Western Spruce Budworm Outbreak, Climate, and Fire Interactions
in the Mixed Conifer Forests of the Interior Pacific Northwest
Aquila Flower*1, Daniel G. Gavin1, Emily K. Heyerdahl2, Russ A. Parsons2
*-Presenting author, 1-Department of Geography, University of Oregon, 2-Rocky Mountain Research Station, Fire Science Laboratory, Missoula
western Montana are subject to frequent extensive fires and damaging
through the creation of a similarity index based on the total Euclidian
standard dendroentomological methods that use periods of divergent radial
confidence that can be placed in this particular outbreak’s identification.
distance in climate space between the PDSI records.
growth patterns in host and non-host trees to identify outbreaks (Swetnam
These results show a high level of agreement with the outbreaks reported
et al. 1985; figure 3).
in Swetnam et al.’s (1995) regional reconstruction for the Blue Mountains.
outbreaks of insect defoliators. These forests are particularly susceptible to
of western North America.
widely considered to be the most damaging defoliator insect in the forests
Figure 3: Dixie Butte host and non-host master chronologies.
Standard dendropyrochronological methods (Arno and Sneck 1977) were
used to create a preliminary record of fire occurrence by dating fire scars
preserved in trees in the host stand. Pearson’s correlation analysis was
used to determine the radial growth response of the host and non-host
trees to climatic variability. Monthly 1895-2009 climate data for Oregon
Climate Division 8 was obtained from the US Historical Climatology
Figure 1: cumulative years of WSBW outbreaks in the interior Pacific
Northwest, 1977-1993 (data from Williams and Birdsey 2003).
During the late 20th century the extent and severity of fires and insect
outbreaks have increased in this forest type, with both climate change and
Network (USHCN 2009). WSBW outbreak and fire occurrence records were
also compared to Cook et al.’s (2004) PDSI reconstruction for the nearest
outbreaks identified during 1897-1911, 1941-1961, and 1982-2000.
grid cell.
Preliminary Results
mechanisms governing interactions among insects, fire, and climate and
a reasonable approximation of the climatic signal within the host ring-width
their effects on fuel and fire behavior.
series.
!""
the causal relationships between climatic variability, wildfires, and WSBW
!""
%
&
and standardize the cores (Stokes and
Figure 2: Dixie Butte host
tree sampling site.
0.38
outbreaks across much of
2-3
-0.11
1.12
western North America (figure 7).
4-5
0.55
1.61
Higher synchroneity appears to Table 1: Average PDSI and Climatic
Similarity Index values during years
be positively associated with with 0 to 1, 2 to 3, or 4 to 5 regional
both wetter average climatic dendroentomological reconstructions
indicating outbreak conditions.
conditions and with periods in
which large-scale climatic forcing leads to an above average level of
similarity between climatic conditions throughout western North America
(table 1).
Figure 6: preliminary WSBW outbreak periods (grey bars) and fire
occurrence dates (orange lines), with a five-year running mean of
PDSI (blue line).
An additional 14+ sites will be sampled in mountainous regions of
northeastern Oregon, northern Idaho, and western Montana during summer
2010 and 2011. At each site along the transect, dendrochronological
methods will be used to reconstruct records of WSBW outbreaks, fire
occurrences, climatic variability, and tree establishment dates. A suite of
Methods
statistical techniques will be used to assess these records for spatial and
To assess the level of inter-regional synchrony of WSBW outbreaks in
temporal synchroneity within each disturbance type at different sites and
western North America, five regional dendroentomological reconstructions
between different disturbance types at each site, as well as to quantify the
of WSBW outbreaks in southeastern British Columbia (Campbell et al.
relationship between climatic variability and disturbances. Data obtained
2006), western Montana (Anderson et al. 1987), northern New Mexico
through this research will also be used to parameterize a mechanistic,
(Swetnam and Lynch 1993), northeastern Oregon (Swetnam et al. 1995),
process-driven stand dynamics model (FIRE-BGC) to examine how
and Colorado (Ryerson et al. 2003) were collected and converted to a
interactions of historical climate and disturbances influenced fuel profiles
binary classification scheme of either “outbreak” or “no outbreak”. The total
through time.
Cook et al.’s (2004) dendroclimatic Palmer Drought Severity Index (PDSI)
reconstruction records from the five grid points closest to the WSBW
reconstructions were obtained for the period between 1700-2000 and a
Figure 4: radial-growth response of host and non-host chronologies
to climatic variability. Dotted lines indicate 95% confidence level
threshold.
single inter-regional average PDSI index was calculated. The associations
The preliminary reconstruction of WSBW outbreaks at Dixie Butte (figure 5)
PDSI values for all years in which 0 or 1, 2 or 3, and 4 or 5 regional WSBW
indicates three distinct stand-level outbreaks during the periods from
outbreak reconstructions indicated outbreak conditions. The level of
1897-1911, 1941-1961, and 1982-2000. An additional outbreak is apparent
similarity between the five regional PDSI records was further explored
were used to collect cores using increment
borers and to prepare, measure, cross-date,
Standard dendrochronological methods
were tallied and plotted for visual assessment.
non-host (ponderosa pine) species.
