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.