Back to the Future: Restoring Northern Drained Forested Peatlands for Climate Change Mitigation

Abstract

Draining peatlands for forestry in the northern hemisphere turns their soils from carbon sinks to substantial sources of greenhouse gases (GHGs). To reverse this trend, rewetting has been proposed as a climate change mitigation strategy. We performed a literature review to assess the empirical evidence supporting the hypothesis that rewetting drained forested peatlands can turn them back into carbon sinks. We also used causal loop diagrams (CLDs) to synthesize the current knowledge of how water table management affects GHG emissions in organic soils. We found an increasing number of studies from the last decade comparing GHG emissions from rewetted, previously forested peatlands, with forested or pristine peatlands. However, comparative field studies usually report relatively short time series following rewetting experiments (e.g., 3 years of measurements and around 10 years after rewetting). Empirical evidence shows that rewetting leads to lower GHG emissions from soils. However, reports of carbon sinks in rewetted systems are scarce in the reviewed literature. Moreover, CH4 emissions in rewetted peatlands are commonly reported to be higher than in pristine peatlands. Long-term water table changes associated with rewetting lead to a cascade of effects in different processes regulating GHG emissions. The water table level affects litterfall quantity and quality by altering the plant community; it also affects organic matter breakdown rates, carbon and nitrogen mineralization pathways and rates, as well as gas transport mechanisms. Finally, we conceptualized three phases of restoration following the rewetting of previously drained and forested peatlands, we described the time dependent responses of soil, vegetation and GHG emissions to rewetting, concluding that while short-term gains in the GHG balance can be minimal, the long-term potential of restoring drained peatlands through rewetting remains promising.

Pros and Cons of Strategies to Reduce Greenhouse Gas Emissions from Peatlands: Review of Possibilities

“…For effective peatland restoration, it is necessary to consider both the hydrology of peatlands and species diversity and conservation. Peatland restoration is aimed at restoring degraded peatlands to their natural status and hydrological conditions, recovering their natural habitats, and, thus, increasing biodiversity [1,[106][107][108][109].…”

“…After rewetting, ditch systems or networks of plugged or backfilled canals are left open, and the rise in the water table results in CH 4 emissions [128]. Few studies have shown that CH 4 emissions from peatlands in northern regions have been rewetted for more than ten years [106]. For this reason, changes in GHG emissions from these peatlands remain to be discovered.…”

“…In the short term, rewetting may have negative consequences, such as degraded water quality in downstream areas or aggravating GHG emissions [106,117,131]. Rewetting is also reported to cause slight changes in the chemical composition of the peat.…”

“…Growing trees in degraded peatlands causes tree transpiration and lowers the water table [106]. Fire is a risk in peatlands after afforestation.…”

A large-scale and long-term experiment to identify effectiveness of ecosystem restoration

“…For instance, in Finland alone there are up to one million hectares of mire forests and fens where the drainage has not been successful in increasing the tree growth to a profitable level 18 , mainly due to lack or imbalance of nutrients in these sites. Hence, restoring forestry-drained ecosystems could result in long-term biodiversity 14 and climate benefits 19 . Indeed, restoration of forestry-drained ecosystems often succeeds in rising the water table to pristine levels 20,21 .…”