Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 201

In 1999, a gridded network of 154 Palmer Drought Severity Index reconstruc-
tions for the continental United States was generated from a set of 388 tree-ring
chronologies (Cook et al. 1999 ) . More recently, the spatial and temporal coverage
of the PDSI reconstructions was expanded, including 286 points in a 2.5 ◦ × 2.5 ◦

grid covering most of North America (Cook et al. 2004 ; Box 7.4 ). The new PDSI
reconstructions are based on an expanded network of 835 tree-ring chronolo-
gies. The temporal coverage was also expanded to the maximum permitted by
the available tree-ring data, extending back nearly 2000 years for some locations.
Finally, the process of variance restoration applied to the grid point reconstruc-
tions allows for updates of those records to AD 2003 with instrumental PDSI
data. In a previous work (Villalba et al. 2001 ) , the temporal evolution of the
PDSI in four cells located in the midwestern-southwestern United States were
compared with precipitation-sensitive chronologies from central Chile. For the com-
mon interval 1700-1978, the correlation coefficient between the first Principal
Component (PC) from the four PDSI reconstructions and tree-ring variations at
El Asiento, central Chile, is r 0.001), which was considered as

an indication of common modes of variations in these series. With the increas-
ing number of PDSI reconstructions across North America, the use of a spatial
approach to study large-scale atmospheric variations connecting mid- to high-
latitude precipitation changes in North and South America now appears to be
feasible.
A major advance in the effort to expand the spatial coverage of tree-ring
records across the Americas has been the recent development of Polylepis tara-
pacana chronologies in the Bolivian Altiplano (Argollo et al. 2004 ) . These records,
located between 17 ◦ and 20 ◦ S and above 4500 m elevation, represent the closest-to-
equator tree rings in the Andes and the highest-elevation chronologies worldwide
(Box 7.7 ). A careful examination of interannual variations in ring width and
climate in the Altiplano indicate that the growth of Polylepis is remarkably asso-
ciated with summer water balance. Most Polylepis records cover the past three
to four centuries, but some of them extend over seven centuries. Two of the
longest chronologies (Caquella and Soniquera) were merged in a single record and
used for comparison with precipitation-sensitive records in other regions of the
western Americas. For central Chile, a ring-width chronology from Austrocedrus
chilensis D. Don at El Asiento (32º40 S), which represents the northernmost
extent of this species in central Chile, was used for comparison with North
American records (Box 7.9 ). Recently, the site was revisited, and series from the
new cores were merged with the original data collected in 1974 by LaMarche
( 1975 ) .

= 0.32 ( p <