Fire and Climate History of Mixed Conifer Woodlands
in the Great Basin, USA
Franco Biondi - fbiondi@unr.edu
Scotty Strachan - strachan@unr.edu
Megan Bradley
Jonathan Cheek
Leia Jamieson
Mackenzie Kilpatrick
Jason Sibold
Abstract GC51D-0774
DendroLab
Department of Geography
University of Nevada, Reno
Background
Fire History
Fire, climate, and species dynamics were investigated before and after Euro-American settlement at two locations
in Lincoln County, Nevada. Both the Mt. Irish and Clover Mountains sites are isolated high ranges in the southern
Great Basin Desert, not far from the northern extent of the Mojave Desert.
At Mt. Irish, non-scarred ponderosa pines and singleleaf pinyon were used to develop a tree-ring reconstruction
of drought (mean PDSI for May-July, NV Clim. Div. 3) from 1396 to 2003. Fire-scarred ponderosa found at both study
areas were then sampled, and crossdated fire-scar records were used to generate the fire history. Fire frequency was
analyzed through time at each site, and compared to a previously-developed “pyroclimatic” hypothesis drawn strictly
from the wet/dry climatic episodes found in the Mt. Irish chronologies.
Fuels loads and reconstructions were also developed at each site along with a study of species dynamics. Based
on a combination of this work, a potential change in fire behavior from a surface to a crown fire occurred after
settlement at the Clover Mountains but prior to settlement at Mt. Irish.
Fire scar samples from both study areas were collected two years after the completion and presentation of the
Mt. Irish PDSI reconstruction and hypothetical fire history. The primary species sampled was ponderosa pine, and
all fire history analysis was performed using this species. Partial cross sections were taken using standard methods,
and scars were crossdated with the master tree-ring chronologies using the visual method.
Ring width was also measured on sections where possible for quality-control purposes and crossdating verification.
When charred scars occurred at the boundary between two rings, the year was assigned to the inner (older) ring
in accordance with fire records from the region that indicated most fire occurrence takes place in the later part of
the year after the growing season.
Numerical and graphical analysis of the fire regime, before and after Euro-American settlement, was based on
FHX2 software (Grissino-Mayer 2001) to compute the mean fire return interval (MFRI). An arbitrary threshold was
used to evaluate larger fires, those scarring 10% of recorder trees.
FHX2 plots of all fires scarring at least two trees
Part of the catastrophic 1946 fire in the Clover Mountains, showing the slow recovery of a once continuous ponderosa stand.
Tree-Ring Chronologies and Climate Analysis
Mt. Irish
Master tree-ring chronologies from Pinus ponderosa scopulorum (Rocky Mountain ponderosa) were developed
at both sites using undisturbed trees. At Mt. Irish, an additional chronology from Pinus monophylla (singleleaf pinyon
pine) was also constructed. These chronologies were used to crossdate fire scars as well as provide climatic information.
Crossdating was possible within and between species. Ring widths were small (PIPO mean of 0.6 mm, PIMO
mean of 0.4 mm), with 234 locally-absent rings out of 17,502 total. Mean segment length for the PIPO and PIMO was
457 and 439 years, and mean sensitivity was 0.426 and 0.508, respectively.
After collecting, processing, crossdating, and measuring cores from the two sites, chronologies were computed
from the ring widths using standard dendrochronological techniques. Latewood and earlywood chronologies were
also computed for PIPO at Mt. Irish. These chronologies were then compared to monthly mean temperature and
total precipitation from the PRISM dataset using a 12-month water year (Oct-Sept) to evaluate tree response to
climatic variables. Both species showed positive relationships with winter-spring precipitation and negative
relationships with spring-summer temperature, a reflection of response to hydrological drought.
The best combination of predictors were the pinyon ring width and the ponderosa latewood width, which together
explained 41% of the mean summer (May-July) PDSI from 1930 to 2003.
Acknowledgements
Left: All scars collected from Mt.
Irish, including those excluded
from numerical summaries
because of dating confidence or
bracketing issues.
This work was supported, in part, by Cooperative Agreement
No. FAA070002 with the Bureau of Land Management under the
Great Basin Cooperative Ecosystem Studies Unit Agreement, and
by NSF Cooperative Agreement EPS-0814372 to the Nevada System
of Higher Education.
