Short‐lived peaks of stem methane emissions from mature black alder (Alnus glutinosa (L.) Gaertn.) – Irrelevant for ecosystem methane budgets?

Tree stems can be a source of the greenhouse gas methane (CH(4)). However, assessments of the global importance of stem CH(4) emissions are complicated by a lack of research and high variability between individual ecosystems. Here, we determined the contribution of emissions from stems of mature black alder (Alnus glutinosa (L.) Gaertn.) to overall CH(4) exchange in two temperate peatlands. We measured emissions from stems and soils using closed chambers in a drained and an undrained alder forest over 2 years. Furthermore, we studied the importance of alder leaves as substrate for methanogenesis in an incubation experiment. Stem CH(4) emissions were short‐lived and occurred only during times of inundation at the undrained site. The drained site did not show stem emissions and the soil acted as a small CH(4) sink. The contribution of stem emissions to the overall CH(4) budget was below 0.3% in both sites. Our results show that mature black alder can be an intermittent source of CH(4) to the atmosphere. However, the low share of stem CH(4) emissions in both investigated stands indicates that this pathway may be of minor relative importance in temperate peatlands, yet strongly depend on the hydrologic regime.