Epsilon Open Archive

Abstract

Boreal forests harbour diverse fungal communities with decisive roles in decomposition and plant nutrition. Difficulties in studying soil fungi have limited knowledge about how fungal communities are shaped. The objective of this thesis was to study factors influencing soil fungal communities, aiming for increased understanding of their effect on environmental processes.

Using next generation sequencing, responses of fungal communities to their physical-chemical environment, and responses of ectomycorrhizal (ECM) fungi to logging, were investigated. In a trenching experiment, this technology, combined with measurements of decomposition and vertical nitrogen distribution, enabled evaluation of direct and indirect involvement of ECM fungi in humus decomposition.

Fungal community composition was found to be significantly related to soil fertility, with ascomycetes dominating in less fertile forests, whereas basidiomycetes increased under more fertile conditions. ECM fungi were found to more or less disappear with complete clear-cutting and reestablishment of ECM diversity took several decades. However, a clear positive relationship between the amount of retention trees and ECM fungal species richness and abundance was found. By excluding ECM fungi, nitrogen limitation of saprotrophic fungi was released, increasing litter decomposition rates. However, this effect was overshadowed by an almost complete loss of oxidative enzyme activities in deeper humus layers, associated with removal of ECM fungi by trenching.

Our results indicate ECM fungi to be the principal decomposers of boreal forest humus layers. This, together with the predictability of soil fungal communities, reinforces the importance and ability of integrating rhizosphere microorganisms, in particular ECM fungi, in forest ecosystem models.