Acacia Koa Research Articles

The influence of soil resources and plant traits on invasion and restoration in a subtropical woodland

It has been shown in some cases that nitrogen (N) addition to soil will increase abundance of plant invaders because many invaders have traits that promote rapid growth in response to high resource supply. Similarly, it has been suggested, and sometimes shown, that decreasing soil N via carbon (C) additions can facilitate native species recovery. Yet all species are unlikely to respond to resource supply in the same way. We asked how soil nutrients and competition affect native and exotic woody species in a restoration experiment where we added N or C, and crossed soil manipulation with the manipulation of dominant exotic grass abundance in a Hawaiian subtropical woodland. We related changes in survival and growth of outplanted individuals to native/exotic status and plant traits. As a group, N-fixers showed reduced survival compared to non-fixers in response to added N, with Morella faya (exotic) and Acacia koa (native) having dramatic negative responses. Among non-fixers, species with greater foliar %N had more positive survival responses to increasing soil N. Specific leaf area was not predictive of responses to nutrients or competition. In general, responses to carbon addition were weak, although reducing competition from existing exotic grasses was beneficial for all outplanted species, with N-fixers showing the most positive response. We conclude that commonly used restoration strategies to clear exotic species or lower soil resources with C addition will most greatly benefit N-fixing species, which themselves may be unwanted invaders. Thus statements about the influence of increased soil N on invasions should be carefully dissected by considering the traits (such as N-fixation status) of the regional species pool.

Stable isotope ratios and reforestation potential in Acacia koa populations on Hawai’i

Stable carbon and nitrogen isotopes can be influenced by a multitude of factors including elevation, precipitation rate, season, and temperature. This work examined the variability in foliar stable carbon (δ 13 C) and nitrogen (δ 15 N) isotope ratios of koa ( Acacia koa ) for 17 sites on Hawai’i Island delineated by elevation and precipitation gradients. Sites were identified and grouped with respect to mean annual precipitation (MAP), mean annual temperature (MAT) and position along three elevation ranges. Analysis of the resultant δ 13 C and δ 15 N isotope ratios from multiple individuals at these sites indicated that certain sites showed a demonstrated correlation between carbon and/or nitrogen content, isotope ratios, precipitation, and elevation however many sites showed no correlation. We used publicly available temperature and moisture data to help eliminate confounding effects by climatic drivers and capture possible points of contention. At sites where the temperature, precipitation, and elevation data were not significantly different we compared stable isotope information to determine if additional variables could have contributed to the lack of more correlative data. Our results note several areas within the Waiakea Forest Reserve and Volcanoes National Park where, based on isotope results, reforestation efforts could be most successfully initiated.

Thigmomorphogenesis: changes in morphology, biochemistry, and levels of transcription in response to mechanical stress inAcacia koa

Acacia koa A. Gray, an economically important timber-wood tree growing in the Hawaiian Islands, is affected by many environmental stresses, including drought, strong wind, heavy rain, and infection by fungal pathogens. Previous studies have shown that some morphological and biochemical changes that take place as a result of environmental stresses in plants can be also induced by mechanical stresses such as touching and bending. We studied morphological and biochemical changes and levels of gene transcription in A. koa plants due to mechanical stress. For a mechanical stress treatment, A. koa seedlings were gently bent in four cardinal directions daily for 2–6 months, after which morphological and biochemical changes were quantified. The stressed A. koa had significantly increased stem diameter, number of xylem cells, and anthocyanin and lignin contents and significantly reduced stem length. The gene expression analyses showed that 53 genes, including the genes for calcium signaling, ethylene biosynthesis, abscisic acid degradation, stress-related transcriptional regulation, and disease resistance, were induced more than twofold within 10–60 min following mechanical stress. The observation that the genes for disease resistance such as NBS-LRR can be induced by mechanical stress suggests that strong wind and rain in the natural forest may also induce disease resistance in trees.

