Current Research
Current research areas:
1) 'No-Analog' plant associations and environments
2) Late-Quaternary land-cover change & surface-atmosphere feedbacks
3) Ecological responses to abrupt climate change
4) Plant diversity in space and time
'No-Analog' Plant Associations and Environments
Quaternary paleoecological records convincingly show that plant communities are ephemeral, and that plant taxa individualistically track shifts in climate. Late-Quaternary plant communities appear to have no counterpart in the modern vegetation (hence 'no-analog') and, moreover, it is quite possible that novel plant associations will develop in response to current climate trends. We have recently shown that novel climates are also likely to develop by the end of the 21st-century, with the risk proportional to end-century greenhouse gas concentrations. Learning how and why the late-glacial 'no-analog' plant communities developed can thus aid efforts to forecast ecological responses to future climate scenarios. On-going research investigations include 1) modeling the role of seasonality and other environmental factors for promoting the development of no-analog plant associations in eastern North America and 2) re-coring and redating key ENA pollen records to more precisely constrain the development and disaggregation of the late-glacial no-analog communities.
PUBLICATIONS
Williams, J. W. and Jackson, S. T. (2007) Novel Climates, No-Analog Plant Communities, and Ecological Surprises: Past and Future. Frontiers in Ecology and Evolution 5: 475-482. doi:10.1890/070037. (PDF)
Williams, J. W., Jackson, S. T., Kutzbach, J. E. (2007) Projected distributions of novel and disappearing climates by 2100AD. Proceedings of the National Academy of Sciences 104: 5738-5742. (PDF) (Suppl. Figures) (Suppl. Tables) (Powerpoint Slides)
Jackson, S. T. and Williams, J. W. (2004) Modern analogs in Quaternary paleoecology: Here today, gone yesterday, gone tomorrow? Annual Review of Earth and Planetary Sciences 32: 495-537. (PDF)
Williams, J. W., Shuman, B. N., Webb III, T. (2001) No-analog conditions and rates of change in the climate and vegetation of eastern North America. Ecology 82: 3346-3362. (Abstract and Figure PDFs) (PDF)
Late-Quaternary land-cover change & surface-atmosphere feedbacks
Vegetation and atmospheric dynamics are coupled by the exchanges of energy, water, and trace gases at the land surface; surface-atmosphere feedbacks have been shown to be important modulators of climate at landscape to global scales. However, the reciprocal interactions between the atmosphere and vegetation are asymmetrical in scale, because the atmosphere 'sees' the vegetation as an aggregate physical entity (with properties of albedo, surface roughness, etc.) but each plant species reacts differently to climate change, and vegetation characteristics emerge from species-level responses.
Understanding the role of atmosphere-vegetation interactions during the Quaternary thus requires data about vegetational change across multiple levels of ecological organization. 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. My collaborators (from the University of Minnesota, Purdue University, and the University of Oregon) and I are currently applying these reconstructions to investigate sensitivity of a regional climate model (RegCM) to Holocene shifts in the prairie-forest border in eastern North America.
PUBLICATIONS
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. (PDF (subscription required))
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 (PDF)
Notaro, M., Liu, Z., and Williams, J. W. (2006) Observed vegetation-climate feedbacks in the United States. Journal of Climate 19: 763-786. (PDF)
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. (Abstract and Figure PDFs) (PDF)
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. (Abstract) (PDF)
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. (Abstract) (PDF)
Ecological Responses to abrupt climate change
Abrupt climate changes in the past offer a natural experiment for study the rates and types of ecological response under times of rapid change - a critical issue given that anthropogenic modifications of atmospheric chemistry are increasing the risk of future rapid climate change. Multiproxy lake-sediment records are ideal for examining vegetational responses to abrupt climate change, because indices of past climate change (e.g. oxygen and hydrogen isotopes, chironomids and ostracodes) and past vegetational change (pollen and larger plant fossils) can be extracted from the same sediments. High-resolution paleoecological and paleoclimatological records in North America and Europe consistently show that vegetation response times to late-glacial climate change were less than 200 years, and may have been on the order of decades or less. This finding is consistent with reports that plant distributions may already be shifting in response to recent climate changes. Analyses of the response of plant taxa and communities in eastern North America show that plant changes during the Younger Dryas Chronozone (Shuman et al., 2002) were spatially complex and do not indicate a regionally coherent cold event. These studies need to be followed with the collection of more high-resolution records in regions distant from treeline.
PUBLICATIONS
Williams, J. W., Post, D. M., Cwynar, L. C., Lotter, A. F., Levesque, A. J. (2002) Rapid vegetation responses to past climate change. Geology 30: 971-974. (Abstract) (PDF)
Shuman, B. N., Webb III, T., Bartlein, P. J., Williams, J. W. (2002) The anatomy of a climatic oscillation: Vegetation change in eastern North America during the Younger Dryas chronozone. Quaternary Science Reviews 21: 1777-1791. (PDF)
Plant diversity in space and time
Plant diversity is threatened at present by climate change, habitat loss and fragmentation, and the introduction of invasive species, while the mechanisms that promote and maintain diversity remain poorly understood. As part of the Knowledge Network for Biocomplexity project at the National Center for Ecological Analysis and Synthesis, I have been studying patterns of species richness across modern and pre-European-settlement gradients of ecosystem productivity and environmental favorability. I am also pursuing research into temporal variations in the latitudinal gradient of plant diversity.
PUBLICATIONS
Kaufman, D. M. and Williams, J. W. (accepted, in revision) Latitudinal gradients of diversity, past and present: plants in eastern North America for the last 16,000 years. Journal of Biogeography.
Williams, J. W., Seabloom, E. W., Slayback, D., Stoms, D. M., Viers, J. H. (2005) Anthropogenic impacts upon plant species richness and net primary productivity in California. Ecology Letters 8: 127-137. (PDF)
Chalcraft, D. R., Williams, J. W., Smith, M. D., Willig, M. R. (2004) Scale dependence in the relationship between diversity and productivity: assessing the role of spatial and temporal turnover. Ecology 85: 2701-2708. (PDF)
Jennings, M. D., Williams, J. W., Stromberg, M. R. (2005) Diversity and productivity across plant communities of the inland northwest, USA. Oecologia 143/144: 607-618. (PDF)
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