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

Abstract

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.

This work was supported by the Portuguese Foundation for Science and Technology [PTDC/ASP\u2013SIL/28593/2017 to ALNUS project, 2021.05147.BD to A.C. SFRH/BD/133162/2017 and COVID/BD/152459/2022 to I.G.M. IF/00059/2015 and 2020.03356.CEECIND to P.M.R.G. UID/00239/2025 to Forest Research Centre, LA/P/0092/2020 to Associate Laboratory TERRA], UID/00329/2025 to Centre for Ecology, Evolution and Environmental Changes. Molecular, morphological and physiological identification of isolate LI109 was performed within the Project Phytophthora Research Centre Reg. No. CZ.02.1.01/0.0/0.0/15_003/0000453 co-financed by the Czech Ministry for Education, Youth and Sports and the European Regional Development Fund. The authors are grateful to Josef Janou\u0161ek and Marilia Horta Jung (both Mendel University in Brno, Czech Republic) for molecular sequencing and sequence analysis of P. \u00D7multiformis isolate Li109. C.V.B. was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Sk\u0142odowska-Curie grant agreement No 101034288. We thank the following colleagues for the seed collection: Roland Jansson in Fyris (Sweden), Tamara Corcobado in Riederbach (Austria), Bohumil Mand\u00E1k in Otava (Czech Republic), Yann Laurent, Ivan Bernez and Simon Dufour in S\u00E9lune (France), Hassan Ennouni, Abdelouahab Sahli, Mohammed Ater in Oued Laou (Morocco) and Javier Dorado in Jerte (Spain). We thank Marta V\u00E1zquez Rodr\u00EDguez, Ant\u00F3nio Albuquerque and Vasileios Kotinas for their kind help to collect seeds in Furelos (Galicia, Spain), Bertiandos (Portugal), and Amitis (Naxos Island, Greece), respectively; and Agust\u00EDn Merino for support to access field sites in Amitis (Naxos Island, Greece) and Arno (San Rossore, Italy). Seed collection in Amitis (Naxos Island, Greece), and Arno (San Rossore, Italy), was conducted in the framework of the project Land Degradation and Rehabilitation in Mediterranean Environments (LANDCARE), which belongs to the ERASMUS +\u2009Program (2015\u20131-ES01-KA203\u2013016214).