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Mediterranean forest ecosystems in Croatia are of very high significance because of the ecological functions they provide. This region is highly sensitive to abiotic stresses such as air pollution, high sunlight, and high temperatures alongside dry periods; therefore, it is important to monitor the state of these forest ecosystems and how they respond to these stresses. This study was conducted on trees in situ and focused on the four most important forest species in the Mediterranean region in Croatia: pubescent oak (Quercus pubescens Willd.), holm oak (Quercus ilex L.), Aleppo pine (Pinus halepensis Mill.) and black pine (Pinus nigra J. F. Arnold.). Trees were selected and divided into two groups: trees with defoliation of >25% (defoliated) and trees with defoliation of ≤25% (undefoliated). Leaves and needles were collected from selected trees. Differences in chlorophyll content, hydrogen peroxide content, lipid peroxidation and enzyme activity (superoxide dismutase, catalase, ascorbate peroxidase, non-specific peroxidase), and nutrient content between the defoliated and undefoliated trees of the examined species were determined. The results showed that there were significant differences for all species between the defoliated and undefoliated trees for at least one of the examined parameters. A principal component analysis showed that the enzyme ascorbate peroxidase can be an indicator of oxidative stress caused by ozone. By using oxidative stress indicators, it is possible to determine whether the trees are under stress even before visual damage occurs.

Climate change continues to present global challenges for farmers’ livelihoods. Celtis australis (kharik) is one of the versatile multipurpose tree species, that provides quality fodder, particularly during the lean periods and fulfils the needs of rural communities for sustenance. Unfortunately, owing to significant defoliant attacks, tree phenology and forage availability og this species have changed in recent years. Therefore, the present study was conducted during four seasons (spring, summer, rainy, and winter) in three diameter classes (< 10 cm, 10–20 cm, 20–30 cm) under defoliated and undefoliated tree conditions to assess the alteration in phenology, nutritive value and leaf biomass productivity of kharik. The study revealed that under undefoliated trees, the leaf nutrients (dry matter, ether extract, crude fibre, total ash content, acid insoluble ash, acid fibre content and neutral detergent fibre content) were highest during the summer in 20–30 cm diameter class, except for crude protein, nitrogen-free extract, organic matter and total carbohydrate contents. In defoliated trees, the tree nutrients, namely phosphorus (0.28%) and calcium (12.50%) were highest in winter. However, the leaf antinutrients, i.e., tannins and phenolic content were maximum in the spring season in 20–30 cm diameter class under defoliated and undefoliated tree conditions. Overall, the defoliation of kharik resulted in changed phenology, a marked decline in the crude protein, neutral detergent fibre, organic matter content, and leaf and branch biomass productivity vis-à-vis undefoliated ones. Simultaneously, defoliated trees retain their leaves until winter thus providing fodder availability during scarcity season. However, continued insect attacks may have a negative impact on kharik’s health; thus, a timely effective pest management strategy is required to be devised.