Castle Mound Pine Forest State Natural Area: Fire history at the edge of the Driftless Area
in central Wisconsin
By: Martha A. Green, Advisor – Dr. Evan Larson
Introduction
Fire is an integral process in forest systems around the globe (Bowman et. al., 2011).
Historical patterns of fire activity varied with respect to vegetation type, climate, and human
activities. In dry forests, such as the mixed conifer forests of the American Southwest, surface
fires occurred frequently due to ignitions by both lightning and people (Swetnam and Baisan
2003). This maintained open forests of widely spaced trees and low fuel loads. In more mesic
systems, such as the boreal forests of Canada, fires were less frequent, but due to high fuel loads
were typically severe, stand-replacing events (Johnson 1992). A gradient in frequency and
severity spans these examples.
Perceptions and management of fire as an ecological process has changed dramatically
over the 20th century. Early efforts to suppress and exclude fire from forests led to dramatic
changes in forest structure that included increased forest density and fuel loads, and which in
turn resulted in increasingly severe wildfires over the late 1900s (Falk et. al, 2011). Recognition
of these changes led to research on the historical role of fire in forests around the world, but with
particular emphasis on locations where old forests remained and the ecological effects of fire
suppression were most evident. In the United States, this resulted in much of the early fire
history research being largely focused on western forests (Figure 1).
In Wisconsin, located in the Upper Midwest, fire-dependent communities include the
mixed hardwood, savanna, and prairie systems of the Driftless Area and the mixed pine forests
of the north and central sand plains (Curtis 1959). Despite a relative dearth in fire history
research across this area, increasing awareness of fire’s ecological value has led many land
managers to use prescribed fire to protect biodiversity and prevent fuel accumulation that can
lead to more severe and catastrophic fire events (Stephens and Ruth 2005). A strong and growing
community of land owners and managers are promoting the restoration of fire on Wisconsin
landscapes (http://www.tposfirescience.org/), but because fire has had different ecological effects
on individual ecosystems, knowledge of local historical fire regimes is necessary to create
effective land management plans for specific areas (Taylor and Skinner, 2003; Falk et al., 2011).
Here, we report an absolutely-dated fire history reconstruction for Castle Mound Pine
Forest State Natural Area, a site at the intersection of the Driftless Area and Central Sand Plains
of Wisconsin. This study represents the first tree-ring-based fire history for central Wisconsin
and provides insight into historical patterns of fire for a region where no such data exist but
where managers are increasingly utilizing prescribed fire in restoration efforts. Our specific
research objectives were to: 1) reconstruct a fire history for Castle Mound Pine Forest State
Natural Area, 2) test for temporal changes in fire occurrence, 3) determine the relationship
between climate and fire occurrence at our site, and 4) compare the fire history to vegetation
structure and composition to better understand the modern vegetation patterns on Castle Mound
and their historical causes. We then placed our results in the context of human land-use patterns
to examine the role of people in driving ecological change in this State Natural Area.
Methods
Study Site
Castle Mound Pine Forest State Natural Area (CMPF) is a 118 acre preserve located in
Jackson County, Wisconsin, near the city of Black River Falls and within the Black River State
Forest. The mound is composed of Cambrian sandstone that contributes to the texture of the
dominate soil series of the area, Boone-Tarr sands. The mound itself is a relatively sharp ridge
oriented with a generally east-west axis (Figure 2). The climate of the region is strongly
continental, with relatively short, hot summers and long, cold winters. Mean annual precipitation
is 833 mm (32.8 inches), mean temperature of the warmest month, July, is 21.5° C (70.7° F), and
mean temperature of the coldest month, January, is -9.6° C (14.72° F).
