COS 105-7 - Estimating changes in prehistoric forest composition from sediment pollen in the Upper Midwestern United States

Wednesday, August 9, 2017: 3:40 PM
C120-121, Oregon Convention Center
Andria Dawson1, Christopher J. Paciorek2, Jason S. McLachlan3, Simon Goring4, John W. Williams4 and Stephen T. Jackson5, (1)Department of Geosciences, University of Arizona, Tucson, AZ, (2)Statistics, University of California, Berkeley, Berkeley, CA, (3)Biological Sciences, University of Notre Dame, Notre Dame, IN, (4)Geography, University of Wisconsin-Madison, Madison, WI, (5)Southwest Climate Science Center, U.S. Geological Survey, Tucson, AZ

Terrestrial ecosystems play an important role in Earth systems
processes, yet we still do not understand how they respond to changes
in climate. While it has been argued that terrestrial ecosystems were
fairly stable (by Quaternary standards) in the millennia before major
anthropogenic disruption, others have emphasized vegetation response
to environmental variability during this time. These competing
perspectives are not necessarily in conflict, but argue for a
quantitative assessment of forest ecosystem variability over the last
several millennia.

Here we reconstruct maps of forest composition for the last two
millenia, with uncertainty. To do this, we use a network of fossil
pollen records - the most reliable paleoecological proxy for forest
composition. We link the fossil pollen records to public land survey
forest composition using a Bayesian hierarchical model which accounts
for key processes including pollen production and dispersal (Dawson et
al., 2016; Paciorek et al., 2009). The model is calibrated using data
from the pre-settlement time with the hope of minimizing anthropogenic
impacts. Process parameters are estimated in the calibration phase,
and are subsequently used in the prediction phase to generate
spatially explicit maps of relative species composition across the
upper Midwestern US over the last 2000 years, with robust uncertainty


Estimates of forest composition and uncertainty improve the
spatio-temporal resolution of our previous understanding of past
forest change in the upper midwestern US. First, results show
significant increases in hemlock composition within the established
hemlock range in Wisconsin. Second, results indicate little movement
in ecotone position (between temperate hardwood forests and northern
mixed hardwood/conifer forests), predominantly as a

result of changes in pine comoposition. Third, significant
increases in elm and ash indicating the emergence of the distinct
Minnesota Big Woods region at approximately 500 years before
present. These changes are significant in both a statistical and
ecological sense, but the scale of these changes is small relative to
changes in the early Holocene. Our novel spatio-temporal composition
estimates will be used to improve the forecasting capabilities of
ecosystem models.

Dawson, A., Paciorek, C. J., McLachlan, J. S., Goring, S., Williams, J. W., & Jackson, S. T. (2016). Quantifying pollen-vegetation relationships to reconstruct ancient forests using 19th-century forest composition and pollen data. Quaternary Science Reviews, 137, 156-175. ;

Paciorek, C. J., & McLachlan, J. S. (2009). Mapping ancient forests: Bayesian inference for spatio-temporal trends in forest composition using the fossil pollen proxy record. Journal of the American Statistical Association, 104(486), 608-622.