Tree Rings and Climate: Sharpening the Focus - Dendroclimatology - page 341

Dendroclimatology has experienced explosive growth since the late 1980s, with
a great number of published papers documenting regional- to hemispheric-scale cli-
matic variability over the past several centuries. Its results and insights have been
applied to scientific and practical problems in ecology and hydrology; for example,
issues with important policy and management implications.
Swetnam and Brown ( Chapter 9 , this volume) review the parallel strategy of net-
work development that ecological tree-ring studies have pursued in recent decades,
building from local scales to networks of ecological histories at regional to con-
tinental scales. At these broader scales, chronology networks reveal emergent
patterns of spatial and temporal synchrony of forest fires, insect outbreaks, and
tree demography (natality and mortality) reflecting climatic drivers (e.g., Swetnam
and Betancourt 1990 , 1998 ; Kitzberger et al. 2007 ) . The existence of independently
derived climatic proxies from ring-width and density chronologies has greatly facil-
itated this new development in 'macroecology.' Moreover, the remarkably strong
association between extreme years in the regional fire history networks and the
drought and ENSO reconstructions, for example, provide significant assurance that
both types of networks have captured the most extensive and highest magnitude cli-
mate and fire events (e.g., Fig. 9.3 in Swetnam and Brown, Chapter 9 , this volume;
Swetnam and Betancourt 1990 , 1998 ) .
Although tree-ring ecological and climate networks have, so far, been assem-
bled and used jointly primarily at regional scales, hemispheric and global scales
are feasible. Following the work of Villalba and colleagues regarding evidence
for synchronicity of dendroclimatic responses to Pacific Ocean SST changes on
multiple timescales, Kitzberger et al. ( 2001 ) showed that forest fire histories in
North and South America shared similar histories and relationships to ENSO. The
newest and perhaps most promising approach for reconstructing regional to conti-
nental (and perhaps global) forest fire histories involves an adaptation of the classic
dendroclimatic regression methods of calibration and verification. Combinations
of modern-area-burned time series (from forest fire records and atlases), indepen-
dent networks of tree-ring width chronologies, and tree-ring dated fire-scar based
fire chronologies are used to reconstruct and test multicentury histories of wildfire
at regional and continental scales (Westerling and Swetnam 2003 ; Girardin 2007 ;
Girardin and Sauchyn 2008 ) .
The Girardin ( 2007 ) and Girardin and Sauchyn ( 2008 ) studies have been particu-
larly valuable in evaluating the rise in area burned in Canada in recent decades in the
context of multiple centuries, and in association with warming trends, as observed in
twentieth-century data for both Canada and the western United States (Gillett et al.
2004 ; Westerling et al. 2006 ) . The values and uses of these ecological disturbance
and climate studies from tree rings have expanded from improved basic understand-
ing of large-scale processes to new insights with applications in management. For
example, applications have included the development of predictive models of sea-
sonal fire activity based on the state of the ENSO and prior season rainfall and
drought conditions (Corringham et al. 2008 ) .
During past few decades, a significant amount of work regarding hydrologic
aspects of climatic variability throughout the world on interannual to century