Tree stem methane emissions: Global patterns and controlling factors

Growing evidence suggests that methane (CH4) emissions from tree stems are a potentially significant atmospheric source. However, a comprehensive global assessment of the general pattern and the controlling factors of stem CH4 emissions is still unavailable. Here, we present a global data-synthesis of 1081 observations from 62 studies to explore the patterns of stem CH4 emission rate in upland and wetland ecosystems and their controlling factors. The global average stem CH4 emission rate in uplands was 2.04 ± 0.61 nmol m − 2 s − 1, which was significantly lower than in wetlands (87.08 ± 19.49 nmol m − 2 s − 1). Angiosperm stem CH4 emission rates were significantly higher than gymnosperm in both upland and wetland ecosystems. Meanwhile, stem CH4 emission rate in inland wetlands (101.78 ± 27.44 nmol m − 2 s − 1) was significantly higher than in coastal wetlands (56.21 ± 19.23 nmol m − 2 s − 1). In both wetland and upland ecosystems, stem CH4 emission rate decreased with increasing sampling height, but increased as soil CH4 emission rate increased. However, the relationship between stem CH4 emissions and soil CH4 fluxes was much weaker for uplands, but very strong for wetlands. In wetlands, stem CH4 emission rate was also regulated by plant properties, as it was negatively correlated with wood specific density, but positively correlated with stem lenticel density. In uplands, tree diameter negatively affected stem CH4 emission rate. Overall, our results shed light on the magnitude and controlling factors of stem CH4 emissions in upland and wetland ecosystems and could facilitate their upscaling to ecosystems and identify the role of stem CH4 emissions in global CH4 budgets.