Birch Transpiration Is Not the Main Reason for Insufficient Water Levels in a Rewetted Bog

Natural bogs have high water level (WL) that prevents peat decomposition and promotes bog‐specific vegetation. Especially in rewetted bogs, increasing tree encroachment is observed in combination with low WL. Simultaneously, a general shift in vegetation and microform distribution is visible. The effects of this development on evapotranspiration (ET) are not clear. Two sites were established at a former peat extraction site, one with high WL and a Sphagnum ‐dominated vegetation (open site), one with varying and lower WL and a dense birch population (tree site). Energy and CO 2 exchange were measured with eddy covariance towers. For one growing season, ET of the different microforms (hummocks and hollows) and birches was measured with closed chambers to model their contribution to growing season ET. ET was lower at the tree site in all months and consequently in all three investigation years (620 and 387 mm at the open and tree site, respectively, averaged over 3 years). ET of birches was 104 mm, which was 28% of growing season site ET, while microform ET was strongly reduced at the tree site. ET at the open site was high due to high Sphagnum cover and WL. The open site generally had a higher proportion of latent heat, leading to a lower Bowen ratio, while the water use efficiency was higher at the tree site. Consequently, the birches are not the primary cause of the lower WL, but rather the additional water loss due to site conditions and incomplete rewetting. Therefore, despite the higher ET, the open site is in a better state of conservation.