Peatlands are the largest natural terrestrial carbon store. They store more carbon than all other vegetation types in the world combined, thereby providing a long-term net-cooling effect according to the International Union for Conservation of Nature. Under natural conditions, peat accumulates when photosynthesis-driven carbon dioxide (CO2) sequestration exceeds the decomposition-driven CO2 and methane (CH4) emissions. The balance between these two processes determines the peatland net carbon budget and subsequently the overall climate feedback, which is mainly decided by peatland water-table position. Previous studies have found that water-table drawdown results in a net increase of greenhouse gas emissions, while water-saturated peatlands are large CH4 sources. Therefore, the topical question is whether peatlands will get drier or wetter under climate changes.
Predicting the peatland moisture balance using climatic parameters only may not be reasonable due to the complex interactions with precipitation-evapotranspiration, runoff, permafrost dynamics, and autogenic processes. Hydrologically sensitive bioindicators archived in peats, especially testate amoebae, provide an opportunity to study the contemporary moisture conditions over the period when the peat they occupied was accumulating at the surface, while with past climatic data, it is possible to link peatland hydrological change patterns to climate change. This approach was previously applied to document the widespread drying of central-European peatlands. However, it is uncertain whether drying extends to the subarctic-arctic ecosystems, which are characterized by amplified warming and permafrost.
In our paper published in Nature Communications, we compiled 103 peat records from the high latitudes, with most located within the northern permafrost zones. We found that 54% of these peatlands are drying out, 32% have become wetter, and the remainder (14%) showed no clear trend with fluctuating hydrological conditions over the past 400 years. Most of the hydrological changes have occurred since the 19th century, which is in line with post-Little Ice Age warming. However, the pattern of diverse timing of the hydrological shifts between the individual coring points indicates the variability in sensitivity of different regions/peatlands to climate changes.
The comparison between reconstructed hydrological data and climatic data suggests that climate, especially summer temperature, has played an important role in determining the position of the peatland water table. For example, a wetting trend has been observed more often in northeastern Canada, which also experienced less warming (during our study period) than other regions. In addition, climate change can also indirectly impact the peatland water table by directly impacting permafrost dynamics. For instance, permafrost initiation could cause a peat surface uplift (Fig. 1) and, consequently, dry conditions, while permafrost thawing could result in wet conditions. It is a challenge to estimate a specific tipping point of warming that might trigger permafrost thawing, as the local conditions vary considerably, e.g., soil conditions, hydrology and vegetation. The consequent wetting or drying depends on evapotranspiration and on ice richness etc., which further challenges the prediction of hydrological conditions of permafrost peatlands.
Overall, our study highlights the pronounced effect of recent climate change on determining the peatland moisture balance, and by extension, driving their carbon balance. The observed divergent pathways of peatland hydrological succession clearly challenge the projection of peatland carbon sink and source dynamics. We propose that determining the strongly heterogenic successional pathways in peatlands must be a key research focus in the future.