Final report for GW24-007
Project Information
In Hawaiʻi, endemic sandalwood (Santalum) species (“ʻiliahi” in the Hawaiian language), were over-exploited for their fragrant oils, but landowners are interested in their reforestation and restoration of associated dry forests. Because sandalwood species are root hemiparasites, restoration and silviculture of these mixed-species forests present novel challenges. Field experiments initiated in 2019 reforested abandoned pastures with ʻiliahi alongside native host species. In one experiment, ʻilahi was outplanted simultaneously with either koa (Acacia koa), a fast-growing nitrogen-fixing tree or ʻaʻalili (Dodonaea viscosa), a fast-growing shrub. All three species have important economic and cultural values. In another experiment, ʻiliahi was underplanted in a 10-year old koa plantation. We used a mixed stand management approach to find optimal planting designs and overstory thinning to balance growth and survival of all species. Objectives were: (1) to estimate the optimum ratio of ʻiliahi and host plants to balance long-term survival and growth, and (2) to evaluate the influence of overstory host thinning on growth of ʻiliahi saplings. Plant size and growth were measured along with foliar nutrient concentration. Competition, facilitation, and parasitism were estimated from models using species composition, comparative growth of individual plants within a plot, and overall growth in the plot. Individual and stand growth models were used to project long-term outcomes of planting designs and effects of overstory thinning. Results provided recommendations for planting designs and mid-rotation management. They also were used as the basis of a field day and workshop on ʻiliahi establishment and stand management.
(1) to estimate the optimum ratio of ʻiliahi and host plants in balancing competition, facilitation, and parasitism related to their long-term survival and growth, (2) to evaluate the influence of host thinning on the growth of ʻiliahi at three years post-establishment, and (3) develop a model that accounts for the relative effects of silvicultural strategies on the establishment and growth of ‘iliahi in restored tropical dry forests.
Cooperators
- - Producer
- (Educator and Researcher)
- - Technical Advisor
Research
WSARE 2024 GSRE Final Report Sections
Materials and Methods
Objective 1: Optimize species composition and ratio of ʻiliahi and host plants
The first experiment estimated the optimum ratio of ʻiliahi and host plants to balance competition, facilitation, and parasitism related to long-term survival and growth. We monitored the growth of ʻiliahi in a host suitability experiment in which seedlings of ʻiliahi were planted in 2019 alone or with seedlings of either koa or ʻaʻaliʻi at one of four spacings (0-2 m) to evaluate the suitability of host species and spacing (9 total treatments). Experimental units were planted randomly in 40 rows with 9 experimental units per row. Each row was 3 m apart, and ʻiliahi were planted 6 m apart within a row. In total, there were total 360 seedlings of ʻiliahi, 160 of koa, and 160 of ‘a‘ali‘i planted in this experiment.
Results of this initial experiment were published in Thyroff et al. (2023). Three years after planting, ʻiliahi survival and growth were significantly greater when paired with koa and at a closer distance. Given the size of surviving plants at the end of this experiment, we expected that the root networks of ʻiliahi and host plants extended beyond the distance between adjacent rows and experimental units within a row. There was also mortality of some of the ʻiliahi and host plants and natural regeneration of koa and other woody plants within the site. This provided an opportunity to investigate interactions among plants at the community level, which we assume is becoming more important over time.
We combined the experimental units within and across rows into adjacent 20 x 20-m plots containing four of the original experimental units within a row and seven rows. There was one experimental unit or one row between adjacent plots, resulting in 10 total plots. Plant composition in each plot varied depending on previous treatments, plant mortality, and plant natural regeneration. The layout of the original experiment was mapped over these plots to indicate the position of plants that died in the original experiment and to differentiate between the original experimental plants and natural regeneration (Figure 2). The map of all current plants was used to identify the nearest neighborhood plant and to calculate plant competition.
Ground-line diameter (D) and plant height (H) were measured for all plants in every plot to evaluate the growth of individual plants and the plots as a whole. For koa saplings with a stem diameter > 1 cm at breast height (~1.35 m), diameter at breast height (DBH) was also measured. Additional parameters were also measured for ʻiliahi, such as crown radius (CR), leaf chlorophyll concentration (CHL), leaf nitrogen (N) concentration (%N), and leaf water potential (LWP). Five fully expanded leaves were used to estimate foliar chlorophyll, %N, and pre-dawn leaf water potential. Chlorophyll was estimated using a atLEAF chlorophyll meter (FT Green, Wilmington, DE). Foliar %N was analyzed by the University of Hawaiʻi at Hilo (UH-Hilo) Analytical Laboratory using a high temperature elemental analyzer. Leaf water potential was estimated using a leaf pressure chamber. Soil samples were also collected from every plot at a depth of 0−20 cm to identify the environmental characteristics of the plot. Soils were analyzed for pH, total nitrogen, and soil organic matter, also by the UH-Hilo Analytical Laboratory.
