PARENT SESSION
Oral Session #33: Climate Change. Presiding: A. Peterson.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas E.

Late-Quaternary land cover change in boreal and eastern North America.

Williams, John¹, ¹

ABSTRACT- Land cover has changed dramatically during the late Quaternary, driven by the retreat of the Northern Hemisphere ice sheets and the biogeographic responses of individual plant taxa to the climate changes accompanying deglaciation. Past changes in land cover have had a major impact upon terrestrial carbon sequestration and may have modulated past climate change. By calibrating modern pollen samples against AVHRR-derived maps of tree cover, quantitative estimates of absolute tree cover for simple plant functional types have been produced for boreal and eastern North America for the past 21,000 years at a one-degree resolution and a thousand-year time step. An 'iterated analog technique' enables tree cover reconstructions even for regions where fossil pollen samples are compositionally unlike any modern pollen samples, by condensing pollen taxa to plant functional types and rechecking for analog matches. When applied to modern pollen samples, r²=0.87 for broadleaved tree cover and r²=0.70 for needleleaved tree cover. The precision of the IAT estimates decreases when the pollen data are condensed into functional categories. The calibrated tree cover maps show that eastern North America was covered by needleleaved forests and woodlands during the full glacial. Broadleaved tree cover began to increase in the southeastern US 16,000 years ago and deciduous broadleaved forests were well established in eastern North America by the end of the Pleistocene. The modern distribution of needleleaved and deciduous tree cover began to form by the early Holocene and was well established by the mid-Holocene. The tree cover maps are consistent with biome reconstructions produced by the Prentice biomization technique and represent a useful alternative to categorical classifications of past land cover.

KEY WORDS: land cover, Quaternary, North America, pollen