Individual and combined effects of a simulated heatwave and the emerging Phytophthora ×multiformis on European alder species

The decline of alder forests induced by increasing temperatures and the “ Phytophthora ×alni species complex”, a group of emerging non-native pathogens, represents a major threat to riparian ecosystems across the Northern Hemisphere. The intensification of heatwaves and the pathogen spread and disease expansion to Southern Europe underscores the importance of studying the responses of alder species to these stressors. This study assesses individual and combined effects of a simulated extreme heatwave and infection by the pathogen P. × multiformis on Alnus glutinosa and A. lusitanica to identify more resilient genotypes and thus help to mitigate alder decline. 1760 two-year-old alder saplings from 11 populations and 6 genetic groups from Sweden to Morocco were subjected to individual and combined treatments of heatwave and subsequent pathogen infection. Mortality, stem and leaf necrosis, morphological and water-exchange parameters were periodically assessed. Alnus lusitanica showed higher tolerance to heat stress than A. glutinosa . Responses to the combined stressors differed significantly between populations and genetic groups of both alder species. Plant mortality was highest in the treatment with combined stressors, particularly for the southern populations of A. lusitanica . High genetic diversity and previous exposure to extreme temperatures and Phytophthora may provide alder populations an adaptive advantage when responding to individual or combined stressors. Our results suggest that P. × multiformis infection after heatwave events may aggravate alder decline in riparian ecosystems, particularly in A. lusitanica . Further research on the disease effects in riparian forests of A. lusitanica is essential for the conservation and management of this endemic species. • Combined stress increased tree mortality in both alder species. • Performance of Alnus glutinosa was more affected by heat than that of A. lusitanica. • Variation in susceptibility to P. × multiformis occurred between and within species. • lusitanica was more impacted by combined stress than A. glutinosa. • Alder populations adaptable to combined stress were identified.