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Spider (O: Araneae) Responses to Fire and Fire Surrogate Fuel Reduction in a Piedmont Forest in Upstate South Carolina
Michael E. Vickers and Joseph D. Culin

Southeastern Naturalist, Volume 13, Issue 2 (2014): 396–406

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Southeastern Naturalist M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 396 2014 SOUTHEASTERN NATURALIST 13(2):396–406 Spider (O: Araneae) Responses to Fire and Fire Surrogate Fuel Reduction in a Piedmont Forest in Upstate South Carolina Michael E. Vickers1,* and Joseph D. Culin2 Abstract - The two forest-management practices of prescribed burning and thinning are techniques used to reduce heavy fuel loads that have resulted from years of fire suppression. Therefore, the National Fire and Fire Surrogate (NFFS) study was conducted to determine the effects of prescribed burning and thinning on different environmental factors. This study was conducted in the Clemson University Experimental Forest in South Carolina, one of the 13 NFFS sites in the United States, and examined the impacts of these management practices on spider populations. We used pitfall traps to sample ground-dwelling spiders to determine if changes in population levels had occurred one year following implementation of these practices. We collected a total of 1220 specimens of Araneae, representing 13 families. Results indicated that by 1-year post-treatment, spider populations had recovered following the initial (2001) burning and thinning. However, in 2002, when we compared the first post-burn samples to pre-burn samples in thin+burn plots, we found a significant decrease in the mean abundance of Agelenidae and Linyphiidae after the prescribed burn. Introduction During the 20th century, fire-exclusion policies were put into practice by federal land managers, resulting in the accumulation of an extraordinary amount of fuel (i.e., dead trees, logs, leaves, and litter; Agee and Lolley 2006, Lasko 2010, Schwilk et al. 2009, Stephens 1998). High levels of fuel accumulation results in fires that burn hotter, move slower, and have more profound ecological effects than fires in areas with lower fuel accumulation (McCullough et al. 1998). Such high-fuel fires create unnaturally severe wildfires and lead to an overall deterioration of forest ecosystem integrity (Stephens 1998). Management strategies have been developed to reduce fuel loads and are practiced across the United States. The two primary practices are prescribed burning and thinning. A prescribed burn has characteristics similar to naturally occurring fires but is controlled by forest managers so it burns material mainly at ground level and moves slowly through a forest. Thinning is used to remove smaller trees that have the potential to spread fire into forest canopies (Converse et al. 2006). However, it is not clearly understood if controlled burning, thinning, or the combination of thinning and burning harm (decrease shelter and food resources, cause heightened disturbance to the area) or benefit (increase niche diversity and habitat complexity) forest ecosystems. 1Department of Biology, Millikin University, 1185 West Main Street, Decatur, IL 62521. 2School of Agricultural, Forest and Environmental Sciences, Clemson University, 101 Barre Hall, Clemson, SC 29634. *Corresponding author - mvickers-alum@millikin.edu. Manuscript Editor: Vincent A. Cobb Southeastern Naturalist 397 M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 Therefore, the National Fire and Fire Surrogate (NFFS) study was established to quantify consequences that fuel-load–reduction treatments have both on economics and on ecological factors such as, vegetation, wildlife, arthropods, and pathology (Youngblood et al. 2005). Our study assessed the effects of forest-management strategies on the spider community. The few studies conducted in the United States that have examined the effects on spiders of prescribed burning, thinning, or thinning combined with burning have reported highly variable results ranging from spider populations being: unaffected (Haskins and Shaddy 1986), decreasing