Precommercial release thinning of potential Acacia koa crop trees increases stem and crown growth in dense, 8-year-old stands in Hawaii

An 8-year-old dense monotypic stand of naturally regenerated koa (Acacia koa A. Gray) on the Island of Hawaii was selected to determine the effects of precommercial release thinning, phosphorous (P) fertilization and herbaceous weed control on growth of potential crop trees over approximately 30months. Thinning consisted of cutting down all stems within a 4.5-m radius of the crop tree. Phosphorus was added at a rate of 300kgha−1 over two years. Herbaceous weeds were sprayed once with imazapyr, a broad-spectrum herbicide. Thinning alone or in combination with P fertilization significantly increased stem diameter increment and allometric estimates of the growth of leaf area and aboveground biomass. There was no significant increase in stem diameter, leaf area or biomass in the absence of thinning. Within the thinned treatment, P fertilization resulted in significant increases in tree height over time. Herbaceous weed control had no effect on tree growth. The atmospherically resistant vegetation index (ARVI), which was derived from spectral analyses of high-resolution satellite imagery (GeoEye1), was significantly higher for thinned than unthinned trees 25months after study initiation, suggesting greater light absorbance and a possible explanation for overall greater growth of thinned trees. When considered with results from previous studies, these findings indicated that crop tree selection and precommercial release thinning in dense, even-aged koa stands should be done early in stand development to prevent loss of crown vigor and growth potential. Additional interventions like P fertilization or herbaceous weed control may not be necessary until trees are older or site conditions suggest important soil resource limitations.

Botany, Ecology and Diversity of <i>Acacia koa</i> in the Hawaiian Islands

Acacia koa (koa) is a valuable leguminous timber-wood tree endemic to the Hawaiian Islands. Over the past century, conversion of some native forests for agriculture and ranching, uncontrolled spread of invasive plant species and outbreak of a deadly fungal wilt disease severely reduced A. koa forests. However, recently there has been a growing interest in re-establishing A. koa as an agroforestry tree because the tree has significant importance in economy, ecology and culture of Hawai‘i. This review presents the current knowledge on A. koa, including its ecological roles, various morphological forms, genetics, evolution and development of methods for seedling selection and propagation. A better understanding and awareness of the nature of A. koa will help successful development of wilt-resistant A. koa trees with high wood quality that can support Hawaii’s ecology, economy and culture.

First Report of the Root-Rot Pathogen, Armillaria gallica, on Koa (Acacia koa) and ‘Ōhi‘a Lehua (Metrosideros polymorpha) on the Island of Kaua‘i, Hawai‘i

Koa (Acacia koa) and 'ohi'a lehua (Metrosideros polymorpha) are the two most dominant native tree species in Hawai‘i. Their populations are continuously decreasing, primarily because of forest disease (Dudley et al. 2007; Keith et al. 2015) and other biotic disturbances. In April 2015, Armillaria rhizomorphs were collected from woody hosts on the island of Kaua‘i, Hawai‘i.

Wood Quality of Old-Growth Koa Logs and Lumber

Abstract Acacia koa trees are ecologically, economically, and culturally significant to the Hawaiian Islands. Koa wood is one of the most valuable species in the world and sale of koa products represents a majority of all the Hawaiian wood products sold by Hawaiian retailers. Today, there is concern in Hawaii among foresters, forest landowners and managers, wood products manufacturers, and the public that the remaining old-growth koa resource has become scarce, is suffering from declining health and diseases, and is characterized by poor growth form. Current practices require harvest of only dead and dying trees and using downed material in wood products manufacturing. We examined lumber volume and value recovery from logs sawn from dead and dying trees and from relic logs (logs that have been on the ground) from four sites on the island of Hawaii. Gross lumber recovery from all study logs was 71 percent. Log size did not significantly influence lumber volume recovery. Forty-five percent of the lumber manufactured from relic logs was below grade compared with 26 percent of the lumber sawn from standing dead and dying trees. Variables that affect lumber quality, such as different defects and heartwood proportion, were measured. Decay-type defects were the most prominent.

