TWO CENTURIES OF FIRE IN A SOUTHWESTERN VIRGINIA PINUS PUNGENS COMMUNITY
E. K. Sutherland, H. Grissino-Mayer, C. A. Woodhouse, W. W. Covington,
S. Horn, L. Huckaby, R. Kerr, J. Kush, M. Moorte and T. Plumb
(Author information in final section of paper)
ABSTRACT: Fire exclusion in fire-dependent forest communities can alter stand
structure and composition. The objective was to construct a fire history of
two Pinus pungens Lamb. communities growing in southwestern Virgina. Tree-
ring analysis of fire-scarred P. pungens specimens and a tree survey were used
to determine species composition and age distributions. From 1798-1944, fires
burned approximately every 10 years. After acquisition by the United States
Department of Agriculture Forest Service (ca. 1935) the study area burned only
once (1944). Most of the population derives from two large cohorts
established in the 1850's and the 1930's, but some trees established during
nearly every decade before 1950. Few, if any, trees have established since
then. There appears to be a linkage between tree establishment and major fire
occurrence. Recent regeneration failure appears to be coincident with fire
exclusion. Continuing fire exclusion will probably result in decline in the
P. pungens communities, as they succeed to Quercus-dominated communities.
INTRODUCTION
The exclusion of fire from fire-adapted communities eventually results in
catastrophic, highly destructive fires (Mutch, 1970). Management of
fire-adapted communities requires an understanding of the role fire plays in
the community, and characterization of the fire regime. Age structure
analysis is a common method of determining fire frequency in long fire-return
interval communities where high-intensity stand-replacing fires occur (e.g.,
Arno et al, 1993). However, in communities with short fire-return intervals
where fires are low in intensity, few trees are killed and some trees may
develop fire scars. In these systems, tree-ring analysis of the scars is the
appropriate method to determine fire history (Baisan and Swetnam, 1990).
Combining both these approaches would reveal the relationship between age
structure and historical fire patterns.
The purpose of this study was to reconstruct the historical relationship
between fire and community structure using both the age and species
composition approach in combination with tree-ring fire history analysis. The
fire-adapted community chosen to study is Pinus pungens Lamb., commonly called
Table Mountain pine and sometimes hickory pine, mountain pine, or prickly
pine. P. pungens is endemic to the central and southern Appalachian
Mountains. It occupies dry, steep, exposed ridge tops and slopes throughout
its range, although it will occupy more mesic sites following fire (Zobel,
1969).
The regeneration and maintenance of Pinus pungens depends on stand and site
disturbance that exposes mineral soil to light and warmth (Della-Bianca,
1990). McCune (1988) classified P. pungens as a fire-resilient species. Fire
appears most effectively to promote regeneration (Zobel, 1969; Harmon, 1982)
although Whittaker (1956) hypothesized that populations are maintained in the
absence of fire in dry pine-oak forests, as components of a topographic or
edaphic climax (extremely dry and sterile rock outcrops and steep shale
slopes), or both. Barden (1977) examined age structures of stands near
Brevard, North Carolina, and concluded that populations there have maintained
themselves since the last fire, which had occurred nearly a century earlier
(1889). However, on Brush Mountain in southwestern Virginia, Williams and
Johnson (1990) found an under-representation of P. pungens seedlings and small
trees and concluded that maintenance of the populations in this area was
"unlikely under current disturbance regimes." Williams and Johnson (1990)
hypothesized that the populations of P. pungens that they studied originated
from seed trees left on the sites following heavy logging and fires at the
turn of the century.
Pinus pungens has many physical adaptations to fire (Della-Bianca, 1990);
interestingly, these adaptations are consistent with both long- and short-
interval fire regimes. For example, trees that are adapted to catastrophic,
stand-replacing fire often have serotinous cones that open when heated.
