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Seasonal drought can severely impact leaf photosynthetic capacity. This is particularly important for Mediterranean forests, where precipitation is expected to decrease as a consequence of climate change. Impacts of increased drought on the photosynthetic capacity of the evergreen Quercus ilex were studied for two years in a mature forest submitted to long‐term throughfall exclusion. Gas exchange and chlorophyll fluorescence were measured on two successive leaf cohorts in a control and a dry plot. Exclusion significantly reduced leaf water potential in the dry treatment. In both treatments, light‐saturated net assimilation rate (A_max), stomatal conductance (g_s), maximum carboxylation rate (V_cmax), maximum rate of electron transport (J_max), mesophyll conductance to CO₂ (g_m) and nitrogen investment in photosynthesis decreased markedly with soil water limitation during summer. The relationships between leaf …