Impacts of invasive plants on soil fungi and on above- and belowground plant diversity in temperate forests

Urbanization is considered as a major driver for biotic homogenization and also promotes the dispersal of non-native species. In the last decades, non-native plant species have increasingly invaded Central European forests. The spread of alien plants is generally assumed to be one of the most important threats for native biodiversity and this in turn could affect forest ecosystem functions and services. Understanding how invasive species affect native biodiversity, both above- and belowground, and their impact on ecosystem functioning is of central importance in conservation biology. Within the scope of this thesis, four studies were conducted to examine the introduction pathways of non-native plant species into natural forest habitats and to better understand potential effects of non-native plants on ecosystems by changing plant, soil bacterial and fungal communities.
The aim of the first study was to examine the roles of suburban settlements and of the surrounding landscape composition for the spread of non-native plants into adjacent mixed deciduous forests in Southern and Northwestern Switzerland. For this purpose, I conducted field surveys and recorded the number and abundance of native and non-native species in forest sites situated adjacent to settlements and in control sites far from settlements. In both study regions, non-native species were found in higher number and larger abundance in forests adjacent to settlements than in forests far away from settlements. These findings highlight the role of settlements as an effective source for the spread of non-native plant species into suburban forests. In addition, the occurrence of non-native plants was positively affected by the proximity of roads and by the percentage cover of gardens around the study sites, showing that the composition of the surrounding landscape matrix also affects the establishment of non-native plants.
Apart from understanding the pathways of the introduction of non-native species into natural habitats, assessing and understanding their impact on biodiversity and ecosystem functioning is of crucial importance. Several studies investigated the impacts of non-native, invasive species on aboveground biodiversity, whereas belowground diversity and its role for ecosystem functioning are much less studied in this respect. Therefore, the aim of the second study was to assess the potential impacts of the annual invasive plant species Impatiens glandulifera on soil fungal and bacterial communities in forests of Northwestern Switzerland. To do this, I collected soil samples in coniferous and deciduous forest areas invaded by I. glandulifera and in forest areas, which were not yet invaded. The diversity and composition of the soil fungal community was assessed using terminal restriction fragment length polymorphism (T-RFLP) analysis. Biolog Ecoplates were used to assess the activity of soil bacterial communities. Forests invaded by I. glandulifera were characterized by a higher diversity and an altered composition of the soil fungal community, and by an overall lower soil bacterial activity in late spring. These can be indirect effects of altered soil properties induced by the presence of the invasive plant, combined with the release of allelopathic compounds into the soil.
Soil fungi fulfil a variety of ecosystem functions. Among them, mycorrhizal fungi develop mutualistic partnerships with the roots of the majority of plant species and play a crucial role for soil nutrient and water uptake by plants, the diversity of the plant communities and forest ecosystem functioning. The third study aimed to investigate the effects of I. glandulifera on hyphal growth of ectomycorrhizal fungi, their genetic diversity and the diversity of other soil fungi in deciduous forests. Pairs of transect lines were established at the edge of I. glandulifera patches. Using ingrowth mesh bags, I assessed hyphal length along the transects, and applied the T-RFLP analysis to examine fungal genetic diversity. I. glandulifera drastically reduced hyphal growth and affected the composition of the soil fungal community, but did not alter their diversity. This may negatively affect nutrient cycling and soil stability.
Disregarding the fact that the majority of ecosystems have more than 50% of plant biomass belowground, most studies investigating the effects of invasive species on plant diversity focused only on the aboveground vegetation. The aim of the fourth study was to assess the potential impact of invasive plants on belowground plant species richness in deciduous forests. This has not yet been examined in any study. I established plots in forests invaded by I. glandulifera and in forests, which were not yet invaded. In each plot, I conducted vegetation surveys to assess aboveground plant diversity. Belowground species richness was determined by collecting root samples and by applying the fluorescent amplified fragment length polymorphism (FAFLP) technique for two regions of the chloroplast DNA. Plant species richness was reduced above- but not belowground in invaded forests, whereas plant species composition differed between invaded and uninvaded forests both above- and belowground. Root biomass was strongly reduced in forests invaded by I. glandulifera, and this may negatively affect their soil stability and productivity. These results show that the impact of invasive plants may differ between above- and belowground plant communities.
The findings of this thesis highlight the role of settlements for the spread of non-native plant species into adjacent forests. By focusing on belowground diversity, my studies show that annual invasive plants have the potential to disturb belowground fungal, bacterial and plant communities. As a consequence, ecosystem services and functions of invaded forest habitats, such as nutrient cycling, productivity and soil stability, may also be influenced. Removal of existing I. glandulifera populations and preventing further spread of this invasive plant may be a worthy investment for the conservation of native species diversity and the functioning of forest ecosystems.