Climatic Inferences from Dendroecological Reconstructions - Dendroclimatology - page 283

Fig. 9.11 Fire-scar chronologies from ponderosa pine and mixed-conifer forests in the Gila
Wilderness, New Mexico (Swetnam and Dieterich 1985 ; Abolt 1997 ) . Horizontal lines are tree-
ring records from individual fire-scarred trees along an elevational gradient, and the vertical tick
marks are the fire-scar dates recorded on the trees. Note the 'gap' in fire occurrence in the period
1811-1837. The lack of fire scars after 1904 was due to livestock grazing and fire exclusion by
government firefighters, and is a common feature of many surface fire chronologies in the western
United States
which might have been associated with volcanic eruptions, and is clearly indicated
in Northern Hemisphere temperature reconstructions (e.g., Mann et al. 1998 , 1999 ) .
Additional detailed analysis of the Southwest regional fire-scar series (Fig. 9.2 ) ,
and a subset of this series, offers further insight on the nature and possible causes of
the gap. Swetnam and Baisan (1996) found that an index of regional fire occurrence
in the Southwest based on 63 fire-scar sites was highly correlated with tree-ring-
width-based Palmer Drought Severity Index reconstructions from the region during
the early to mid-1700s and after the mid-1800s (Pearson's r > 0.8). However, this
correlation declined precipitously during the late 1700s and early 1800s ( r <0.3)
before rising again after about 1850. This finding seems to support the interpre-
tation that interannual climate variations were reduced, perhaps associated with a
quiescent ENSO, and this may have caused an uncoupling from the fire occurrence
pattern in the Southwest (see also Fig. 9.8 , and Brown and Wu [ 2005 ] findings of
reduced 'biennial' oscillations during this period).
Other dendroecological evidence also points to possible shifts in fire regimes
during the late eighteenth- and early nineteenth-century period. An analysis of a sub-
set of fire-scar chronologies from the El Malpais area of west-central New Mexico