dramatically (Coyle 1981, Merrott 1976, Reichert and Reeder 1972, Willett 2001), or increasing (Vogl 1993). The goal of our study was to determine if the fuel-load–management practices of prescribed burning and thinning has an effect on spider populations in a southern piedmont forest. Methods During 2002, we used pitfall traps to sample spider populations (Araneae) after three forest-management treatments: prescribed burning (burn-only), thinning (thin-only), and thinning followed by prescribed burning (thin+burn). Control areas were established as a fourth treatment to allow comparisons with un-impacted areas. Replicated plots of the 4 treatments were located in the Clemson Experimental Forest in Anderson, Oconee, and Pickens counties, SC (Fig. 1). Burning of burn-only plots was conducted in the spring 2001. Thinning in the thin-only plots was conducted in the winter of 2001. Thinning and burning in the thin+burn plots were conducted in the winter of 2001 and spring 2002, respectively. Initial design setup Because the Clemson Experimental Forest was one of 13 sites participating in the National Fire and Fire Surrogate (NFFS) study, the initial plot design and establishment were completed by USDA Forest Service researchers prior to our sampling. In that design, the burn-only, thin+burn, and control treatments were replicated 3 times, and thin-only treatments were replicated 5 times. Thin-only treatments were replicated 5 times due to the herbicide for two established thin+herbicide treatments not being applied prior to our sampling; therefore, those two treatments were designated as thin-only in this study. Within each treatment area, 40 permanent grid points were established with 50-m spacing between each grid point in one of the cardinal directions. At randomly selected grid points, four 20-m x 50-m plots were established. Each 20-m x 50-m plot was divided in half, and a coin was tossed to determine which 10-m x 50-m plot was used during each of the 6 sampling periods. The 10-m x 50-m plot was further divided into five 10-m x 10-m subplots, and a Figure 1 (following page). Replicated treatments, shaded areas within highlight boxes, located in the Clemson Experimental Forest in Anderson, Oconee, and Pickens counties, SC. Inset: Expanded view of one treatment area with replicated plots showing the 10-m x 50-m area containing the five subplots that were sampled. Southeastern Naturalist M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 398 Southeastern Naturalist 399 M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 pitfall trap was placed in the center of each subplot (Fig. 1). A total of 280 pitfall traps were deployed on each sampling date. Sampling methods We conducted pitfall-trap sampling every 2 months for 1 year beginning in January 2002. Pitfall traps were active for approximately 48 hrs during each sampling period. Traps were constructed using a 473-ml (16-oz) plastic cup placed into the ground with a 266-ml (9-oz) cup, placed inside. The 473-ml cups remained in the ground during the entire study, while the 266-ml cups were only used during the trapping periods. To kill and preserve organisms that fell into the traps, we placed approximately 80 ml of 70% EtOH into the 266-ml cup. After we brought samples to the laboratory, we rinsed and stored them in 70% EtOH. Using published taxonomic keys (Dondale and Redner 1990, Kaston 1972, Roth 1993), we then sorted spiders to family and placed them into separate containers labeled with date, treatment, and plot. We deposited voucher specimens in the Clemson University Arthropod Collection (CUAC), Clemson, SC. Statistical analysis To determine spider family diversity and richness in the 4 treatments, we plotted the proportional abundance against rank in the spider community for the 4 treatments. We used the plotted rank-abundance curves to determine which treatments supported more-diverse spider communities, so as to focus on the most-abundant families. To determine if spider populations were affected by the management practices, we used a general linear model (P = 0.05) to analyze numbers of individuals per family collected at the plot level (SAS 1999) to obtain least square means. We made comparisons 1) between individual treatments (burn-only, thin-only, and thin+burn) and the control, and 2) among treatments (burn-only, thin-only, and thin+burn). Also, to exclude seasonality effects on spider families, we analyzed sampling dates versus treatment interactions. Sampling dates were January/February, March/April, June, August, October, and December. All statistical analyses were performed using SAS (SAS 1999). Also, we compared pre-burn samples (January/February) to post-burn samples (March/April, June, August, October, and December) in thin+burn plots to determine if burning had an immediate effect on spider populations in these plots. Results Plot rank-abundance We collected a total of 1220 specimens of Araneae, representing 13 families (Table 1). After comparing each family’s proportional abundance versus rank, we considered 7 families to be abundant enough for analyses (Fig. 2). Effects of treatment on mean abundance of spider populations Four of the 7 families identified—Agelenidae, Gnaphosidae, Pisauridae, and Thomisidae—were significantly affected by one or more treatment. The other 3 Southeastern Naturalist M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 400 families—Clubionidae, Linyphiidae, and Lycosidae—were not significantly affected by any treatment (Table 2). Effect of sampling period versus treatment on mean abundance of spider populations The families Agelenidae and Linyphiidae exhibited significant effects related to treatment over the 6 sampling periods, while the other 5 families analyzed did not Table 1. Total number of individuals collected for each of the 13 spider families during the six sampling periods in the Clemson Experimental Forest. Family # collected Control Burn-only Thin-only Thin+burn Agelenidae 144 59 33 23 29 Araneidae 5 1 1 1 2 Atypidae 3 1 1 0 1 Clubionidae 44 17 9 14 4 Gnaphosidae 212 61 59 68 24 Hahniidae 34 6 3 22 3 Linyphiidae 283 66 46 108 63 Lycosidae 321 33 64 166 58 Oxyopidae 16 2 2 10 2 Pisauridae 73 22 4 34 13 Salticidae 33 6 11 8 8 Theridiidae 6 1 1 3 1 Thomisidae 46 20 10 14 2 Figure 2. Rank-plot abundance of spiders collected during the six sampling periods in the four treatment plots. A total of 13 families were collected. Families are ranked from highest to lowest in proportional abundance. Southeastern Naturalist 401 M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 (Table 3). In addition, there was a significant difference in pre- versus post-burn mean numbers of agelenids (Table 3, Fig. 3) and linyphiids (Table 3, Fig. 4) following the March 2002 burn in the thin+burn treatment. Table 2. Total number of individuals collected for each of the 7 abundant spider families during the 6 sampling periods in the Clemson Experimental Forest and LSmean totals per treatment. Superscripted letters indicate families that exhibited a significant difference (general linear model: df = 3, P = 0.05): Abetween control plots and either burn-only, thin-only, or thin+burn plots; and Bbetween burn-only compared to thin-only or thin+burn plots. LSmean Family # collected Control Burn-only Thin-only Thin+burn Lycosidae 321 0.5322 0.9411 1.4310 0.8055 Linyphiidae 283 1.0645 0.6764 0.9310 0.8750 Gnaphosidae 212 0.9838 0.8676 0.5862A 0.3333A,B Agelenidae 144 0.8709 0.6176 0.1637A,B 0.4166A Pisauridae 73 0.3548 0.0588A 0.2931B 0.1805 Thomisidae 46 0.3225 0.1470A 0.1206A 0.0273A Clubionidae 44 0.2741 0.1323 0.1206 0.0555 Figure 3. LSmean number of Agelenidae (O: Araneae) collected during each of the six sampling periods in the four treatment plots. The burn indicates when the burn occurred in thin+burn plots in 2002. Southeastern Naturalist M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 402 Discussion Burn-only plots Plots burned in April 2001 had significantly lower mean numbers of Pisauridae and Thomisidae compared to control plots, suggesting that populations of Table 3. Total number of individuals collected for each of the 7 abundant spider families during the six sampling periods in the Clemson Experimental Forest and LSmean totals per sampling period by treatment. Superscripted letters