Moisture and temperature changes associated with the mid-Holocene Tsuga decline in the northeastern United States

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• 作者：Jeremiah P. Marsicek ; Bryan Shuman ; Simon Brewer ; David R. Foster ; W. Wyatt Oswald
• 关键词：Lake levels ; Hemlock (Tsuga canadensis) decline ; Fossil pollen ; Climatic change ; Abrupt change ; Holocene ; Northeastern U.S.
• 刊名：Quaternary Science Reviews
• 出版年：15 November, 2013
• 年：2013
• 卷：80
• 期：Complete
• 页码：129-142
• 全文大小：2362 K

文摘

A decline of hemlock (Tsuga) populations at ca聽5.5聽ka (thousands of calibrated radiocarbon years before 1950聽AD) stands out as the most abrupt vegetation change of the Holocene in North America, but remains poorly understood after decades of study. Recent analyses of fossil pollen have revealed a concurrent, abrupt oak (Quercus) decline and increases in the abundance of beech (Fagus) and pine (Pinus) on Cape Cod in eastern Massachusetts, but the replacement of drought-tolerant oaks by moisture-sensitive beeches appears inconsistent with low lake levels in the region at the same time. The oak and beech changes are also limited to coastal areas, and the coastal-inland differences require an explanation. Here, we develop a new lake-level reconstruction from Deep Pond, Cape Cod by using a transect of sediment cores and ground-penetrating radar (GPR) profiles to constrain the past elevations of the sandy, littoral zone of the pond. The reconstruction shows that a series of multi-century episodes of low water coincide with the abrupt hemlock and oak declines, and interrupt subsequent phases of hemlock recovery. The lake-level variations equal precipitation deficits of 鈭?00聽mm superimposed on a Holocene long moisture increase of >400聽mm. However, because moisture deficits do not easily explain the oak and beech changes, we also evaluate how the climate preferences of the regional vegetation changed over time by matching the fossil pollen assemblages from Deep Pond with their modern equivalents. Reconstructions of the precipitation requirements of the vegetation correlate well even in detail with the lake-level record (r聽=聽0.88 at Deep Pond), and indicate close tracking of effective moisture (precipitation minus evapotranspiration) by the vegetation despite the abrupt species declines, which could have decoupled climate and vegetation trends. Reconstructions of the temperature preferences of the vegetation indicate that coastal sites may have cooled by 0.5-2.5聽掳C after ca聽5.5 ka, while inland sites warmed by 0.5-1聽掳C. The change in coastal temperature preferences agrees with sea surface cooling in the western Atlantic Ocean of >1聽掳C. Consequently, the persistence of low hemlock abundance after 5.5 ka in the northeast U.S. may have resulted from oceanic changes that produced multi-century droughts and thus delayed the post-decline recovery of hemlock populations.