Acacia Koa Research Articles

Influence of Native Woody Understory on Invasive Grasses and Soil Nitrogen Dynamics Under Plantation and Remnant Montane Tropical Trees

While the influence of canopy trees on soils in natural and restored forest environments is well studied, the influence of understory species is not. Here, we evaluate the effects of outplanted native woody understory on invasive grass biomass and soil nutrient properties in heavily grass-invaded 30 + year-old plantations of a native N-fixing tree Acacia koa in Hawai‘i. We analyze soils from under A. koa trees with versus without planted woody understory and compare these to soils from under remnant pasture trees of the pre-deforestation dominant, Metrosideros polymorpha where passive recruitment of native woody understory has occurred since the cessation of grazing. Simultaneously, we experimentally planted understory species at three times the density used by managers to see if this could quickly decrease grass biomass and change soil nutrient dynamics. We found that invasive grass biomass declined with understory planting in surveyed and experimental sites. Yet, woody understory abundance had no effect on N cycling. Short-term N availability and nitrification potential were higher under A. koa than M. polymorpha trees regardless of understory. Net N mineralization either did not differ (~ 1 mo) between canopy species or was higher (171 day incubations) under remnant M. polymorpha where organic matter was also higher. The only influence of understory on soil was a positive correlation with loss-on-ignition (organic matter) under M. polymorpha. We also demonstrate differential controls over N cycling under the two canopy tree species. Overall, understory restoration has not changed soil characteristics even as invasive grass biomass declines.

Tradeoffs in access to light and root networks of established host trees for restoration planting of the root hemiparasite Santalum paniculatum

Restoration of root hemiparasite trees, such as Hawaiʻi's endemic Santalum species (ʻiliahi), may benefit from underplanting in stands of suitable hosts like the nitrogen‐fixing native tree, Acacia koa (koa). At a pasture site on Hawaiʻi Island previously reforested with koa, we underplanted seedlings of the island‐endemic sandalwood species, Santalum paniculatum, to examine the tradeoff between access to an established root network (distance to the nearest koa tree) under variable overstory shading (8.8–90.1% canopy openness range) during regeneration establishment. We hypothesized that there is an optimal parasite–host spacing and canopy openness that balance parasitic resource transfer with light availability. ʻIliahi seedling survival was 96% with no survival treatment differences. ‘Iliahi seedling growth was positively related to canopy openness but negatively related to the distance to the nearest koa tree, and the slope of these relationships increased over time. Leaf photosynthetic light compensation points, light saturation points, and stomatal density mostly followed similar trends as growth. These results demonstrate that ‘iliahi can be successfully underplanted in an established koa stand, which benefits ‘iliahi plantings and contributes to diversifying initial restoration and reforestation plantings. There appears to be a significant tradeoff in planting distance between benefits from and competition with the host; however, the improvement in growth with increased canopy openness appeared to be much greater than the effect of planting distance. Underplanting into an established host stand with sufficient canopy openness can help restore functionally compatible and abundant ‘iliahi regeneration into forests.

In-situ one-pot synthesis, characterization of magnetic hydrochar and its application as Fenton-like catalyst for the degradation of methylene blue dye

The widespread use of dyes in various industries has led to alarming levels of water pollution, posing significant environmental and health risks. This study presents biochar being a carbon-rich source, produced from Acacia koa pod covers has been used as a sustainable source to prepare Ak/BC-Fe3O4 magnetic nanocomposite, subsequently employed for Fenton-like degradation of methylene blue (MB) dye. FESEM-EDS revealed that the synthesized nanocomposite has a morphology of spherical magnetic nanoparticles being embedded on the biochar base and EDS further confirmed the synthesis of magnetic biochar by detecting only C, O, and Fe. The nanocomposite is mesoporous with a pore volume of 0.03204 cm3/g and a surface area of 7.11 m2/g as shown by the BET results. XRD substantiated the presence of an amorphous structure of biochar and the presence of ultra-small crystallites of Fe3O4 nanoparticles. FTIR signified the presence of specific functional groups for Fe–O and biochar whereas, XPS showed the peaks at 712.39 and 725.78 eV indicated the coexistence of Fe2+ and Fe3+. The nanocomposite is superparamagnetic with a saturation magnetization of 8.64 emu/g having remarkable magnetic separation capacity. Fenton-like degradation of MB dye in the presence of H2O2 revealed a first-order reaction with removal efficiency ranging between 82 and 96% in 120 min with convenient recycling ability for 7 cycles. Hence, this study underscores the outstanding characteristics and MB degradation efficiency of Ak/BC–Fe3O4, emphasizing its synthesis from sustainable plant biomass as a promising solution to address water pollution challenges associated with dye contamination.