Historical vegetation for the region included mixed pine and hardwood forests, oak
openings, and prairie intermixed with lowland hardwood forests and wetlands (Figure 2). The
primary pine species included Pinus resinosa, P. strobus, and P. banksiana while hardwoods
included Quercus rubra, Q. alba, Q. ellipsoidalis, Q. velutina, Acer saccharum, and Tilia
Americana. Aerial imagery from 1939 documented a closed canopy forest on the north slope of
CMPF and a relatively open canopy of widely spaced trees intermingled with forest openings on
the south slope (Figure 3a). A 2010 aerial photograph of the site depicted substantial changes in
stand structure as both the north and south slopes exhibited a closed canopy (Figure 3b). The
modern forest communities on Castle Mound include a canopy of P. resinosa on the north slope,
many individuals of which exhibit fire scarring, over a subcanopy of mixed hardwoods. The
south slope supports a more mixed forest with P. resinosa dominating the upper slopes and
mixed Pinus Strobus and hardwoods on the lower slopes. A stand of even-aged Populus
tremuloides is evident on the western end of the ridge in the 2010 aerial imagery which
established following a stand replacing crown fire from the Airport Fire of 1977 (WI DNR
2007).
European settlers were first drawn to the Castle Mound area in the early 1800s by the
name of another mound north of CMPF that was referred to as Silver Mound by local Native
American tribes. The name Silver Mound referred to flint found at the site, rather than the silver
sought by Europeans, and economic interests in the region quickly shifted to timber. The first
saw mill on the Black River was constructed in 1819 and led to growing tensions between
settlers and local Native American groups, who burned the structure later that year (Wisconsin
Historical Society, 1903). The United States federal government purchased land titles for the
region in 1838 and Black River Falls was founded in 1839 by J.D. Spaulding. Spaulding built a
saw mill on the north bank of the Black River in the same year.
Logging quickly became the dominant industry for the region and continued through the
1800s, with P. Strobus being the species of primary interest. The region was largely cut over by
the early 1900s (Pyne, 1982). Similar to other post-settlement logged landscapes in the Great
Lakes states, the land was then utilized for agriculture (Rhemtulla et al., 2007). Poor soils,
sometimes compounded by adverse weather such as the 1930s Dust Bowl, led many of these
properties to be turned back to the state on back taxes (Porath, 1994). During the logging era
slash fires were common throughout this area and led to a policy of total fire suppression. A fire
tower midway along the crest of the mound reflects the long-term fire suppression policies
practiced at Castle Mound.
Castle Mound has been an important cultural landmark for the town of Black River Falls
since the establishment of the community. Wisconsin’s first Children’s Forest was established on
the mound in 1934 and was subsequently the site of tree planting efforts (Porath, 1994). The
Wisconsin Department of Natural Resources designated Castle Mound as a State Natural Area in
the fall of 1952 and has since passively managed the site as part of a dry-mesic forest
(http://dnr.wi.gov/topic/Lands/naturalareas/index.asp?SNA=16).
Field Methods
Our study design included the collection of fire history, stand structure, stand
demographics, and regeneration data. We also developed a master tree-ring-width chronology
from P. resinosa to aid in crossdating fire history samples. Designation of the site as a State
Natural Area precluded the collection of fire-scar samples from living trees. We therefore
conducted a thorough inventory of fire-scarred stumps throughout the study area and used a
chainsaw to collect cross sections from all viable samples, many of which displayed multiple fire
scars. Stand structure and demographic data were gathered using the point-quarter approach at
10m intervals along four transects that extended from the ridge to the toe slope of the mound
(Cottam and Curtis 1956). Two transects were placed on the north slope in February of 2014,
followed by the placement of two transects on the south slope in June of 2014. The starting point
of each transect was located near the center of distinct slope aspects identified in the field and
extended perpendicularly to the slope contours to the base of the slope. At each sample location,
the angle and distance to the nearest four trees was recorded. The trees were inventoried by
species, diameter at breast height (dbh), health, and canopy class. Tree-ring samples were
collected from each tree greater than 3cm dbh using Swedish increment borers (Stokes and
Smiley, 1996). We developed a master ring-width chronology by identifying and coring the
largest and oldest-appearing P. resinosa across the study area. We recorded dbh, health
condition, canopy position, and evidence of fire scarring for each of these trees. Tree
regeneration was documented by species within 1m × 10m intervals along each transect.
