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

Abstract

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.