-0.22
Future Research
number of regional reconstructions indicating an outbreak in each year
sites for host (Douglas-fir and true fir) and
0-1
#$
chronologies were developed at nearby
Similarity
Index
Figure 7: Number of regional chronologies recording an outbreak,
with total number of chronologies indicated by the dashed grey line.
Inter-regional Outbreak Synchrony
!""
knowledge gap, we are employing dendroecological methods to elucidate
Methods
fieldwork in summer 2009. Separate
Average
PDSI
mentioned in journal articles, little is actually known about the causal
2) in northeastern Oregon during preliminary
western spruce budworm
One site was sampled at Dixie Butte (figure
PDSI of 0.03 during all outbreak years and 0.44 during the three stand-level
records (figure 4), indicating that the non-host chronology can be treated as
Outbreak & Fire Reconstructions
occurred during periods with above average PDSI (wet), with an average
between insect outbreaks, climatic variability, and wildfires is often
northeastern Oregon to northwestern Montana.
with an average PDSI of -1.9 during fire years. WSBW outbreaks generally
outbreaks in mixed conifer forests along a transect running from
(figure 6) reveals a pattern of fire occurrences during low PDSI (dry) years,
growth responses to mean monthly precipitation, temperature, and PDSI
disturbances have been completed in this region. To address this
is apparent in the patterns of
# of Regional
Outbreaks
reconstruction for Dixie Butte with Cook et al.’s (2004) PDSI reconstruction
theory that there are complex causal relationships and feedback loops
few long-term, annually resolved paleoecological reconstructions of
Comparison of the WSBW outbreak reconstruction and fire occurrence
The host and non-host master chronologies exhibit fairly consistent radial
provide us with a sound understanding of these ecological interactions, and
Figure 5: WSBW outbreak reconstruction for Dixie Butte in NE
Oregon. Stand-level outbreaks identified using a threshold (dark grey
line) of 33% of cores indicating outbreak conditions.
fire suppression often invoked as potential causes. However, while the
The observational record is too short in much of western North America to
outbreaks of the western spruce budworm (WSBW; figure 1), which is
A moderate level of synchroneity
Preliminary Results
#"
The mixed conifer forests of northeastern Oregon, northern Idaho, and
in the record from 1827-1841, but the low sample depth limits the
Introduction
Smiley 1968). Construction of the WSBW outbreak record followed
between outbreak synchrony and PDSI values were explored by averaging
Works Cited
Arno, Stephen F., and Kathy M. Sneck. 1977. A method for determining fire history in coniferous forests
of the mountain west. USDA Forest Service General Technical Report INT-42.
Anderson, Leslie, Clinton E. Carlson, and Ronald H. Wakimoto. 1987. Forest fire frequency and western
spruce budworm outbreaks in western Montana. Forest Ecology and Management 22, no. 3-4
(December): 251-260.
Campbell, Rochelle, Dan J. Smith, and André Arsenault. 2006. Multicentury history of western spruce
budworm outbreaks in interior Douglas-fir forests near Kamloops, British Columbia. Canadian
Journal of Forest Research 36, no. 7: 1758-1769.
Cook, E.R., Woodhouse, C.A., Eakin, C.M., Meko, D.M., and Stahle, D.W. 2004. "Long-Term Aridity
Changes in the Western United States". Science 306, No. 5698 (November 5): 1015-1018.
Ryerson, Daniel E., Thomas W. Swetnam, and Ann M. Lynch. 2003. A tree-ring reconstruction of western
spruce budworm outbreaks in the San Juan Mountains, Colorado, U.S.A. Canadian Journal of
Forest Research 33, no. 6: 1010-1028.
Stokes, M.A., Smiley, T.L. 1968. Tree-Ring Dating. The University of Chicago Press, Chicago, 73 pp.
Swetnam, Thomas W., and Ann M. Lynch. 1993. Multicentury, Regional-Scale Patterns of Western Spruce
Budworm Outbreaks. Ecological Monographs 63, no. 4 (November): 399-424.
Swetnam, Thomas W., Marna Ares Thompson, and Elaine Kennedy Sutherland. 1985. Using
Dendrochronology to Measure Radial Growth of Defoliated Trees. Agriculture Handbook 639.
U.S.D.A. Forest Service.
Swetnam, T. W, B. E Wickman, H. G Paul, and C. H Baisan. 1995. Historical patterns of western spruce
budworm and douglas-fir tussock moth outbreaks in the northern Blue Mountains, Oregon, since
a.d. 1700. Forest Service research paper PNW-RP-484. Pacific Northwest Research Station:
U.S.D.A. Forest Service.
USHCN. The United States Historical Climatology Network (USHCN) Main Page. http://cdiac.ornl.gov/
epubs/ndp/ushcn/ushcn.html.
Williams, D.W., and R.A. Birdsey. 2003. Historical patterns of spruce budworm defoliation and bark
beetle outbreaks in North American conifer forests: an atlas and description of digital maps.
USDA Forest Service, Northeastern Research Station, General Technical Report NE-308, 33 pp.
Thanks to: