2008 SOUTHEASTERN NATURALIST 7(1):61–68
Infl uence of Competition on the Density of the Federally
Endangered Michaux’s Sumac (Rhus michauxii) at
Fort Pickett, Virginia
Verl Emrick1,* and Jeffray Jones2
Abstract - Rhus michauxii (Michaux’s sumac) is a federally endangered species
native to the lower piedmont and upper coastal plain of the Southeastern United
States. Fort Pickett, VA, has the largest known population and is unusual because the
majority of the colonies are located in habitats maintained by frequent fire caused
by military training. Our objectives were to investigate the relationship between
woody competition and Michaux’s sumac density. The strongest correlations were
between woody competition and pistillate density. In the 2–5 m strata, the infl uence
of woody density, total density, and DBH on pistillate Michaux’s sumac density
were significant (P < 0.05) and strongly correlated with r2 of 0.558, 0.488, and 0.511,
respectively. Based upon the results of our study, reducing woody competition in the
2–5 m strata would improve habitat conditions for sexual reproduction, which could
increase the potential for recovery of Michaux’s sumac.
Introduction
Rhus michauxii Sargent (Michaux’s sumac) is a 1.5–4.0 dm tall, densely
pubescent, dioecious, rhizomatous shrub of the Anacardiaceae family
(Radford et al. 1968). The narrow winged or wingless rachis supports 9–13
sessile, oblong leafl ets that are 4–9 cm long, 2–5 cm wide, and acute to
acuminate. Small, 4–5 parted, greenish-yellow fl owers are borne in a dense,
erect terminal cluster. The red fruit is borne on a 5–6 mm, broad, densely
pubescent panicle. Individual plants may have multiple fl owering stems that
are all the same sex. Michaux’s sumac was first described by Sargent (1895)
who considered it one of the most poisonous plants in North America. Subsequently
it has been found not to cause allergic reactions in humans, hence
one of its colloquial names, false poison sumac. Michaux’s sumac is closely
related to Rhus glabra L. (smooth sumac), and the two will occasionally
hybridize (Hardin and Phillips 1985). Michaux’s sumac is generally much
shorter than smooth sumac, has a winged rachis, and is covered with dense
hairs on both the leaves and stems.
Michaux’s sumac was listed as endangered by the US Fish and Wildlife
Service in 1989 (USFWS 1989) and was discovered on Army National Guard
Maneuver Training Center-Fort Pickett (Fort Pickett) in 1993 (Fleming and
Van Alstine 1994). Currently, 46 populations occur in North Carolina, 6 in
1Conservation Management Institute, Virginia Tech College of Natural Resources,
1900 Kraft Drive Suite, 250 Moss Building, Blacksburg, VA 24061. 2Conservation
Management Institute, Virginia Tech College of Natural Resources, 1100 Confroy
Drive - Box 3, South Boston, VA 24592. *Corresponding author - vemrick@vt.edu.
62 Southeastern Naturalist Vol.7, No. 1
Virginia, and 2 in Georgia (Eric Davis, USFWS, Pleasantville, NJ, pers.
comm. 2006). Habitat loss and degradation through fire suppression, agricultural
conversion, herbicide applications, and road construction were the main
factors that led to the loss of Michaux’s sumac habitat and the primary cause of
the species’ decline and reason for listing (Russo 1993, USFWS 1993). Other
potential threats are hybridization with R. glabra; geographic isolation of
small, single-sex populations; fungal disease; stem borers; and damage caused
by off-target herbicide drift (Russo 1993). In addition, Wilkinson et al. (1996)
found that Eurytoma rhois (Crosby) (a Chalcid wasp) parasitized seeds at Fort
Pickett and reduced numbers of viable seed.
Michaux’s sumac is endemic to the inner coastal plain and piedmont of
the southeastern United States, where it occupies sandy or rocky savannas
and open woods (USFWS 1993). These sites typically have low cation exchange
capacities and depend on some form of disturbance to maintain the
open character of the habitat (Boyer 1993). Primary disturbances are fire
(accidental or prescribed), right-of-way maintenance, and agricultural brush
clearing (USFWS 1993). Many authors concluded that Michaux’s sumac
requires a combination of soil disturbance and occasional fire to maintain a
healthy population (Emrick and Hill 1997, 1998; Hardin and Phillips 1985;
Russo 1993; Savage and Bucher 1991; Wilkinson et al. 1996; and USFWS
1993). Soil disturbance appears to stimulate rhizomatous growth, while periodic
fire keeps other scrub/brush species from out-competing Michaux’s
sumac for sunlight and soil nutrients.
