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tested with the expanding network of tree-ring chronologies for South America. The
discovery of the dendroclimatic value of Polylepis tarapacana , a small arid-site tree
of the Andes, which grows at the highest elevations of any tree species on earth, is
one of the most interesting recent developments in dendrochronology (Argollo et al.
2006 ) . These Polylepis chronologies may help test the 1524 climate-bartonellosis
hypothesis if time series of sufficient length can be developed.
10.3 Social Impacts of Climate Extremes During the Historic Era
The gridded tree-ring reconstructions of the summer Palmer Drought Severity
Index for North America recently produced by E.R. Cook and colleagues pro-
vide an exactly dated, spatially detailed record of the hydroclimatic conditions
attending many tumultuous events in American history and prehistory (Cook et al.
2007 ) . To highlight the selected climatic extremes, we used the time series of
tree-ring-reconstructed summer PDSIs (e.g., Cook et al. 2004 ) averaged from all
286 individual grid point reconstructions across North America for the past 500
years (Fig. 10.6 ) . This reconstruction highlights the most important continent-
wide annual to decadal dry and wet regimes and is highly correlated with the
continent-wide average of summer PDSIs based on the instrumental data ( r
=
0.84
for 1900-1978). We then mapped the patterns of reconstructed PDSIs during the
specific time periods of interest to estimate the intensity and spatial distribution
of these climate extremes. Multiyear droughts with severe social impacts in the
new North American PDSI reconstructions include, or are suspected of includ-
ing, the late nineteenth-century drought over the central and northern Great Plains,
the mid-nineteenth-century drought focused over the central Great Plains, the late
Fig. 10.6 The time series of tree-ring-reconstructed summer PDSI averaged each year for all 286
North American grid points (from Cook et al. 2004 ) , illustrating continent-wide wet and dry spells
from AD 1500 through 1990. Selected severe droughts of the past 500 years are highlighted and
discussed in the text
 
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