North American Tree Rings, Climatic Extremes, and Social Disasters - Dendroclimatology - page 301

Fig. 10.3
Tree-ring-reconstructed
summer PDSI averaged at
each of the 286 grid points
and mapped for the 10 most
extreme consecutive years of
the early twentieth-century
pluvial (1907-1916; same
mapping conventions as in
Fig. 10.2 ) . Note the two cells
of reconstructed wetness over
the central Rocky Mountains
and Mexico
the Bureau of Reclamation from discharge data compiled during the twentieth-
century pluvial (Hundley 1975 ; Fye et al. 2003 ; see Woodhouse et al. 2006 for a
recent reanalysis). Streamflow reconstructions for the Salt, Verde, and Gila Rivers
(Graybill 1989 ; Graybill et al. 2006 ) show a similar positive anomaly for the
Colorado River drainage below Lees Ferry. The early twentieth-century period of
elevated flow was certainly not sustained, but it coincided with the negotiations that
led to the Colorado River Compact, which over-allocated the flow of the Colorado
River among the basin states and later included Mexico (Brown 1988 ) . This wet
period also coincided with massive ecological changes on the forest and range-
lands of the West, and even in the absence of human activities would have favored
reduced fire and a pulse of forest regeneration (Swetnam and Betancourt 1998 ;
Westerling and Swetnam 2003 ) . These favorably wet conditions may have con-
tributed to the 'unhealthy' overstocked forests with elevated fire risks in the West
that also have been encouraged by overgrazing, deliberate fire exclusion, and anthro-
pogenic warming associated with warmer spring temperatures and earlier snowmelt
(Westerling et al. 2006 ) .
This chapter cites a selection of tree-ring studies of climate extremes with
demonstrated or suspected societal impacts, including both moisture and temper-
ature extremes. We then use the gridded tree-ring reconstructions of the summer
PDSI for North America (Cook et al. 1999 , 2004 ; Cook and Krusic 2004 ) and
other regional climate reconstructions to estimate the intensity and spatial extent
of selected drought and wetness extremes that are related at least chronologically, if
not causally, to major societal changes in parts of North America. This retrospective
discussion begins with the data-rich modern era, for which we know much more
about the impacts of climatic extremes on society; it then extends back in time to