Greenhouse Gas Fluxes (CO₂, CH₄ and N₂O) in Forests and Wetlands of Boreal, Temperate and Tropical Regions

Abstract

Current estimates of GHG budgets of forests indicate that these ecosystems act, over all, as C sinks, regardless of climatic region. The sequestering of GHGs by forests is the result of the balance between a substantial uptake of CO₂ by vegetation and CO₂ release through soil respiration, a weak consumption of CH₄ by methanotrophic bacteria, and a more or less significant emission of N₂O from soil, a by-product of nitrification and denitrification reactions. According to the estimated GHG budgets, the forest C sink is weaker in the boreal region (mean −236 mgC·m⁻²·d⁻¹) than in the temperate and tropical regions (−398 and −632 mgC·m⁻²·d⁻¹, respectively), in agreement with the NEE (Net Ecosystem Exchange in CO₂) pattern observed with latitude. The increase in solar radiation and growing season length from north to south could be responsible for such a pattern. The C sinks typical of tropical forests can be offset by nearly 30% by N₂O emissions from the soils. In northern forests, some sites have been found to be net sources of CO₂, particularly during warm and dry years.

In wetlands, water-saturated soils are conducive to anaerobic decomposition of organic matter and methane production. CH₄ largely dominates the GHG budget of wetlands. Overall, northern peatlands are sources of GHGs (mostly as CH₄) (173 gCO₂-eq.·m⁻²·yr⁻¹), while accumulating small quantities of CO₂ as peat (−22 gC·m⁻²·yr⁻¹). Our GHG budgets for peatlands do not generally take into account the high CO₂ and CH₄ emissions from ponds. Tropical wetlands, dominated by marshes and swamps, emit large amounts of CH₄ into the atmosphere (mean 71 mgCH₄·m⁻²·d⁻¹) compared to northern peatlands (34 mgCH₄·m⁻²·d⁻¹).

The estimated GHG budget values for forests depends on a) the error in NEE measurements, which dominate the forest GHG budgets and whose percent correction for calm nights can represent up to 50% of their initial value; b) the small number of published data on NEE for boreal and, especially, tropical forests, for which two out of the three available NEE data sets are contested; and c) the lack of NEE data in certain stands, such as in boreal forests post-fire stands, to which a certain potential for GHG emission is attributed. In wetlands, the popular use of the chamber method for CH₄ flux measurements, along with a sampling period of less than a year, make it difficult to estimate an annual GHG budget that would integrate these systems' considerable spatial and temporal variability. The use of flux towers in forests, and recently in peatlands, allows for estimations of annual CO₂ fluxes (NEE) typical of the local variability of environmental conditions within these systems.