Laboratory Analyses
All field notes were transcribed into electronic spreadsheets and tree-ring samples were
air dried. Increment core samples were glued into core mounts and surfaced by hand sanding.
Fire-scarred samples were mounted to plywood, given a preliminary surface using a band saw,
and sanded using hand-held belt sanders with progressively finer grit sandpaper until the rings
could be seen clearly without magnification. All tree-ring samples were counted and rings were
marked at decadal (single dot), bi-century (double dot), and century years (triple dot) under 3×–
45× magnification using a binocular microscope (Arno and Sneck, 1977, Stokes and Smiley,
1996). Core samples from living trees were visually crossdated, calendar dates were assigned to
the inner-most rings, and inner-ring dates were combined in decadal bins to account for
uncertainty in rings to pith and age-to-coring height. The oldest cores from the transects and
targeted sampling were scanned at 2400 dpi and their ring-widths were measured using
WinDENDRO v2014 (Regent Instruments 2014). Ring-width series were standardized using a
30-year spline and combined into a ring-width-index chronology using a robust bi-weight mean
in the program Arstan v44 (Cook and Krusic 2013).
Fire-scarred samples were first marked from pith or inner ring to last complete outer ring
or bark (if present), noting the annual ring and intra-ring position of all fire scars and injuries.
We employed skeleton plots and visual crossdating to develop a floating chronology among the
fire-scarred samples (Spear 2012). Fire-scarred samples with over 100 rings were scanned and
measured to compare with the master ring-width chronology for the site. Intra-ring fire-scar
position, latewood thickness, and latewood color were subjectively described and noted on each
sample to aid in crossdating. Once crossdated, calendar years were assigned to all positively
identified fire scars and entered into FHAES v2.0 to create a fire chart, conduct a sample size
analysis, and calculate the mean fire return interval (MFI) of a composite fire chronology for
CMPF. The fire chronology was then compared to the age-class histograms to compare fire years
with regeneration and tree-recruitment patterns.
Fire-climate relationships were visually examined by plotting fire occurrence against
instrumental (NCDC 2015) and reconstructed (Cook et al 2004) Palmer’s Drought Severity
Index (PDSI) values. We used superposed epoch analysis to test for current and lagged fireclimate relationships for an 8-year window extending from 5 years prior to 2 years following
each fire event (Swetnam and Betancourt 1990). SEA was conducted in FHAES using all fires
and stratified for only those fires that scarred 3 or more trees across the study area.
Results
Of the 24 cross-sections collected we were able to crossdate 12 (Figure 4). Many of the
samples that we did not date contained fewer than 50 growth rings; some contained no fire scars
but were used for cross-dating. The dated samples contained 55 fire scars that represented a total
of 14 event years in which a fire was recorded in our study area. Sample size analysis indicated a
robust fire chronology when four or more samples were included, resulting in a period of
reliability from the first widespread fire event in 1841 until 1960. The mean fire return interval
from the first fire event to the last fire event in 1923 was 6 years. The current fire-free period in
our samples is 91 years and counting. We collected tree-ring samples from a total of 92 trees
across the four transects and the P. resinosa sampled for chronology development. The earliest
inner dates among the samples collected along the transects indicated a cohort of red pine
established on the north slope in the 1880s, closely corresponding to a fire in 1881. These trees
universally exhibited wide early growth rings, suggesting that they established under open
canopy conditions. In several instances for both P. resinosa and other species, trees of the same
species establishing at relatively the same period were often vastly different in size. Sporadic P.
resinosa establishment occurred in the 1940s, and no P. resinosa have established in our study
area since 1960s (Figure 5). Since the last fire was record at our site tree establishment has been
dominated by P. strobus, A. saccharum, and Quercus spp. Our regeneration surveys documented
no P. resinosa seedlings or saplings and an abundance of relatively more shade-tolerant species
such as P. strobus, A. saccharum, and other hardwood species (Figure 6). The master P. resinosa
ring-width chronology developed for the site extended from 1819 to 2013 with a 0.51 series
intercorrelation and a 0.25 mean sensitivity and aided crossdating efforts.