Co-limitation of resources reveals adaptations of a tropical tree to heterogeneous environments along an elevational cline

IntroductionPlant species often exhibit significant variation in functional traits in populations along elevational gradients to cope with varying stress conditions. While plant development has been assumed to be most limited by a single resource, growing evidence suggests the potential for interactions of co-limiting resources to impact plant performance. Here, we aimed to determine how light, nitrogen, and water availability influence the growth and physiology of different populations of koa (Acacia koa), a tree species of concern that occurs across a large elevational gradient in tropical Hawaii, United States.MethodsPopulations from three seed sources [low (L), mid (M), and high (H) elevation] were grown in a controlled greenhouse experiment and exposed to co-limiting light, water, and nutrient (nitrogen) conditions. Light response, gas exchange, water status, resource use efficiency, nutrients and shoot non-structural carbohydrate concentrations, and growth and biomass allocation responses were quantified.ResultsWe found that resource co-limitation sometimes interacted to determine responses of the measured parameters. In general, the low elevation (L) koa population was more sensitive to conditions where both moisture and nutrients were limiting, while the high elevation (H) koa population was more sensitive to conditions where either light and moisture or light and nitrogen were co-limiting. The M population performed well overall regardless of resource limitation.DiscussionOur findings lend support to the theory that multiple resources limit growth and physiology of populations rather than the traditional view of a single resource limiting performance. Therefore, the possibility that multiple resource limitations drive population differences should be considered when developing population-based guidelines for forest and tree species restoration.

Integrative analysis of physiology and genomics provides insights into freeze tolerance adaptations of Acacia koa along an elevational cline.

Natural selection for plant species in heterogeneous environments creates genetic variation for traits such as cold tolerance. While physiological or molecular analyses have been used to evaluate stress tolerance adaptations, combining these approaches may provide deeper insight. Acacia koa (koa) occurs from sea level to 2300 m in Hawai'i, USA. At high elevations, natural koa populations have declined due to deforestation, and freeze tolerance is a limiting factor for tree regeneration. We used physiology and molecular analyses to evaluate cold tolerance of koa populations from low (300-750 m), middle (750-1500 m), and high elevations (1500-2100 m). Half of the seedlings were cold acclimated by exposure to progressively lowered air temperatures for eight weeks (from 25.6/22.2°C to 8/4°C, day/night). Using the whole plant physiology-freezing test and koa C-repeat Binding Factor CBF genes, our results indicated that koa can be cold-acclimated when exposed to low, non-freezing temperatures. Seedlings from high elevations had consistently higher expression of Koa CBF genes associated with cold tolerance, helping to explain variation in cold-hardy phenotypes. Evaluation of the genetic background of 22 koa families across the elevations with low coverage RNA sequencing indicated that high elevation koa had relatively low values of heterozygosity, suggesting that adaptation is more likely to arise in the middle and low elevation sources. This physiology and molecular data for cold tolerance of koa across the elevation gradient of the Hawaiian Islands provides insights into natural selection processes and may help to support guidelines for conservation and seed transfer in forest restoration efforts.

Twenty‐five years of change in forest structure and nesting behavior of Hawaiʻi ʻelepaio

Long‐term ecological studies are invaluable for detecting changes over time. Forest restoration has been a conservation priority in Hawaiʻi, where invasive species have negatively impacted native bird habitat. During 1993–1994, a study was conducted of Hawaiʻi ʻelepaio (Chasiempis sandwichensis) nest site selection and forest composition in mesic montane forest along Mauna Loa Road in Hawaiʻi Volcanoes National Park. We returned to the site and repeated the methods used in the earlier study to record Hawaiʻi ʻelepaio nest site selection (tree species, tree and nest height, and tree girth [diameter at breast height, DBH]) from 2015 to 2017, and measured forest composition (tree density, relative abundance, height, and DBH) in 2019. Three times as many nests were found in ʻaʻaliʻi (Dodonaea viscosa) as in koa (Acacia koa) in both time periods. Heights of koa and ʻaʻaliʻi did not change, but their DBH increased over time. The relative height of nests in ʻaʻaliʻi trees did not change, but nests in koa were placed higher in the crown during the later study. Overall tree density increased from 1,619 (95% confidence interval [CI] = 1,499–1,740) trees/ha to 2,583 (95% CI = 2,176–3,096) trees/ha. Koa relative abundance was 53% of total trees earlier and 45% later, ʻaʻaliʻi remained at 47%, and māmane (Sophora chrysophylla) increased from 0 to 8% over time. Our results indicate that changes in forest composition affect nesting behavior, but in ways that are not necessarily simple or consistent.

