REGENERATION OF HEMIPARASITIC HAWAIIAN SANDALWOOD (SANTALUM PANICULATUM HOOK. & ARN.): THE ROLE OF SEEDLING NUTRITION AND PLANT HOSTS

Santalum spp., known globally as sandalwood, are highly sought after for their aromatic oil-rich heartwood and have been exploited throughout their range. Six of the 19 Santalum species are endemic to the main Hawaiian Islands, where they are known locally as ʻiliahi. Excessive harvesting led to the extirpation of Hawaiian sandalwood from 90% of its historic range by 1840. There is limited peer-reviewed literature concerning the propagation of Hawaiian sandalwood, and methods developed for other non-Hawaiian species cannot be directly adopted due to differences in sandalwood species physiology, available host species, and growing environment. This, combined with increased interest in growing Hawaiian sandalwood, prompts the need for the development of propagation protocols based on empirical research. The primary knowledge gaps in propagation include best practices for producing high-quality seedlings in the nursery (e.g., fertilizers and hosts) and silvicultural practices for maximizing outplanting survival and growth. The Hawaii Island endemic Santalum paniculatum has the largest remnant population and distribution and high commercial value, making it an ideal species to focus our study on. We conducted two experiments to evaluate the response of S. paniculatum seedlings to propagation methods employed with Australian and Indian sandalwood, although with species of hosts native to Hawaii. The first experiment was a nursery growth trial that evaluated the quality of S. paniculatum seedlings in response to nutrient availability (controlled-release fertilization, control), chelated iron fertilizer (applied, control), and species of pot host (Acacia koa, Dodonaea viscosa, control). The quality of seedlings was determined by measurements of height, root collar diameter, dry mass, root shoot ratio, chlorophyll index, and nutrient status (N & Fe concentration). Nutrient availability had the greatest impact on seedling quality and increased height, root collar diameter, dry mass, chlorophyll index, and nutrient status. Chelated iron fertilizer effectively improved seedling quality (height, collar, dry mass, chlorophyll content, and Fe concentration) in a nutrient-limiting environment, although a nutrient-rich environment diminished its effect with sufficient iron levels. The host species treatment had the least influence on seedling quality and only influenced haustoria formation by causing more haustoria in A. koa-paired compared to D. viscosa-paired and control seedlings. Although the pot host had the lowest effect on seedling quality during nursery propagation, it provided benefits in the field planting phase of the project.

The second experiment of the project assessed the survival and performance of field-planted S. paniculatum in response to (1) nursery fertilization, (2) an A. koa pot host, and (3) an A. koa field host. Nursery fertilization had the greatest effect on performance and enhanced survival, height, height growth, collar, collar growth. The survival rate of unfertilized seedlings was 43.3% (± 5.9) compared to 86.9% (± 4.2) for fertilized seedlings. The pot host improved height, height growth, collar, collar growth of seedlings, but it did not influence survival. The intermediate field host significantly improved survival from 52.7% (± 7.8) to 78.0% (± 5.6) and only affected the height measurements and not the collar. There was a significant interaction between the field host and nursery fertilizer treatment associated with the fertilized seedlings planted with field hosts having lower water potential than the fertilized seedling planted without a field host. Our results demonstrated that supplemental nursery nutrition, pot hosts, and intermediate A. koa field hosts benefited S. paniculatum regeneration establishment in different ways. Furthermore, the effect of the hosting treatments may become more pronounced in the field over time as more haustoria connections are formed. This research project provides essential baseline information that helps to enhance the current methodology and inform future decision-making concerning the propagation of S. paniculatum and other Hawaiian Santalum species.