Conclusion
Our results show that diversity-driven water source partitioning did not overcome the intense drought stress trees can experience in Mediterranean forests. When soil moisture decreases in the summer, oak species could shift their water sources to deeper soil layers. In contrast, the shallower-rooted pines were constrained to more superficial layers, leading to increasing water source partitioning. As a result, oaks mixed with pines could sustain slightly higher gas exchange rates than those in monospecific plots, probably because of the lower competition in deeper layers. On the contrary, pines’ shallow root system and water-saving strategy induced earlier stomatal closure as the drought progressed, resulting in low gas exchange, especially in mixtures with oak. Thus, the combination of species with contrasting stomatal regulation strategies and root systems only benefited the most drought-tolerant species at the expense of drought-sensitive ones, highlighting that diversity could end up in a general decline of less competitive Pinus species (Gea-Izquierdo et al., 2021). While diversity may promote a range of services, promoting diverse forests may not prevent pine decline due to the worsening of droughts in Mediterranean areas. Over the long term, interactions between species could be further disrupted within mixed oak-pine forests, potentially leading to cascading effects on the entire ecosystem, including shifts in species composition and reduced biodiversity. Continued research is necessary to inform evidence-based management practices to foster the adaptability of those climate change hotspots.