Does aridity modulate species mixing effects on growth response to drought in pine-oak forests in Mediterranean mountains?

Abstract

Forest management faces important challenges for the adaptation of Mediterranean ecosystems to more frequent and severe extreme droughts. Tree species mixing is seen as a key strategy to enhance long-term forest resilience. However, the evidence of positive effects on tree response to droughts is limited and context dependent. Here, we analyzed whether the effect of species mixing on tree growth response to extreme drought events was modulated by site aridity and drought intensity. For this, we selected mixed and monospecific stands of Quercus pyrenaica Willd. and Pinus sylvestris L. along an environmental gradient in mountains in central Spain and quantified growth stability to extreme droughts using dendrochronological methods. Oaks showed greater growth resilience and recovery in mixed than in monospecific stands, while pines showed greater resistance but lower recovery. The effect of species mixing was not related to site aridity, yet we found a trend toward more positive effects of admixture on pine resistance to drought in more arid sites. Both species showed a strong trade-off between resistance and recovery, but recovery cannot compensate for low resistance in pines pointing to the importance of ameliorating drought impacts. Our results emphasize that diversifying monospecific stands with functionally different species can enhance growth stability in drought prone forests. This study can also help forest managers to prioritize areas where diversifying monospecific stands could generate more positive outcomes.