Alternative field fertilization techniques to promote restoration of leguminous Acacia koa on contrasting tropical sites

Field fertilization can promote early growth and survival of planted trees on degraded pastures and agricultural lands where low soil fertility and high herbaceous competition inhibit regeneration success. Controlled-release fertilizers (CRF) may improve the effectiveness of fertilization relative to that of immediately available fertilizers (IAF) because CRF gradually release nutrients directly to the root zone, thereby limiting nutrient losses. Despite past research in boreal and temperate landscapes, few studies have tested the efficacy of similar applications in tropical systems where year-round high temperatures can increase release rates of CRF and intensity of competing vegetation. On two contrasting sites on the Island of Hawaii, USA, we evaluated early growth and survival responses of koa (Acacia koa Gray), a fast-growing legume, using ten treatments: a control, four IAF formulations, and five rates of polymer-coated CRF (15N-9P-12K; 15–75g). At Pahala, a productive site, we detected no significant growth, survival, or foliar nitrogen (N) or phosphorous (P) responses to the fertilizer treatments. At Volcano, a rockier and cooler site on younger soil, height increased by 36–49% for the highest performing CRF and IAF relative to the control; diameter likewise increased by 55–92%. Growth responses appeared to be a result of P fertilization rather than N. The highest performing IAF had a reduced survival rate relative to the lowest CRF (46% vs. 83%). Although total nutrient application rates were much lower for CRF, our results suggest that on tropical restoration sites, CRF may promote seedling performance at least equally to that of IAF. There is a need to more carefully evaluate the effects of site-specific interactions that may determine field fertilizer responses, across a range of genera and functional groups.

Linking dominant Hawaiian tree species to understory development in recovering pastures via impacts on soils and litter

AbstractLarge areas of tropical forest have been cleared and planted with exotic grass species for use as cattle pasture. These often remain persistent grasslands after grazer removal, which is problematic for restoring native forest communities. It is often hoped that remnant and/or planted trees can jump‐start forest succession; however, there is little mechanistic information on how different canopy species affect community trajectories. To investigate this, I surveyed understory communities, exotic grass biomass, standing litter pools, and soil properties under two dominant canopy trees—Metrosideros polymorpha (‘ōhi‘a) and Acacia koa (koa)—in recovering Hawaiian forests. I then used structural equation models (SEMs) to elucidate direct and indirect effects of trees on native understory. Native understory communities developed under ‘ōhi‘a, which had larger standing litter pools, lower soil nitrogen, and lower exotic grass biomass than koa. This pattern was variable, potentially due to historical site differences and/or distance to intact forest. Koa, in contrast, showed little understory development. Instead, data suggest that increased soil nitrogen under koa leads to high grass biomass that stalls native recruitment. SEMs suggested that indirect effects of trees via litter and soils were as or more important than direct effects for determining native cover. It is suggested that diverse plantings which incorporate species that have high carbon to nitrogen ratios may help ameliorate the negative indirect effects of koa on natural understory regeneration.

Stocktype and grass suppression accelerate the restoration trajectory of Acacia koa in Hawaiian montane ecosystems

Restoring degraded mesic-montane forests represents a major challenge in maintaining functioning ecosystems throughout the tropics. A key example of this lies in Hawai'i, where restoring native koa (Acacia koa, A. Gray) forests are a top conservation and forestry priority because of the critical habitat and high-value timber products that they provide. Efforts to restore koa forests, however, are directly impeded by extensive, non- native kikuyu grass (Pennisetum clandestinum Hochst. ex Chiov.) swards occupying deforested montane landscapes. In this study, we implemented a combination of grass suppression and selection of koa seedling stocktypes to measure outplanting performance in a naturalized site on the island of Maui. Seedlings were grown in a nursery in two root container sizes (111 and 207 cm 3 ) and subsequently outplanted into grass-dominated plots that were either untreated or suppressed with a high-rate herbicide combination of imazapyr and glyphosate (1.7 kg a.i. ha -1 , respectively), 30 days prior to planting. Across all treatments, seedling survival was high ((95 %). Thirty months after planting, trees from the larger stocktype had significantly greater growth in height and root-collar diameter. Initial grass suppression resulted in trees that were 34 % taller with 66 % larger root-collar diameters after 30 months. Herbicide treated plots also had significantly higher leaf area indices (2.6 vs. 1.8 m 2 m -2 ), indicative of higher photosynthetic capacity and canopy closure. Grass suppression increased soil temperature along with soil moisture in the first year followed by a dramatic drop in moisture corresponding to large growth