Allometric relationships for eight species of 4-5 year old nitrogen-fixing and non-fixing trees.

Allometric equations are often used to estimate plant biomass allocation to different tissue types from easier-to-measure quantities. Biomass allocation, and thus allometric equations, often differs by species and sometimes varies with nutrient availability. We measured biomass components for five nitrogen-fixing tree species (Robinia pseudoacacia, Gliricidia sepium, Casuarina equisetifolia, Acacia koa, Morella faya) and three non-fixing tree species (Betula nigra, Psidium cattleianum, Dodonaea viscosa) grown in field sites in New York and Hawaii for 4-5 years and subjected to four fertilization treatments. We measured total aboveground, foliar, main stem, secondary stem, and twig biomass in all species, and belowground biomass in Robinia pseudoacacia and Betula nigra, along with basal diameter, height, and canopy dimensions. The individuals spanned a wide size range (<1-16 cm basal diameter; 0.24-8.8 m height). For each biomass component, aboveground biomass, belowground biomass, and total biomass, we determined the following four allometric equations: the most parsimonious (lowest AIC) overall, the most parsimonious without a fertilization effect, the most parsimonious without canopy dimensions, and an equation with basal diameter only. For some species, the most parsimonious overall equation included fertilization effects, but fertilization effects were inconsistent across fertilization treatments. We therefore concluded that fertilization does not clearly affect allometric relationships in these species, size classes, and growth conditions. Our best-fit allometric equations without fertilization effects had the following R2 values: 0.91-0.99 for aboveground biomass (the range is across species), 0.95 for belowground biomass, 0.80-0.96 for foliar biomass, 0.94-0.99 for main stem biomass, 0.77-0.98 for secondary stem biomass, and 0.88-0.99 for twig biomass. Our equations can be used to estimate overall biomass and biomass of tissue components for these size classes in these species, and our results indicate that soil fertility does not need to be considered when using allometric relationships for these size classes in these species.

Isoscapes of remnant and restored Hawaiian montane forests reveal differences in biological nitrogen fixation and carbon inputs.

Deforestation and subsequent land-use conversion has altered ecosystems and led to negative effects on biodiversity. To ameliorate these effects, nitrogen-fixing (N2-fixing) trees are frequently used in the reforestation of degraded landscapes, especially in the tropics; however, their influence on ecosystem properties such as nitrogen (N) availability and carbon (C) stocks are understudied. Here, we use a 30-y old reforestation site of outplanted native N2-fixing trees (Acacia koa) dominated by exotic grass understory, and a neighboring remnant forest dominated by A. koa canopy trees and native understory, to assess whether restoration is leading to similar N and C biogeochemical landscapes and soil and plant properties as a target remnant forest ecosystem. We measured nutrient contents and isotope values (δ15N, δ13C) in soils, A. koa, and non-N2-fixing understory plants (Rubus spp.) and generated δ15N and δ13C isoscapes of the two forests to test for (1) different levels of biological nitrogen fixation (BNF) and its contribution to non-N2-fixing understory plants, and (2) the influence of historic land conversion and more recent afforestation on plant and soil δ13C. In the plantation, A. koa densities were higher and foliar δ15N values for A. koa and Rubus spp. were lower than in the remnant forest. Foliar and soil isoscapes also showed a more homogeneous distribution of low δ15N values in the plantation and greater influence of A. koa on neighboring plants and soil, suggesting greater BNF. Foliar δ13C also indicated higher water use efficiency (WUEi) in the plantation, indicative of differences in plant-water relations or soil water status between the two forest types. Plantation soil δ13C was higher than the remnant forest, consistent with greater contributions of exotic C4-pasture grasses to soil C pools, possibly due to facilitation of non-native grasses by the dense A. koa canopy. These findings are consequential for forest restoration, as they contribute to the mounting evidence that outplanting N2-fixing trees produces different biogeochemical landscapes than those observed in reference ecosystems, thereby influencing plant-soil interactions which can influence restoration outcomes.

