Reproduction and age composition of a population of woodland salamanders (Plethodon albagula) after a prescribed burn in southwestern Arkansas.
Plethodontid salamanders are important to function and stability of ecosystems (Wyman and Hawksley-Lescault, 1987; Wyman, 1998; Davic and Welsh, 2004; Walton and Steckler, 2005). They are indicators of integrity of ecosystems because of their reliance upon litter and moisture in soil, forest-floor microhabitats, and canopy of forest (Feder, 1983; Welsh and Droege, 2001). Species of Plethodon are among the most abundant vertebrates in North American forests (Burton and Likens, 1975; Hairston, 1987; Petranka, 1998) and they play a vital role in breakdown of litter by regulating communities of invertebrates (Wyman and Hawksley-Lescault, 1987; Wyman, 1998; Walton and Steckler, 2005). Thus, studying the response of plethodontids to burns could provide guidance for development of management plans for use in national forests. Examining the impact of these organisms in the context of prescribed burning is particularly important considering the prescribed-burning initiatives of the united states Forest Service (United States Department of Agriculture Forest Service, in litt.).
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Previous reports offer contradictions concerning the impact of fires on salamanders of the genus Plethodon, primarily because these reports were derived from anecdotal accounts in studies focusing on other taxa of amphibians and reptiles. For example, Keyser et al. (2004) detected no affect of fire on populations of Plethodon cinereus in Virginia and Greenberg and Waldrop (2008) noted no response of Plethodon to prescribed burning in the Appalachian Highlands. However, potentially positive effects from fire were reported by Kirkland et al. (1996), Cole et al. (1997), and Ford et al. (1999); there were more Plethodon in burned areas in eastern forests. Moseley et al. (2003) observed no change in overall abundance of amphibian species in burned plots compared to unburned plots, but they detected fewer Plethodon in areas recently burned. McLeod and Gates (1998), Means and Campbell (1981), Mitchell (2000), and Schurbon and Fauth (2003) reported similar results where Plethodon was present in unburned forest more often than in burned areas. These contradicting reports concerning relationships between Plethodon and prescribed burning emphasize the need for additional research that focuses specifically on these potential bioindicators.
In Texas, Missouri, Oklahoma, and Arkansas, P. albagula is a large woodland salamander that typically inhabits forested hillsides and seepages (Anthony, 2005). For an individual female, ovipositing typically occurs every other year during August-September in rotting logs, under large rocks, or in underground rock crevices, and occasionally in mine shafts, at which time females brood eggs until hatching (Conant and Collins, 1998; Trauth et al., 2004, 2006; Anthony, 2005; Fig. 1). A long-term dataset (Milanovich et al., 2006) provided a framework of ecological relationships of P. albagula against which we compared post-burning data. our goal was to investigate affects of a prescribed burn on a population of P. albagula in south-central Arkansas.
Materials and Methods--Our study site was an abandoned mine and adjacent forested habitat in the Ouachita National Forest, Garland County, Arkansas. The horizontal shaft of the mine (ca. 2.0 m high, 2.0 m wide, 149 m long) was set within a moderately sloping, southeast-facing hillside. Ridges within the Ouachita Mountains run east and west; thus, long, exposed north-facing and south-facing slopes are common within the 728,434-ha Ouachita National Forest. North-facing slopes tend to be dominated more by hardwoods than south-facing slopes and south-facing slopes are dominated by pines and are drier (T. L. Foti and S. M. Glenn, in litt.). The area adjacent to the mine was dominated by Quercus stellata (post oak) and Pinus echinata (shortleaf pine). During spring and autumn, P. albagula uses the area adjacent to the mine, but takes refuge from heat and desiccation in the mine during summer. The mine is used for brooding clutches in autumn and winter (Milanovich et al., 2006; Trauth et al., 2006).
