Tree Harvest Decisions Modulate the Climate Impact of Rewetting in a Low-Productive Peatland Forest in Boreal Sweden.

Over the past century, extensive areas of northern peatlands have been drained for forestry. Today, concerns about their role as significant sources of greenhouse gases (GHG) have sparked growing interest in peatland rewetting as a climate mitigating strategy. However, empirical evidence for rewetting effects on ecosystem carbon (C) and GHG balances is still limited, particularly for minerogenic boreal peatland forests. Rewetting of peatland forests also involves decisions about tree harvest, which can have important but understudied consequences for the C cycle. In this study, we quantified tree growth and estimated carbon dioxide (CO2) and methane (CH4) fluxes in both peatland areas and ditches over 2 years before (2019-2020) and after (2021-2022) rewetting a low-productive, minerogenic peatland forest in boreal Sweden. We also assessed effects of tree removal during rewetting by comparing harvest and non-harvest areas. Our results suggest that the peatland forest was, on average, C-neutral at the ecosystem-scale during the drained years. After rewetting, the harvested area became a C source (79 g C m-2 year-1), while the treed area acted as a small C sink (-24 g C m-2 year-1), with the difference due to diverging responses in net CO2 exchange. Furthermore, CH4 emissions doubled after rewetting, resulting in a two- to threefold increase in total GHG emissions (expressed in CO2 equivalents) over both 20- and 100-year timeframes. While ditches functioned as significant CO2 sinks and moderate CH4 sources during the drained years, they became CO2-neutral and CH4 emission hotspots after being infilled. Altogether, our findings suggest that rewetting low-productive boreal peatland forests may have a negative short-term climate impact. However, rewetting without tree harvest considerably meliorates ecosystem C and GHG balances. Overall, our study highlights the importance of tree harvesting decisions and the need for a deeper understanding of rewetting as a climate mitigation strategy.