Ontogenetic consistency in oak defence syndromes

1. Plant species allocate resources to multiple defensive traits simultaneously, which often leads to so-called defence syndromes, i.e. suites of traits that are co-expressed. While reports of ontogenetic variation in plant defences are commonplace, no research to date has tested for ontogenetic shifts in defence syndromes. In addition, we still know little about how ecological and evolutionary factors concurrently shape plant defence syndromes via ontogeny. 2. We tested for ontogenetic variation in plant defence syndromes by measuring a suite of defensive and nutritional traits on saplings and adult trees of 29 oak (Quercus, Fagaceae) species distributed across Europe, North America, and Asia. In addition, we investigated if these syndromes exhibited a phylogenetic signal to elucidate their macro-evolutionary nature, whether they were associated with levels of herbivore pressure and climatic conditions, and if such evolutionary and ecological patterns were contingent on ontogeny. 3. Our analyses revealed three distinct defence syndromes: the first one including species with high defences, the second one including species with high defences and low nutrient levels, and the third one including species with high nutrients and thinner leaves. Interestingly, these defence syndromes remained virtually unchanged across the two ontogenetic stages sampled. In addition, our analyses indicated no evidence for a phylogenetic signal in oak syndromes, a result consistent for both ontogenetic stages. Finally, with respect to ecological factors, we found no effect of climatic conditions on defences for either ontogenetic stage, whereas herbivory levels differed among defence syndromes in adults but not saplings suggesting an association between herbivore pressure and syndrome type that is contingent on ontogeny. 4. Synthesis. Together, these findings indicate that defence syndromes remain remarkably consistent across oak ontogenetic stages, are evolutionarily labile, and while they appear unrelated to climate they are associated with herbivory levels in an ontogenetic-dependent manner. Overall, this study builds towards a better understanding of ecological and evolutionary factors shaping multivariate plant defensive phenotypes.