Ontogenetic consistency in oak defence syndromes

1. Plant species allocate resources to multiple defensive traits simultaneously, which often leads toso-called defence syndromes, i.e. suites of traits that are co-expressed. While reports ofontogenetic variation in plant defences are commonplace, no research to date has tested forontogenetic shifts in defence syndromes. In addition, we still know little about how ecological andevolutionary factors concurrently shape plant defence syndromes via ontogeny.2. We tested for ontogenetic variation in plant defence syndromes by measuring a suite ofdefensive and nutritional traits on saplings and adult trees of 29 oak (Quercus, Fagaceae) speciesdistributed across Europe, North America, and Asia. In addition, we investigated if thesesyndromes exhibited a phylogenetic signal to elucidate their macro-evolutionary nature, whetherthey were associated with levels of herbivore pressure and climatic conditions, and if suchevolutionary and ecological patterns were contingent on ontogeny.3. Our analyses revealed three distinct defence syndromes: the first one including species withhigh defences, the second one including species with high defences and low nutrient levels, andthe third one including species with high nutrients and thinner leaves. Interestingly, these defencesyndromes 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 consistentfor both ontogenetic stages. Finally, with respect to ecological factors, we found no effect ofclimatic conditions on defences for either ontogenetic stage, whereas herbivory levels differedamong defence syndromes in adults but not saplings suggesting an association between herbivorepressure and syndrome type that is contingent on ontogeny.4. Synthesis. Together, these findings indicate that defence syndromes remain remarkablyconsistent across oak ontogenetic stages, are evolutionarily labile, and while they appear unrelatedto 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 shapingmultivariate plant defensive phenotypes.