Competition, facilitation and parasitism were estimated using mixed-species growth models as in Forrester et al. (2013). We used spatially explicit competition indices for the individual level due to the irregular spacing in this site (Maleki et al. 2015). Competition was calculated using the basal area and distances of the nearest neighboring plants surrounding each ʻiliahi. Stand-level competition was calculated from the total basal area of all plants in the plot. Greater basal area was assumed to equal greater competition, which reduced growth of all plants. Facilitation and parasitism were calculated using the ratio of the number of host to ʻiliahi plants and the relative growth rate of ʻiliahi compared to host plants. The importance of facilitation was indicated by greater growth of ʻiliahi when the ratio of host to ʻiliahi plants was greater at the same competition level. The importance of parasitism was indicated by greater relative growth of ʻiliahi to host plants at lower host:ʻiliahi plant ratios, since the parasitic resource drain on the hosts was expected to be greater
Hypothesized outcomes of the growth of ʻiliahi and host plants under these different scenarios are illustrated in Figure 3. Of note is the hypothesized lower growth of the host under a 1:1 as opposed to a 2;1 ratio of host:ʻiliahi. Without parasitism, host plants would be expected to grow faster at a lower ratio, since this reduces intraspecific competition (Forrester et al. 2013); however, parasitic resource transfer is likely to be much greater at this ratio. With two host species in this experiment (a shrub and an N-fixing tree), optimizing the species mixture needed to take into account the growth rate of each species, the differing architecture of the plants, and the effect of increased N transfer to ʻilahi from koa as compared to ʻaʻaliʻi.
Classification and regression tree analysis (CART) using a random forest algorithm was used to assess the influence of plant composition, competition, parasitism, facilitation, and soil quality on the growth and leaf physiology of ʻiliahi and host plants. This identified the importance of soil resources on growth and the driving forces for parasitism for both ʻiliahi and host in mixed stands. The relationship between selected factors and the growth of ʻiliahi and host was then evaluated using regression analysis. Linear, power, exponential, polynomial, and threshold models were tested for best fit using significance of fit parameters, including the significance of ANOVA F-test, the adjusted coefficient determination (R2adj), the residual standard error (RSE), and the Akaike information criterion (AIC) (Rozendaal et al. 2020). The Normality of residuals and heteroscedasticity assumptions were also used to evaluate these models (Meng et al. 2021). The validity of each model was assessed using the mean absolute bias (MAB), and the root mean square error (RMSE) generated from the Leave-One-Out Cross-Validation (LOOCV) method (Tetemke et al. 2019). Finally, a scatter diagram was created based on the relationships among host density, competition, facilitation, and parasitism to find an optimum planting mixture to balance the growth of ʻiliahi and host plants. This research provided new information and a deeper understanding of how ʻiliahi and potentially other Santalum species perform in mixed-species stands. It also provided a novel test of the application of mixed species growth modeling to include hemiparasitic trees or other plants. This directly informed recommendations for silviculture and restoration of dry forests with ʻiliahi and provided a foundation for application to other Santalum species.
Objective 2: Overstory Thinning
The second experiment evaluated the response of ʻiliahi saplings to the thinning of overstory koa trees. Our aim was to increase growing space for ʻiliahi (reduce competition), recognizing this may have resulted in the loss of parasitic resource transfer from nearby hosts.
A koa plantation was established in 2010 at KMR in a fenced 0.81 ha area. Trees were planted on approx. a 5.5-m spacing. Survival was 77% after one year, which resulted in heterogeneous canopy cover. In 2020, one-year-old nursery-grown ʻiliahi seedlings were planted between the koa trees on a similar 5.5-m spacing. Survival of ʻiliahi after 3 years was > 95%, and growth was been positively related to canopy openness. Results from this experiment were published in Thyroff et al. (2024).
For the proposed experiment, a randomized design was set up to facilitate a thinning trial with three levels and eight replicates. Thinning levels included a no-thin control, 2-sided release, and 4-sided release (Table 1). A circular area 5.5 m in diameter was measured around select ʻiliahi saplings, with the sapling designated as the center point of the circle. Koa tree stems that existed within the circular area were subject to thinning based on the treatment level (Figure 4). After thinning, the natural regeneration of koa seedlings or sprouts within the thinned areas was removed.
