Potential lasting effect of opportunistic parasitic fungi on coarse wood decomposition

Abstract Deadwood is a crucial component of the global carbon budget, storing a substantial amount of carbon in forests. Understanding factors influencing deadwood turnover is therefore vital for predicting carbon cycling under climate change. While climate is an important driver, biotic factors, including wood and fungal traits, also play significant roles in deadwood decomposition. How parasitic fungi affect deadwood decomposition is likely important for global forest carbon turnover but is poorly understood. As opportunistic parasitic fungi, Armillaria species are capable of degrading wood components and altering wood traits. However, how (much) Armillaria drives deadwood decomposition remains unclear. To address this, we studied black pine ( Pinus nigra ), susceptible to Armillaria infection. We hypothesized that branches from infected stands would exhibit higher decay rates, even at given similar wood density. We collected naturally fallen Pinus nigra branches from infected and uninfected stands, sorted them by wood density and incubated them for 1 to 1.6 years under standardized conditions in a common garden experiment. Mass loss was measured and decomposition dynamic was modelled using a relative wood density approach. Our findings support the hypothesis that branches from infected Pinus nigra stands experience higher decay rates, backed up by molecular evidence. This difference may arise from Armillaria itself being a wood decay agent and from its effects via wood traits. These findings have broader implications for estimating deadwood stocks in forests impacted by climate‐induced fungal pathogens. Read the free Plain Language Summary for this article on the Journal blog.