Impacts of restoration via rewetting of previously drained wetlands and climate variability on peatland greenhouse gas fluxes

Abstract:

Disruptions to natural conditions in peatlands, which are Earth's largest terrestrial carbon (C) store, threaten to liberate large C stocks by increasing emissions of carbon dioxide (CO₂) and, in some cases, methane (CH₄). Shifts in vegetation, hydrology and temperature resulting from disturbance are likely to alter greenhouse gas (GHG) dynamics, with potential for peatlands to act as a positive (i.e. promote warming) or negative feedback to the global climate system. Restoration efforts such as re-wetting drained wetlands could play an important role in climate change mitigation or adaptation by reducing CO₂ emissions, increasing their ability to sequester atmospheric CO₂. However, this may come at the cost of increased CH₄ release, i.e. restoration often leads to a biogeochemical compromise between net CO₂ uptake and net CH₄ emissions. There is high uncertainty regarding how well these ecosystems are able to regain their function as C and GHG sinks following restoration, thus the necessity to investigate GHG dynamics within these novel ecosystems.

Eddy covariance (EC) flux measurements, normalised difference vegetation index (NDVI) data, and abiotic variables (e.g. soil temperature and water table depth) from two restored sites at different stages of ecological development located in the metro Vancouver area were analysed. GHG flux measurements were compared between the two sites alongside abiotic variables thus allowing identification of the key biophysical drivers of GHG fluxes within multiple timeframes and between different stages of ecosystem development.

Results show that during the growing season, the site dominated by a higher abundance of vascular vegetation such as shrubs, characteristic of a later successional stage, had higher CO₂ uptake than the site dominated by sedges and Sphagnum mosses. Higher rates of photosynthesis, and thus increased CO₂ uptake in the more developed site were confirmed through differences in NDVI between the two sites. The site representing an ecosystem at an earlier stage of succession however, was a greater CH₄ source to the atmosphere. These findings suggest that restoration and management practices need to focus on creating ecological conditions that maximise C uptake, whilst minimizing CH₄ emissions, if we are to reinstate the GHG sink status of these peatlands.