Climatic Inferences from Dendroecological Reconstructions - Dendroclimatology - page 267

1851

Largest Fire Years
(>20 sites)

1748

60

40

1752

1684-85

1773

1879

1729 1806

1819

1801

1715 − 16

Intensive Livestock Grazing
and Fire Suppress i on Era

1631 − 32

1735
1738

1765

1664

1763

1 70 9

1 6 22 16 4 8

20

0

1

1600 1650 1700 1750 1800 1850 1900 1950 2000

6

5

4

3

1885

1620

2

1680

1687 1712 1783 1791 1827

1815 1888 − 89

1618

1

1839 1844

1634 1649 1741

Smallest Fire Years (<4 sites)

1609

0

1600 1650 1700 1750 1800 1850 1900 1950 2000

Year

Fig. 9.2 The number of sites recording fire scars in the southwestern United States from 1600
to 2000. This chronology of regional fire occurrence is based on the 120-site network shown in
Fig. 9.1 . The largest regional fires are labeled in the upper plot, and the dashed lines show the
threshold of >20 sites and <4 sites used to identify 'large' and 'small' regional fire years, respec-
tively. The lower plot shows the smallest fire years. Note that the values in the upper plot are
shown as black vertical bars on a white background, and in the lower plot the values are white
vertical bars on a black background. Because these were mainly surface fires burning through
grass and herbaceous fuels, the widespread introduction of livestock grazing that accompanied
Euro-American settlement in the late 1800s led to the decline of fires in virtually all sites

occurred during relatively dry years of a set of gridded, independently derived recon-
structions of summer Palmer Drought Severity Indices (PDSIs; Cook et al. 2004 ;
Fig. 9.3 , top panel). In contrast, years during which almost no fires were recorded
around the region tended to occur during relatively wet years. Regional fire years
also tended to occur during years of cool sea surface temperature (SST) conditions
(low Niño-3, an index of ENSO, which are La Niña years), while all but one of the
small fire years occurred during warm sea surface temperature conditions (Fig. 9.3 ,
bottom panel; El Niño years).
These patterns are supported by superposed epoch analysis (SEAs; Fig. 9.4 ) .
Superposed epoch analysis is used to compare average annual climate anomalies for
the set of regional fire years to climate for the entire period of the climate reconstruc-
tions (Swetnam 1993 ) . Superposed epoch analysis also is used to compare climate