Season over plant sex: drivers of leaf damage and plant defence in a dioecious Mediterranean shrub

In dioecious plants, females often prioritize reproduction over growth, potentially investing more in defence, while males grow faster but allocate fewer resources to defence, making them more susceptible to herbivory. Recent studies challenge this view, showing that males may grow more slowly and sometimes invest equally or more in defence. Variability in sex-specific herbivory and defence strategies may stem from seasonal shifts in resource allocation, with females prioritizing growth early in the season and reproduction later. These changes complicate herbivory patterns, necessitating research that considers temporally dynamic factors. This study investigated plant sex influence on herbivory and defence mechanisms in Pistacia lentiscus over the course of a year in Doñana National Park. We assessed insect herbivory and leaf traits linked to herbivore resistance, including phenolic compounds and specific leaf area (SLA), in 100 P. lentiscus plants (53 female, 47 male) at two sites during early and late seasons. Herbivory was higher in males than females and increased late in the season. A significant interaction between plant sex and season revealed that males experienced more herbivory late in the season, while there was no significant difference in the early season. Leaf phenolic concentration and SLA were higher early in the season, but these traits were not influenced by plant sex or the interaction between plant sex and season. Moreover, plant sex and season effects on herbivory remained significant even after controlling for leaf phenolics and SLA as covariates, indicating that these traits do not fully explain the observed differences in herbivory across sexes and seasons. Overall, our findings highlight the complex interplay between seasonality and plant sex in shaping herbivory and defence strategies, emphasizing the need to consider temporal dynamics when studying plant-herbivore interactions in dioecious species.