Conservation management in fens: Do large tracked mowers impact functional plant diversity?

Cessation of traditional mowing management has been the main threat to fen biodiversity for many decades; shrubs and trees expand and target plant and animal species decline. In Biebrza National Park (BNP), the EU’s largest conservation area of alkaline fens, abandonment has affected more than 300 km2. To combat unwanted succession, tracked mowers were developed based on snow grooming vehicles and are increasingly used in BNP and some other European fens. While effects on bird populations seemed promising, the impact on botanical diversity remained yet unknown. We assessed the effects of this management on vegetation by comparing 12 pairs of mown and unmown plots (percentage cover recorded from 4 m2 for all species and 25 m2 for rare species) spread over several km2. We found that tracked vehicles suppressed some species, which are likely to facilitate others by providing shelters above groundwater level (hummock mosses, tussock sedges and shrubs) and enhanced species with effective clonal spread and high specific leaf area. An even stronger increase was observed in forbs, which share effective clonal spread and flood tolerance (high Ellenberg F value). Analysis of functional diversity indices based on all measured traits showed a decrease of functional divergence due to mowing (indicating convergence of traits) and no response of functional richness, evenness and functional dispersion. Mowing has also decreased the number of rare species. We associate these changes with the reduction of micro-topography by caterpillar tracks. Tracked mowers should therefore be avoided in those fens, in which microtopography contributes to unique botanical diversity.