Vegetation composition and structure is a fundamental state variable in the earth system, and accurate descriptions of past land cover are essential to e.g. understanding the role of vegetation feedbacks to past climate change. Paleoecological datasets such as networks of fossil pollen records provide information about the distribution and abundance of plant taxa - the challenge is to scale upwards to inferences about vegetation structure and the physical properties of past landscapes.
Much of my research has focused on developing late-Quaternary land-cover maps that are well grounded in the data, which can be used to construct accurate carbon budgets, to prescribe more realistic land surface parameterizations for general circulation model experiments, and to evaluate the results of coupled atmosphere-vegetation models. In the 1990's and 2000's I was involved in the Biome 6000 project, and in more recent years we have been developing transfer functions that link paleoecological datasets with remotely sensed observations of vegetation structure (e.g. the relative abundance of plant functional types, leaf area index (LAI), and using these transfer functions to reconstruct past vegetation. Applications include the quantitative reconstruction of Holocene shifts in the prairie-forest ecotone in eastern North America and the boreal forest ecotone in the Northern Hemisphere.
Now, we are partnering with the PalEON project to develop new methods for inferring past forest composition from pollen data and using this information to improve the predictions made by terrestrial ecosystem models.
Williams J.W., Tarasov P.A., Brewer S. and Notaro M. (2011) Late-Quaternary variations in tree cover at the northern forest-tundra ecotone. Journal of Geophysical Research - Biogeosciences 116: G01017, doi: 01010.01029/02010JG001458.
Williams J.W., Shuman B. and Bartlein P.J. (2009) Rapid responses of the Midwestern prairie-forest ecotone to early Holocene aridity. Global and Planetary Change 66: 195-207.
Gonzales, L.M., Williams, J.W, and Kaplan, J.O. (2008) Variations in leaf area index in northern and eastern North America over the past 21 000 years: A data-model comparison. Quaternary Science Reviews. 27: 1453-1466.
Williams, J.W., Gonzales, L.M., Kaplan, J.O., (2008) Leaf area index for northern and eastern North America at the Last Glacial Maximum: a data-model comparison. Global Ecology and Biogeography. 17: 122-134.
Diffenbaugh, N. S., Ashfaq, M., Shuman, B., Williams, J. W., Bartlein, P. J. (2006) Summer aridity in the United States: Response to Mid-Holocene changes in insolation and sea surface temperature. Geophysical Research Letters 33, DOI: 10.1029/2006GL028012
Notaro, M., Liu, Z., and Williams, J. W. (2006) Observed vegetation-climate feedbacks in the United States. Journal of Climate 19: 763-786.
Williams, J. W., Shuman, B. N., Webb, T., III, Bartlein, P. J., Leduc, P.(2004) Quaternary vegetation dynamics in North America: Scaling from taxa to biomes. Ecological Monographs 74: 309-334.
Williams, J. W. and Jackson, S. T. (2003) Palynological and AVHRR observations of modern vegetational gradients in eastern North America. The Holocene 13: 485-497.
Williams, J. W., Webb III, T., Richard, P. J. H., and Newby, P. (2000) Late Quaternary biomes of Canada and the eastern United States. Journal of Biogeography 27: 585-607.
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