REGENERATION OF ʻILIAHI (SANTALUM PANICULATUM): HEMIPARASITIC RELATIONSHIPS IN HAWAIIAN TROPICAL DRY FORESTS

Abstract

Santalum (sandalwood) species, including the endemic Hawaiian species known as ʻiliahi, are ecologically distinct as root hemiparasitic, evergreen trees requiring suitable hosts for long-term resource acquisition. Restoration and sustainable forestry of ‘iliahi at degraded sites, including threatened tropical dry forests, therefore, require supporting host plant populations. In this Ph.D. dissertation, I aimed to improve the survival and establishment of planted ‘iliahi, Santalum paniculatum, seedlings by better understanding the hemiparasitic relationship between ‘iliahi and native tropical dry forest host species. Three projects were implemented to address this overarching goal with the intention of integrating results into current stewardship strategies. The first project compared host dynamics for ʻiliahi paired with different species at varying planting distances. In June 2019, 360 one-year-old container ‘iliahi seedlings were planted in a completely randomized experimental design. ‘Iliahi seedlings were randomly assigned one of nine treatments: ‘iliahi with (1) no host (control), (2) koa at <0.2 m, (3) koa at 0.5 m, (4) koa at 1.0 m, (5) koa at 2.0 m, (6) ‘a‘ali‘i at <0.2 m, (7) ‘a‘ali‘i at 0.5 m, (8) ‘a‘ali‘i at 1.0 m, and (9) ‘a‘ali‘i at 2.0 m. After three years, ‘iliahi grew more and had greater foliar nitrogen concentration when paired with koa (Acacia koa) at a close distance than with ʻaʻaliʻi (Dodonaea viscosa) at any distance or the control with no host. In the second project, I assessed potential tradeoffs between competition and parasitism of ʻiliahi planted under an established koa host canopy. In August 2020, 324 nine-month-old container ‘iliahi seedlings were planted under the canopy of ten-year-old koa trees. Canopy openness above each ‘iliahi seedling varied throughout the planting site with a range of 8.8 % to 90.1 %. ‘Iliahi seedling height and diameter were greater with larger canopy openness, and physiological measurements also increased with greater canopy openness. In the third project, I examined the transfer of resources between ʻiliahi and its host. In April 2021, 2-pot and 3-pot experimental units were established with the roots of ‘iliahi and koa split between containers. The 2-pot and 3-pot units allowed the segregation of roots to examine root exudate transfer and haustoria transfer between ‘iliahi and koa. In April 2022, 15N and 13C stable isotope tracers were applied to specific units and species. We detected 15N root exudate transfer and 15N transfer from the host koa to the hemiparasite ‘iliahi. One- and two-way transfer of 13C were detected in the roots; however, we are cautious as to the biological significance of this result. These results contribute to our understanding of hemiparasitic plants and improve efforts to restore native Hawaiian dry forests, form a sustainable ‘iliahi silviculture basis, and contribute to sustaining functionally compatible and abundant forests.