Herbivory legacy modifies leaf economic spectrum and drought tolerance in two tree species.

Abstract

The concurring effect of herbivory by wild ungulates and drought events is experiencing a notable increase in Mediterranean and temperate forests. While many studies have addressed the influence of drought on plant susceptibility to herbivory, it appears crucial to comprehend the impact of prolonged browsing on the physiological response of plants to increasing water deficit. To this end, we analyzed the effect of long-term recurrent herbivory by ungulates on physiological, biochemical, anatomical and morphological variables of Ilex aquifolium and Fagus sylvatica saplings during the growing seasons of 2018 and 2019 in a mixed sub-Mediterranean forest. We compared plants growing within an exclosure fence since 2006 (unbrowsed) with plants growing outside (browsed) that were also fenced during the study to investigate herbivory legacy. Twelve years of herbivory pressure modified significantly plant functional performance. Independently of the species, browsed plants showed higher root-to-shoot ratio, stem cross-sectional area-to-leaf area ratio, predawn leaf water potential, leaf nitrogen concentration and leaf gas exchange rates than unbrowsed plants. Moreover, browsed plants had lower leaf bulk modulus of elasticity, and higher osmotic potential at full turgor and turgor loss point. Thus, herbivory modified the leaf economic spectrum towards a more resource-acquisitive and less water stress tolerant type. These results suggest that, once browsing has subsided, plants continue to reflect some legacy effects that make them more vulnerable to further abiotic and biotic stresses, which has implications for forest regeneration.