Slope exposure and species identity modulate positive effects of admixture on drought impact in Mediterranean mountain forests.

In the last decades, promoting mixed stands is an ongoing trend in management for the adaptation of forest ecosystems to climate change. However, whether tree diversity might ameliorate extreme drought impacts is still an open question. Positive effects of admixture seem to depend on species identity, environmental conditions and drought severity. Thus, increasing our knowledge on the response of different species to admixture along environmental gradients could be very helpful in the design of adaptation strategies. In this study, we evaluated drought impact and growth recovery in mixed and monospecific stands of two functionally different species (Pinus sylvestris L. and Quercus pyrenaica Willd.) at northern and southern exposures in Sierra de Guadarrama National Park (center of the Iberian peninsula). For this, we quantified radial growth patterns in 180 individuals along twelve different sites (six on each exposure) and evaluated growth responses to extreme droughts in the last 30 years. Our results show lower drought impact in pines growing in mixed than in monospecific stands. Admixture positive effects on pine trees response to drought were greater in sites with northern than southern exposure. On the other hand, oak trees showed better growth recovery after drought in mixed stands, being recovery higher at northern than at southern exposures. Our results confirm that admixture effects on growth response to drought are species-specific and shift with environmental conditions. Admixture may contribute to increase resilience of drought-sensitive pines, but positive effects would be increased with water availability conditions associated to exposure. On the other hand, oak showed greater growth recovery in mixed than in monospecific stands. Overall, admixture can increase the stability of forest productivity in a context of more frequent extreme droughts through species-specific responses, which can be modulated by local environmental conditions.