P. pungens has serotinous cones throughout much of its range (McIntyre, 1929;
Zobel, 1969; Barden, 1979), particularly on western and southern exposures,
which tend to be dry and steep and are prone to burning. Seeds are medium to
heavy in weight compared to other Pinus species, and not adapted to carriage
by wind, but rather are adapted to regeneration in situ and to the typically
dry conditions. While P. pungens does shed seeds each year, viable seeds on
serotinous cones can persist for up to 11 years (Barden, 1979). P. pungens
also can reproduce vegetatively after fire from basal bud sprouts, which
allows recovery of saplings after injury (Zobel, 1969). However, P. pungens
also has characteristics of trees that are adapted to frequent ground fires,
such as medium-thick bark, a deep rooting habit, self-pruning limbs, and pitch
production to seal wounds. Zobel (1969) noted healing ridges and charcoal on
many trees, evidence typical of fire scars. On the study sites, living trees
and dead snags with multiple fire scars frequently were encountered.
STUDY AREA
The study area was on the northern flank of Brush Mountain, a strike-ridge
mountain in the Valley and Ridge Province in the Appalachian Mountains of
southwestern Virginia (USA), north of Blacksburg, and apparently near those
studied by Williams and Johnson (1990, 1992). The sites selected were on the
upper west- and southwest-facing slopes of the many hollows that dissect Brush
Mountain. Elevations within the sample areas range from approximately 750-840
meters.
Regionalized climatic data for southwestern Virginia are displayed in
Figure 1. The climate is strongly seasonal, with summer months the warmest
and wettest. The autumn (September to November) is the driest season, and
February and April are dry compared to preceding months.
Williams and Johnson (1990) found age distributions on Brush Mountain to be
bimodal at three sites, with 10-, 50-, 75- and 80-year age classes at each
site, and few trees over 100 years old. They interpreted these results to
mean that the populations either were not self-maintaining or were
episodically recruited. They found that regeneration was limited by thick
litter, because P. pungens requires mineral soil exposure for successful
seedling establishment. They asserted that the P. pungens communities on
Brush Mountain probably originated at the end of the 19th century during a
period of heavy logging and fire, and presently are succeeding to Quercus
communities dominated Q. prinus and Q. coccinea. Pinus pungens seedlings and
saplings were uncommon. They further noted copious evidence of fire in the
form of fire-scarred trees and charcoal in the soils. The species composition
of the communities we encountered were nearly identical to those described by
Williams and Johnson (1990). The entire north face of Brush Mountain is steep
and difficult for logging equipment to reach, and has been designated as
unsuitable for timber by the U.S. Department of Agriculture Forest Service.
Besides fire suppression, there is no active forest management in this
management unit, and the area is little disturbed except some fuelwood cutting
(Personal communication, 5/28/93, E. Leonard, USDA Forest Service, Jefferson
National Forest, Blacksburg Ranger District).
METHODS AND RESULTS
We visually examined three of the dissected sideslopes to locate fire-
scarred trees. On Sites 1 and 3, we found several size classes of P. pungens
and many fire-scarred living and standing dead trees. On Site 2, the P.
pungens trees were all small and there was little fire-scarred material,
indicating that the site had experienced a stand-replacing fire that consumed
overstory trees and dead material. Due to time constraints, we concentrated
only on Sites 1 and 3.
Development of a Master Chronology
We developed a master chronology of common ring-width variation for the
area (Stokes and Smiley, 1968; Fritts, 1976; Swetnam et al., 1985) using two
cores from each of 11 dominant trees on Site 1 and from 15 dominant trees on
Site 3. Diameter at breast height (1.45 m; dbh) of each tree was recorded.
The patterns were easily discernable, and the master chronolgy readily matched
most of the cores.
Fire History Development
We located fire-scarred living and dead P. pungens trees at Sites 1 and 3,
cutting down living trees only if they had at least three fire scars. Small
wedges were taken from the fire-scarred face of living trees that were too
large to sample (Arno and Sneck, 1977). Dead material included standing snags
and downed logs. The cross-sections wre collected as outlined by Arno and
Sneck (1977) and Baisan and Swetnam (1990). We first applied calendar-year
dates to the living trees using the master dating chronology developed from
increment cores (Dieterich, 1980; Swetnam and Dieterich, 1985) to determine
the year of occurrence for the most recent fires. The season of fire
occurrence was learned by noting the intra-annual position of the fire scar
within the annual ring (Baisan and Swetnam, 1990). These seasonal
designations included dormant season fires (scars between the latewood of one
year and the earlywood of the following year) and fires occurring in the
early, middle, and late portion of the growing season. We expected the
majority of fires to be classified as dormant season scars because fires are
historically known to occur predominantly during the months of February,
March, and April (personal communication, 6/1/93, S. B. Stephenson, Fairmont
College, West Virginia).