Fires throughout the mid-1800s largely occurred during drier years, while in the late
1800s and early 1900s this relationship appears to weaken (Figure 7). Our SEA analyses
identified significantly wet conditions 5 years and 3 year prior to fire events, and significantly
dry years during the year of fire (Figure 8).
Discussion
Our fire chronology strongly indicates that the structure and composition of the
vegetation communities at CMPF in the 1800s were driven by fire. In the mid-1800s post fire
years are followed exclusively by cohorts of P. resinosa, one of the most fire-tolerant species in
eastern North America (Starker 1934). A MFI of 6 years is extremely frequent for forest systems
in the Midwestern United States and likely was a determining factor in maintaining the relatively
open forest depicted in the 1939 aerial photographs. This is reflective of other mid-latitude North
American forests prior to European settlement, where seasonally dry-forests experienced
frequent low-intensity surface fires that caused limitations in the recruitment of trees (Bowman
2011). Though absence of evidence does not denote a true absence of less fire-adapted species,
historical photographs from the early 1900s suggest a substantially more open forest on Castle
Mound with an abundance of fine fuels (Figure 9).
The fire history of CMPF clearly illustrates a period of extremely high fire frequency for
the upper Midwest followed by a sharp reduction in fire frequency that coincides with a distinct
change in forest composition. Factors that drove fire at CMPF during European settlement may
have included slash fires or ignitions from the railroad lines that quickly followed the
establishment of the lumber industry in the mid-1800s in Jackson County. Following the
cessation of fire at CMPF in the early 1900s tree recruitment changed to hardwood species and
shade tolerant P. strobus, thus exhibiting similar changes in species composition and species
functional traits as observed in the western United States (Cooper 1960). Post 1930s, during fire
suppression, there is a substantial increase in hardwood recruitment across the entire site, though
some differences existed between the north and south slopes. The establishment of A. saccharum
was common on the north slope, while this relatively mesic species is absent from the south
slope where instead more drought-tolerant Quercus species were abundant (Peet and Loucks
1977). These differences indicate that the south-facing slope is more xeric than the north slope,
and although our data indicate most fire years were common to both slopes, it is possible that the
south slope experienced more fires than the north slope that may have limited establishment in
the 1800s (Heyerdahl et al 2001). The recruitment of P. resinosa in the 1940s on both slopes is
an interesting detail that at first appears anomalous, given the absence of fire from the site during
that time. It is possible that low-severity fires burned across the study area that simply were not
recorded by our samples, however, this also coincides with planting efforts carried out annually
by the 4H as part of a reforestation effort when the land had been managed as a county children’s
forest (Porath, 1994). The modern forest composition of CMPF therefore reflects both natural
and human processes.
Fire-suppression has led to high densities of juvenile trees that, if similar changes
elsewhere can serve as models, translate into increased fuel loads and potential for severe crown
fires in the future (Bowman et. al., 2011). The Airport fire of 1977 was a stand replacing crown
fire; the last fire-scar recorded fire in our samples occurred in 1923, marking a 54 year interval
between these events. According to the nearby Black River Falls Forest Fire historical marker,
strong germination of Aspen and Oak resulted; however, pines were planted as they are a more
valuable species (http://www.hmdb.org/marker.asp?marker=3321).
The historical context of the Airport Fire suggests that the stand replacing disturbance
may be outside of the historical range of variability of CMPF (Morgan et. al., 1994). Kipfmueller
and Kupfer (2005) found that fire scars form primarily in areas where trees survived low-severity
fires and that once scarred, individual trees are more likely to record passing fires. The
abundance of fire-scarred P. resinosa on CMPF strongly indicates that the site was dominated by
frequent surface fires, though high-intensity fires may have occurred and evidence of these
events may not be recorded. Though studies have indicated that not all fires are recorded, even
though fire-scarred trees exist (Baker and Ehle, 2001, Piha et. al., 2013), other research has
indicated fire scars to be an accurate and robust estimate of past fire occurrence and may in fact
underestimate past fire frequency if errors exist (Farris et al 2013). The modern fire free interval
represents an ecologically important shift in fire regime. Western fire histories have
demonstrated changes in density through the exclusion of fire (Biondi, 1999). However, local
controls are limiting factors after ignition in the extent and severity of fires (Falk et. al., 2011).
