John W. (Jack) Williams

Research

University of Wisconsin-Madison
Room 421 Science Hall
550 North Park St
Madison, WI 53706
jww@geography.wisc.edu
608-265-5537

My research interests center on ecological responses to climate change and the two-way interactions between the terrestrial biosphere and atmosphere.  Most of my work has focused upon late-Quaternary vegetational and environmental change.  This is a particularly exciting time to study because 1) it includes large and in some cases quite rapid changes in climate, atmospheric CO₂, solar radiation, and ice sheet extent, 2) these environmental changes transformed the distribution of plant taxa and the structure and function of terrestrial ecosystems, 3) numerous well-dated paleoclimatic and paleoecological records enable spatially explicit reconstructions of vegetation and climate history, and 4) Quaternary history sets the context for present-day biogeographic patterns and current challenges in global change research. 

My training and research approach are fairly broad.  I work at the interface between data and models, with an emphasis upon quantitative analyses of ecological datasets and mechanistic models of the vegetation and climate system.  Much of my recent work has focused on mapping late-Quaternary vegetation history and land cover change in North America, using networks of paleoecological records, with the goal of developing benchmark datasets for refining and testing earth system models.  These maps also generate new questions and hypotheses, thus serving as a starting point for further field-based research. 

Current research areas include:
    1)    Late-Quaternary land-cover change and surface-atmosphere feedbacks
    2)    ‘No-Analog’ plant associations and environments
    3)    Ecological responses to abrupt climate change
    4)    Plant diversity in space and time

Late-Quaternary land-cover change and 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:

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) (Request Reprint)

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) (Request Reprint)

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) (Request Reprint)

‘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.   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) understanding the role of seasonality and other environmental factors for promoting the development and ultimate collapse of the late-glacial no-analog plant associations and 2) determining whether the no-analog fossil pollen assemblages represent novel mixtures of species within stands, stands within landscapes, or other spatial arrangements.

 

Publications:
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. (Request Reprint)

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) (Request Reprint)

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 <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 theYounger 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) (Request Reprint)

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. (Request Reprint)

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:

Chalcraft, D. R., Williams, J. W., Smith, M. D., Willig, M. R. (in press) Scale dependence in the relationship between diversity and productivity: assessing the role of spatial and temporal turnover.  Ecology.