The views and conclusions contained in this document are those
of the authors and should not be interpreted as representing the
opinions or policies of the U.S. Government.
We thank the additional DendroLab personnel who helped with
field and laboratory work, particularly Kelli Hoover, Kurt Solander,
Michael Koch, Katie Mann, and Alex Mensing.
Right: Time-series graph from Mt.
Irish representing sampled and
“recorder” trees, together with
the number of fire scars per year
from 1200 to 2006.
These graphs show limitations of sample depth
and preservation, but also illustrate strongly
the peak of fire in the 1800’s at Mt. Irish.
Vegetation Dynamics, Fuel Studies, and Discussion
PILO
CELE
MTI (PIMO)
MIR (PIPO)
PIPO
PIMO
JUOS
Above: Plot showing synchrony between normalized
chronologies from pinyon and ponderosa pine at Mt.
Irish.
Below: Correlation maps between the ponderosa and
pinyon pine chronologies and all tree-ring chronologies
publicly available from the International Tree-Ring Data
Bank (ITRDB), clearly showing a Great Basin signal.
Above: Bootstrapped moving correlation functions
between the chronologies and PRISM monthly
temperature and precipitation.
Below: Reconstructed summer Palmer Drought Severity
Index (PDSI) for Nevada Climate Division 3 with a
“pyroclimatic hypothesis” consisting of “wet” and “dry”
sets of a-priori rules.
Mt. Irish Study Area
Clover Mountains Study Area
The Pyroclimatic Hypothesis is an approach using a-priori assumptions about the
relationship of fire and climate, derived from published correlations between PDSI
and total area burned in different ecoregions of the western USA. The “dry” algorithm
emphasizes the role of drying fuels in the current and preceding years, while the
“wet” algorithm assumes that the fire occurs in a dry year preceded by a wet period.
Clover Mountains
ABCO
A total of 12 random plots, each 0.1 ha in size, was sampled at each site to quantify
stand structure, age of surviving trees, and fuel loads. Additional information on species
dynamics were collected at regularly spaced grid points.
At Mt. Irish, we see a pulse of establishment in pinyon pine starting in the mid-1800’s
and tapering off through the 1900’s, with the other species remaining relatively stable.
In the Clovers, we see two pulses of ponderosa pine establishment, one in the late 1700’s
and one in the late 1800’s, with an increase in pinyon pine establishment into the mid1900’s. These are simply live-tree data, and do not include age information from dead
trees or trees that have been completely burned in fires.
The canopy coverage of each site was unique: Mt. Irish is home to reasonably contiguous stands of ponderosa pine, whereas
the Clover Mountain populations studied are very patchy in nature. Subsequent analysis of the fire scar records explain these
differences, with the Clover conditions likely the result of a catastrophic fire in 1946 that was essentially stand-replacing in nature.
Mt. Irish scars and population statistics, in contrast, did not indicate any stand-replacing events occurring within the record.
Another contrast between the sites was the species mixing at Mt. Irish opposed to the Clover Mountains. At Mt. Irish, in
addition to the ponderosa and pinyon, white fir (Abies concolor) and juniper (Juniperus osteosperma & scopulorum) were frequently
mixed together, although some separation on specific aspects did occur. At the Clovers, white fir was not present, and the pinyonjuniper was often separate from the ponderosa stands.
Examination of the fire history showed a remarkable decrease in fire at Mt. Irish around 1850, which predates the settlement
of the local mining districts by sizable Euro-American populations. It is possible that pre-settlement fire occurrence was influenced
by local native populations, although there is no archaeological evidence to support this. The “pyroclimatic hypothesis” developed
prior to the fire history study clearly indicates a shift in possible climatic influences starting in the early 1800’s.
Fire history at the Clover Mountains indicates that widespread fire is a regular event, more typical of other western ponderosa
regimes. The evidence seems to indicate that while fire occurrence was slightly reduced in the late 1800’s following regional
settlement, fuels buildup continued to occur until the catastrophic event of 1946 altered the landscape severely. Fire was afterwards
removed from the landscape, as the scar records show and the presence of a BLM fire tower on the summit of Ella Mountain
underscores.