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.