The population of Michaux’s sumac at Fort Pickett is the largest known
population, is comprised of numerous colonies, and is genetically more
diverse than other known populations (Burke and Hamrick 2002, Sherman-
Broyles et al. 1992). Before discovery of the Fort Pickett population, most
known Michaux’s sumac populations clung to disturbed edges along open
brushy fields, power lines, railroads, agricultural clearings, and pine plantations.
On Fort Pickett, 94% of the colonies occur within a 4400-ha restricted
access area (RAA) that serves as a buffer zone for a variety of military livefire ranges. Military live-fire training has occurred consistently since 1942
and results in frequent, low intensity wildfires throughout the entire RAA
(Emrick and Murray 2006). Fire frequencies have historically ranged from
1–3 years and caused a series of fire-adapted communities to develop over
time (V. Emrick and J. Jones, unpubl. data). Emrick and Hill (1998) found
that Michaux’s sumac colonies occurred in two distinct fire-adapted community
types at Fort Pickett: oak woodland and open shrubland.
Current management strategies for Michaux’s sumac emphasize the need
to eliminate woody competition through the use of fire or other mechanical
means (Emrick et al. 2003, USFWS 1993). However, the impact of woody
competition may vary depending upon the sex of an individual because
dioecious plants typically have different physiological requirements for
staminate and pistillate fl owering (Antos and Allen 1999). In this paper,
we report findings on the effects of woody competition on the density of
2008 V. Emrick and J. Jones 63
Michaux’s sumac in a “natural” fire-maintained system at Fort Pickett and
relate these findings to current management strategies. Specifically we had
two questions: (1) does the size and type of woody competition have differential
effects on Michaux’s sumac density? and 2) does the effect of woody
competition differ among non-fl owering, staminate, and pistillate stems?
Field-Site Description
Fort Pickett is located in the predominantly rural piedmont of southeastern
Virginia 5 km east of the town of Blackstone and approximately 25 km west of
the fall line demarcating the Coastal Plain. Fort Pickett encompasses 16,592
ha of land in three counties: Nottoway (8647 ha), Brunswick (2645 ha), and
Dinwiddie (5300 ha). The mission of Fort Pickett is to provide a maneuver
training center capable of handling the live-fire and maneuver training requirements
for brigade-sized combat, combat support, and combat service
support elements of the active, guard, and reserve components of all services.
Training doctrine requires military installations to maintain large acreages of
natural and semi-natural landscapes to simulate a variety of potential combat
scenarios. Though there are areas of extreme disturbance, these areas are small
compared to the vast majority of the lands, which are maintained in a relatively
“natural’ state (V. Emrick and J. Jones, unpubl. data).
The Virginia piedmont has a temperate climate with hot, humid summers
and mild winters with frequent short cold spells (Emrick and Murray 2006).
The mean annual temperature is 14.4 °C, with a mean maximum temperature
of 20 °C and a mean minimum temperature of 8.8 °C. Precipitation is well-distributed
throughout the year, with mean annual precipitation of 115 cm. Fort
Pickett soils generally consist of a quartz sandy loam surface layer, ranging in
depth from 15–46 cm, over a micaceous clay loam, and have a frost depth of
61 cm. The majority of the upland soils found on Fort Pickett are non-hydric,
infrequently fl ooded, and have a slow to moderate infiltration rate. Loams
and sandy loams are the most common soil types with organic matter fraction
ranging from 2–10%. The majority of these soils support forest vegetation
under natural conditions (Emrick and Murray 2006). Many of the fl oral and
faunal species are typical of the southeastern piedmont, with some coastal
plain infl uences (Fleming and Van Alstine 1994). Furthermore, because of the
unusual land use of Fort Pickett, there are several occurrences of rare and endangered
species and rare biologic communities (Emrick and Murray 2006).
Methods
Plot allocation and data collection
Because Michaux’s sumac colonies occur within the RAA at Fort Pickett,
there were time constraints on field data collection due to safety concerns.
Therefore, field sampling occurred over a two-year period from 2004–2005.
In 2004, we established research plots in colonies occurring within the RAA
at Fort Pickett. Using a geographic information system (GIS), we randomly
64 Southeastern Naturalist Vol.7, No. 1
allocated the maximum number of 100-m2 plots within Michaux’s sumac
colonies using a 10-m buffer between plots and a 5-m buffer from the colony
boundary. This approach resulted in the establishment of 25 plots encompassing
the full range of Michaux’s sumac colonies at Fort Pickett.