Intervals for fires in this area averaged <10 years in the pre-settlement Ouachita Mountains region with fires occurring predominantly in late-growing and dormant seasons (T. L. Foti and S. M. Glenn, in litt.). However, current natural intervals range from 40 to >120 years, and prescribed burns are conducted during the dormant season in october-March (Masters et al., 1995). Total number of hectares burned within the Ouachita National Forest during 2001-2006 was 195,265 (mean/year = 32,544 ha; median/year = 35,746 ha; range = 17,439-54,384 ha; United States Department of Agriculture Forest service, in litt.). For our study, a moderate burn (fuel model = 9, mean length of flame = 0.91 m) was implemented by the United States Forest Service on 14 March 2005. The burn was 136 ha encompassing the entire ridge immediately surrounding the mine.
Reproductive and age-composition data from before the burn (August 1983-January 2005) were available for our analysis (Milanovich et al., 2006) with 30 post-burn visits during March 2005-october 2007. During each visit, we counted number of clutches, number of eggs per clutch (fecundity), total number of eggs in all clutches per year, and measured diameter of eggs (embryo and surrounding membrane; to the nearest 0.1 mm) for each available clutch. We took measurements of clutches within 14 days of oviposition (usually within 7 days), except for 2007, when some measurements were not taken until ca. 30 days after the typical oviposition period for clutches. Because this was a long-term study, some reproductive data before and after burning may include repeated measures of individual females across years preventing data from being considered independent. Although ovipositioning females only have 1 clutch/year, and because measurements were made across multiple years, multiple clutches from a single female across years may be included in the analysis. Females typically oviposit every other year; therefore, effects from the burn may not be apparent until subsequent years, as females acquire resources for reproduction the year preceding oviposition (Milanovich et al., 2006). Milanovich et al. (2006) analyzed long-term data on fecundity from this population and suggested that there was no relationship between size of body and fecundity; therefore, reproductive data from before and after the burn were tested with individual and year of study as independent variables. We also recorded numbers of salamanders and their life stages (juvenile < 50 mm snout-vent length or adult > 50 mm snout-vent length) in the mine during June-August. if the mine was visited more than once during those months, only the visit with the greatest number of animals was analyzed.
Given the importance of precipitation to Plethodon (Jaeger, 1972, 1980a; Fraser, 1976a; Jaeger et al., 1995; Grover, 1998; Milanovich et al., 2006), we measured precipitation adjacent to the mine to help identify potential causes of change after the burn. We obtained levels of precipitation before (1983-2004) and after (2005-2007) the burn for 12 months prior to ovipositioning, because precipitation from this period has been significantly correlated with reproductive output in this population of P. albagula. For example, precipitation for the 2008 reproductive year would encompass 1 August 2007-31 July 2008. Data on precipitation were from the weather station at Blakely Mountain Dam, Garland County, Arkansas, and from Milanovich et al. (2006).
We used general-linear models to examine the influence of the prescribed burn and precipitation on fecundity, diameter of eggs, total number of eggs per year, and number of clutches per year for the 12 years before the burn and 4 years after the burn (1983-2008). The reproductive-output variable was used as the response variable, whereas pre-burn or post-burn was the categorical predictor and precipitation was the continuous predictor. Differences in pre-burn and post-burn age-composition of the population (ratio between juveniles and adults) for animals seeking refuge in the mine during summer were compared for 8 years (1983-2006) using a Mann-Whitney U-test ([alpha] = 0.05). Ratios were not transformed because Liermann et al. (2004) reported no significant difference between observations where ratios were transformed compared to non-transformation. Non-parametric Mann-Whitney U-tests were used because variables were not independent. There were additional clutches in the mine, so number of eggs and diameter of eggs did not represent every egg in the mine. Analyses were performed using Statistica 6.0 (Statistica, Cary, North Carolina).
Results--Pre-burn, post-burn, or precipitation were not significant predictors of number of eggs in the mine each year or of diameter of eggs among years. However, there was a significant interaction between burn and precipitation for number of eggs (Table 1). Precipitation influenced mean yearly fecundity, and the interaction between burn and precipitation was a significant predictor of the annual number of clutches in the mine (Table 1). Despite the decrease in fecundity, number of eggs, and number of clutches in 2006, our overall analysis did not indicate that the prescribed burn had a measurable negative influence on reproductive output of P. albagula (Figs. 2 and 3). The ratio of juveniles to adults seeking refuge in the mine did not differ during years before and after the burn (W = 30, P = 0.40; Fig. 4). However, the ratio was lowest in 2006 when compared to previous years.