Understanding fire’s historical effects on the vegetation community at CMPF is an essential
component to creating accurate reference conditions, wildfire preparedness, and contemplation
of management objectives.
The fire-climate relationships we identified at CMPF indicate that climate is an important
factor in driving fire occurrence at our site, though the weakening relationship between drought
and fire over the early 1900s may indicate a gradual increase in human influence at our site. The
identification of lagged fire-climate relationships, where fires were typically preceded by wetter
than average conditions, is indicative of a fuel-limited fire regime similar to the American
Southwest, where abundant rainfall promotes the growth of fine fuels that are necessary to carry
surface fires in subsequent years (Swetnam and Betancourt 1998). Our data represent only one
site, but if similar relationships are identified elsewhere in central Wisconsin it creates the
potential for synoptic-scale climate to have synchronized the historical fire regimes of the region.
Understanding the role of climate in the fire regimes of this region could provide important
information for fire management and suppression planning and resources.
As elsewhere, fire regimes have been influenced by human interactions in multifaceted
ways including: changing fuel types, modifying fuel structures and continuity, and creating
sources of ignitions (Bowman et. al., 2011). There still remains some uncertainty as to the extent
that humans have changed patterns from “natural”, those “independent of humans”, fire regimes
and what influence climate has contributed to these changes (Bowman et. al., 2011). Regardless
of sources and causes, fire played an important ecological role at CMPF by influencing forest
composition and creating an appropriate seed bed for P. resinosa. The MFI of 6 years we
calculated for our study area during the 1800s and early 1900s likely resulted in such a high
frequency of fire that many red pine seedlings may not have been mature enough by the onset of
another fire to survive and likely produced the open forest structure evident in the early historical
aerial photographs (Drobyshev et. al. 2011).
Our results clearly illustrate a transition in species composition from pine to hardwoods.
As documented throughout the Great Lakes Region, P. resinosa depends on fire to regenerate
(Heinselman, 1973). European settlement greatly impacted all of these community types, with
conversion of most prairie and savanna sites to agriculture and near-complete logging of pine
forests throughout the Great Lakes states (Pyne, 1982; Loope and Anderton, 1998). These
changes have been exacerbated since the early 20th century through fire suppression policies and
have led to an overall mesophication that is evident at our site (Abrams, 2008). Based on ageclass data CMPF has had successional stages that led this historically xeric site to become more
mesic. Our data suggest that the conditions that enabled P. resinosa to dominate this site have
changed. If exclusion of fire continues at CMPF it is likely that successional species will
continue to dominate and replace P. resinosa, thus initiating a bifurcation of this system from its
historical conditions (Folke et al. 2004).
Conclusions
We successfully developed the first crossdated fire history reconstruction for central
Wisconsin. The nearest tree-ring based fire history record available through the International
Multiproxy Paleofire Database is over 560 km from our site, the nearest charcoal based fire
record is 200 km. This is an exciting development that indicates the feasibility of establishing
additional tree-ring-based fire history research in this ecologically complex region at the
intersection of the Driftless Area and Central Sand Plains of Wisconsin. The key to
understanding the drivers of fire frequency here and elsewhere in central Wisconsin will be to
develop longer records of fire that predate European settlement. Information of this type would
provide more complete reference conditions for land managers. Jackson county and neighboring
counties share in a common land-use heritage set in similar geologic landscapes of sandstone
ridges with roughly congruent vegetation communities as that of CMPF. It therefore seems likely
that a network of fire history sites could be established that would enable a more comprehensive
and nuanced understanding of the historical role of fire across this region.
