Recent Gray Birch (Betula populifolia) Encroachment in Temperate Peatlands of Eastern North America

Abstract

Numerous peatlands in eastern North America are currently experiencing an accelerated process of tree encroachment. Deciduous species, particularly gray birch, can sometimes form large, dense stands. Our objectives were to document the periods during which gray birch established in temperate peatlands of southern Quebec (Canada), and examine the factors that may have contributed to such massive establishment. We hypothesized that the abundant presence of gray birch is a relatively recent phenomenon associated with human activities that are creating conditions favorable to the establishment of this species. We analyzed dendrochronological, plant-macrofossil, aerial photographs and climatic data for 10 sites. All of the peatlands recently experienced a major increase in forest cover. The first sign of gray birch establishment dates back to the 1950s, although at over half of sites, establishment took place after 1980. Abundant macroremains of the species were found only in the upper portion of peat deposits. Birch encroachment most often occurred after a fire had spread through the peatland. Increasing agricultural and draining activities on the periphery of the peatlands, that coincides with drier climatic conditions, likely induced a drying out of the peat, thereby creating conditions propitious for fire and subsequent germination and growth of birch seeds.