Climatic Inferences from Dendroecological Reconstructions - Dendroclimatology - page 287

periods of maximum coherence (cross correlations) among the fire-scar chronolo-
gies during warm phases of the AMO, and minimum coherence during cool phases.
The longest and coldest AMO phase occurred during the period 1750-1849, encom-
passing the fire-scar gap observed in many of the sites in the network. These findings
corroborate and expand a growing list of tree-ring studies demonstrating fire-climate
teleconnections at mountain range to subregional scales in the western United States
involving ENSO (e.g., Swetnam and Betancourt 1990 ; Veblen et al. 1999 , 2000 ;
Donnegan et al. 2001 ; Heyerdahl et al. 2002 ; Norman and Taylor 2003 ; Schoennagel
et al. 2005 ) , PDO (e.g., Norman and Taylor 2003 ; Westerling and Swetnam 2003 ;
Hessl et al. 2004 ; Taylor and Beaty 2005 ; Schoennagel et al. 2005 ) , and AMO
(Brown 2006 ; Sibold and Veblen 2006 ) .
9.4 Ecologically Effective Climate Change
It appears likely that changes in both fire regimes and increases in tree recruitment
during the early nineteenth century in western North America and South America
are related to changes in timing and strength of ENSO events (Brown and Wu 2005 ) ,
with further possible modulation of the effects of ENSO (at least in North America)
by the AMO (Kitzberger et al. 2007 ) . The early 1800s regional cohort synchrony
(Fig. 9.10 ) may be the best example of the contingency of favorable regional cli-
mate and fire conditions occurring together to promote successful establishment;
i.e., wetter conditions and longer fire intervals (e.g., Figs. 9.8 , 9.11 , and 9.12 ) . We
suggest that overall, these findings and the widespread occurrence of the 'gap' in
fire-scar chronologies in North and South America are compelling examples of an
'ecologically effective climate change' deserving of more focused analysis by pale-
oclimatologists and others. In particular, we posit that these ecological responses
that occurred at the turn of the eighteenth to nineteenth centuries reflect one of the
most important and ecologically effective climatic changes in the past three cen-
turies in western North America and southern South America, causing changes in
forest structure that persist to the present.
Most dendroecological studies involving climatic analysis, and most of the pre-
ceding examples in this chapter, focus primarily on the ecological implications of
the responses of disturbances or demographic processes to climate variations and
changes. This is natural and appropriate as a focus of dendroecological research.
However, now that numerous case studies have been conducted demonstrating and
evaluating ecological responses to climate, and broadscale network approaches
using dendroecological data are increasingly feasible (and with regional datasets
starting to become available), we propose that such dendroecological datasets and
analyses be used more broadly for identifying important climatological events
and processes. Glacier- and lake-level fluctuations are commonly used by den-
droclimatologists and paleoclimatologists as corroborating lines of evidence for
identifying climate events and changes. Fire and other dendroecological evidence
are used less frequently for such validation (but see Cook et al. 2004 for an example