Tree Encroachment Varies by Plant Community in a Large Boreal Peatland Complex in the Boreal-Temperate Ecotone of Northeastern USA

Large peatland complexes at the boreal-temperate ecotone are essential habitats for boreal species at their southern range limits where they are threatened by tree encroachment accelerated by climate change and nitrogen deposition. To inform vascular plant and biodiversity conservation, we studied tree encroachment patterns in a large (> 400 ha) boreal peatland complex in the northeastern United States across vegetation types and environmental gradients. We characterized vascular plant composition, environmental drivers and tree demography on 50 plots (each 25 m(2)). We used non-metric multidimensional scaling (NMS) to identify two main drivers of vascular plant composition in the herbaceous layer-pH and tree canopy openness-that described three broad plant community types (open bog, forested bog, and fen). Tree demography suggested that woody encroachment (i.e., tree seedling recruitment) varied across these community types; open bog was colonized byPicea marianaseedlings, while forested bog and fen (dominated by evergreen conifers,Picea marianaandThuja occidentalis, respectively) were colonized by deciduous tree species (Acer rubrumandBetula alleghaniensis). Our findings provide early warning signs of vegetation change in boreal peatlands near their southern range limits caused by the encroachment of temperate tree species into forested peatlands and expanding tree cover in open bogs.