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Prescribed Fire and the Abundance of Soil Microarthropods in Northeast Georgia
Matthew W. Hutchins, Barbara C. Reynolds, and Steven P. Patch

Southeastern Naturalist, Volume 10, Issue 3 (2011): 489–500

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2011 SOUTHEASTERN NATURALIST 10(3):489–500 Prescribed Fire and the Abundance of Soil Microarthropods in Northeast Georgia Matthew W. Hutchins¹,*, Barbara C. Reynolds¹, and Steven P. Patch² Abstract - We examined the effects of prescribed fire on the abundance of soil microarthropods in a southeastern pine-hardwood forest in northeast Georgia. Using soil cores, the soil microarthropod groups Prostigmata, Oribatida, and Collembola were examined before and after a low-intensity prescribed fire intended for fuels reduction and wildlife habitat improvement. A post-burn evaluation found 100% duff layer coverage and 80% of the understory vegetation consumed. Prostigmata numbers were significantly reduced four months after the burn, with numbers returning to pre-burn levels more than one year later. Although Oribatida and Collembola fluctuated from year to year, we found no significant effects from the burn on those taxa. These results suggest that low-intensity prescribed burning has no lasting negative effects on soil microarthropod populations. The results from this study add to the evidence suggesting the adaptability of southeastern forests to low-intensity prescribed fire. Introduction Natural resource managers recognize fire as an important element in promoting wildlife habitat, species composition, and protecting biodiversity (Masters et al. 1996, Vose 2000). In addition to promoting habitat and species, fire assists in reducing fuels along forested and urban interfaces. The use of prescribed fire for stand maintenance and management is increasing in the southeastern United States as managers observe benefits of fire and understand the historical role that fire has played in southeastern ecosystems (Van Lear 2000, Vose et al. 1999). The southeastern mixed pine-hardwood ecosystem type is particularly dependent on fire and may be negatively affected by fire suppression (Vose et al. 1997, 1999). Recent studies suggest low-intensity prescribed burning as an effective method for restoring desirable species in southeastern pine-hardwood communities without harmful effects on carbon and nutrient cycling (Hubbard et al. 2004, Vose et al. 1999). However, even with desirable results of prescribed fire on nutrient cycling, impacts on decomposer communities after fire could counterbalance any positive results. Prescribed burning alters crucial habitat and resources for soil decomposer organisms through changes in above-ground vegetation, litter, and organic matter (Haimi et al. 2000). With low-intensity prescribed burning, direct effects to mobile soil invertebrates are of less concern than effects on immobile ¹Department of Environmental Studies, University of North Carolina at Asheville, Asheville, NC 28804. ²Department of Mathematics, University of North Carolina at Asheville, Asheville, NC 28804. *Corresponding author - 490 Southeastern Naturalist Vol. 10, No. 3 microorganisms. On the other hand, indirect effects of burning, such as litter reduction, have shown to dramatically reduce abundance of mobile and immobile soil-dwelling invertebrates (Certini 2005). Despite the impact fire has on the forest floor habitat, soil fauna often display resilience to this type of disturbance. Resilience may be defined not only as the stability of a system following a disturbance, but how quickly the system is able to return to equilibrium after disturbance (Holling 1973). The resilience of the soil system to prescribed burning depends on factors such as climate, vegetation, and topography of the burned area (Certini 2005). In addition, fire severity and depth of burn has been suggested as the determining factor in the effects of fire on soil fauna (Malmstrom 2010). Microarthropods are among the most abundant organisms in forest soil and serve vital functional roles in connecting the litter layer to mineral soil (Susilo et al. 2004). Many microarthropods occupy multiple feeding guilds in the soil food web as primary and secondary decomposers and feed on a range of materials as shredders, scavengers, detritivores, and fungivores (Coleman et al. 2004, Schneider et al. 2004). Other microarthropods are predators and feed on other soil organisms, such as nematodes (Coleman et al. 2004). By breaking down organic material, soil microarthropods increase the surface area of organic matter for colonization of fungi and bacteria, indirectly impacting primary production (Coleman et al. 2004, Crossley et al. 1992, Susilo et al. 2004). Fungal-feeding microarthropods are also thought to be important in production of dissolved organic matter (Osler and Sommerkorn 2007). Soil mites and Collembola make up the majority of microarthropods in most soil systems (Coleman et al. 2004). While the important role of soil microarthropods is clear, there is still limited understanding of how the soil microarthropod community is affected by prescribed burning or fire in the southeastern US. The few investigations on this subject in the Southeast are inconsistent in their findings. Studies on the impacts of fire on abundance of mites, such as prostigmatids and oribatids, show variable results, with some reports of lasting negative effects and other reports of no apparent effects (Barratt et al. 2006, Coleman and Rieske 2006, Dress and Boerner 2004, Haimi et al. 2000). A few studies indicate Collembola to be resilient to or possibly benefit from disturbance, including fire (Coleman and Rieske 2006, Lindberg and Bengtsson 2005), while other studies have implicated fire to have lasting negative effects on abundance of Collembola (Berch et al. 2007, Brand 2002, Collett 1998). In this study, we examined the response of soil microarthropod populations to a low-intensity prescribed burn in a southeastern pine-hardwood forest. Populations of the mite suborders Prostigmata and Oribatida and the hexapod