Fungi are ubiquitous inhabitants of all forest types and important for the proper functioning and health of forests. Fungi controls decomposition of plant litter and later stage recalcitrant organic matter as well as nutrient recycling via symbiotic mycorrhizal associations. Some fungi are also detrimental tree/plant pathogens but may be combated by other beneficial fungi. Fungi perform a range of ecosystem services, e.g., minimizing leaching of nutrients from below the rooting zone, providing nutrition of trees, regulating ecosystem carbon sequestration, and by producing an extensive mycelial network, they integrate and stabilize soil structure.

The high taxonomic and functional diversity of fungi also provide a buffer for forest ecosystems against climate changes. Furthermore, there are important applied aspects of fungi e.g. as producers of enzymes for biofuel-utilization of forests products, as bioremediation agents for especially PAH and heavy metal contaminated soils, and as harvesting products as edible mushrooms.

In spite of the central importance of forest fungi for a variety of forest services, fungal ecology is still poorly explored, primarily due to the difficulty of studying microscopic organism residing below ground. New molecular methods, such as high throughput sequencing of DNA and RNA, now allow us to rapidly expand our knowledge both on the diversity and functional roles of subterranean and wood-inhabiting fungi.