Due to climate change, drier summers have been observed over the last ten years in Mediterranean areas. Increasing drought levels may have a different weight in influencing the stomatal versus photosynthetic activity of forests, altering the water-use efficiency (i.e., WUE, the amount of carbon gain per water lost) and, consequently, the global carbon balance. By combining leaf gas exchanges and leaf carbon/oxygen isotope measurements, we tested under Mediterranean conditions a semi-quantitative dual isotope model to track adjustments in stomatal conductance (g_s) and maximum CO₂ assimilation at saturating light (A_max) in response to changes in air and soil water availability. The experiment was established at Allumiere site (Rome, Italy) over the course of two consecutive years. There, we modified the amount of precipitation reaching the soil on water depleted (D) and watered (W) replicate plots (~100 m²) of an Arbutus unedo L. forest using a system of rain gutters and sprinklers, respectively. Changes in soil water availability affected g_s and A_max in parallel. As an application of the model, we found that, in response to reduced air and soil water availability, constant carbon (δ¹³C) and increasing oxygen (δ¹⁸O) isotope values were consistent with a parallel decline of either g_s and A_max. As a result of parallel decline, WUE did not differ or only slightly differed between treatments, in contrast with most of the studies that found a wide-spread increase of WUE in response to enhanced drought. This study highlights the potentiality of the dual isotope model to provide insights of forest ecosystem functioning in Mediterranean environments.