Applied disease screening and selection program for resistance to vascular wilt in Hawaiian Acacia koa

Acacia koa is a valuable tree species economically, ecologically and culturally in Hawai'i. A vascular wilt disease of A. koa resulting from infection by the fungal pathogen Fusarium oxysporum f. sp. koae (FOXY) causes high rates of mortality in field plantings and threatens native A. koa forests in Hawai'i. Landowners are reluctant to consider A. koa for reforestation and restoration in many areas due to the threat of FOXY. Producing seeds or propagules with genetic resistance to FOXY is vital to successful A. koa reforestation and restoration. Virulent FOXY isolates were used in seedling inoculation trials to evaluate resistance levels among A. koa families in greenhouse experiments. Seedling survival varied by family, ranging from 3% to 92%, with an overall average of 46%. One clonal and three seedling field trials were established in 2012 and 2013 using selected families based on the inoculation trials. The greenhouse screening method serves as a powerful tool to rapidly evaluate A. koa families prior to outplanting, but the field trial data are needed to further validate the results and to monitor the durability over time. The field trials will also serve as a source of germplasm for selection of other commercial traits.

Pollen Carried by Native and Nonnative Bees in the Large-Scale Reforestation of Pastureland in Hawai‘i: Implications for Pollination

Abstract: Due to habitat loss, disease, and introduction of nonnative species, many native species in Hawai‘i have gone extinct or are at risk of extinction. As a result of interspecific interactions such as pollination, the decline, loss, or introduction of species can have cascading effects on island ecosystems. We studied Hylaeus spp., Hawai‘i's yellow-faced bees, the only native bees in Hawai‘i. This group of potentially important pollinators has been largely overlooked until recently, and its conservation status and ecological role are virtually unknown. We investigated how native (Hylaeus spp.) and nonnative (Apis mellifera) bees interact with flowering plants in a large-scale pasture-to-forest restoration system. We used pan traps and nets to collect bees in mature forest, remnant corridors, planted Acacia koa tracts, and open pastureland. We removed pollen from each specimen and identified it using pollen samples collected on-site. We found that Hylaeus spp. were more likely to carry less pollen a...

Illumina-Based <i>De Novo</i> Transcriptome Analysis and Identifications of Genes Involved in the Monolignol Biosynthesis Pathway in <i>Acacia koa</i>

Acacia koa is a leguminous timber tree endemic to the Hawaiian Islands. For breeding projects involved in improving wood quality of A. koa, understanding of genes influencing wood quality is crucial. Therefore, the objective of this study was to identify A. koa genes in the monolignol biosynthesis pathway, which is involved in wood formation and development. In this study, whole transcritpome sequencing of A. koa seedlings was performed through Illumina-based sequencing and over 88 million high-quality paired-end raw reads were generated. Trinity de novo assembly of those reads yielded 85,533 unigenes with an average length of 641 bp. Based on sequence similarity search with known proteins, we annotated 47,038 of the unigenes. Using the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, 149 unigenes were assigned to ortholog groups of enzymes involved in the monolignol biosynthesis pathway. In addition, we identified complete coding sequences of genes for all the ten identified enzymes of the pathway. Future studies on expression levels of these genes in A. koa with different wood qualities will provide a tool for selection of desirable types. Comprehensive sequence resources of A. koa generated through this study will contribute to genomic studies and improvement programs for this tree.

Review of wood properties of Acacia koa A. Gray (koa)

Koa wood is highly valued primarily because it has unique color, figure, and favorable working characteristics; however, little is known about its properties. Current information is inconsistent and is limited to few sources. Understanding the properties of diminishing old growth koa is important so that comparisons to planted koa can be made. Planted koa will serve as a substitute only if the value adding properties are present. This review reveals how difficult it is to characterize old growth and potentially plantation grown wood.