Co-planting of a fast-growing, nitrogen-fixing host tree facilitates regeneration of the root hemiparasitic ‘iliahi (Hawaiian sandalwood)

Root hemiparasitic trees can photosynthesize yet also acquire resources from host plants, which may benefit the long-term survival and growth of the hemiparasite. Experimental evaluation of planting distance between tree hemiparasites and their hosts can lead to biological insights and contribute to practitioner decision making. On an abandoned pasture site in a historically tropical dry forest in Hawai‘i, we studied the effects of two host species and four planting distances on the survival and growth of ‘iliahi (Hawaiian sandalwood, Santalum paniculatum), an endemic root hemiparasitic tree. Treatments included a control with no host or one of two native host species: ‘aʻaliʻi (Dodonaea viscosa), a fast-growing shrub, or koa (Acacia koa), a fast-growing nitrogen-fixing tree. ‘Iliahi and host seedlings were planted at a distance of <0.2, 0.5, 1.0, or 2.0 m from each other. After three years, survival of ‘iliahi seedlings was greatest for ‘iliahi paired with koa at 1.0 m (88 %) compared to the control with no host (53 %) and when paired with koa at 2.0 m (55 %). Height and ground line diameter of ‘iliahi seedlings were greater when paired with koa at closer distances than with ‘aʻaliʻi at any distance or the control with no host. Specifically, ʻiliahi growth was greatest when paired with koa at <0.2 m distance. Foliar nitrogen concentration was greater for ‘iliahi paired with koa at <0.2, 0.5, and 1.0 m distances, whereas the other foliar nutrient concentrations were typically greater for the ‘iliahi control with no host. For sites with few or no pre-established hosts, such as abandoned pastures, ‘iliahi growth can be greatly improved by co-planting in proximity to koa. As with other Santalum spp., early and abundant parasitic root connections with a host, especially a nitrogen-fixing host, are likely important for ‘iliahi establishment and early growth.

Crossing the threshold: Invasive grasses inhibit forest restoration on Hawaiian Islands.

Forest removal for livestock grazing is a striking example of human-caused state change leading to a stable, undesirable invasive grass system that is resistant to restoration efforts. Understanding which factors lead to resilience the alternative grass state can greatly benefit managers when planning forest restoration. We address how thresholds of grass cover and seed rain might influence forest recovery in a restoration project on Hawai'i Island, USA. Since the 1980s, over 400,000 Acacia koa (koa) trees were planted across degraded pasture and invasive grasses still dominate the understory with no native woody-plant recruitment. Between this koa/grass matrix are remnant native Metrosideros polymorpha ('ōhi'a) trees beneath which native woody plants naturally recruit. We tested whether there were threshold levels of native woody understory that accelerate recruitment under both tree species by monitoring seed rain at 40 trees (20 koa and 'ōhi'a) with a range of native woody understory basal area (BA). We found a positive relationship between total seed rain (but not bird-dispersed seed rain) and native woody BA, and a negative relationship between native woody BA and grass cover, with no indication of threshold dynamics. We also experimentally combined grass removal levels with seed rain density (6 levels) of two common understory species in plots under koa (n = 9) and remnant 'ōhi'a (n = 9). Few seedlings emerged when no grass was removed despite adding seeds at densities 2-75 times higher than naturally occurring. However, seedling recruitment increased 2 to 3 times once at least 50% of grass was removed. Existing survey data of naturally occurring seedlings also supported a threshold of grass cover below which seedlings were able to establish. Thus, removal of all grasses are not necessary to achieve system responses: even moderate reductions (~50%) can increase rates of native woody recruitment. The non-linear thresholds found here highlight how incremental changes to an inhibitory factor lead to limited restoration success until a threshold is crossed. The resources needed to fully eradicate an invasive species may be unwarranted for state change, making understanding where thresholds lie of the utmost importance to prioritize resources. This article is protected by copyright. All rights reserved.

Nutrient Availability Has a Greater Influence than Pot Host on Seedling Development of Hemiparasitic Hawaiian Sandalwood (Santalum paniculatum Hook. and Arn.)

