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Interpretation of Fire Scar Data from a Ponderosa Pine Ecosystem
in the Central Rocky Mountains, Colorado1
R. D. Laven, P. N. Omi, J. G. Wyant and A. S. Pinkerton2
Abstract.--Fire scar data from twenty ponderosa pine
. trees located in the Central Rocky MOuntains, Colorado, were
used to determine fire frequencies by historical era, fire
size, topographic position and slope aspect. The average
mean interval between fires is somewhat larger than in other
parts of ponderosa pine's range.
INTRODUCTION
litter is usually meager, canopy coverage is
generally no more than 25% (Daubenmire 1943),
and fine fuels appear to be less than in other
parts of its range. In the only quantitative
study to date, Rowdabaugh (1978) indicates an
average fire frequency of 84 years prior to the
initiation of fire suppression, and 110 years for
the entire fire chronology.
Prior to the arrival of European man to North
America, creeping surface fires were a common
occurrence in ponderosa pine (Pinus ponderosa
Laws.) forests (Kotok 1·934, Wagener 1961, Biswell
1972). These fires had a significant effect on
stand structure and composition by eliminating
understory reproduction and enhancing regenertion within forest openings. The result was an
uneven-aged forest with trees growing in evenaged groups (Biswell 1972, Gartner and Thompson
1972, Wright 1978). Previous studies indicate
that fire frequency varies in these forests depending on study area location. Weaver (1951)
found the frequency in Arizona and New Mexico to
be between 5 and 12 years. Show and Kotok (1924)
and Wagener (1961) determined an average frequency of 8 to 10 years in California. Arno
(1976) reported an average frequency of 6 to 11
years for mature ponderosa pine in western Montana
and eastern Idaho.
In this study we have determined the mean
time interval between fires in several ways as
our data tend to support Kilgore and Taylor's
(1979) viewpoint that a single mean time interval is misleading. We calculated fire frequencies
based on the historical time period, the size of
the fire, the topographic position, and the slope
aspect.
Study Area
The study area ig a 50 ha ponderosa pine
forest located in Wintersteen Park in the Front
Range of the Colorado Rocky MOuntains, approximately 2 km northeast of Rustic, Colorado in the
Roosevelt National Forest. Lying in a level to
steeply sloping tertiary drainage of the Cache
la Poudre River, the study area ranges from 2500
m to 2800 m in elevation. Numerous outcroppings
of Precambrian granite occur throughout the area.
The soils are rocky loams of the Wetman-BoyleMOen complex. These are shallow, well-drained
soils of steep to gentle mountain slopes (USDA
Soil Conservation Service, unpublished).
Little is known, however, about natural fire
frequencies in the ponderosa pine forests of the
central Rocky Mountains. Daubenmire (1943) indicates that slow spreading surface fires were
quite common but does not attempt to quantify
their frequency. Wright (1978) speculates that
fire may be less frequent in this region since
lpaper presented at the Fire History Workshop, University of Arizona, Tucson, October 2024, 1980. The research reported here was approved
and financially supported by the Eisenhower Consortium for Western Environmental Forestry Research.
2Assistant Professor, Associate Professor,
Graduate Research Assistant and Forestry Technician, respectively, Dept. of Forest & Wood Sciences, Colorado State University, Fort Collins ,CO.
METHODS
The primary objective was to determine sitespecific fire history for the study area. For
this reason, all intact fire-scarred trees in the
location were sampled. Lightning-scarred trees,
and trees that showed evidence of previous scarring but were healed over, were not included in
46
the sample. As a result, we examined 20 trees
that encircled the study area.
inception of fire suppression at the turn of the
century, 65.4% of the scarring occurred. The remaining 13.5% of the scars were recorded during
the suppression era.
Wood sections were removed from the base of
each scarred tree using the method described by
McBride and Laven (1976). Each tree was aged
and its location was recorded on a topographic
map. Fire dates were determined for each tree
by counting the number of annual growth rings
from the sampling year (1980) to the last (most
recent) scar, and between previous scars on each
wood section.