The effectiveness of thinning was evaluated every year. Similar measurements as in Experiment 1 were used, e.g. D, H, CR, CHL, %N, and LWP. Similar measurements were also made for the koa trees remaining in the thinning circle. Data were assessed for homogeneity of variance using the Bartlett test (Beyene 2016). Treatment levels were compared and evaluated using analysis of variance (ANOVA) and followed by Tukey’s multiple range test. The normality of residuals was examined using the Shapiro-Wilk test (Ghasemi and Zahediasl 2012). The nonlinear least-squares (NLS) regression analysis was also applied to identify ʻiliahi response to thinning level. The best-fit model was evaluated based on the significance of the fit parameter, the significance of ANOVA, the coefficient of determination (R2adj), and the Akaike information criterion (AIC) (Guillemot et al. 2015).
Objective 3: Develop models to compare strategies for establishment of ʻiliahi
Finally, we used multivariate path analysis to compare silvicultural strategies for establishing ʻiliahi as part of mixed-species reforestation. Planting time, planting mixture and proportion of nitrogen-fixing host plants were the independent variables. Major plant interactions (competition, facilitation, and parasitism) and ʻiliahi growth were the dependent variables. Plant interactions were also treated as intermediation variables to serve as causal links between silvicultural strategy and ʻiliahi growth. The model was simulated and evaluated using the maximum likelihood method. The validity of variables was reviewed using average variance extraction (AVE), while reliability was evaluated using Cronbach's alpha (CA). The model also had to meet a minimum goodness of fit with the field data. The relative effects of silviculture strategies on the growth of ‘iliahi were verified using a standardized path coefficient (Liang and Wei, 2021). Variables with a significant path coefficient were considered as important strategies.
Objective 1: Parasite-host ratio
Results have shown that facilitation (increased growth of ʻiliahi due to the presence of hosts) and parasitism (reduced growth of host due to the presence of ʻiliahi) are optimized at a host:ʻiliahi ratio of 2:1 (Fig. 1).
Figure 1. Tradeoff in facilitation and parasitism with proportion of host in the species mixture.
Objective 2: Overstory thinning
We implemented thinning of the overstory host in August 2024. One-year results will be taken in August 2025. Results prior to thinning show that facilitation and parasitism are balanced at total basal area of ~24 m2 ha-1 (Fig. 2), which is the key measure of competition. We decided to thin experimental plots to 50% of existing basal area using a thin from below approach (thin the smallest trees first). This will allow us to evaluate the changes in facilitation and parasitism to the modification in competition. All thinned plots now have less than the optimal basal area; thus, we expect parasitism to increase significantly. Because the goal of this project is to restore mixed-species native forests with a root hemiparasite, the effect on long-term growth of remaining overstory trees is as important to monitor as the effect on ʻiliahi growth.
Objective 2: Overstory thinning effects
Because canopy openness in the underplanting study was on average less than optimal for ʻiliahi growth and because the koa stem density was approaching the need for thinning based on average stem diameter, we conducted an additional study on the effects of overystory thinning on canopy openness and growth of koa and ʻiliahi. Results one year after thinning showed an increase in growth of ʻiliahi in thinned plots but a decrease in growth of koa in thinned plots. This suggests that ʻiliahi benefited from the increase in canopy openness (reduction in competition) but remaining koa suffered from the increase in parasitism due likely to both greater ʻiliahi growth as well as a lower proportion of host:ʻiliahi ratio.
This was confirmed by an exponential increase in ʻiliahi growth with canopy openness across all of the plots in the thinning study.
Objective 3: Model comparing silvicultural strategies
Objective 3: Model comparing silvicultural strategies for establishment of ʻiliahi
Outcomes of the path model showed that competition, facilitation, and parasitism significantly affected the growth of ‘iliahi (Figure 4.2). Growth was positively correlated with competition and facilitation but negatively correlated with parasitism. Planting time and planting mixture had significant effects on the growth of ‘iliahi with a standardized path coefficient of 0.73 and 0.62, respectively (Table 4.5); however, the relationship between both variables and ‘iliahi growth showed different patterns. Planting time showed a significant direct effect on ‘iliahi growth, while planting mixture had a significant indirect effect on ‘iliahi growth through the balance of ‘iliahi-host interactions. This suggests that both variables are important silviculture strategies for ‘iliahi restoration at the study sites.
