Experiment 1: ploidy effects on tree growth traits and their response to stress
When averaged across genotypes, diploid and triploid trees differed in several growth traits, but the magnitude of the differences was small (Fig. 1, left panel in each box). Across both stress treatments, diploid trees were 7% taller than triploids. Diploids also had 6% higher SWRs and slightly larger SLAs than triploids. No difference between diploid and triploid trees was found for any growth trait when incorporating genotypic effects in our model (Fig.1, right panel). Genotype significantly influenced most traits, as shown by the log-likelihood ratio test results (Appendix 6). Tree growth traits were affected by drought stress, defoliation or their interaction (Fig. 1). Drought-stressed trees had 5% higher SWR and 8% lower LWR. Similarly, defoliation increased SWR and reduced LWR and SLA. For total tree weight we observed significant drought × defoliation interactions. Undefoliated, watered trees had 25% greater total weight when compared with undefoliated, drought-stressed trees. However, when trees were defoliated, drought-treatment differences disappeared. Interactions between drought and defoliation treatments were also observed for height growth. When not defoliated, drought-stressed trees grew slightly taller than watered trees. These height differences disappeared in trees that were defoliated.
In most cases, diploid and triploid growth responses were similar when experiencing stress (no significant ploidy × stress interaction). The only exception was final height, for which we observed an interaction between ploidy and defoliation when not accounting for genotypic effects (Fig. 1, left panel). However, follow-up analyses using stress treatment ratios revealed no significant difference between diploid and triploid trees.