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

## TL;DR

This study compares how two tree species from Hyrcanian forests respond to drought, finding that Quercus atropatena is more drought-tolerant than Pyrus boisseriana.

## Contribution

The study provides new insights into the drought tolerance of two endemic Hyrcanian forest species during the seedling stage.

## Key findings

- Quercus atropatena showed greater drought tolerance with smaller declines in photosynthesis and growth compared to Pyrus boisseriana.
- Drought caused significant reductions in photosynthesis, transpiration, and carotenoids in both species.
- Quercus atropatena had higher proline levels under drought, indicating better stress adaptation.

## Abstract

The diverse Hyrcanian relic forests in the Caspian region are increasingly threatened by more frequent and severe climate change-related droughts. This study investigated morphophysiological 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, representing the 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 CO2 concentration (Ci) (-54.8, -8.3%), mesophyll conductance (gm) (-58.5, -49.5%) and RWC (only Q. atropatena: -22%), 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, Ci, gm, 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 involved in afforestation and reforestation initiatives aimed at cultivating these two species. Since tree species selection is essential to maintaining forest sustainability, our research adds to the ongoing discussion about the drought tolerance of tree species during the critical seedling stage, particularly in the context of climate change.