Resource availability drives microevolutionary patterns of plant defences

Abstract

Abstract The resource availability hypothesis (RAH) was formulated to disentangle patterns of variation among species in plant antiherbivore defences. A novel theoretical framework was recently proposed to expand the RAH among populations within species (RAHintra), but unresolved conceptual issues and considerable gaps surrounding the new hypothesis still need to be addressed. Here we tested the RAHintra predictions in a pine tree species using population‐level correlations among plant traits and climate as a proxy of resource availability. We analysed genetic variation among 10 maritime pine populations in growth, constitutive and induced chemical defences and resistance to herbivory in a glasshouse experiment. We used a clonally replicated collection with family structure and accounted for the neutral genetic differentiation with Single Nucleotide Polymorphism (SNPs) data. Most of our results agree with the predictions of the RAHintra. As postulated, growth showed a positive relation with resource availability at population origin. Constitutive investment in phenolics (but not in terpenes) and antiherbivore resistance was also positively related to resource availability at population origin, as theory predicts. Conversely, we did not detect relationships between resource availability and the inducibility of any of the traits. A trade‐off between constitutive investment and inducibility of chemical defences was observed for phenolics, as expected, but not for terpenes. A positive relationship between growth and constitutive resistance was also found likely due to reduced physiological constraints under suitable conditions in the glasshouse experiment. Our study provides evidence and support for the RAHintra, highlighting the strong influence of resource availability in shaping genetically based patterns of local adaptive variation among populations. We detected clinal patterns in phenolic investment along a resource availability gradient, suggesting that population local adaptation to resource availability has more implications for investment in phenolics than for terpenes. Most patterns emerged only after accounting for the neutral processes in the species, not initially conceived in the RAHintra. Neutral differentiation, as a result of non‐adaptive demographic processes and population isolation, may obscure the influence of resource availability as a driver responsible of local adaptation of populations. A free Plain Language Summary can be found within the Supporting Information of this article.