Sphagnum-dominated peatlands occupy extensive tracts of land throughout the Boreal and Subarctic regions of North America, extending north onto the Low Arctic of the Canadian Shield and south along the west coast of Oregon, Rocky Mountains of Wyoming, and Appalachians of West Virginia. In addition, short pocosins found along the southeastern coast also can be considered as Sphagnum-dominated peatlands, even though they differ significantly from traditional concepts of boreal peatlands. Along the southern limit of Sphagnum-dominated peatlands, where climate is limiting, edaphic factors allow for the development of outliers. As the current distribution of Sphagnum-dominated peatlands is related to Sphagnum spore rain, past distributions of Sphagnum-dominated peatlands can be constructed from spore records preserved in lakes and peatlands. Here we present six time slices extending back to the Last Glacial Maximum to determine how Sphagnum-dominated peatlands have varied in both time and space. The spore record indicates that Sphagnum-dominated peatlands were present in North America during the Last Glacial Maximum although they were spatially limited to central Alaska, the Olympic Peninsula and Puget Trough of Washington, and to a narrow band in the eastern states of Kentucky, North Carolina, Tennessee, and Maryland. During the Late Wisconsinan Sphagnum-dominated peatlands shifted northwards in eastern North America and expanded farther into Alaska and the Midwest. The Late Wisconsinan/Holocene transition marks a time of overall increase in the area supporting Sphagnum-dominated peatlands, while extending farther in eastern Canada and western continental and coastal regions, they almost completely disappear in the Midwest where they were extensive earlier. Sphagnum-dominated peatlands generally reach their current extent about 2,000–3,000 years ago. Sphagnum-dominated peatlands have dramatically changed their distribution and abundance since the Last Glacial Maximum, and hence the carbon that is stored in these present-day important sinks has also changed dramatically. When compared to the estimated 220 Pg of carbon stored in North American peatlands today, less than 10% of this carbon was present in these peatland during the LGM.
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Vol. 103 • No. 2