As part of his MS studies at Purdue University (completed at end of 2022), Tawn Speetjens evaluated fertilizer and hosting treatments required to grow healthy ‘iliahi seedlings in the nursery and what types of hosting schemes are necessary to maximize ‘iliahi seedling survival when planting in in barren monotypic pasture landscapes. Tawn has been working to understand and develop the systems required to grow ‘iliahi in Hawai‘i during his past four years working for the Hāloa ‘Āina reforestation project. He has experimented with various strategies to grow healthy ‘iliahi seedlings and has unearthed several findings that have helped to direct his research while at Purdue University.
Santalum paniculatum has been shown to regenerate vigorously with root suckers and coppicing following a harvest disturbance, although methods of artificial regeneration are not very well established or corroborated with empirical studies. Effective methods of propagation must be established in order to reliably grow this species for restoration, commercial production and also for future research endeavors. Identifying effective propagation techniques for S. paniculatum could also guide propagations efforts for other Hawaiian Santalum species that are more scarce or federally listed. The goal of Tawn’s project was to quantify the effects of the practices that have produced healthy S. paniculatum seedlings in the nursery and in field establishment. Tawn has leaned on his several years of experience growing S. paniculatum for Hāloa ‘Āina and the substantial literature pertaining to the cultivation of Australian and Indian species of sandalwood to guide his research project.
Phase 1
The first stage of Tawn’s research examined the effects of fertilizers and host treatments that have produced healthy S. paniculatum seedlings during nursery culture. His pilot studies have established that S. paniculatum seedlings can be grown without a host if the seedlings are treated with controlled release fertilizer (CRF) and chelated iron. Additionally, he had found that S. paniculatum seedlings attached to a nitrogen fixing A. koa pot host, display healthy growth under limited fertilizer regimes. The objective of this experiment is to quantify the independent and interacting effects of CRF, chelated iron and various pot host species on the seedling quality of S. paniculatum.
The experiment design was a randomized complete block with four repetitions and a full factorial combination of CRF (2 levels, applied (+) vs control (−), chelated iron (2 levels, applied (+) vs control (−) and host species (3 levels, A. koa vs Dodonaea viscosa vs control) (Table 1) as the predictor variables, and shoot height, root collar diameter, dry mass, shoot root ratio, chlorophyll index, foliar nutrients concentrations, and haustoria abundance as response variables. Twelve unique treatments were created by combining the three predictor variables (Table 1). Each of the twelve treatments are represented within each repetition by 32 sub-samples within the same tray, 12 of which were destructively sampled at the end of the growing period, while the remaining 20 were left for use in the field planting trial. Seedlings were destructively sampled beginning on June 14, 2021 when the seedlings were 312-day-old (Figure 2). The seedling chlorophyll index, shoot height, and root collar diameter were measured at the time of destructive sampling. Soil was washed from roots and the seedlings were dried in a tissue drying oven at 70°C for 72 the mass of the seedling stems, leaves, and roots were measured (Figure 3). Haustoria present on the roots of each seedling were counted and leaf tissue samples were collected for analysis of foliar iron (Fe) and nitrogen (N) concentration.
The results showed that increased nutrient availability (CRF) led to greater dry mass, root collar diameter, shoot height, chlorophyll index, and foliar nutrient (Fe and N) concentration, confirming that the hemiparasitic S. paniculatum can be successfully grown in early stages of cultivation by providing adequate mineral fertilizers (Figure 4). There was a significant interaction between nutrient availability and chelated iron treatments associated with increases in height, root collar diameter, dry mass, chlorophyll index, and Fe concentrations and contents when chelated Fe was applied (vs not) in a nutrient-limiting environment (Figure 5). The pot host treatment did not affect any growth metrics, but it did affect the total count of haustoria, with A. koa-hosted seedlings developing 60.3% more haustoria individuals than D. viscosa-hosted and control seedlings. Our results demonstrate that high-quality S. paniculatum seedlings can be grown in containers by providing adequate nutrition and that S. paniculatum in a nutrient-limiting growing environment may benefit from chelated iron fertilizers.
Phase 2
The second stage of Tawn’s research was a field planting trial that examined the effects of nursery culture practices and site preparation practices on S. paniculatum seedling performance following field planting. The specific objective was to quantify the effects of nursery fertilization, an A. koa pot host, and an established A. koa field host on the survival and growth of field-planted S. paniculatum seedlings. The field trial used a complete randomized block design with a split plot and four blocks. A full factorial combination of nursery fertilizer (2 levels, applied vs control), pot host (2 levels, applied vs control), and field host (2 levels, applied vs. control) created eight unique treatments (Table 2). The whole-plot factor was pre-established field host (FH+ vs FH−) and the within plot factors were nursery fertilization (NF+ vs NF-), and pot host (PH+ vs PH−). The treatments selected to represent the fertilized seedlings that received CRF and chelated iron during the nursery trial, and the treatments representing the host factor were paired with an A. koa pot host during the nursery trial (Figure 4). For each block, ten seedlings from each of the four treatments from the nursery trial were planted in a sub-plot with pre-established field host and ten were planted in the sub-plot with no field hosts. The sub-plots were 24 m x 20 m with seven rows spaced 2 m apart and each row had ten planting sites. Planting sites were spaced 2 m apart within the row. The sub-plots were 3 m apart within the block and 4 m from sub-plots in neighboring blocks. Exterior rows (#1 & #7) and exterior planting positions (#1 & #10) on interior rows served as a buffer (Figure 7 & 8).
The field hosts were 15 months old when the S. paniculatum seedlings were planted on August 9th – 10th, 2021. Survival and growth were assessed approximately one year later on August 16th – 17th, 2022. Weed control was done four months after planting and every other month as needed to reduce competition from surrounding Kikuyu grass. After the one-year grow-out period, living seedlings were counted to determine mean survival per treatment, and baseline and final morphology (height (cm) and groundline diameter (mm)) measurements were collected to determined growth.Leaf samples were collected and analyzed for foliar N concentration and predawn water potential was measured to assess drought stress.
The results showed that nursery fertilization had the greatest effect on field planting S. paniculatum and enhanced survival, height, height growth, root collar, and root collar growth. The survival rate of unfertilized seedlings was 43.3% (± 5.9) compared to 86.9% (± 4.2) for fertilized seedlings (Figure 9). The pot host improved height, height growth, ground line diameter (GLD), and GLD growth of seedlings, but it did not affect survival. The field host significantly improved survival from 52.7% (± 7.8) to 78.0% (± 5.6) and increased height, but not the GLD (Figure 9). 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 those planted without a field host. These results demonstrated that supplemental nursery nutrition, pot hosts, and intermediate A. koa field hosts benefited S. paniculatum regeneration establishment differently (Figure 10).
