Domestication provides the key to conservation ofSantalum yasi– a threatened Pacific sandalwood

Abstract

Santalum yasi is a high-value hemiparasitic tree endemic to Fiji, Niue and Tonga. It has been overexploited for its oil-yielding heartwood and is now threatened. Remaining stands lack genetic diversity and are likely to be suffering from inbreeding depression, although the species still has significant genetic diversity overall. We argue that the best way to conserve this species is through an active domestication program that will adequately sample and conserve the genetic base in ex situ and circa situm plantings. The approach to S. yasi tree-breeding can be characterised as a low-input strategy involving the early use of molecular markers for population parameter determination. Longterm success will have strong interdependent links with the conservation of the remaining genetic resources. A strategy based on recurrent selection and breeding for key traits-including heartwood volume and oil yield per year, oil quality and environmental adaptability related to cyclone resistance and the tolerance of pests and diseases-is recommended. The establishment of genetic conservation stands based on collections of the species throughout its natural range in Fiji and Tonga has commenced. Challenges associated with the conservation and domestication of S. yasi are discussed. These include the advanced age required before oil characterisation can be undertaken; the need to assess genotype-host-plant interactions; and the need for comparatively sophisticated equipment and destructive harvesting to carry out oil assessments. Capacity development of professional staff in the Pacific Islands is an additional prerequisite for implementing an effective strategy. Research into the variation and heritability of heartwood formation and oil characteristics, and a better understanding of the breeding biology of S. yasi and geneflow between it and exotic Indian sandalwood (S. album), are high priorities. It will be more than a decade-probably around 20 years-before S. yasi individuals in planned, well-designed trial plantings have sufficient heartwood development to enable oil-trait assessment. Establishment of such trials is an immediate priority. In addition to this long-term activity, we recommend a simple interim strategy that promotes high genetic diversity of seedling-based planting stock. This can be implemented using a combination of gene conservation stands, progeny trials that can be culled to seedling seed orchards, and genetically diverse community-based seed stands. The strategy will both provide a safeguard against the further loss of diversity and promote wide outcrossing. Releasing fragmented populations from inbreeding depression is expected to increase general vigour.

Oil yield and composition of young Santalum yasi in Fiji and Tonga

“…Applying Equation 1, these are trees that have a basal DOB between 150 mm and 210 mm. They are likely to be at ages between 12 and 20 years, assuming a growth rate of approximately 10 mm per year, which is the growth rate recorded for a wider sample of 200+ trees, including those studied here, inventoried in Fiji and Tonga (Bush et al 2020). Very similar growth rates have been recorded for other Pacific sandalwood species, including Santalum insulare Bertero ex A.DC.…”

“…The differences in these studies may be attributable to subpopulation genetic variation, differences among the sites, or interaction between these. The establishment of common-garden trials, where tree growth and heartwood development can properly be studied, and which will also serve as conservation stands, is an objective of the breeding and conservation strategy for S. yasi (Bush et al 2020) and also a high priority for other sandalwood species, including S. austrocaledonicum (Page, Doran et al 2020).…”

“…Santalum yasi has been selected for domestication in Fiji and Tonga (Bush et al 2020) because it has the potential to make a significant economic contribution to these countries (Thomson 2020;Thomson et al 2020). The species is reputed to grow more slowly than both S. album and the hybrid S. album × S. yasi, both of which are also commonly planted in Fiji and Tonga, although data that confirm this are lacking.…”

Restoration of Vanuatu sandalwood (Santalum austrocaledonicum) through participatory domestication

“…The species is a root parasite and ideal for planting in areas where other trees serve as hosts, such as home gardens and for enriching agricultural fallows and forest areas. Sandalwood woodlots and enrichment plantings can function as circa situm populations and provide a source of seed for further natural recruitment (Bush et al 2020). The effectiveness of this approach to restoration will depend on the genetic diversity and quality of the germplasm used (Dawson et al 2013;Jalonen et al 2018).…”

Management and Utilization of Plant Genetic Resources for a Sustainable Agriculture

“…Some wild species are threatened by overexploitation by communities living in the periphery of conservation areas. The domestication of such species and propagation through seed gardens with the participation of users has been proposed as an approach for the conservation of genetic diversity of such species [ 140 ]. Studies on the changes in the diversity of landraces on farms have been critically analysed and recommendations for further studies and conservation measures needed have been proposed in a recent review by Khoury et al [ 132 ].…”