Sandalwood (Santalum spp.) has been overharvested throughout its range, including the Hawaiian Islands, where 6 of the 19 species Santalum spp. are endemic. As hemiparasitic plant species, Hawaiian sandalwoods require a host plant for optimal forest establishment, yet the importance of a host during seedling development is unclear. Furthermore, understanding interactions between pot hosts and nutrient availability on sandalwood seedling development during nursery culture will help to promote the production of high-quality sandalwood seedlings for restoration and commercial purposes. We evaluated the effects of controlled-release fertilizer (CRF), chelated Fe treatments, and two pot host species (Acacia koa and Dodonaea viscosa) on the seedling development of Hawaiian sandalwood (Santalum paniculatum). Increased nutrient availability (CRF) led to increased dry mass, root collar diameter, shoot height, chlorophyll index, and nutrient status values, confirming that the hemiparasitic S. paniculatum can be successfully grown in early stages of cultivation by providing adequate mineral fertilizers. There was a significant interaction between the nutrient availability and chelated iron treatments associated with increased height, root collar diameter, dry mass, chlorophyll index, Fe concentration, and Fe content when chelated Fe was applied (vs. not) in a nutrient-limiting environment. The pot host treatment did not affect any growth metrics, but it did affect the total count of haustoria, with A. koa-hosted seedlings developing 60.3% more haustoria than D. viscosa-hosted and control seedlings. Our results demonstrate that high-quality S. paniculatum seedlings can be grown in containers by providing adequate nutrition and that S. paniculatum in a nutrient-limiting growing environment may benefit from chelated iron fertilizers.

Tree symbioses sustain nitrogen fixation despite excess nitrogen supply

AbstractSymbiotic nitrogen fixation (SNF) is a key ecological process whose impact depends on the strategy of SNF regulation—the degree to which rates of SNF change in response to limitation by N versus other resources. SNF that is obligate or exhibits incomplete downregulation can result in excess N fixation, whereas a facultative SNF strategy does not. We hypothesized that tree‐based SNF strategies differed by latitude (tropical vs. temperate) and symbiotic type (actinorhizal vs. rhizobial). Specifically, we expected tropical rhizobial symbioses to display strongly facultative SNF as an explanation of their success in low‐latitude forests. In this study we used 15N isotope dilution field experiments in New York, Oregon, and Hawaii to determine SNF strategies in six N‐fixing tree symbioses. Nitrogen fertilization with +10 and +15 g N m−2 year−1 for 4–5 years alleviated N limitation in all taxa, paving the way to determine SNF strategies. Contrary to our hypothesis, all six of the symbioses we studied sustained SNF even at high N. Robinia pseudoacacia (temperate rhizobial) fixed 91% of its N (%Ndfa) in controls, compared to 64% and 59% in the +10 and +15 g N m−2 year−1 treatments. For Alnus rubra (temperate actinorhizal), %Ndfa was 95%, 70%, and 60%. For the tropical species, %Ndfa was 86%, 80%, and 82% for Gliricidia sepium (rhizobial); 79%, 69%, and 67% for Casuarina equisetifolia (actinorhizal); 91%, 42%, and 67% for Acacia koa (rhizobial); and 60%, 51%, and 19% for Morella faya (actinorhizal). Fertilization with phosphorus did not stimulate tree growth or SNF. These results suggest that the latitudinal abundance distribution of N‐fixing trees is not caused by a shift in SNF strategy. They also help explain the excess N in many forests where N fixers are common.

Agronomic qualities of Pennisetum clandestinum in association with Acacia koa for the mitigation of the effects of ranching and promotion of conservation in a silvopastoral context on the island of Maui, Hawaii

