Invasion biology of forest Phytophthora species in Sweden : pathways, traits, climate and host adaptation

Abstract: New introductions of Phytophthora species pose a threat to forest ecosystems globally. This thesis aims to increase our understanding of the invasion process of forest Phytophthora species, and of the long term consequences of these invasions on host populations. Phytophthora species were obtained from nurseries, rivers, and forests, by isolation and by a newly developed metabarcoding approach to investigate the factors involved in Phytophthora introduction, establishment and spread. In vitro inoculations were performed on progenies of alder trees from invaded and uninvaded sites to study whether alder populations have the potential to adapt to species of the P. alni complex. Five Phytophthora species were widespread in Sweden, namely P. plurivora, P. cambivora, P. cactorum, P. x alni, and P. uniformis and they were considered invasive. The occurrence of three of these invasive Phytophthora species and the alpha diversity of Phytophthora communities were higher in urban settlements than in natural forests, pointing at human activities as pathways during invasion. Both the distribution of single Phytophthora species, and the diversity of Phytophthora communities were associated with climatic factors. The cold sensitive P. x alni was restricted to the southern areas of Sweden with milder winters, whereas the more cold tolerant P. uniformis was found across the studied region. The diversity of communities containing species that develop most of their life cycle in soil was associated with total annual precipitation, whereas the diversity of communities containing species mostly developing their life cycle in water was associated with mean annual temperature. The functional diversity of communities revealed a convergence of traits in areas with low temperature and precipitation, where species able to create survival structures and displaying low cardinal temperatures dominated the communities. Adverse climatic conditions seemed to act as an environmental filtering on 20% of the terrestrial Phytophthora communities, although this effect was only of 3% for aquatic communities. In vitro inoculations on progenies of alders invaded by P. uniformis revealed a lower susceptibility to the pathogen than uninvaded populations, pointing to an effect of natural selection. By contrast, no signs of natural selection were observed in P. x alni invaded populations. The broad sense heritability of resistance against P. uniformis was higher than against P. x alni, suggesting that low genetic variation in resistance might slow natural selection, and therefore adaptation.