Fire Frequencies
Table 1 presents a summary of our determinations of fire frequency. The average mean interval
between fire scars for all sampled trees throughout the entire chronology is 45.8 years. This
frequency compares to Rowdabaugh's {1978) figure
of 110 years. The apparent disparity in results
is considerably lessened if the data are uniformly
analyzed. For example, if we use Houston's (1973)
formula for fire frequency {used by Rowdabaugh)
our frequency becomes 129.9 years. On the other
hand, if we use Rowdabaugh's data and calculate
the frequency as the mean interval between fire
scars (our method), his frequency becomes 37.9
years. Rowdabaugh's pre-suppression figure of
84.0 years likewise becomes 38.9 years. Either
way, the results are much closer. Houston (1973)
incorporates the entire age of the tree in determining the frequency, whereas we consider only
the intervals between fire scars. The difference,
then, is that Houston's (1973) formula includes the
time it takes for initial scarring and the time
Cross-dating techniques were used to reduce the possibility of dating errors and to
account for potential complications such as missing rings, double or false rings or anomalous
rings associated with fire scarring. Skeleton
plots (Stokes and Smiley 1968) were constructed
for each specimen and used to develop the master
fire chronology. Since scars, however, are not
caused by every fire and more recent fires can
consume the evidence of earlier fires, the recorded scars represent a conservative estimate
of the number of previous fires.
For each individual tree the mean interval
between scars was determined. These values were
used to calculate the average mean interval for
all of the trees as a function of aspect, topographic position, historical era and fire size.
Additionally, we compared the average mean intervals using our formulation with the results
generated using Houston's (1973) formulation.
Table 1. Average Interval Between Fire Scars in
Years {Wintersteen Park, Colorado)
Ring widths for each individual for three
years prior to each scar were compared to the
mean ring width of the previous ten years to investigate the supposition that fires in this region are preceded by several years of drought.
An optical micrometer mounted to the eyepiece of
a dissecting microscope was used to measure the
ring widths to .0005 mm.
X
(min-max)
Entire Chronology {n
Finally, a stand-age analysis was performed
to determine if the age-class destribution found
on our study site was a typical ponderosa pine
mosaic of even-aged groups. Two increment cores
taken at ground level were prepared in the lab
and examined under a dissecting microscope to determine tree age; tree ages were compared to fire
scar dates to assess the degree of correspondence.
Slope Aspect
South (n
a
18)
North (n
a
2)
a
20)
34.9
{ 3-161)
64.3
( 5-157)
Topographic Position
Lower Third ~D 5)
Middle Third (n
a
5)
{n
a
5)
Upper Third
RESULTS AND DISCUSSION
Fire Chronology
The range in fire scar dates covered a period
from 1708 to 1973. Because the mean age of the
sampled trees was 248 years and the mean age of
the trees when initial scarring occurred was 81
years, relatively few scars (21.1%) occurred prior
to the mid-1800's. Mortality, and consumption of
scars by subsequent fires would also contribute
to this low percentage. Between the beginning of
the settlement in the mid-1800's until the
47
Historical Era
Pre-1840 (n
a
6)
1840-1905 (n
a
12)
Post-1905 (n
a
3)
Fire Size
Small (n
a
9)
Large (n
a
12)
45.8
{ 3-161)
37.5
( 5-157)
35.7
( 3-105)
37.9
( 5-114)
66.0
( 5-157)
17.8
( 3-161)
27.3
8-46
20.9
(11-145)
41.7
( 5-63 )
since last scarring (in our case an average of
81.3 and 91.6 years, respectively). Since unscarred trees are not as sensitive to scarring
by low intensity fires as previously scarred trees
(Kilgore and Taylor 1979, Gill 1974, Lachmund
1921), including this relatively insensitive
period when calculating fire frequency may be
misleading.
fires every 41.7 years. It is noteworthy that
during the settlement era, the mean incidence of
large fire occurrence was 16.3 years.
Tree Ring Widths
Analysis of average tree ring widths three
years prior to a fire scar did not reveal any
significant narrowing. If a prolonged drought
period is a precursor to fire occurrence in this
region, the narrower ring widths would most likely
precede large fires prior to the influence of
settlement. However, regardless of historical
era or fire size, ring width narrowing was not
exhibited.
As previously suggested, we feel that a
single mean interval is not an appropriate representation of fire frequency. We therefore calculated the mean interval for a variety of eire~
stances. We emphasize, however, that by so doing
we reduce an already small sample size with considerable inherent variability. Consequently the
results should be viewed as only indications of
trends.
Stand-Age Analysis
The mean fire frequency for trees occurring
on south-facing slopes is 34.9 years which co~
pares to 64.3 years for north-facing slopes. The
north-facing interval is based on a sample of
only two trees; the dearth of sample trees in
itself is indicative of the variation in frequency associated with slope orientation.
Analysis of age-class distribution does not
support the classic concept that ponderosa pine
is found in small even-aged groups that are part
of a larger uneven-aged·forest. Our study area
is indeed uneven-aged, but the suspected corresponpence of stand age with time elapsed since fire
is not found. Because of the open nature of this
forest, successful seedling establishment may not
be as strongly tied to the exposure of mineral
soil (as a result of fire) as ponderosa pine
forests with heavy litter loadings.
In spite of a 300 m elevational change, fire
frequencies are essentially the same (less than 2
years difference) for the lower third, middle
third, and upper third of the slope. Analysis of
fire sizes (discussed subsequently) indicates that
these similarities are not due to individual fires
burning entire slopes.
CONCLUSIONS
Based on the history of land use in the general area (Swanson 1975), three eras were distinguished which provide a more realistic indication of past fire frequency. The pre-settlement era (pre-1840) revealed a mean interval of
66.0 years; the settlement era (184Q-1905) had a
mean interval of 17.8 years; and the suppression
era (post-1905) has a mean interval of 27.3 years.
Since these eras encompass different spans of
time, another approach is to compare the number
of fire scars per 100 annual rings of growth
(Kilgore and Taylor 1979). For these three historical eras, the respective figures are 8.3,
52.3, and 9.3. The large differences in frequencies associated with these historical eras
only serve to underscore the need to interpret
average fire frequencies with caution.
Fire history studies are often conducted to
provide guidelines for fire management planning.
We assume that the resultant fire frequencies
reflect the 'natural role of fire' that helped
shape the structure and composition of the area
being studied. This assumption may not be valid
and we are therefore compelled to interpret our
results with caution.
Each historical era of our fire chronology
is confounded to such an extent that interpretation is difficult. The pre-1840 portion of the
chronology is not complete; the data become less
reliable as you move back in time, and the role
that Indians played cannot be adequately quantified. The settlement era is confounded by the
impact of settlers, lumbermen, cattlemen, and
others, and obviously the fire suppression era is
impacted by our control technologies. Averaging
over these time periods is not the solution; in
fact averaging tends to further obscure the results.
In order to compare frequencies by fire size,
two general size categories were distinguished.
The "small" fire size category represented fires
that scarred single or small groups of trees in
close proximity. Such fires were on the order of
1 ha in size. The nlarge" fire size category
represented fires that scarred trees on at least
two opposing slopes, and were approximately 25 ha
in size. Approximately half of the fires fell
into each category. There is no evidence to suggest that a single fire burned the entire study
area. Using fire incidence as our measure of
frequency in this case, the small fires occurred
on the average of every 20.9 years and the large
Since most studies of this kind are fraught
with similar difficulties, inter-study comparisons
of results may be the most valuable aspect of this
work. Our results indicate that the average mean
interval between fires is somewhat longer in ponderosa pine forests of the central Rockies than
in other geographical regions. Additionally, the
fire regime is variable: longer interval "large"
fires are superimposed on more frequent ''small"
48
fires. Finally, successful regeneration of ponderosa pine does not appear to be correlated with
past fire occurrence.
McBride, J. R. and R. D. Laven. 1976. Scars as
an indicator of fire frequency in the San
Bernardino Mountains, California. J. of
For. 74(7):439-442.
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49
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