Discussion--After accounting for precipitation, we did not detect a direct affect of prescribed burning on reproductive output of P. albagula (Table 1). This differs from previous studies that elucidated deleterious effects of fire (Means and Campbell, 1981; McLeod and Gates, 1998; Mitchell, 2000; Moseley et al., 2003; Schurbon and Fauth, 2003; Cummer and Painter, 2007). Our study supports past studies where there were minimal effects of prescribed burning on Plethodon (Kirkland et al., 1996; Cole et al., 1997; Ford et al., 1999; Keyser et al., 2004). Although parameters we measured were not negatively influenced, number of eggs per clutch, total number of eggs, and number of clutches did decline 1.5 years after the prescribed burn. This reduction in reproductive output in 2006 suggests that the prescribed burn had an impact on the reproductive biology of P. albagula immediately following the burn, but our overall data suggest that the population rebounded (reproductively) from this effect. oviposition by individual P. albagula occurs every other year (e.g., most females ovipositing during 2005 would not oviposit again until autumn 2007). The amount of resources obtained 1 year prior to oviposition influenced fecundity in P. albagula (Milanovich et al., 2006). This suggests that any impact from the burn may not be detectable until the following year. Thus, unfavorable conditions created by the burn might affect females ovipositing 1 full year post-burn (i.e., 2006), rather than directly following the burn. This could explain the lower reproductive output in 2006. one would expect that reproductive output in 2007, 2008, or both, would be negatively impacted because reproductive output was lowest in 2006. Because reproductive and population parameters changed little in years prior to the burn (Milanovich et al., 2006), we suggest this decline (2006) was caused, in some way, by the prescribed burn. These results suggest that repeated burning over shorter intervals may have a more significant negative effect on reproductive output.
Ratios of juveniles to adults did not decline significantly after burning when both post-burn years were combined. However, a decline in 2006, 1.5 years post-burn, did occur (Fig. 4). Juvenile Plethodon are less desiccation resistant than adults (Fraser, 1976a; Jaeger, 1980a, 1980b). Therefore, our data contradict other studies that reported a decline in number of juveniles following a burn (Ash, 1997; Ash et al., 2003). Cummer and Painter (2007) also contradicted past studies that reported a decline in smaller life stages following a disturbance, as they reported an increase in juvenile Jemez Mountain salamanders (Plethodon neomexicanus) following a wildfire in New Mexico. Although data on ratios of juveniles to adults were not available for 2007, the increased number of clutches and eggs in 2007 further supports the hypothesis that terrestrial plethodontids are adapted to resist ecological stochasticity, which could be important in maintaining long-term stability of ecosystems (Davic and Welsh, 2004). The full effect of the prescribed burn on age composition of the population cannot be fully understood until juveniles from post-burn years become reproductively active. our data suggest that this effect could be minimal. one consideration is the mine itself, which offers refuge for salamanders and could have increased survival. However, if the mine was used to increase survival or for seeking refuge from unfavorable conditions post-burn, then one would expect to find a significant increase in use of the mine. in surveys during the weeks following the burn, we observed no increase in use of the mine as a refuge site. Presumably, Plethodon can retreat to subterranean refuges to escape unfavorable conditions (Taub, 1961; Heatwole, 1962; Spotilia, 1972; Petranka and Murray, 2001). Fraser (1976b) hypothesized that Plethodon can remain under subterranean refuge for days. Thus, we suspect the mine provided little to no additional refuge to salamanders when compared to subterranean refuge sites of other populations.
