Tropical wetlands: A missing link in the global carbon cycle?

Abstract

Tropical wetlands are not included in Earth system models, despite being an important source of methane (CH4) and contributing a large fraction of carbon dioxide (CO2) emissions from land use, land use change, and forestry in the tropics. This review identifies a remarkable lack of data on the carbon balance and gas fluxes from undisturbed tropical wetlands, which limits the ability of global change models to make accurate predictions about future climate. We show that the available data on in situ carbon gas fluxes in undisturbed forested tropical wetlands indicate marked spatial and temporal variability in CO2 and CH4 emissions, with exceptionally large fluxes in Southeast Asia and the Neotropics. By upscaling short-term measurements, we calculate that approximately 90 ± 77 Tg CH4 year−1 and 4540 ± 1480 Tg CO2 year−1 are released from tropical wetlands globally. CH4 fluxes are greater from mineral than organic soils, whereas CO2 fluxes do not differ between soil types. The high CO2 and CH4 emissions are mirrored by high rates of net primary productivity and litter decay. Net ecosystem productivity was estimated to be greater in peat-forming wetlands than on mineral soils, but the available data are insufficient to construct reliable carbon balances or estimate gas fluxes at regional scales. We conclude that there is an urgent need for systematic data on carbon dynamics in tropical wetlands to provide a robust understanding of how they differ from well-studied northern wetlands and allow incorporation of tropical wetlands into global climate change models.

Root oxygen loss from Raphia taedigera palms mediates greenhouse gas emissions in lowland neotropical peatlands

“…This supports hypothesis (i), predicting a reduction of CH 4 emissions due to radial oxygen loss of R. taedigera into the peat matrix. The in situ greenhouse gas fluxes from the undisturbed palms forest are in the range previously reported from peatlands in the area ; Jauhiainen et al 2012;Sjögersten et al 2014;Wright et al 2013b). Land use change strongly increased CH 4 and N 2 O emissions; with CH 4 fluxes being 17-fold higher than pre-disturbance fluxes and N 2 O fluxes doubling after vegetation removal (Fig.…”

“…By contrast, in the unvegetated monoliths, CO 2 fluxes were either close to zero or positive. This suggests that even relatively small palm seedlings can mitigate CO 2 release during peat decomposition, highlighting the critical role of NPP for the C balance of tropical peatlands (Sjögersten et al 2014). In parallel, ex situ CH 4 fluxes were lower during daylight in monoliths with R. taedigera seedlings than in the control monoliths (Fig.…”

“…Tropical peatlands are an important component of the global carbon cycle, playing a central role in global warming by exchanging greenhouse gases with the atmosphere (e.g., CO 2 , CH 4 , and N 2 O) Lähteenoja et al 2009;Page et al 2011;Sjögersten et al 2014). At the global scale, wetlands are estimated to be the most important natural source of CH 4 to the atmosphere (Ciais et al 2013), emitting 0.1-0.23 Gt of CH 4 yr. −1 , equivalent to 17 to 40 % of the global CH 4 emissions (IPCC 2013;Laanbroek 2010).…”

“…Such adaptations included aerial roots, pneumatophores, and aerenchyma designed to transport oxygen to roots growing in anoxic conditions (Cronk and Fennessy 2001). The productivity of tropical peatland forests is variable, but can be high under favourable nutrient conditions (Sjögersten et al 2014;Sjögersten et al 2011). Large areas of Neotropical peatlands support productive palm forests that form extensive mixed and monodominant stands (Lähteenoja et al 2009;Myers 1981;Phillips et al 1997;Roucoux et al 2013).…”

“…oxygen release from roots into the peat matrix), influence greenhouse gas emissions in tropical wetland forests (Sjögersten et al 2014). To address this, we tested the following hypotheses: i) R. taedigera palm seedlings supress CH 4 emissions due to large radial oxygen loss from their root systems; ii) clearance of R. taedigera forest increases CH 4 and N 2 O emissions as peat oxygen concentrations decline; iii) peat surface CO 2 emissions decrease as root respiration ceases following clearance; and iv) land use change affects the relative contribution to radiative forcing of greenhouse gas emissions from soil, increasing the relative importance of CH 4 emissions with respect to CO 2 and N 2 O.…”