2.6 Data analysis
For each species, we analysed three performance metrics that have fitness consequences: overall survival, height increase after DRO, and Thiaki respectively), and the inherent growth rate. Analysis was limited to just 17 months given a) partial canopy closure by some trees at the DRO was beginning to lead to neighbour shading b) this allowed us to exclude the impacts of ex-Tropical Cyclone Jasper, which made landfall close to the DRO and Cow Bay sites on the 13th Dec 2023 causing extensive regional flooding.
Probability of survival was assessed using a generalised linear model with a binomial response with a ‘log-link’ family. Survival data (1 = alive, 0 = dead) was the response variable. To identify the impact of provenance and site on growth for each species, we fitted a generalised linear model with a Gaussian distribution. We ran parallel analyses of absolute fitness (growth increment over ~17 months) as well as the inherent growth rate (Stanton & Thiede 2005, Lamontet al. 2023), \(IGR=ln\mathrm{\Delta}H/lnH\), where ∆H is the change in height and H is the initial height. For each model, we looked at the interaction effects of provenance (factor, two levels) and site (factor, three levels), and included covariates for initial height (numeric, continuous) and the underlying spatial autocorrelation as the autologistic term (numeric, continuous) (Dormann 2007).
To determine if species-level wood density could explain the strength and direction of local adaptation, we performed ordinary least squares regression with wood density as the independent variable and local adaptation as the dependent variable. Local adaptation was calculated as the difference between fitness in sympatry and in allopatry averaged across the three sites (Blanquart et al. 2013), with local adaptation present if fitness in sympatry is higher than in allopatry. Fitness of each population at each site was derived from the estimated marginal means of either survival probability, height increment, or inherent growth rate from the analyses described above. All analyses were performed using R Statistical Software (R Core Team 2022).
Two species – Alstonia scholaris and Toona ciliata – were found to have a complex interaction between initial growth and insect herbivory in that once individuals (irrespective of provenance) obtained a certain size they were subject to devastating insect damage from either an unidentified caterpillar or native Cedar tip moth (Hypsipyla robusta ) respectively. This damage caused loss of the apical meristem and a reduction in overall height in these species at sites where they first gained a larger height - as such these species were excluded from growth analyses (but retained in survival analyses).