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Because reproductive and age-composition parameters after the burn were not lower than years before the burn, and all parameters tested in 2007 and 2008 were similar to years before the burn, it appears if there were affects from the burn they were short-lived. Given our limited data on habitat and age composition of the population, we can neither speculate on attributes of the burn that may have caused harm, nor on how the burn may influence long-term ecological relationships of P. albagula. The burn also was conducted during mid-March, which is considered a less active period for P. albagula in Arkansas (Trauth et al., 2004). Results of our study may have been different if the burn was conducted in a time of high activity for P. albagula (i.e., April-May or September-November). Moreover, in the Ouachita National Forest, areas undergoing prescribed burns are small (mean/year = 32,544 ha during 2001-2006) in comparison to the 728,434-ha forest (United States Department of Agriculture Forest Service, in litt.). Much of this area may not be suitable for Plethodon; thus, outcomes from prescribed burns in the Ouachita Mountains should be location specific. Results from our study might be best interpreted as an example of how a prescribed burn might influence ecology of salamanders and how our observations might be used by resource managers in the Ouachita Mountains or in similar habitats elsewhere. We studied salamanders at one location, which means that our data violate assumptions of experimental design (e.g., no control, no replication or independence) and, thus, leads to pseudoreplication (Hurlbert, 1984). We believe that the long-term nature of our study shows continuing trends in reproductive output and age composition of this population. We believe that future research dealing with effects of prescribed or natural fires should incorporate information addressing specific effects of fire on a multitude of habitat variables associated with Plethodon.
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We are grateful to M. McCallum, R. Jordan, B. Wheeler, and many others for assistance in the field. We also thank R. B. Bury and J. W. Petranka for providing helpful comments on early drafts of the manuscript and J. C. Maerz, A. K. Davis, and K. Barrett for helpful advice. This study was conducted under authority of Arkansas Game and Fish Commission scientific permit (041420042) issued to SET. Funding was provided by a Challenge Cost-Share Agreement (03-CS-11080901-090) between Arkansas State University and the United States Department of Agriculture Forest Service (Ouachita National Forest). Appreciation is extended to personnel of the United States Army Corps of Engineers, Lake Ouachita Field office, for allowing access and entry into the mine and to personnel of the United States Forest Service for conducting the prescribed burn.
Submitted 15 May 2009. Accepted 15 October 2010. Associate Editor was Paul A. Stone.
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JOSEPH R. MILANOVICH, * STANLEY E. TRAUTH, AND DAVID A. SAUGEY
Department of Biological Sciences, Arkansas State University, P.O. Box 599, State University, AR 72467 (JRM, SET) United States Forest Service, 8607 Highway 7 North, Jessieville, AR 71949 (DAS) Present address of JRM: Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602
* Correspondent: email@example.com
Table 1--Results from the general-linear model used to investigate influence of prescribed burning and precipitation on reproductive variables of the woodland salamander Plethodon albagula in Ouachita National Forest, Garland County, Arkansas. Pre-burning and post-burning are categorical predictors, and precipitation is the continuous variable. Dependent variable Effect df MS Fecundity Burn 1 0.644 Precipitation 1 3.51 Burn by precipitation 1 1.180 Error 12 0.678 Number of eggs Burn 1 19,938.870 Precipitation 1 25,412.800 Burn by precipitation 1 17,365.620 Error 12 6,977.531 Number of clutches Burn 1 315.114 Precipitation 1 158.096 Burn by precipitation 1 317.977 Error 12 58.871 Diameter of eggs Burn 1 0.024 Precipitation 1 0.001 Burn by precipitation 1 0.045 Error 5 0.095 Dependent variable Effect F P Fecundity Burn 0.949 0.349 Precipitation 5.177 0.042 Burn by precipitation 1.741 0.212 Error -- -- Number of eggs Burn 2.857 0.117 Precipitation 3.642 0.081 Burn by precipitation 2.489 0.141 Error -- -- Number of clutches Burn 5.353 0.0392 Precipitation 2.685 0.127 Burn by precipitation 5.401 0.0385 Error -- -- Diameter of eggs Burn 0.252 0.637 Precipitation 0.009 0.927 Burn by precipitation 0.471 0.523 Error -- --
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|Author:||Milanovich, Joseph R.; Trauth, Stanley E.; Saugey, David A.|
|Date:||Jun 1, 2011|
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