Here, we investigated the physiological and structural leaf responses of seedlings of two evergreen and two deciduous Quercus species, grown in a glasshouse and subjected to contrasted conditions of light (low, medium and high irradiance) and water (continuous watering vs 2-months drought). The impact of drought on photosynthetic rate was strongest in high irradiance, while the impact of shade on photosynthetic rate was strongest with high water supply, contradicting the hypothesis of allocation trade-off. Multivariate causal models were evaluated using d-sep method. The model that best fitted the dataset proposed that the variation in specific leaf area affects photosynthetic rate and leaf nitrogen concentration, and this trait determines stomatal conductance, which also affects photosynthetic rate. Shade conditions seemed to ameliorate, or at least not aggravate, the drought impact on oak seedlings, therefore, the drought response on leaf performance depended on the light environment.