We systematically placed nine 2.5-m2 subplots within each plot, tallied
the number of Michaux’s sumac stems in each subplot, and categorized each
as staminate, pistillate, or non-fl owering. Within each 100-m2 plot, we tallied
all woody individuals, identified each to species, and categorized them
as either shrub/sapling (2–5 m) or tree (5+ m) based upon the location of
the crown. We collected diameter-at-breast height (DBH) for each woody
species tallied and measured total vegetative cover for the entire plot using
a concave stand densiometer.
Data analysis
We calculated total plot density as stems/m2 for all four categories of
Michaux’s sumac stems (i.e., non-fl owering, pistillate, staminate, and total
stems), summarized woody species density and DBH by height class, and
determined total woody density for each plot. Prior to statistical analysis,
we used a Shapiro-Wilks test to test for normality of all data. All Michaux’s
sumac density and woody competition data were non-normal. Each variable
was square root transformed following the recommendations of Gotelli and
Ellison (2004) prior to analysis. We used linear regression to determine if
there were correlations between woody competition on all four measures
(total, non-fl owering, staminate, pistillate) of Michaux’s sumac density.
Multiple linear regression was used to examine the relationship of linear
combinations of key woody competition variables to Michaux’s sumac density.
Only woody competition variables that did not vary collinearly were
used in the multiple regression analysis. All analyses were performed using
SYSTAT 11.0 (SYSTAT 2004).
Results
Overall, woody competition negatively correlated with stem density of
Michaux’s sumac at Fort Pickett. However, the impact of woody competition
differed substantially, depending on the type of competition and the sex of
the Michaux’s sumac stem (Table 1). There were negative effects of woody
competition on total Michaux’s sumac density for five of the six variables
measured. Woody density and DBH in the 2–5 m strata showed significant
negative correlations with total Michaux’s sumac stem density at P < 0.10
level with r2 of 0.129 and 0.117, respectively. There were also significant
negative correlations at the P < 0.05 level between total Michaux’s sumac
stem density and woody density in the 5+ m strata (r2 = 0.200), total density
(r2 = 0.249), and total aerial vegetative cover (r2 = 0.199).
Significant negative correlations were detected at the P < 0.10 level
between the density of non-fl owering Michaux’s sumac stems and woody
species density in the 5+ m height class (r2 = 0.145), DBH in 5+ meter height
2008 V. Emrick and J. Jones 65
class (r2 = 0.141), total aerial vegetative cover (r2 = 0.200), and total woody
stem density (r2 = 0.118). Woody density in the 2–5 m height class and DBH
in the 2–5 m height class showed no effect on non-fl owering stem density.
There were significant negative correlations at the P < 0.05 level between
staminate Michaux’s sumac density and woody density in the 5+ m
height class (r2 = 0.283), DBH in 2–5 m height class (r2 = 0.180), total aerial
vegetative cover (r2 = 0.237), and total woody stem density (r2 = 0.222).
Woody density in the 2–5 m height class and DBH in the 5+ m height class
were not correlated with staminate Michaux’s sumac stem density.
The strongest correlations were between woody competition and pistillate
Michaux’s sumac density. In the 2–5 m strata, the infl uence of woody
density, total density, and DBH on pistillate Michaux’s sumac density were
significant (P < 0.05) and strongly correlated, with r2 of 0.558, 0.488, and
0.511, respectively. Total cover showed a significant (P < 0.05) correlation
(r2 = 0.237) with pistillate Michaux’s sumac density. Woody stem density in
the 5+ m strata was also significantly correlated with pistillate Michaux’s
sumac stem density (P < 0.05), but only explained a small percentage of the
variation (r2 = 0.149).
Two multiple linear regression models showed significant (P < 0.05) negative
relationships between woody competition and Michaux’s sumac density.
Male density was negatively correlated (r2 = 0.333) with a linear combination
of mean DBH in the 2–5 m strata and total aerial vegetative cover. Female
density showed a negative correlation (r2 = 0.635) with a linear combination of
mean DBH in the 2–5 m strata and total aerial vegetative cover.
Discussion
At Fort Pickett, all measures of Michaux’s sumac density negatively
correlated with woody competition. This result was consistent with known
Table 1. Regression coefficients and p values for relationships between Michaux’s sumac stem
density and woody competition at Fort Pickett, VA.
