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 fire-

climate 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 age-

class 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 Wisconsin-

Platteville 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).