Broadleaf species expansion in forested peatlands: the case of <i>Thuja occidentalis</i> – <i>Larix laricina</i> peatland

The impacts of human disturbances on temperate forested peatlands remain poorly understood. What are their past and present dynamics under pressure from human activities? How will their forest cover evolve in the future? These questions were examined through a study of the contemporary (20th–21st centuries) and past (several millennia) dynamics of a forested peatland dominated by Thuja occidentalis and Acer rubrum (minerotrophic sectors) and Larix laricina and A. rubrum (ombrotrophic sectors). We use paleoecological, dendrometric, dendrochronological, and aerial photographs analyses to define the reference condition of the vegetation by describing its long-term evolution, documenting its transformation in response to anthropogenic disturbances, and propose a scenario for forest evolution in the coming decades. The reference condition of the minerotrophic sectors is a coniferous forest ( T. occidentalis, L. laricina, Picea sp.) that has existed for at least 5000 years. Acer rubrum established in recent decades and is now abundant in the regeneration (seedlings and saplings). The reference condition of the ombrotrophic sectors is a shrub-dominated peatland, with tree encroachment initially by L. laricina, followed by A. rubrum since the mid-20th century. Coniferous species currently face a regeneration deficit in both sectors, and Acer rubrum could eventually become the dominant species, which would represent an alternative state for the peatland. Forest dynamics have been influenced by conifer logging and possibly by a lowering of the peatland water table.