Nurse species play a key ecological role in the success of forest tree regeneration particularly in semi-arid ecosystems. However, the influence of nurse tree size on both microclimatic conditions and seedling development has been insufficiently explored. This study aimed to evaluate the effect of nurse oak trees dimensions (Quercus brantii Lindl.) on light and soil moisture availability as well as on the morphological, biochemical and ecophysiological characteristics of oak seedlings in oak forests in western Iran. Twenty-four oak trees were selected according to three DBH classes: <20 cm (small class), 20-50 cm (medium class) and ≥ 50 cm (large class). Three oak seedlings were randomly selected beneath the canopy of each tree. Seedling morphological traits, mesophyll conductance, leaf ion leakage, concentration in Ca, P, K, chlorophyll and carotenoids, as well as in several enzymes were measured. Beneath each tree, soil moisture and light availability were also measured. We found that the soil moisture, concentration in photosynthetic pigments and leaf area increased along the three DBH classes. The highest values for transpiration (5.15 mmol H2O m-2), photosynthesis rate (5.50 μmol CO2 m-2 s-1), mesophilic conductivity (0.042 mmol CO2 m-2S-1), total seedling dry weight (1.67 g) and relative leaf moisture (75.58 %) were observed in seedlings under the medium DBH class. The photosynthetically active radiation (PAR) decreased across the DBH classes, and the same trend was recorded in seedlings for leaf temperature, intercellular carbon dioxide concentration, leaf dry weight, calcium and potassium concentrations, proline concentration, malondialdehyde concentration, ion leakage, phenol content and activity of catalase and peroxidase enzymes. Using a principal component analysis (PCA) including all environmental factors and seedling characteristics, we showed that the three DBH classes offered contrasted conditions of microclimatic conditions and growth for the seedlings. We concluded that the microhabitat prevailing under the cover of tree oaks of the medium class was the most favorable to the early development stage of a natural or introduced oak regenerations in these water-limited areas.
