was determined to be between 0.68 to 0.72 grams H2O per gram of dry weight. The onset of seed head tangling provided a visual cue which coincided with the optimum seed yield and range of spike moisture content. A decline in seed production over four harvest cycles (spanning two years) was recorded with possible causal factors such as stand age, cutting height and thatch accumulation.