ABSTRACT Upper-elevation mesic Hawaiian forests have been transformed by cattle ranching and invasive ungulates within the past 200 years. Although the flora and fauna of the Hawaiian Islands have been radically altered by the introduction of invasive species, there is a movement of agriculturalists within the islands interested in reintroducing native species and reforestation in agricultural spaces. One of the keystone species in the Hawaiian Islands is the endemic tree species Acacia koa, known for its ecosystem services and cultural importance. This species is valuable for nitrogen fixation, aquifer recharge, habitat for biodiversity and usefulness in agroforestry systems. This investigation is focused on the relationship between the tree species A. koa and the forage grass species Pennisetum clandestinum (kikuyu) in a silvopastoral context, at an elevation of 5,200 feet above sea level (1585 masl) on the island of Maui. The site of investigation is called Pu’u Makua in Hawaiian and is situated on the southwestern flank of Haleakalā (10,023 feet above sea level–3055 masl) and is part of Ulupalakua Ranch. A total of n = 28 forage samples of P. clandestinum were taken in dry season, 21 of which were taken from underneath the canopies of koa trees (which had been planted in densities of 3, 4, and 6 m apart). The other seven samples were taken in full sun (where no koa trees were growing). This same protocol (sampling n = 28) was repeated in the wet season. Forage samples were then sent to the Agricultural Diagnostics Laboratory at the University of Hawai`i at Manoa for forage analysis to compare attributes of kikuyu under different canopy densities of A. koa. To better understand the leaf area index (LAI) of the canopy of various densities of A. koa, hemispheric (180 degree) photos were taken and then input into Gap Light Analyzer Software for LAI. LAI data of A. koa as well as forage attributes of P. clandestinum were then input into the statistical software InfoStat to investigate relationships between canopy cover and forage quality at each specific location. It was found through Pearson correlation analysis that there are in fact significant relationships between koa and the quality of kikuyu. Measurements of koa trees were also taken to estimate biomass and the amount of carbon sequestered within each tree. These measurements were input into allometric equations specific to koa growing on the Island of Maui that were created by the United States Forest Service. Lastly, to better understand the knowledge of ranchers on Maui, we conducted surveys regarding various ranchers’ perceptions of native and non-native trees and their usefulness for silvopastoral ranching systems. Ranchers also identified various tree species that were of use (or harmful) to their operations. Since A. koa was on this list of helpful trees and conferred various ecosystem services and agroecological benefits, the incorporation of koa into ranching systems on the island of Maui is recommended in zones appropriate for its growth.

Trophic interactions and feedbacks maintain intact and degraded states of Hawaiian tropical forests

AbstractFeedbacks within ecosystems can lead to internal reinforcement of the current state providing ecosystem resilience. Often, multiple biotic interactions across trophic levels play a role in such feedbacks, yet these are generally studied independently, obscuring the relative importance of interactions among different factors. We look at various potential feedbacks in intact and degraded mesic forests on Hawaiʻi Island where managers have planted native Acacia koa (koa) trees in an attempt to jumpstart succession in former cattle pastures. These restoration forests, however, have not undergone secondary succession, instead maintaining a koa overstory with an exotic pasture grass understory. We contrasted different trophic level processes that influence the capacity for natural understory regeneration: feedbacks between bird‐mediated seed rain and fruiting understory (“top‐down”), as well as links between understory composition and microhabitats for native seed germination (“bottom‐up”). We quantified bird‐mediated seed rain under canopy trees along transects spanning intact, fragmented, and restoration forests. Along these transects, we established plots around focal overstory trees to measure abundance of fruiting understory species, ground cover (e.g., exotic grass, bryophyte), and obtained estimates of bird density to evaluate the contribution of each of these factors to seedling abundance. We also used a factorial seed addition/grass removal experiment to directly compare the influence of seed rain and germination substrate. We found evidence of both top‐down and bottom‐up feedbacks that reinforced the current state of each forest type. In the intact and fragmented forests, the combination of comparatively high seed rain and ample germination substrate is likely critical for maintaining a diverse forest system. In contrast, exotic grasses exhibit priority effects in restoration forests, inhibiting seed germination and effectively negating any benefits that could be derived from bird‐mediated seed rain. Such internal reinforcement suggests that active, rather than passive, restoration would be beneficial to increase forest diversity in restoration areas.

Hypotheses and lessons from a native moth outbreak in a low‐diversity, tropical rainforest