All fire information was analyzed using the FIRE2 fire history analysis
program (Grissino-Mayer, Henri. 1993. FIRE 2: Unpublished computer program
developed at the Laboratory of Tree-Ring Research, University of Arizona,
Tucson). This program provided information on fire frequency, Mean Fire
Intervals (MFI), and the dominant seasonality of fire for various percentage-
scarred classes, and helped to identify temporal and spatial changes in fire
occcurrence at the Brush Mountain sites.
The fire scar record is displayed in Figure 2. We found that major fires
(over 50 percent of trees scarred) occurred at the Brush Mountain sites during
the years 1882, 1883, 1893, 1900, 1910, 1926, and 1934. These fires occurred
exclusively during the dormant season. Other, perhaps minor, fire years were
noted for 1885, 1907, 1911, 1914, and 1944. In
Figure 3, we compare major and minor fire years to the Palmer Drought Severity Index (PSDI), an index of
drought intensity (Palmer, 1965). Negative PSDI values indicate drought
conditions relative to regional norms and positive values indicate relatively
wet conditions. Although major fire years do not necessarily coincide with
the most intense drought years, all have negative PDSI values. Notably, no
major fires occurred after Forest Service fire suppression policy was enforced
in the late 1930's, even though droughts occurred after 1940 of intensity
equal to or greater than those of the major fire years before 1940.
We found that the majority of trees collected, both living and dead,
germinated during the interval 1855 to 1865, perhaps indicating the occurrence
of a stand-initiating fire some time before 1855. Only one sample (BR1-01)
extended before 1855, and contained fire dates for 1847 and 1853. We believe
that the 1853 fire is most likely the stand-initiating fire because it
occurred during the latter portion of the growing season unlike all the other
fires, which occurred during the dormant season.
Fires were synchronous between both Brush Mountain sites for all major
fires. Fires in 1882 and 1883 are classified as minor and were
asynchronous; Site 1 burned in 1882 and Site 3 burned in 1883. In Figure 3, all major fires that occurred during the
period of climate record happened during relative dry periods (negative
PDSI, usually < -2). Synchrony of fire events probably requires a
combination of climate and fuel conditions that promotes fire spread.
Sample BR1-01 is the only scarred specimen that pre-dates the 1855 to 1865
cohort establishment. However, many other samples were collected from dead
material, and possibly some of these samples will verify the fire dates
obtained from BR1-01. Fires are indicated on this sample during the years
1798, 1805, 1810, 1819, 1847, and 1853.
Age Distribution analysis
To determine species composition and size distribution of the community on
Site 1, we ran a 5 m-wide belt transect from the top of a spur ridge down the
slope for a distance of 100 meters to the point where there were no more P.
pungens. At the end of this transect we turned and ran a 60-meter transect
parallel to the contour. Measurements were recorded in blocks, 5 m by 5 m. We
recorded d.b.h. of all trees. The frequency of encountered species ( Figure 4)
indicates that Quercus coccinea and Q. prinus dominate the community in
numbers. Only one each of Q. alba, Pinus strobus, and Acer rubrum were
encountered. However, the size structure of the community with respect to
species in Figure 4 clearly is apparent. Most of the Quercus sp. trees are
very small; in the 0 category (trees under 1.45 m height, d.b.h. not
measured), there are many Q. coccinea, and size classes less than 20 cm d.b.h.
are dominated by Q. prinus. However, all the size classes larger than 20 cm
d.b.h. are dominated by P. pungens. Note that there are few small P. pungens.
We determined the age structure of the P. pungens trees by coring trees
> 10 cm and < 20 cm as they were encountered by chance along the slope. Cores
were taken at breast height and parallel to the slope contour. The cores were
brought to the laboratory where the number of rings at d.b.h. was determined.
To examine the age structure of larger trees, we used the cores taken for
master chronology development. In cores that did not intersect the pith, we
estimated the number of rings to the inside by visually interpolating from
the curvature of the rings. The age structure of the sampled P. pungens
reveals a bimodal age distribution, with only three trees established before
1851 ( Figure 5). Clearly recruitment was episodic with a significant
recruitment event in the 1850's, and another important event in the 1930's and
1940's, with minor recruitment occurring at other periods and little or no
establishment in recent years.
