Mowing of the moss layer triggers the loss of endangered fen bryophytes

Fens are the predominant peatlands in temperate regions and provide key ecosystem services such as water retention and carbon sequestration. Under threat of drainage, discharge of polluted water, and subsequent eutrophication, fen conservation is a priority. A key feature of fens is their high biodiversity, particularly of specialist bryophytes that play a pivotal role in peat formation. Since many mosses are poor competitors, increased growth of vascular plants following eutrophication is often remediated by annual mowing with litter removal. The precise mowing method favoring bryophytes is rarely investigated, yet creation of “recruitment gaps” by mowing of the bryophyte layer has been recommended. We performed a 3‐year field experiment with three annual treatments: (1) mowing of the bryophyte and vascular plant layer to create gaps, (2) only mowing of vascular plants above the bryophyte layer, and (3) not mowing. Our results show that mowing of bryophytes is detrimental for endangered bryophytes, specifically Sphagnum contortum, S. teres, Warnstorfia exannulata, Calliergon giganteum, and Hamatocaulis vernicosus. Only the generalist species S. palustre and Calliergonella cuspidata benefited from bryophyte mowing. We also found that accretion of the bryophyte layer and development of micro‐topography are impeded by bryophyte mowing, potentially hampering future water retention and carbon sequestration. Nevertheless, mowing of vascular plants proved necessary as fen specialists S. teres, S. contortum, S. subsecundum, W. exannulata, and Bryum pseudotriquetrum declined in the unmown treatment. We conclude that, if mowing is necessary to remediate eutrophication, it should be restricted to vascular plants, particularly in fens with endangered bryophytes.