Contrasting Water-Use Strategies of Ailanthus altissima and Crataegus orientalis Under Drought Stress Reveal Restoration Potential in Mediterranean Drylands

Water availability is the primary environmental factor limiting plant establishment in arid and semi-arid ecosystems, and increasing drought-stress due to climate change is making forest restoration efforts in the Mediterranean basin increasingly critical. The Eastern Mediterranean region of Türkiye, particularly Kahramanmaraş Province, is characterized by increasingly severe drought stress resulting from low natural precipitation and rising temperatures under global warming. This study comparatively evaluates the ecophysiological responses of Ailanthus altissima (Mill.) Swingle and Crataegus orientalis Pall. ex M. Bieb., under adequate irrigation and drought-stress conditions. To determine plant water status, pre-dawn and midday leaf water potentials were measured, and leaf gas exchange was characterized by measuring net photosynthesis (A), stomatal conductance (gs), and transpiration rate (E); water-use efficiency (WUE) was then derived from these parameters as the ratio of net photosynthesis to transpiration (A/E). Both species sustained positive net photosynthesis under drought, although at rates significantly below those of the well-watered controls in the driest months, and both attained higher WUE than the controls in most months, with marked seasonal variation. However, significant differences in physiological adaptation strategies emerged between the species. Ailanthus altissima exhibited a water-saving strategy, restricting stomatal conductance to maintain a stable leaf water status, whereas Crataegus orientalis adopted a water-spending strategy, sustaining high leaf gas exchange while tolerating pronounced tissue dehydration. Modeling results, supported by spatial drought stress indices, have shown that both species have high adaptation potential in arid and semi-arid rehabilitation areas. The findings reveal that ecophysiological characteristics play a critical role in the selection of drought-tolerant woody species and provide a scientific basis for sustainable forest restoration under climate change.