Open access publications in the SLU publication database

Abstract

Forest trees host a plethora of microorganisms (bacteria, viruses, fungi) whose roles and diversity are still poorly understood despite the increased scientific interest for the past decades. The thesis focuses on the diversity of endophytic and epiphytic fungi in the aerial tissues of broadleaved trees. It tests a basic hypothesis that the diversity and frequency of endophytic and epiphytic fungi vary depending on the vitality and disease susceptibility of the host. Additionally the thesis explores if the chemical variation in trees may relate to differences in fungal community. Particularly, the aim was to describe how the fungal communities of three broadleaved species relate to the general vitality or specific pathogen resistance of the trees, and if herbivory or fertilization influence the fungi through altered levels of potentially antimicrobial metabolites, condensed tannins. Culture-based and culture-independent (NGS) techniques were used to capture the fungal community in the twigs of pedunculate oak (Quercus robur L), in the leaves of aspen (Populus tremula L), and in the leaves and twigs of European ash (Fraxinus excelsior L). The secondary metabolites were studied with HPLC, LC-MS, and GC-MS analyses.

The results showed that the fungal assemblages are influenced by a complex network of factors related to the status of the host (internal factors: health, chemotype) and its environment (season, site, nitrogen, herbivory). Trees with different vitality or different phenotypic response to pathogen hosted quantitatively and qualitatively diverse fungal communities. Tissue type and seasonal variation were confirmed to be highly selective factors in shaping fungal communities of forest trees. The endophytic communities associated with xylem seemed to be shaped by the tree vitality more readily than the fungi associated with leaf or bark. Condensed tannins, nitrogen fertilization, and herbivory did not explain the structure of fungal communities in aspen leaves. Leaf phenolic metabolites reflected well the general vitality phenotype of the trees, but the relationship between fungi and phenolics may not be straightforward.

The technological advances and the use of different methods to survey fungal communities may help disclosing the unknown fungal biodiversity hosted by forest trees. Further studies on fungal communities are needed to reveal the ecological relevance that fungal assemblages have in the regulation of major ecological cycles. Understanding the mechanisms regulating the establishment of fungal communities may contribute to the possibility of using fungal assemblages in forest practices to help forest coping with sudden changes and be able to provide different ecosystem services.