indicate families that exhibited a significant difference (general linear model: df = 3, P = 0.05): Abetween control plots and either burn-only, thin-only, or thin+burn plots; Bbetween burn-only compared to thin-only or thin+burn plots; and Cbetween thin-only compared to burn-only and thin+burn plots. LSmean Family # collected Jan/Feb Mar/Apr June Aug Oct Dec Lycosidae 321 Control 0.0909 0.2857 1.3636 0.8181 0.4545 0.0909 Burn-only 0.5000 0.8333 0.8333 1.8333 1.1666 0.2500 Thin-only 1.3888 0.6842 1.2500 1.7368 3.3000 0.2000 Thin+burn 1.4166 0.4166 1.0000 0.9166 0.8333 0.2500 Linyphiidae 283 Control 3.2727 3.7142 0.2727 0.0909 0.0000 0.0000 Burn-only 2.0000A, B 0.7500A 0.9166 0.1666 0.0000 0.0000 Thin-only 4.8888A 0.5263A 0.2500 0.1578 0.0000 0.1000 Thin+burn 4.1666A, B 0.6666A 0.0833 0.2500 0.0000 0.0833 Gnaphosidae 212 Control 0.5454 0.8571 2.1818 0.5454 1.7272 0.0000 Burn-only 0.0833 1.5000 1.7500 0.7500 0.6666 0.2500 Thin-only 0.2777 0.2631 1.6500 0.3684 0.9000 0.0000 Thin+burn 0.0000 0.5000 0.8333 0.2500 0.4166 0.0000 Agelenidae 144 Control 3.5454 0.0000 0.4545 0.5454 0.0909 0.2727 Burn-only 1.0833A 1.5000A 0.3333 0.2500 0.1666 0.2500 Thin-only 0.4444A, C 0.3684B 0.0500 0.0526 0.0000 0.1000 Thin+burn 1.7500A 0.2500B 0.3333 0.0000 0.0000 0.1666 Pisauridae 73 Control 0.0000 0.2857 0.3636 0.7272 0.6363 0.0909 Burn-only 0.0833 0.0000 0.0833 0.1666 0.0000 0.0000 Thin-only 0.0555 0.1052 0.4500 0.7894 0.3500 0.0000 Thin+burn 0.0000 0.0000 0.5833 0.0833 0.3333 0.0833 Thomisidae 46 Control 0.1818 0.0000 1.2727 0.1818 0.0909 0.0909 Burn-only 0.1666 0.0833 0.5000 0.0833 0.0000 0.0000 Thin-only 0.1111 0.2105 0.3000 0.0526 0.0000 0.0500 Thin+burn 0.0000 0.0000 0.0000 0.0769 0.0833 0.0000 Clubionidae 44 Control 0.0000 0.1428 0.1818 0.9090 0.3636 0.0000 Burn-only 0.0000 0.0000 0.0000 0.2500 0.5000 0.0000 Thin-only 0.0000 0.1052 0.0000 0.4736 0.1500 0.0000 Thin+burn 0.0000 0.1666 0.0000 0.0833 0.0833 0.0000 Southeastern Naturalist 403 M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 these two families had been reduced by burning and had remained impacted for at least 1 year (Table 2). Both pisaurids and thomisids are wandering spiders that actively hunt prey. Although thomisids are generally considered to be sit-and-wait predators, Uetz (1975) has observed them running and pouncing on prey. The decrease in numbers of these 2 families could be due to either direct mortality, prey reduction, or reduction in habitat complexity (Phillips et al. 2004, Reichert and Reeder 1972). The results for the five families not having significantly different mean numbers in burn-only versus control plots (Table 2) may be due to those spiders’ ability to escape the fire by burrowing into the ground, hiding in protected locations, or rapidly recolonizing impacted habitats (Merrott 1976, Reichert and Reeder 1972, Vogl 1993). Thin-only plots The families Agelenidae, Gnaphosidae, Linyphiidae, and Thomisidae were negatively affected by thinning (Table 2). Agelenids and linyphiids are commonly found on vegetation, and a reduction in understory habitat may have caused this reduction. Correspondingly, Coyle (1981) has suggested that a decrease in number of ground-level web-building spiders was a result of decreased forest canopy and Figure 4. LSmean number of Linyphiidae (O: Araneae) collected during each of the six sampling periods in the four treatment plots. The burn indicates when the burn occurred in thin+burn plots in 2002. Southeastern Naturalist M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 404 its corresponding effects on ground-level microclimate and reduction of litter. Both gnaphosids and thomisids are wandering spiders, and a reduction in litter depth, habitat complexity, and prey abundance could result in higher mortality (Uetz 1979), or in a high number of spiders being captured in pitfall traps due decreased vegetation (Phillips et al. 2004). Thin+burn plots The families Agelenidae, Gnaphosidae, and Thomisidae were negatively affected by the combination of prescribed burning and thinning (Table 2). For these three families, the drastic change in environment or potential changes in prey density could have caused