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Holocene. Increased precipitation and cooler temperatures
would have meant higher moisture availability to plants
nearly everywhere during the late Holocene. An increase
in available soil moisture could have resulted in a change in
the dominance of plants toward those better adapted to
cooler, moister conditions. Indeed, red
fir, the dominant
tree growing at Coburn Lake today, grows in the Sierra
Nevada where the winter snow pack is the deepest [Barbour
et al., 1991].
Increased soil moisture and consequent changes in vege-
tation would have altered fire regimes at Coburn Lake and in
other areas. Light fuels, like grass, may have increased in the
generally depauperate understory of red
fir stands resulting in
more frequent low-severity surface fires. On the other hand,
moister and cooler conditions could have resulted in an
increase in heavy fuels within red
fir stands, along with fewer
opportunities for those heavy fuels to dry out. When severe
droughts occurred, those heavy fuels would have dried out.
This would have resulted in severe stand-replacing
fires, of
the type I hypothesize are represented in the Coburn Lake
charcoal record (section 5.1.3).
5.5. Controversy
The results from Coburn Lake were presented at science
meetings from August of 2005 [Wathen, 2005] through May
of 2007 [Wathen, 2007]. Attempts to publish results from
Coburn Lake began in November of 2005 and have contin-
ued since. Opposition to the findings from Coburn Lake have
come primarily from three concerns: (1) that I did not use
traditional approaches to analyzing the charcoal record from
Coburn Lake, (2) that I did not provide enough evidence for
climate teleconnections between California and the North
Atlantic, and (3) that I did not provide an explanation for
those teleconnections.
Figure 12.
Increase in Greenland climate variability over the past
8500 years. (a) A reconstructed ice accumulation record from the
GISP2 ice core serves as a proxy for precipitation [Meese et al.,
1994; NSIDC and WDC-A, 1997]. (b) A reconstructed
δ
5.5.1. The study of sediment charcoal. Researchers have
used sediment charcoal to study the relationships between
past climates and past
18
O curve
from the GRIP ice core serves as a proxy for air temperature
[Johnsen et al., 1995; NSIDC and WDC-A, 1997]. The two large
vertical arrows highlight the beginning of increases in the magni-
tude of centennial-scale oscillations of precipitation and tempera-
ture at 3000 and 3700 years ago, respectively.
fire frequencies. The standard meth-
odology for interpreting lake sediment charcoal records was
designed to study fire frequencies at the millennial time scale
[Long et al., 1998]. This methodology has been used with
success to analyze
charcoal records that contain
only marginally higher charcoal peaks relative to high levels
of background charcoal [Cwynar, 1977; Gajewski et al.,
1985; Clark, 1990; Whitlock and Anderson, 2003]. With this
standard methodology, a line of demarcation between char-
coal peaks and what is considered to be the background level
of charcoal for a given record is determined. Peaks that are
higher than this predetermined background level of charcoal
are assumed to represent individual severe
“
typical
”
increase in the Sierra Nevada for the long-term beginning
approximately 2000 years ago [Mensing et al., 2004].
Increased precipitation, cooler temperatures, and an in-
crease in the frequency of ACCEs in the Northern Hemi-
sphere could explain the increase in severe fires at Coburn
Lake, and perhaps at similar environments, during the late
re events.
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