Mangrove forests are celebrated as highly efficient “blue carbon” ecosystems, capable of absorbing and storing atmospheric carbon dioxide (CO2) at remarkable rates. However, these same forests also release methane (CH4), a greenhouse gas far more potent than CO2 over shorter time scales. New research reveals that methane emissions from mangrove tree stems —previously overlooked—significantly reduce the climate benefits of these vital ecosystems.
The Overlooked Role of Tree Stems
For years, scientists have focused on methane release from mangrove soils and water surfaces. The role of tree stems as an emission pathway remained largely unexamined. Recent findings published in Nature Geoscience challenge this narrow view, revealing that stems act as a substantial conduit for methane, offsetting a considerable portion of the carbon mangroves bury in sediments.
How the Study Was Conducted
Researchers from the South China Botanical Garden of the Chinese Academy of Sciences conducted a global assessment of methane emissions from mangrove tree stems. They combined long-term, on-site monitoring from multiple Chinese mangrove forests with existing global data and machine learning models. This approach allowed them to systematically evaluate the drivers of stem emissions and quantify how much these emissions reduce the overall carbon sequestration benefit.
Key Findings and Global Impact
The study revealed that mangrove tree stems are a significant and previously underestimated source of methane. The team found that methane produced by anaerobic microbes in mangrove soils is transported upward through specialized tissues within the trees, called aerenchyma. These tissues facilitate gas exchange, allowing methane to escape directly from the stems.
Field observations and isotopic analysis confirmed that methane flux is highest near the base of the stem and decreases with height. This gradient suggests that the soil is the primary source of methane, with the stems acting as a direct pathway to the atmosphere.
At a global level, the researchers estimate that mangrove tree stems release approximately 730.6 gigagrams (Gg) of methane annually. This offsets roughly 16.9% of the carbon buried in mangrove sediments. When combined with methane emissions from the soil, total methane losses could offset up to 27.5% of the blue carbon sequestered by mangroves.
Why This Matters
This discovery has significant implications for carbon accounting and climate mitigation strategies. Current estimates of blue carbon sequestration often fail to account for stem emissions, leading to an overestimation of mangroves’ net climate benefit. Accurate accounting is crucial for policy decisions and investment in mangrove restoration projects.
The findings also raise questions about the effectiveness of mangrove-based carbon offset programs. If methane emissions are not adequately addressed, these programs may not deliver the promised climate benefits. Further research is needed to refine carbon accounting methods and develop strategies to reduce methane emissions from mangroves.
The study underscores the need for a more holistic approach to blue carbon accounting, recognizing that methane emissions from tree stems can substantially reduce the net climate benefits of mangrove ecosystems.
In conclusion, while mangroves remain valuable ecosystems, their climate mitigation potential is lower than previously thought. Acknowledging the role of methane emissions from tree stems is essential for accurate carbon accounting and effective climate policy