AbstractOutbreaks of defoliating insects in low‐diversity tropical forests occur infrequently but provide valuable insights about outbreak ecology in temperate environments and in general. We investigated an extensive outbreak of the endemic koa moth (Scotorythra paludicola), which defoliated endemic koa trees (Acacia koa) over a third of their range on Hawai‘i Island during 2013 and 2014. At Hakalau Forest National Wildlife Refuge, we observed the dynamics of the outbreak and its effects on host trees, nutrient cycling, and insectivorous consumers in reforestation stands of densely planted koa and in natural forest stands of mixed koa and ‘ōhi‘a (Metrosideros polymorpha). Contrary to predictions of the resource concentration hypothesis, caterpillar biomass and defoliation severity were greater in the natural forest sites, where koa density was relatively low. Caterpillars preferentially consumed the most palatable koa foliage type (phyllodes), and koa initially refoliated with the least palatable foliage type (true leaves). Lightly defoliated small trees refoliated more quickly than did heavily defoliated ones but the opposite was true for large trees, which also produced a greater proportion of phyllodes. Mortality was greatest for heavily defoliated small koa. Caterpillar frass caused larger increases in soil nitrogen (N) than phosphorus (P) availability, with the greatest N increases in fine‐textured soils. Foliar N increased in alien grasses under koa canopies compared to grasses away from koa and to native woody understory species. Bird activity was influenced by ‘ōhi‘a flower abundance and the severity of koa defoliation; birds switched to outbreaking caterpillar prey, and they gained weight during the outbreak. Bat foraging times decreased during the outbreak, apparently because they became satiated quickly each night. Parasitoid wasps increased with caterpillar abundance but had little influence on outbreak dynamics. Reducing alien grass cover and increasing tree diversity would likely reduce the impacts of insect outbreaks and similar perturbations to native forests.

Can the impact of canopy trees on soil and understory be altered using litter additions?

Trees can have large effects on soil nutrients in ways that alter succession, particularly in the case of nitrogen-(N)-fixing trees. In Hawai'i, forest restoration relies heavily on use of a native N-fixing tree, Acacia koa (koa), but this species increases soil-available N and likely facilitates competitive dominance of exotic pasture grasses. In contrast, Metrosideros polymorpha ('ōhi'a), the dominant native tree in Hawai'i, is less often planted because it is slow growing; yet it is typically associated with lower soil N and grass biomass, and greater native understory recruitment. We experimentally tested whether it is possible to reverse high soil N under koa by adding 'ōhi'a litter, using additions of koa litter or no litter as controls, over 2.5 yr. We then quantified natural litterfall and decomposition rates of 'ōhi'a and koa litter to place litter additions in perspective. Finally, we quantified whether litter additions altered grass biomass and if this had effects on native outplants. Adding 'ōhi'a litter increased soil carbon, but increased rather than decreased inorganic soil N pools. Contrary to expectations, koa litter decomposed more slowly than 'ōhi'a, although it released more N per unit of litter. We saw no reduction in grass biomass due to 'ōhi'a litter addition, and no change in native outplanted understory survival or growth. We conclude that the high N soil conditions under koa are difficult to reverse. However, we also found that outplanted native woody species were able to decrease exotic grass biomass over time, regardless of the litter environment, making this a better strategy for lowering exotic species impacts.

Intraspecific Variation Along an Elevational Gradient Alters Seed Scarification Responses in the Polymorphic Tree Species Acacia koa.

Physical dormancy in seeds can challenge restoration efforts where scarification conditions for optimal germination and seedling vigor are unknown. For species that occur along wide environmental gradients, optimal scarification conditions may also differ by seed source. We examined intraspecific variation in optimal scarification conditions for germination and seedling performance in koa (Acacia koa), which occurs across a wide range of environmental conditions. To evaluate scarification responses, we recorded imbibition percentage, germination percentage, germination time, seedling abnormalities, early mortality, seedling growth, and seedling survivorship. From these, we developed a scarification index (SI) that integrates these measures simultaneously. We hypothesized that seeds from lower elevation sources exposed to higher temperatures would have harder seed coats and would require more intense scarification treatments. To test this hypothesis, we repeatedly exposed seeds to hot water differing in temperature and time until seeds imbibed. Supporting the hypothesis, seeds from lower elevation sources generally required more intense scarification, although we found substantial variation among sources. Koa seeds germinated in about a week following imbibition. Boiling seeds (i.e., maintaining at 100°C) was effective for imbibing seeds but it also substantially reduced germination percentages. Repeated exposure to 90 to 100°C water did not reduce germination percentage but decreased seedling performance and increased early mortality. No seeds remained unimbibed after six attempts of boiling germinated whereas seeds remaining unimbibed after 15 attempts of exposure to 90 to 100°C water showed high germination percentages. Abnormalities in seedling development were rare but increased with treatment intensity. Exposure to 100°C water for 1 min overall generated the best SI values but the best treatment differed by elevation, and the treatment with the best SI was rarely predicted from the highest germination percentages. Seeds that imbibed without any treatment germinated at the same level as manually filed seeds but produced poor seedling quality. Variation in mother tree environments along an elevational gradient can lead to differences in seed coat characteristics, which may explain differing responses to treatments. Scarification treatments affected processes beyond imbibition and germination and using an index like SI may improve efficiency by identifying optimal scarification treatments while reducing seed waste.