increased mortality (Coyle 1981, Huhta 1971, Phillips et al. 2004, Uetz 1979). Agelenidae and Linyphiidae were the only families to show a significant reduction immediately following prescribed burning in thin+burn plots when pre-burn and post-burn mean numbers were compared (Fig. 4). These results suggests that agelenids and linyphiids were negatively impacted due to fire mo rtality. Burn-only versus thin-only plots Mean population numbers of Agelenidae were significantly lower in thinonly compared to burn-only plots (Table 2), suggesting that thinning had a greater negative impact than burning on this family. Agelenids in thin-only plots may have moved to other areas to find more suitable environments. Pisaurids had significantly higher numbers in thin-only plots than in burnonly plots (Table 2), suggesting that this family was able to tolerate the impacts of thinning. Pisaurids are commonly found on ground surfaces where they actively search for prey and potential mates. High pisaurid collection numbers in thin-only plots may have resulted from increased movement of spiders of those species within the pitfall-trapping areas due to habitat disturbance and an increase in habitat complexity. Habitat complexity potentially was increased because of an increase in the amount of woody debris throughout thinned areas compared to burned areas (Waldrop et al. 2004). Sampling period versus treatment When we analyzed treatments over the 6 sampling periods, we found significant seasonal changes in mean population numbers for the families Agelenidae and Linyphiidae (Table 3). When the three treatments burn-only, thin-only, and thin+burn were compared to control plots, we found significant differences in mean population numbers during specific sampling periods. For instance, agelenids and linyphiids had higher numbers during January/February, March/April, June, and August. Additionally, it is important to note that seasonal fluctuations in mean spider populations could have affected number of spiders collected in the 4 treatments during sampling (Coyle 1981, Greenberg and Forrest 2003, Uetz 1979). Multiple-year sampling would have provided a better estimate of familylevel phenology. Southeastern Naturalist 405 M.E. Vickers and J.D. Culin 2014 Vol. 13, No. 2 Conclusion By one year following fuel-load–reduction treatments in the Clemson Experimental Forest, individual spider families exhibited different patterns of impact. Spider families were either not affected, or had recovered quickly from the various fuel-load–reduction treatments. There was an immediate impact following burning of the thin+burn plots on the families Agelenidae and Linyphiidae, which had a significant decrease following the controlled burn. However, by the second postburn (Agelenidae) and third post-burn (Linyphiidae) samples, mean agelenid and linyphiid numbers were not significantly different from those in the control plots. This information corroborates other studies that have examined the impacts of the management practices of prescribed burning and thinning (Haskins and Shaddy 1986, Merrott 1976, New and Hanula 1998). It is important to note, our study was only conducted over 1 year, and it is yet to be determined what the long-term effects of prescribed burning and thinning are on spider populations. Acknowledgments This research was funded in part by the US Joint Fire Science Program, USDA Forest Service (Agreement # SRS 01-CA-11330136-490). In addition, we would like to thank the College of Agriculture, Forestry and Life Sciences, the Clemson Experiment Station, and the Clemson Experimental Forest. Literature Cited Agee, J.K., and M.R. Lolley. 2006. Thinning and prescribed-fire effects on fuels and potential fire behavior in an eastern Cascades forest, Washington, USA. Fire Ecology 2(2):142–158. Converse, S.J., G.C. White, K.L. Farris, and S. Zack. 2006. Small mammals and forest-fuel reduction: National-scale responses to fire and fire surrogates. Ecological Applications 16(5):1717–1729. Coyle, F.A. 1981. Effects of clearcutting on the spider community of a southern Appalachian forest. 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