Approximately 19 sandalwood species (Santalum spp.) occur worldwide and at least six sandalwood species and several varieties are endemic to the Hawaiian Islands, where it is known as ‘iliahi. ʻIliahi is economically and culturally important because of the heartwood’s aromatic essential oil and high value wood for carving. During the early 19th Century, Hawaiian sandalwood was heavily exploited until supplies were depleted. Two species are now relatively common, including S. paniculatum, which is the only species commercially harvested on a limited scale; however, the species have not regained their former abundance or size.
There is increasing interest in restoration and commercial management of ʻiliahi. The aim is to demonstrate that ʻiliahi can be harvested sustainably by ensuring adequate regeneration and responsible forestry practices. While the recent burgeoning interest in growing ʻIliahi for restoration and commercial forestry is an exciting development, in order to transition from interest to successful implementation, major steps towards understanding ʻiliahi cultivation systems are required. Effective cultivation of sandalwood has been established in other sandalwood producing countries (India and Australia), but cultivators in Hawai‘i are unable to directly adopt many of these practices due to differences in cultivation area, host species, sandalwood species and logistical limitations. The development of cultivation systems tailored to Hawaiʻi’s unique environment is vital to the success of ʻiliahi restoration and commercial sandalwood forestry operations.
Santalum spp. are particularly complex because they are hemi-parasitic, meaning that individuals can photosynthesize, yet require host plants to acquire resources for long-term survival. Thus, it is necessary to either interplant ʻiliahi alongside other plant species to act as hosts or underplant among already established hosts. In this new project initiative, we are working collaboratively with different partners on several research phases to answer questions that will help to develop effective management recommendations through a deeper understanding of the mechanisms by which ʻiliahi acts as a parasite to acquire resources from host species, and how this interaction varies across site environments.
ʻIliahi-host plant interactions
Several native Hawaiian species have been recommended to serve as host plant species. However, the optimum spacing and layout of ʻiliahi and secondary hosts is unknown. Dr. Kyle Rose led the establishment of a field experiment in June 2019 at Kealakekua Mountain Reserve to assess the establishment success of ʻiliahi in relation to host plant species and planting density. Two host plant species (koa and ‘a‘ali‘i) were planted in June 2019 at four distances from the planted ʻiliahi (adjacent, 0.5, 1, and 2 m). A control was included, where ʻiliahi was planted without a host plant.
Baseline measurements for seedling morphology and leaf nutrient status were performed at the time of planting. Emily Thyroff has maintained the experiment with regular seedling survival and morphology measurements.After three years, survival was greatest only for ‘iliahi paired with koa at 1.0 m (78 %) compared to the control with no host (53 %) and when paired with koa at 2.0 m (55 %). ‘Iliahi grew more and had greater foliar nitrogen concentration when paired with koa (Acacia koa) at a close distance than with aʻaliʻi (Dodonaea viscosa) at any distance or the control with no host. Specifically, ʻiliahi growth was greatest when paired with koa at <0.2 m distance. For sites with few or no pre-established hosts, such as abandoned pastures, ‘iliahi growth can be enhanced by co-planting in proximity to koa. These results demonstrate that establishment and early growth of reforested ‘iliahi are affected by host species and planting distance. Early and abundant root parasitic connections with the host and transfer of nitrogen are likely important for establishment and early growth of ‘iliahi. Results from this phase will aid in the development of prescriptions to create compatible ‘iliahi and host species plantation designs.
Also as part of her PhD studies at the University of Hawai‘i at Mānoa, Emily Thyroff (advised by Drs. Travis Idol and Douglass Jacobs) designed and implemented a set of greenhouse and field experiments to better understand relationships between ‘iliahi and co-occurring native forest tree species:
With guidance from Dr. Simon Landhäusser (University of Alberta), this study is designed to better understand ‘iliahi-host plant interactions in controlling acquisition of water and nutrients. For Santalum species native to other regions, it is hypothesized that water and nitrogen are important resources acquired from host plant species, but it is unclear whether carbohydrates are also exchanged. Thus, this study helped provide fundamental knowledge on the role of water and nutrient transfer between ‘iliahi and hosts. Building upon the proof-of-concept trial from 2020, we established the 3-pot experiment in April 2021 and applied 15N and 13C stable isotope tracers in April 2022. The detection of root exudate transfer of 15N in the media of 2-pot units highlighted the importance of recognizing other mechanisms of 15N transfer than from haustoria connections. One-way transfer of 15N from the host koa to the hemiparasite ‘iliahi was detected in leaves and roots. One- and two-way transfer of 13C were detected in roots; however, we are cautious as to the biological significance of this result.
Koa is commonly planted in single-aged stands across Hawai‘i, often as a first step in reforestation and restoration plantings, but also to serve as a commercial plantation with economic goals. The resulting overstory gaps (whether through naturally variable survival or pre-commercial crop tree thinning) provide an opportunity to diversify initial planting efforts by enriching the site with other native plants, such as ʻiliahi. Furthermore, established overstory and neighboring hosts may increase the probability that planted ʻiliahi will develop early, and abundant, hemiparasitic connections. In August 2020 the first underplanting site was established by planting 324 ‘iliahi under 10-year-old koa. Hemispherical photos were taken at each seedling to capture the overstory environment and grass competition cut back manually every 3 months for the first year.
Height and diameter were greater with larger canopy openness, and physiological measurements also increased with greater canopy openness. Though significant, the relationships between canopy openness and the measurements were weak to moderate. In April 2021 a second underplanting site was established after crop tree thinning a naturally regenerated koa stand. In addition to testing overstory structure effects on seedling performance, gas exchange and stomatal impressions were captured to create light response curves and better elucidate which management practices will yield high performing ‘iliahi.
In collaboration with Drs. Jeremy Pinto and Kyle Rose, we used four nursery stocktype containers (i.e., SideSlit 150, SC10, D25, D40 – see photo) to raise ‘iliahi for outplanting at two different sites (rocky vs. deeper soil site). In September 2021 a subsample of ‘iliahi seedlings were destructively sampled for baseline stocktype information. The remaining ʻiliahi seedlings were outplanted at the fieldsite with the two different soil substrates. Koa seedlings were outplanted in April 2021 before the ʻiliahi seedlings to act as hosts. One year after planting, trends from the nursery were maintained in the field.