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. 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