Seedling-stage drought responses of two endemic pear and oak species inform climate-adaptive management approaches in Hyrcanian forests

The diverse Hyrcanian relic forests are increasingly threatened by more frequent and severe climate change-related droughts. This study investigated morpho-physiological and biochemical responses to drought in two endemic tree species of the Hyrcanian forests, Pyrus boisseriana Buhse and Quercus atropatena Schwarz. In a 120-day greenhouse experiment, seedlings of each species were exposed to two different irrigation treatments (FC100 and FC40, indicating percentage of field capacity). Under FC100 conditions, all morphological characteristics of Q. atropatena were significantly greater compared to those of P. boisseriana . FC40-exposed P. boisseriana and Q. atropatena seedlings exhibited statistically significant declines in photosynthesis (-55, -49.6%), transpiration (-11.8, -21.7%), intercellular CO ₂ concentration ( C _i ) (-54.8, -8.3%), mesophyll conductance ( g _m ) (-58.5, -49.5%), RWC (only Q. atropatena : -22%), and water use efficiency (-95.3, -57.3%), respectively, and an increase in leaf temperature. Likewise, FC40-treated P. boisseriana and Q. atropatena seedlings displayed an increase in proline (+ 390.8, + 46.5%), and a decline in carotenoids (-19.9, -14.5%), respectively. Drought stress had smaller impacts on radial and height growth, photosynthesis, C _i , g _m , carotenoids, EL and MDA, in Q. atropatena compared to P. boisseriana , indicating greater drought tolerance in the former. These findings imply a prioritization of Q. atropatena in forest management and conservation planning in arid and semi-arid Hyrcanian forests, especially given future climate scenarios. Our results also offer valuable insights for nursery managers facing water scarcity and for stakeholders in afforestation and reforestation projects, thus contributing to the broader discussion on tree species resilience to drought during the critical seedling stage.