Linking fungal communities to wood density loss after 12 years of log decay.

Changes in biodiversity might alter decomposition processes and, consequently, carbon and nutrient cycling. We examined fungal diversity and density loss in experimental Norway spruce logs after 12 years of decay in a hemiboreal forest. Between 28 and 50% of the original wood biomass remained, depending on the fungal community composition in the log, operational taxonomic unit (OTU) richness had only a minor effect on the log biomass. Although the communities were OTU rich (190-340 OTUs per log), the majority of OTUs were infrequent or rare; wood degradation therefore depended mostly on the most abundant OTUs and their decomposing abilities. The least decayed logs were characterized by continuous dominance of an earlier colonizer and by high within-log community diversity, which was significantly related to sample variables (position in log, density and moisture). In the most decayed logs, the earlier colonizers were generally replaced by white-rot species able to exploit the highly decomposed wood. The communities were relatively spatially uniform within whole logs, independent of the sample variables, whereas among-log diversity was high. Importance of fungal community composition in decomposition processes should be taken into account when studying and modeling carbon dynamics in forest ecosystems.