Early growth and ecophysiological responses of Koa (Acacia koa A. Gray) seedlings to reduced water and phosphorus

Early growth and ecophysiological responses of Koa (Acacia koa A. Gray) seedlings to reduced water and phosphorus

Understanding Grass Invasion, Fire Severity, and Acacia koa Regeneration for Forest Restoration in Hawaiʻi Volcanoes National Park

With invasive grasses increasing wildfire occurrence worldwide, a better understanding of the relationships between native plants, fire, and invasive grass is needed to help restoration plans facilitate ecosystem resilience. Invasive grasses are particularly problematic for altering fire regimes in the tropics, yet in Hawaiʻi, restoration sites are often planted with monocultures of the native tree Acacia koa, which can promote grass growth via nitrogen fixation. This, combined with the difficulty of estimating pre-fire grass cover under thick canopies, complicates attempts to restore Hawaiian ecosystems. We studied the 2018 Keauhou Ranch Fire in Hawaiʻi to investigate three questions: (1) at what level of precision can pre-fire grass cover be accurately estimated from oblique aerial photos? (2) how are post-fire A. koa regeneration densities affected by fire severity? and (3) how are post-fire A. koa regeneration densities affected by pre-fire grass cover and its interaction with fire severity? We collected burn severity and post-fire regeneration data from 30 transects stratified across mid-elevation woodland, montane woodland, and montane shrubland communities. We evaluated visual estimates of pre-fire grass cover from oblique aerial imagery with quantitative in situ data from 60 unburned transects of the same cover types. Pre-fire estimates of grass cover categories were 67% accurate in montane woodland (n = 9) and 100% accurate in montane shrubland (n = 11), but only 20% accurate in mid-elevation woodland (n = 10). In montane woodlands with low pre-fire tree densities, A. koa regeneration densities were higher with increased fire severity, but this trend reversed when pre-fire tree densities were high. We detected no effect of pre-fire grass cover, nor its interaction with fire severity, on A. koa regeneration density. This indicates that restoration through the planting of A. koa may be successful in promoting fire-resilient A. koa forest, although there are potential issues to consider regarding the effects that A. koa’s grass promotion may have on other species within the ecosystem.

Cold Acclimation Increases Freeze Tolerance in Acacia koa, a Tropical Tree Species Occurring over a Wide Elevational Gradient

Frost damage is among the major limitations to reforestation and forest restoration projects worldwide. Investigations of environmental and genetic effects on frost resistance have focused on boreal and temperate tree species rather than tropical trees. Koa (Acacia koa A. Gray) is a valuable tropical hardwood tree species endemic to the Hawaiian Islands, USA. Koa occurs across a wide elevational gradient, and newly planted trees are subject to winter frost at high elevations. We sought to determine whether different koa populations show variation in freeze hardiness as a cold-tolerance mechanism, and whether exposure to hardening conditions prior to frost exposure can modify koa cold-tolerance adaptation. Seeds from 13 populations of koa (Acacia koa A. Gray) were collected across an elevational range (603–2050 m) on the Island of Hawai’i. Four-month-old seedlings grown from the 13 population seed sources were divided into control (non-acclimated) and cold-acclimated treatments, maintained at 26 °C/22 °C (day/night) or exposed to gradually decreasing temperatures to 8 °C/4 °C (day/night), respectively. After six weeks, control and cold-acclimated seedlings from each population were tested for freeze tolerance by electrolyte leakage at five test temperatures ranging from 5 °C (control) to −20 °C. Treatment effects were mainly observed at the lowest test temperatures (−15 and −20 °C). A higher index of cold damage occurred in the non-acclimated seedlings for most of the populations. Several of our higher elevation populations showed greater cold tolerance than populations from lower elevations, particularly when cold-acclimated. Our results suggest that cold acclimation may increase frost hardiness in a tropical forest tree species, and that there is likely some adaptive variation in frost tolerance among populations from different elevations. Cold acclimation could be a useful tool to prepare koa seedlings to be planted in high-elevation sites prone to freezing winter temperatures.