Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 215

southwestern United States-northern Mexico for both patterns, with the sign being
positive for El Asiento and negative for the Polylepis records. Overall, the spatial
correlation patterns produced by Niño-3 index reconstructions and the precipitation-
sensitive records from South America are similar, indicating that the tree-ring
estimates are capturing the interhemispheric ENSO teleconnection pattern. The sig-
nificant correlations between El Asiento and PDSI are more extensive and penetrate
northeastward into the central and upper Midwest of the US. This extension in the
correlation pattern also has been observed by using instrumental records, particu-
larly during the first decades of the twentieth century (Cole and Cook 1998 ) . In
contrast, the spatial pattern with the Bolivian chronologies is highly concentrated
to the southwestern United States-northern Mexico region, making it remarkably
similar to the pattern based on the Niño-3 index (Fig. 7.15 ) .

7.4 Future Research

The previous results highlight the potentiality of tree rings as reliable sources
of information concerning past climate variations. However, dendroclimatological
studies are confronted by many challenges in the western Americas. New tree-
ring chronologies in subtropical Mexico ( Pseudotsuga menziesii , Pinus hartwegii ,
Taxodium mucronatum , Pinus cembroides , Pinus lumholtzii ), in the high-elevation
tropics of Bolivia and adjacent areas of Chile and Argentina ( Polylepis tarapacana ),
and subtropical Argentina ( Prosopis ferox ) allow, for the first time, studies of
annual variability in the subtropics. However, these records are still sparse and
difficult to develop (Box 7.9). On the other hand, the development of climate-
sensitive chronologies from the seasonally dry areas of the high- and lowland
tropics remains a major task in the Americas and the world. It was only during
the first years of the twenty-first century that a number of exploratory studies,
mainly in coastal Peru, have established the basis for building reliable and calendar-
dated chronologies from Bursera graveolens . The chronology, which covers the
interval 1950-2002, shows an ENSO signal with an average recurrence period of
about 5 years (Rodriguez et al. 2005 ) . The search for longer records from tropi-
cal species with clear, identifiable annual rings is still a major challenge in modern
dendroclimatology.
An alternative to the reliance on the presence of annual rings in tropical trees is
provided by isotopic studies. An improved mechanistic understanding of controls
on the oxygen isotope ratio (

18 O) of alpha cellulose (Roden et al. 2000 ) and rapid
processing techniques (Brendel et al. 2000 ) make possible the construction of high-
resolution isotope series from tropical trees that can be analyzed to provide both a
chronology and paleoclimatic information, even in trees lacking annual rings (Evans
and Schrag 2004 ) . The seasonal cycle in

δ

18 O of tropical montane precipitation,
primarily controlled by the difference in the amount of precipitation between wet
and dry seasons, is reflected in

δ

18 O of the cellulose of trees. This seasonal rhythm
can then be used for chronological control. Pilot applications of these techniques in
Costa Rica, Peru, and the Amazon show annual isotope cycles as great as 4

δ

6‰,
which permit high-resolution chronological control even in the absence of annual

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