Rapid Broadleave Encroachment in a Temperate Bog Induces Species Richness Increase and Compositional Turnover

ABSTRACT Numerous temperate peatlands are currently experiencing an accelerated process of woody encroachment. Such encroachment may have harmful impacts on peatland biodiversity and functions. Our objective was to investigate the recent history of woody encroachment in a temperate bog using plant macrofossil, testate amoeba, aerial photograph analyses, and plant inventories. We also explored some plausible triggering drivers of the phenomenon and its impacts on plant richness, composition, and beta diversity. We showed that the recent woody encroachment was mainly associated with the native Betula populifolia and the exotic Frangula alnus. It began in the 1960s at the bog margins following a decrease in the water table and recurrent fire events, both likely associated with agricultural activities in the surrounding catchment. We found an increase in species richness with tree cover and basal area, but no effect of tree encroachment on beta diversity. Still, we found a significant compositional turnover from light-demanding bog specialists to terrestrial, exotic or ruderal species with tree basal area. It seems unlikely that the bog will naturally come back to its previous unforested state on a human life's timescale as the observed changes are related to regional factors rather than in situ disturbance.