Are marginal balsam fir and eastern white cedar stands relics from once more extensive populations in north-eastern North America?

Abstract

Marginal stands of balsam fir ( Abies balsamea [L.] Mill.) and eastern white cedar ( Thuja occidentalis L.) are found north of their limits of continuous distribution in eastern North America. Regional-scale paleoecological studies have suggested that fir and cedar populations could have had larger extents in the past. This study aimed at verifying this hypothesis at the local scale. Wood charcoal fragments were collected from the soils of two marginal fir and cedar stands as well as from 15 sites in the surrounding forest matrix where the species are absent currently. Anatomical identification and radiocarbon-dating showed that fir was more extensive in the study area until about 680 cal. BP, representing up to 31% of the charcoal assemblages at sites where it is currently absent. The evidence is less conclusive for cedar, however, although some of the charcoal fragments from the matrix sites could have been either fir or cedar (undistinguishable). Most of the dated fir/cedar charcoal in the matrix were from the ‘Medieval Warm Period’ (ca. 1000 cal. BP), suggesting contraction may have occurred at that time. Marginal fir – and possibly cedar – stands are thus relics of once more extensive populations. Fire is likely the main factor having contributed to the contraction of the species’ distributions. Fir and cedar are now relegated to areas where fires are less frequent and severe, such as the shores of lakes and rivers.