Magnitudes and Controls of Stem Methane Emissions in Amazonian Peatland Forests

Abstract:

Ecosystem CH₄ budgets of Amazonian wetlands are poorly constrained, ranging from 1.7 to 21 Tg CH₄ y^-1, due in large part to a lack of field measurements of CH₄ sources. Recent studies have shown that tree stem CH₄ emissions can contribute major portions of ecosystem CH₄ budgets, particularly in wet tropical forests. Trees may be significant sources of CH₄ in the forested peatlands of the Amazon, particularly from palms whose stem tissue has high porosity. The Pastaza-Marañon foreland basin (PMFB) of the Western Peruvian Amazon contains over 35,000 km² of peatlands and robust CH₄ budgets do not exist for this large wetland complex. Over the past three years we have collected stem CH₄ fluxes from peatland forests in the PMFB using novel flexible temporary chambers and portable trace gas spectrometers.

We found that stem CH₄ emission rates are strongly dependent on tree species, with palms emitting at the highest rates (some over 30 mg C-CH₄ m^-2 d^-1). However, woody species were also capable of emitting at high rates and can contribute significant portions of ecosystem stem emissions when present in high density. Presence of termites also increased rates of CH₄ emission in some species. Correlations were often observed between different trace gas fluxes, indicating stem diffusivity may be a common control of fluxes from many species.

Our findings demonstrate that tree stems can emit large quantities of CH₄ in Amazonian peatlands and suggest that stem anatomy and wood density are key factors controlling emissions of CH₄ across species of Amazonian trees. Ongoing analysis compares the greenhouse gas emissions of soils and trees at the study sites to determine the relative importance of stem emissions in Amazonian peatland budgets and connections between these two sources. If stem anatomy proves to be the primary factor affecting PMFB CH₄ emissions, remote surveys of peatland tree species may be able to provide adequate information to accurately estimate regional CH₄ budgets.