Hemlock (Tsuga canadensis) declines at 9800 and 5300 cal. yr BP caused by Holocene climatic shifts in northeastern North America

Abstract

We present fossil pollen data from a sediment core at Lake Grinnell in northern New Jersey of the northeastern United States. The 12 500-yr chronology of the sediment record was controlled by seven calibrated AMS radiocarbon dates on terrestrial plant macrofossils. Similar to many pollen profiles in the region, our data show that vegetation changed from Picea- and Pinus-dominated woodland/forest at 12.5—11.4 ka (1 ka = 1000 cal. yr BP), through Pinus-dominated mixed forest at 11.4—9.3 ka, to Quercus-dominated forest after 9.3 ka. Some main tree taxa, including Tsuga, Ulmus and Acer, arrived and expanded during the Quercus expansion phase at 11.4—9.3 ka in the early Holocene, while other trees, including Fagus and Carya, established and expanded much later around 9 ka. Pollen data show that major forest shifts were in response to climatic change as independently inferred from oxygen-isotope records in the same core. These responses include major turnovers of tree species at the onset of the Holocene after the Younger Dryas, including the disappearance of Picea, declines of boreal taxa Betula and Alnus, and expansions of most other temperate trees. Our new records show two hemlock ( Tsuga canadensis) decline events during the Holocene that were both likely caused by climatic change. The early-Holocene hemlock decline centering around 9.5 ka was marked by >10-fold decrease in hemlock populations, which was probably caused by summer high temperature as inferred from high oxygen isotope values. The mid-Holocene hemlock decline from 5.3 ka to 3.0 ka corresponds with a pronounced shift in oxygen isotopes at Grinnell and with a dry climate interval documented from other records in northeastern North America. Our results support the notion that a dry climate caused or triggered the mid-Holocene hemlock decline and that the Appalachian Forest has shown sensitive responses to climatic change in the Holocene.