DISCUSSION AND CONCLUSIONS
We have established from the fire scar chronology that fire occurred
frequently (every 9-11 years) in the community, at least through the 19th and
mid-20th century. Most fires occurred during the dormant season, probably in
early spring. Those fire dates were consistent on Site 1 and Site 3,
indicating that fires probably were large and widespread on at least that
portion of the mountainside. During the period of climatic record, PDSI
values indicate dry conditions during major fire years in the fire seasons
before active fire suppression. Except for the minor 1944 fire, the Brush
Mountain fire regime abruptly ends after the 1934 fire. We believe this
cessation of fires is due to the incorporation of the Brush Mountain site into
the Jefferson National Forest by the USDA Forest Service beginning in 1935.
Fire suppression is known to have begun somethime in the late 1930's.
Our species and age structure results are similar to those of Williams and
Johnson (1990) on Brush Mountain. They found abundant oak regeneration
(primarily Quercus prinus and Q. coccinea) and a bimodal distribution of Pinus
pungens establishment that they interpreted as likely to be associated with
some type of disturbance. Given the fire scar analysis results, this
disturbance was almost certainly fire. Our age structure analysis shows that
unusually severe fires may have occurred at least twice, resulting in two
important recruitment events. For example, scar placement of the 1853 fire
during active cambial growth indicates that fire occurred during the growing
season. These fires during the warm period of the year may have been hotter,
more intense, and damaging to existing trees than dormant season fires, and
resulted in the 1850's recruitment event. Many other fires occurred, and
while they too probably promoted some recruitment, the fires probably
functioned by reducing competition from less fire-adapted vegetation, thus
maintaining P. pungens dominance.
Our results are consistent with the acknowledged dependence of P. pungens
upon fire for regeneration and maintenance (Della-Bianca, 1990). Fire
suppression is most likely the cause of a dramatic change in the composition
of the Brush Mountain communities during the last 60 years (Williams and
Johnson, 1990). In the past, fire clearly promoted integrity of the Pinus
pungens community on Brush Mountain. Only by understanding the spatial and
temporal variation of fire in Table Mountain pine landscapes can forests be
managed to ensure the viability of P. pungens communities.
The sum of information from the age structure analysis and fire scar
analysis was more than these analyses would have revealed separately. For
example, the age structure analysis would have led the researcher to conclude
that two or three fires burned through the sites. Fire history alone revealed
the frequency and seasonality of fires. However, taken in combination, we can
infer that the cohorts originated from fire events, and that those fires were
probably higher intensity than most other fires. This approach is highly
informative and is recommended to understand the function and frequency of
fires in fire-adapted plant communities.
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ACKNOWLEDGEMENTS
This research was conducted as a project during the weeklong Fourth Annual
Dendroecological Fieldweek, conducted in June 1992 at the Mountain Lake
Research Station, University of Virginia, Pembroke, Virginia. The Finnnneldweek
was funded by the USDA Forest Service, Southern Global Change Program.
AUTHOR INFORMATION
Elaine Kennedy Sutherland, United States Department of Agriculture, Forest
Service, 359 Main Road, Delaware, OH 43015; Henri Grissino-Mayer, Laboratory
of Tree-Ring Research, University of Arizona, Tucson 85721; Connie Ann
Woodhouse, Laboratory of Tree-Ring Research, University of Arizona, Tucson
85721; W. Wallace Covington, School of Forestry, Northern Arizona University,
Flagstaff, AZ 86011; Sally Horn, Dept. of Geography and Graduate Program in
Ecology, 408 G&G Building, University of Tennessee, Knoxville 37996-1420;
Laurie Huckaby, USDA Forest Service, 240 W. Prospect, Ft. Collins, CO 80526;
Richard Kerr, Dept. of Geography, California State University, Northridge, CA
91403; John Kush, School of Forestry, 108 M. White Smith Hall, Auburn
University, AL 36849; Margaret Moore, School of Forestry, Northern Arizona
University, Flagstaff, AZ 86011; Tim Plumb, Natural Resources Management
Department, California Polytechnic University, San Louis Obispo, CA 93407.