The alien conifer Cupressus arizonica can outcompete native pines in Mediterranean mixed forests under climate change

Exotic species have been introduced in afforestation and reforestation initiatives worldwide. Climate change, including increased aridity and extreme events, can promote the spread of exotic species used in forest plantations while hampering the performance of natives. Evaluating whether climate change may affect the success of biological invasions is key to project dominance shifts in forest ecosystems, yet it requires a comprehensive approach that integrates main demographic rates driving tree population dynamics. Here, we evaluated the performance of co-occurring native pine species (Pinus pinaster, P. nigra and P. sylvestris) and the exotic Cupressus arizonica in mixed forests in Mediterranean mountains by comparing their main demographic rates (regeneration, mortality and growth) and radial growth response to extreme droughts and to climate change scenarios. Overall, the exotic C. arizonica showed less growth dependence to climatic variability, higher growth resilience to drought, lower mortality and higher regeneration capacity than P. sylvestris and P. pinaster. However, P. nigra showed higher regeneration and similar growth response to extreme droughts than C. arizonica. In addition, growth models pointed to better performance of the exotic species under future climate change scenarios than co-occurring natives. Our results suggest that C. arizonica can increase its dominance (relative presence within the forest area), which can enhance its invasive potential and range expansion. Thus, attention is needed to better control the invasive potential of this exotic species in Mediterranean forest ecosystems.