Methane Concentration in the Heartwood of Living Trees and Estimated Methane Emission on Stems in Upland Forests

Abstract

The stems of living trees in upland forests might contribute to the global methane (CH4) source, but the contribution is poorly understood. We investigated CH4 concentration in the heartwood of living trees in dominant upland forests using field campaign and subsequently evaluated the importance of stem CH4 emission in the context of the global total. We found that only 0%, 9.8%, and 1.8% of stems of living trees had substantial CH4 concentration in heartwood of ≥ 10,000 μL L–1 in the boreal, temperate, and tropical and subtropical upland forests investigated, respectively. CH4 concentration in heartwood depended mainly upon tree species and subsequently soil moisture. Relationships fitted indicate that CH4 concentration followed a power function with respect to water content in heartwood, whereas nitrous oxide (N2O) or carbon dioxide (CO2) concentration linearly increased with water content in heartwood. Stem CH4 emission was estimated at approximately 0.2–2 Tg yr–1 globally, corresponding to less than 0.4% of the global total including all natural plus anthropogenic sources. Water content in heartwood is positively associated with soil water content. Soil water content rarely exceeds 40% v/v in upland forests, indicating stem CH4 emission occurs mainly in the areas of low-lying upland forests with occasionally moist soils. More attentions should be paid on low-lying upland forests and forested wetlands in future when stem CH4 emission needs to be estimated in all forests.