Tradeoffs in access to light and root networks of established host trees for restoration planting of the root hemiparasite Santalum paniculatum

Restoration of root hemiparasite trees, such as Hawaiʻi's endemic Santalum species (ʻiliahi), may benefit from underplanting in stands of suitable hosts like the nitrogen‐fixing native tree, Acacia koa (koa). At a pasture site on Hawaiʻi Island previously reforested with koa, we underplanted seedlings of the island‐endemic sandalwood species, Santalum paniculatum, to examine the tradeoff between access to an established root network (distance to the nearest koa tree) under variable overstory shading (8.8–90.1% canopy openness range) during regeneration establishment. We hypothesized that there is an optimal parasite–host spacing and canopy openness that balance parasitic resource transfer with light availability. ʻIliahi seedling survival was 96% with no survival treatment differences. ‘Iliahi seedling growth was positively related to canopy openness but negatively related to the distance to the nearest koa tree, and the slope of these relationships increased over time. Leaf photosynthetic light compensation points, light saturation points, and stomatal density mostly followed similar trends as growth. These results demonstrate that ‘iliahi can be successfully underplanted in an established koa stand, which benefits ‘iliahi plantings and contributes to diversifying initial restoration and reforestation plantings. There appears to be a significant tradeoff in planting distance between benefits from and competition with the host; however, the improvement in growth with increased canopy openness appeared to be much greater than the effect of planting distance. Underplanting into an established host stand with sufficient canopy openness can help restore functionally compatible and abundant ‘iliahi regeneration into forests.