Acknowledgments
We extend our thanks to Peter Bakken and Jed Meunier, Wisconsin DNR, for their support and
help in the collection of fire-scarred samples. Additional gratitude is expressed for the class of
Geography 4120, the students of which began the research at CMPF on the north slope. We
extend our thanks to Elizabeth Tanner, Ben Matthys, Gabriel Brownell, and Danica Larson for
their contributions to the field work of this project and to Sara Allen for helping mentor
undergraduate student researchers. Our research was funded through a Pioneer Academic Center
for Community Engagement grant and a Pioneer Undergraduate Research Fellowship.
Additional support was provided by the TREES Laboratory of the University of WisconsinPlatteville and the Jackson County Historical Society.
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Figure 1. Tree-ring-based fire history sites with data available through the public International
Multiproxy Paleofire Database (IMPD) (accessed May 1, 2015; available:
[http://www.ncdc.noaa.gov/paleo/impd/]). Few studies have been conducted in the upper
Midwest compared with the Western USA.
Figure 2. Original vegetation of Wisconsin based on public land survey data reconstructed by
Finley (1976). The study area aligns at the confluence of three fire-dependent vegetation
communities. Castle Mound Pine Forest now exhibits a closed canopy with a super-canopy of
fire-scarred Pinus resinosa. Tree-ring samples were collected to create a master ring-width
chronology for crossing dating fire scarred cross-sections to reconstruct the fire history of this
site.
Figure 3a and b. Historical aerial photographs taken of Castle Mound Pine Forest in 1939 and
2010, these photographs illustrate dramatic changes in canopy structure at our study area that
correlate with the exclusion of fire from the landscape.
Figure 4. The fire history of Castle Mount Pine Forest State Natural Area. This record is based
on 12 absolutely-dated fire-scarred samples from Castle Mound Pine Forest. Sample depth is
compared to percentage of samples scarred. During the mid-1800s there is a relative pattern in
fire return interval. Younger north slope samples exhibit fire-scarring in common with the south
slope samples beginning in 1891. In the early 1900s fire years are more erratic. The last fire year
to scar multiple samples occurred in 1923, recorded only in north slope samples. The fire chart
was created using FHAES v2.0.
Figure 5. Age structure of the north slope (top), south slope (middle), and combined (bottom)
transects at Castle Mound Pine Forest. Pinus Resinosa cohorts established after fires in 1881 and
1891. When fire suppression began strong recruitment of other species occurred. Establishment
of P. resinosa post 1930s may be part of reforestation efforts. The north slope has mesic species;
the south slope began recruitment with xeric tolerant species, both show a trend toward mesic
species regeneration. Differences in dhb of decadal cohorts indicate shifting factors in
competition as the canopy closed.
Figure 6. Tree regeneration data. Seedlings and saplings were inventoried in 1m × 10m plots
along all transects. Pinus resinosa is absent. Acer saccharum is dominant on the north slope,
xeric species compete with mesic on the south slope, regeneration on the south slope is
significantly less compared to the north slope.
Figure 7. Comparison of PDSI reconstructions from tree-ring data (dashed) and instrumental
climate data (solid) with fire events at Castle Mound Pine Forest. Large diamonds indicate fires
that scarred 3 or more trees in our study area while small diamonds indicate fires recorded by 2
or fewer trees. The number of trees recording each fire event is shown at the bottom of the
figure.
Figure 8. Results of superposed epoch analysis to determine the mean reconstructed Palmer’s
Drought Severity Index conditions for an 8-year window surrounding fire occurrence at Castle
Mound Pine Forest. Bars indicate PDSI anomalies determined through a Monte-Carlo simulation
of 1,000 runs. Bootstrapped confidence intervals are indicated by thin dashed lines (p < 0.05),
and years of significant departure are indicated by darker gray bars. PDSI data were collected for
Grid Point 207 of the North American Drought Atlas (Cook et al. 1999). These results indicate
anomalously wet conditions 5 years and 3 years prior to fire events, which typically occurred
during dry years.
Figure 9. Photos taken in 1926 of Castle Mound Pine Forest near crest of Cambrian sandstone
ridge (above). Postcard from the 1930s show mature pine with similar spacing as Pinus
ponderosa forest in the American southwest prior to fire suppression policies (below).