Ontogenetic consistency in oak defence syndromes

Plant species allocate resources to multiple defensive traits simultaneously, often leading to so-called defence syndromes (i.e. suites of traits that are co-expressed across several species). While reports of ontogenetic variation in plant defences are commonplace, no study to date has tested for ontogenetic shifts in defence syndromes, and we know little about the ecological and evolutionary drivers of variation in plant defence syndromes across ontogeny. 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 the nature of their macro-evolutionary variation, whether they were associated with levels of herbivore pressure and climatic conditions, and if any such evolutionary and ecological patterns were contingent on ontogeny. Our analyses revealed three distinct oak defence syndromes: the first included species with high defences, the second species with high defences and low nutrient levels, and the third 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 across ontogenetic stages. Finally, with respect to ecological factors, we found no effect of climatic conditions on defences for either ontogenetic stage, whereas defence syndromes were associated with differing levels of herbivory in adults but not saplings suggesting an association between herbivore pressure and syndrome type that is contingent on ontogeny. 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 do appear to be associated with herbivory levels in an ontogenetic-dependent manner. Overall, this study builds towards a better understanding of ecological and evolutionary factors underlying multivariate plant defensive phenotypes.