Passive restoration augments active restoration in deforested landscapes: The role of root suckering adjacent to planted stands of Acacia koa

Active forest restoration in Hawaii’s Hakalau Forest National Wildlife Refuge has produced a network of Acacia koa tree corridors and islands in deforested grasslands. Passive restoration by root suckering has potential to expand tree cover and close gaps between planted stands. This study documents rates of encroachment into grassland, clonal stand structure, and tree architecture. Data were collected from random replicate strip transects that started inside 23-year-old koa plantations and ended either in open grassland or in adjacent planted stands. For the former, sucker densities increased from near zero inside planted stands to a maximum of 5–38 stemsm−2 5–14m away from the edge of the plantation canopy, and then decreased to zero–a typical pattern for trees invading grassland. No suckers occurred more than 28m from the canopy edge on east-facing slopes, or more than 18m on south-facing slopes. Rates of expansion into grassland ranged from 0.8 to 1.5myr−1; suckers had already filled gaps between closely spaced plantation stands located on north-facing slopes. Continued suckering should result in the eventual re-establishment of tree cover on deforested areas between planted tree islands and corridors, and without additional active restoration.

Cross-amplification of nonnative Acacia species in the Hawaiian Islands using microsatellite markers from Acacia koa

Thirty-one microsatellite markers, developed from Acacia koa, were evaluated for cross-amplification of seven nonnative species of Acacia in the Hawaiian Islands. The number of alleles per locus ranged from one to seven, with 16 of the 31 markers showing polymorphism in at least two of the nonnative Acacia species. Of the Acacia species examined, A. melanoxylon showed the highest percentage of amplified loci.

Hyperspectral Time Series Analysis of Native and Invasive Species in Hawaiian Rainforests

The unique ecosystems of the Hawaiian Islands are progressively being threatened following the introduction of exotic species. Operational implementation of remote sensing for the detection, mapping and monitoring of these biological invasions is currently hampered by a lack of knowledge on the spectral separability between native and invasive species. We used spaceborne imaging spectroscopy to analyze the seasonal dynamics of the canopy hyperspectral reflectance properties of four tree species: (i) Metrosideros polymorpha, a keystone native Hawaiian species; (ii) Acacia koa, a native Hawaiian nitrogen fixer; (iii) the highly invasive Psidium cattleianum; and (iv) Morella faya, a highly invasive nitrogen fixer. The species specific separability of the reflectance and derivative-reflectance signatures extracted from an Earth Observing-1 Hyperion time series, composed of 22 cloud-free images spanning a period of four years and was quantitatively evaluated using the Separability Index (SI). The analysis revealed that the Hawaiian native trees were universally unique from the invasive trees in their near-infrared-1 (700–1,250 nm) reflectance (0.4 > SI > 1.4). Due to its higher leaf area index, invasive trees generally had a higher near-infrared reflectance. To a lesser extent, it could also be demonstrated that nitrogen-fixing trees were spectrally unique from non-fixing trees. The higher leaf nitrogen content of nitrogen-fixing trees was expressed through slightly increased separabilities in visible and shortwave-infrared reflectance wavebands (SI = 0.4). We also found phenology to be key to spectral separability analysis. As such, it was shown that the spectral separability in the near-infrared-1 reflectance between the native and invasive species groups was more expressed in summer (SI > 0.7) than in winter (SI < 0.7). The lowest separability was observed for March-July (SI < 0.3). This could be explained by the invasives taking advantage of the warmer summer period to expand their canopy. There was, however, no specific time window or a single spectral region that always defined the separability of all species groups, and thus intensive monitoring of plant phenology as well as the use of the full-range (400–2,500 nm) spectrum was highly advantageous in differentiating each species. These results set a basis for an operational invasive species monitoring program in Hawai’i using spaceborne imaging spectroscopy.

AFLP, Pathogenicity, and VCG Analyses of Fusarium oxysporum and Fusarium pseudocircinatum from Acacia koa.

