Birch (<i>Betula pubescens</i> Ehrh.) Encroachment Alters Contribution of Plant Functional Groups to Ecosystem Carbon Cycling in a Rewetted Bog

Rewetted bogs with high water levels (WL) and mire-specific vegetation are crucial carbon (C) sinks, but their function might be threatened by tree encroachment, a phenomenon widespread in the northern hemisphere that often coincides with low WL. This might impact C cycling both at the ecosystem and microform scale in multiple ways, but so far, data are lacking. We established two sites in the same former peat extraction area, one showing permanently high WL and mire-specific vegetation (open site, OS), while the other one has more fluctuating WL and a dense birch (<i>Betula pubescens</i> Ehrh.) population (tree site, TS). We measured the carbon dioxide (CO₂) exchange at ecosystem (eddy covariance) and plot scale (chamber measurements) for 1 year to clarify the differences between the sites and the impact of birch encroachment on the contribution of the different bog-specific microforms and the trees to the ecosystem's CO₂ balance. Overall, the OS had a CO₂ balance of -262.4 ± 7.8 g CO₂-C m^-2 year^-1 indicating CO₂ uptake, while the TS was close to neutral (-28 ± 5.1 g CO₂-C m^-2 year^-1). The smaller uptake at the TS was caused by higher (151%) ecosystem respiration, while gross primary production was 14% higher. However, the microform contributions to C uptake strongly differed: At the OS, both hummocks and hollows showed net uptake, while at the TS, most C (52%) was assimilated by the birches and the understory was a net CO₂ source. This indicates a loss of peat C from the TS, while the successfully rewetted site was accumulating new peat. Accounting for plot-scale CH₄ fluxes, both sites were a weak source of greenhouse gases, but a distinctly stronger C sink occurred at the OS. Our data show the possibility of increasing C removal from the atmosphere by full rewetting and the establishment of mire-specific vegetation.