Leaf traits and insect herbivory levels in two Mediterranean oaks and their hybrids through contrasting environmental gradients

Insect herbivory has attracted enormous attention from researchers due to its effects on plant fitness. However, there remain questions such as what are the most important leaf traits that determine consumption levels, whether there are latitudinal gradients in herbivore pressure, or whether there are differences in susceptibility between hybrids and their parental species. In this work, we address all these issues in two species of Mediterranean Quercus (Quercus faginea subsp. faginea Lam. and Quercus pyrenaica Wild.) and their hybrids. Over 2 years, we analyzed leaf emergence and 11 leaf traits (biomechanical, chemical and morphological), as well as the levels of herbivory by insects in leaves of the three genetic groups in different locations distributed along a climatic gradient. The hybrids showed intermediate values between both species in leaf emergence, chemical traits and structural reinforcement. By contrast, they were more similar to Q. faginea in leaf size and shape. Despite their intermediate leaf traits, hybrids always showed lower losses by consumption than both parental species, which suggests that they possess inherent higher resistance to herbivores, which cannot be explained by their dissimilarities in leaf traits. Within each genetic group, differences in leaf size were the most important determinant of differences in herbivory losses, which increased with leaf size. In turn, leaf size increased significantly with the increase in mean annual temperatures across the climatic gradient, in parallel with herbivory losses. In conclusion, contrary to our expectations, hybrids maintained lower levels of herbivory than their parent species. Given the potential negative effect of leaf herbivory on carbon fixation, this advantage of the hybrids would imply a threat to the persistence of both pure species. More research is needed to elucidate possible alternative mechanisms that allow for maintaining species integrity in the absence of reproductive barriers.