Mid-Holocene climate and the hemlock decline: the range limit of Tsuga canadensis in the western Great Lakes region, USA

Abstract

Eastern hemlock (Tsuga canadensis) expanded rapidly across Upper Peninsula Michigan from ~6500 to 5500 cal. yr BP, followed by a sudden decline of hemlock pollen percentages that lasted over 1000 years. Hemlock declined throughout its range, apparently due to insect/pathogen outbreaks, which may have been affected by regional climatic shifts. Modern analogues were used to reconstruct submillennial climatic trends from fossil pollen sites spanning the western range limit of hemlock. Reconstructions from eight sites inside the range of hemlock at the time of the decline (5400 cal. yr BP) are compared with eight sites outside the range, which should be independent of variation in hemlock pollen percentages and the non-climatic effects of insects/pathogens. Expansion of hemlock before the decline occurred at a time of increasing precipitation and January temperature. Long-term precipitation averages stabilized at ~5500 cal. yr BP and remained fairly constant throughout the period of low hemlock abundance. January temperature reconstructions decrease at the time of the hemlock decline to approximately modern values at sites inside the range, as well as at oak dominated sites just outside the range. The lower-than-modern precipitation of the mid-Holocene may have reduced hemlock's tolerance for cold stress and winter desiccation and perhaps increased its susceptibility to insects or pathogens. January temperature reconstructions for pine-dominated sites outside the pre-decline hem lock range show little change, possibly due to the insensitivity of pine-dominated pollen assemblages to small climatic fluctuations. Hemlock began a new expansion across the region after 4000 cal. yr BP as July tempera ture decreased and precipitation increased.