Figure 4.2. Path diagram demonstrating the relative influences of silviculture strategies on the growth of ‘iliahi in Hawaiian-restored tropical dry forests. Description: PT (planting time), PM (planting mixture), FN (proportion of N2-fixing to non-fixing host), CL (competition), FL (facilitation), PL (parasitism), IG (‘iliahi growth). The symbol * indicated a significant standardized path coefficient.
Table 4.5. The relative effect of silviculture strategies on ‘iliahi growth based on the standardized path coefficient. Description: PT (planting time), PM (planting mixture), FN (proportion of N2-fixing to non-fixing host), CL (competition), FL (facilitation), PL (parasitism), IG (‘iliahi growth). The symbol * indicated a significant result at p-value <0.05
|
Variables |
Direct effect |
Indirect effect |
Total effect |
||
|
CL |
FL |
PL |
|||
|
PT |
0.63* |
0.25 |
0.44 |
-0.59 |
0.73* |
|
PM |
0.10 |
0.64* |
0.71* |
-0.83* |
0.62* |
|
FN |
0.01 |
0.10 |
0.10 |
-0.02 |
0.19 |
A comparison of the two sites shows that PAI of diameter growth of ʻiliahi with pre-established hosts (Site 2) was more than twice as great as with simultaneously planted hosts (Site 1), despite the fully open conditions in Site 1 for at least several years after planting (Figure 4.3A). PAI also increased with the proportion of host-to-‘iliahi at the community level in both sites (Figure 4.3B). The balance of facilitation (FL) and parasitism (PL) also occurred at a higher host:‘iliahi ratio in Site 1 (approx. 2:1) than in Site 2 (slightly greater than 1:1) (Figure 4.4).
Figure 4.3. (A) Growth of ʻiliahi at different planting time: PH (pre-establishing host), SP (simultaneously planting); (B) Correlation between planting mixture and ‘iliahi growth.
Figure 4.4. Projection of an optimum host proportion under two different planting times of ʻiliahi and host: (A) Simultaneous planting, (B) Pre-establishing host.
Research outcomes
Based on our results so far, these are our recommendations, according to the objective.
Objective 1: Optimize species composition and ratio of ʻiliahi to host species
Our results suggest an optimal ratio of 2:1 host:ʻiliahi species composition. We calculated major species interactions, including competition, facilitation, and parasitism. Facilitation (improved relative growth of ʻiliahi due to host presence) and parasitism (reduction in growth of host due to ʻiliahi presence) were balanced at this ratio in the host suitability trial in which seedlings of all species were planted at the same time.
Objective 2: Overstory thinning
We implemented overstory thinning in August 2024, so results have not been measured yet. However, based on past results and ongoing measurements of ʻiliahi growth at the stand level, we recommend an overstory thinning of basal area to approximately 25 m2 ha-1. We hypothesize this will reduce competition for light among the overstory trees and between overstory trees and ʻiliahi. It will also balance facilitation and parasitism.
Joint Recommendation:
Based on results from both studies addressing these objectives, we are also making the recommendation to thin ʻiliahi to maintain both host:ʻiliahi ratio and an optimal basal area of the stand. As the ʻiliahi in the understory planting grow larger, their contribution to stand basal area will increase and their parasitic drain on hosts will likewise increase, so they will likely need to be thinning for long-term survival of native host species.
It is too early to recommend a lower threshold for basal area of overstory trees; however, we hypothesize that if overstory basal area is thinned below 20 m2 ha-1, then, at the ʻiliahi spacing in this study (4.5 x 4.5 m), the increased parasitic stress will cause a decline in the growth and vigor of the overstory trees that they will begin to die, resulting in a downward spiral of survival of koa. The negative effect on facilitation will likewise reduce growth of ʻilahi.
Education and Outreach
Participation summary:
We presented the results of the research so far at the 2025 and 2026 Tropical Hardwood Tree Improvement and Regeneration Center annual meetings and the 2024 National Silviculture Workshop.
The research in this project was the basis of a doctoral dissertation. We are currently writing manuscripts for scientific journal articles based on the dissertation.
We held an on-farm workshop for producers, professionals, and researchers to tour the experimental field sites, present and discuss our results, and discuss implications for management and priorities for future research. A set of workshop handouts was provided to all participants.
We have made connections with many forest researchers and managers around the state of Hawaiʻi through the TropHTIRC meetings. Responses to our presentations have been very positive.