Woody competition
Density Density Total DBH DBH Total
Rhus michauxii 2–5 m 5+ m density 2–5 m 5+ m cover
Non-fl owering p-value 0.356 0.060 0.092 0.614 0.064 0.099
β1 - -3.417 -2.490 - -0.063 -0.037
r2 - 0.145 0.118 - 0.141 0.114
Staminate p-value 0.187 0.038 0.017 0.035 0.145 0.019
β1 - -3.658 -3.342 -0.124 - -0.050
r2 - 0.283 0.222 0.180 - 0.218
Pistillate p-value 0.000 0.057 0.000 0.000 0.643 0.014
β1 -4.498 -1.866 -2.584 -0.115 - -0.028
r2 0.558 0.149 0.438 0.511 - 0.237
Total p-value 0.077 0.025 0.011 0.094 0.114 0.026
β1 -5.270 -5.254 -4.740 -0.134 - -0.063
r2 0.129 0.200 0.249 0.117 - 0.199
66 Southeastern Naturalist Vol.7, No. 1
habitat requirements of Michaux’s sumac and other closely related Rhus
spp. (Emrick et al. 2003, USFWS 1993). However, we found that the correlation
with woody competition was not equal among the different types of
Michaux’s sumac stems. Woody competition in the 2–5 m strata showed no
infl uence on non-fl owering Michaux’s sumac density, and competition in the
in the 5+ m strata only slightly infl uenced density. However, the long-term
viability of Michaux’s sumac is not only contingent upon surviving at current
locations, but also increasing the size of the populations through both
asexual and sexual reproduction.
There were substantial differences in the effect of woody competition on
the density of staminate and pistillate Michaux’s sumac. Woody competition
in the 5+ m strata negatively infl uenced staminate density to a greater
degree than pistillate or non-fl owering density. However, total variation
explained was relatively small. Pistillate density showed the strongest
negative correlations with woody competition in the 2–5 m strata. Presumably,
all of the non-fl owering stems had the potential to fl ower but did not
because of physiological limitations, possibly related to competition from
other woody species. For many dioecious species, fl owering and subsequent
seed set requires a greater physiological investment by pistillate individuals
compared to staminate or non-fl owering plants (Gallant et al. 1998). Antos
and Allen (1999) reported that the level of reproductive effort (RE) differs
between the sexes for at least one dioecious shrub. Staminate RE was dependent
upon available resources (sunlight and soil nutrients) during the
growing season, whereas pistillate RE varied from year to year based upon
resource use and availability over several years. Wildfire caused by military
training at Fort Pickett created and maintained habitat through the reduction
of woody competition that allowed Michaux’s sumac to survive as other
populations decreased and disappeared. Wildfire within the RAA burned
in a mosaic pattern based upon local topography, climate, and fuel availability.
Thus, Michaux’s sumac colonies, which may not burn for several
years, allow woody competition in the 2–5 m strata to increase. We believe
that increased woody competition in the 2–5 m strata negatively infl uenced
pistillate fl owering by reducing available resources over several years. The
differences in response to competition indicate that while the wildfire within
the RAA at Fort Pickett is sufficient to maintain overall Michaux’s sumac
population, it is not ideal for maximizing pistillate fl owering.
In summary, many of the populations of Michaux’s sumac studied
prior to its discovery at Fort Pickett were small, single-sex populations
that existed along roadsides and other rights-of-way (Russo 1993, USFWS
1993). The Fort Pickett population allowed us to study the impact
of woody competition in the only known large, multi-sex population of
Michaux’s sumac in a “natural” fire-maintained habitat. The Fort Pickett
population has the most genetically diverse population and many of the
colonies have both staminate and female individuals (Burke and Hamrick
2002). Woody competition negatively correlated with density of all types
2008 V. Emrick and J. Jones 67
of Michaux’s sumac, but the effects were not equal among non-flowering,
staminate, and pistillate stems. Competition in the 2–5 m strata negatively
correlated with pistillate density, while showing little or no correlation
with staminate density and non-flowering density. Based upon our study,
reducing woody competition in the 2–5 m strata would improve habitat
conditions for sexual reproduction, which could increase the potential for
recovery of Michaux’s sumac.
Acknowledgments
We thank Eric Wolf, Michael St. Germain, Rebecca Murray, and Jessica Dorr
for their assistance in the field, and 1LT Doug Austin of Fort Pickett Range Operations
for arranging access to the restricted areas of Fort Pickett. We also thank Jeff
Waldon and three anonymous reviewers for helpful comments on earlier versions of
this manuscript. The Virginia Army National Guard, Department of Military Affairs,
funded this study.
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