The contribution of wood‐inhabiting fungi and bacteria to dead wood decomposition varies along a regional climatic gradient

Abstract Dead wood decomposition is a crucial ecological function in forests, influenced by climate and facilitated by microbial communities. While fungi are considered the primary decomposers, bacteria also contribute, interacting with fungi in both facilitative and competitive ways. However, it remains unclear how these interactions respond to climate, and how they influence decomposition. To study this, we placed 243 Norway spruce ( Picea abies L.) logs along a 1200‐km latitudinal gradient in Sweden, and under contrasting shade conditions, creating a gradient in microclimate. We characterized microbial communities and quantified wood density loss after 3 years of field exposure. Regional temperature had a positive effect on wood density loss and bacterial and nitrogen‐fixing bacterial richness, while fungi remained unaffected. Shade level had no effect on decomposition rates, but significantly influenced both fungal and bacterial absolute abundances, with higher abundances in sun‐exposed than shaded logs. Nitrogen‐fixing bacterial community composition and fungal richness correlated with wood density loss. Specifically, decomposition rates increased with fungal richness up to a certain level but then decreased. Our modelling revealed a significant three‐way interaction between fungal richness, nitrogen‐fixing bacterial richness and regional temperature, suggesting that in colder regions, decomposition rates increase with fungal richness when richness of nitrogen‐fixing bacteria is high, while in warmer regions, the same combination decreased decomposition. While underscoring fungi as key wood decomposers, our results suggest that nitrogen‐fixing bacteria play a complementary role in the decomposition process. Whereas antagonistic interactions among microbes may reduce decomposition rates in warm regions, facilitative interactions may increase decomposition rates in cold regions. Climate warming due to climate change and forestry operations might lead to an increase of nitrogen‐fixing bacteria, which, in turn, may enhance antagonistic interactions among fungi. Read the free Plain Language Summary for this article on the Journal blog.