Acacia koa (koa), a native tree in Hawaii, suffers from a dieback caused by Fusarium oxysporum. Pathogenicity tests, vegetative compatibility group (VCG) tests, and amplified fragment length polymorphisms (AFLP) analyses were conducted on Fusarium isolates recovered from diseased koa. Koa seedling mortality with individual strains ranged from 0 to 85%, with 42% of the strains killing no seedlings. Thus, strains of F. oxysporum recovered from dying koa trees may or may not be virulent. In addition to F. oxysporum, F. pseudocircinatum strains were isolated from diseased koa; however, they were either nonvirulent or had weak virulence. This is the first report of F. pseudocircinatum in Hawaii. The 46 strains of F. oxysporum and F. pseudocircinatum were grouped into 16 VCGs, but 86% of the highly virulent strains belonged to VCG 2. In AFLP analyses, strains from the same VCG generally clustered with one another. Identification of the same set of strains using VCG, AFLP, and pathogenicity tests showed that the highly virulent strains are genetically close and that high virulence toward koa is not a property of all strains of F. oxysporum. Thus, VCG 2 with the corresponding AFLP data is a significant biological entity for which we propose the name F. oxysporum f. sp. acaciae to reflect its virulence on koa.

Chitinase is a Potential Molecular Biomarker for Detecting Resistance to Fusarium oxysporum in Acacia koa

The populations of Acacia koa (koa), a tree legume native to Hawaii, are currently at risk due to infection by the wilt-inducing fungal pathogen, Fusarium oxysporum. To identify koa families that are resistant to F. oxysporum, it is necessary to develop a screening method through use of a biomarker gene. The purpose of this study was to develop a molecular method for detecting resistance through the use of genes for chitinase that catalyzes the hydrolysis of fungal cell wall chitin. Four distinct chitinase cDNAs corresponding to chitinase genes of classes I, II, and III were isolated from koa, which were designated Akchit1a, Akchit1b, Akchit2, and Akchit3. The expression of these chitinases in Fusarium-wilt resistant and -susceptible koa seedlings following inoculation with virulent F. oxysporum was monitored using quantitative reverse transcription PCR (qRT-PCR). Significant differences (p < 0.05) in chitinase expression between resistant and susceptible seedlings were observed for the Akchit1a and Akchit1b genes at 48, 72, and 96 h post inoculation, in which chitinases were up-regulated among resistant seedlings. The differences between the two groups for expression of Akchit2 and Akchit3 were less pronounced. These results indicate that F. oxysporum-induced expression of the chitinase genes Akchit1a and Akchit1b can be used as a biomarker for selecting for wilt resistance in koa.

Parent tree effects on reestablishment of Acacia koa in abandoned pasture and the influence of initial density on stand development

Increasingly private landholders in Hawaii are considering native forest restoration for their lands, and some public agencies have already started such work. Initial efforts have focused on reestablishing Acacia koa to recover alien-grass-dominated sites. This study was done in Hakalau Forest National Wildlife Refuge, Island of Hawaii, to determine the efficacy of disk plowing to stimulate natural regeneration of koa from buried seeds. Sites with four different koa parent tree configurations were treated-single live overhead koa canopy, multiple live canopies, downed snags, and no parent koa tree. Tree growth and survival were assessed periodically over 21 years. Average initial stand den- sities ranged from 100 to 1,500 trees ha -1 of scarified land, although some open areas had as few as 20 trees ha -1 . The distributions of seedlings with increasing distance from plot center were variable within and between parent tree configurations. Initial seedling density was significantly greater for the multiple-live-parent than for the no-parent configuration. Densities for the single-live and dead configurations differed from the no-parent config- uration only when densities were based on the entire scarified area of each plot. Stand densities declined 10-67 % during the next 20 years. Survival was a negative, non-linear function of initial stand density. Initial stand density exerted strong control over stem diameter and crown size at age 21-years, but had little effect on the proportion of trees with single-stems. The relationships between stand basal area and density at 21 years con- formed to the existing koa stocking guidelines. While moderate to high densities of natural regeneration can be expected from scarifying around live and dead koa trees, single trees or low density stands are likely in open areas.