class Collembola were examined at a burned treatment site, pre and post-burn, as well as at an unburned control site. 2011 M.W. Hutchins, B.C. Reynolds, and S.P. Patch 491 Field-Site Description The study area is located in northern Georgia, in the Lake Russell Wildlife Management Area (USFS) in Habersham County. The area is about 480 m in elevation, with a mean annual precipitation of 150 cm (NOAA 2007). The study area consisted of two sites (burned and unburned) 0.4 km in distance from each other. Both sites comprised slopes with north and south aspects. The unburned south slope was 24°, the unburned north slope was 37°, the burned south slope was 23° and the burned north slope was 27°. Sites were mixed pine-hardwood forests, although the burned site consisted of more pine in the overstory than the control site. The burned area was predominantly Pinus taeda L. (Loblolly Pine) with some P. virginiana Miller (Virginia Pine) in the overstory. Many pines at the burned site had recently died due to pine bark beetle (Ips spp.) attacks, and the site had probably been tilled for agriculture prior to forest growth (B. Boydstun, US Forest Service, Clarksville, GA, pers. comm.). Understory trees at the burned site were mostly Acer rubrum L. (Red Maple), Oxydendrum arboreum L. (Sourwood), and Nyssa sylvatica Marsh. (Blackgum), with a few Cornus florida L. (Flowering Dogwood). The unburned area was thought to be less disturbed even before the fire, with forest that was previously harvested, but not used for agriculture (H.R. Pulliam, University of Georgia, Athens, GA, pers. comm.). The overstory at the unburned site was comprised of Virginia Pine, Quercus prinus L. (Chestnut Oak), Q. alba L. (White Oak), and Carya spp. (Hickory), with some Q. falcata Michx. (Southern Red Oak). The understory at the unburned site consisted of Red Maple, Flowering Dogwood, Sourwood, Blackgum, and Southern Red Oak, with some Kalmia latifolia L. (Mountain Laurel) on the north slope. Young Red Maple saplings were the predominant understory on the south slope at the unburned site. Methods Site treatment and sampling In April 2005, one site was burned by the USDA Forest Service using a mix of hand-set and helicopter-set ignitions on a grid basis as part of a prescribed fire that burned 328 ha. The main purpose of the prescribed fire was fuels reduction along a wildland/urban interface. Secondary benefits of the prescribed fire included wildlife habitat improvement. Surface air temperatures on the day of the burn ranged from 16–18 °C when the burn started in the morning to 24 °C by the afternoon. Relative humidity ranged from 35–40% when the burn started to about 27% by the afternoon. The intensity of the fire was primarily influenced by temperature and relative humidity on that day. The site was burned in the morning at approximately 11:00 am EST; therefore, the intensity of the burn was considered light to moderate due to the temperature and relative humidity at the time of the burn (B. Boydstun, pers. comm.). A post-burn evaluation of the area after the fire found no bare soil exposed, with 100% of the duff layer (layer between 492 Southeastern Naturalist Vol. 10, No. 3 soil and leaf litter) coverage and 80% of the understory vegetation consumed (B. Boydstun, pers. comm.). For our study, a 200-m² area was divided into ten 1- x 2-m plots on both north and south slopes at each site, totaling 40 plots among both burned and unburned sites. North and south plot areas were about 50 m apart at the burned site and about 80 m apart at the unburned site. Plots were divided into an imaginary grid of 12 cells, and collections were made from a randomly selected cell within the grid, using the same cell on a given date. Every plot was sampled on each collection date, for a total of 20 samples/site. Soil cores were taken with PVC piping, 5 x 5 cm (98 cm³) with 1.5-mm mesh screen on one end, through the litter layer (Moldenke 1994). Pre-burn soil core samples were taken on 9 July 2004. Post-burn soil core samples were taken on 20 August 2005 and on 12 September 2006, four and 17 months after the burn. Both burned and unburned sites were sampled each sampling date and the unburned site was considered the control. Soil temperatures were also taken at each plot with soil thermometers to depths of 3–5 cm, and soil moisture was measured using a Hydrosense© (Campbell Scientific, Inc.) water content sensor at depths of 12 cm. The soil cores were wrapped in aluminum foil and transported in a cooler back to the lab the same day and immediately placed screen-side down on modified Tullgren extractors (Mallow and Crossley 1984). Microarthropods were collected into vials containing 70% ethanol over one week with the light intensity gradually increased. Soil microarthropods were sorted into the categories Prostigmata, Oribatida, and Collembola. Other microinvertebrate taxa, such as pseudoscorpions, were separated but not analyzed in this investigation due to their low numbers. To investigate the effects of fire on oribatid life history, we separated the oribatids into two groups: mature and immature. Immature oribatids were considered to be mites that were smaller (<1 μm) and that had weakly sclerotized cuticles. We recognize that many immature oribatids are endophagous, and thus would not respond to the Tullgren extraction (Norton 1994). Therefore, our counts of immature oribatids are probably low. Statistical analysis A linear mixed model was used to analyze the data for each microarthropod taxa: prostigmatids, immature oribatids, mature oribatids, and Collembola. This analysis assumes that the burned treatment site and the unburned control site had the same properties relative to the effects of fire on microarthropods. Because there was only one burn, it was not possible to test this assumption. However, the two sites were adjacent to each other and there were no obvious differences in the geographical features between the two sites. The responses were the natural log counts of taxa. A natural log transform was used to make the residuals homoskedastic and normally distributed (Littell et al. 2006). Taxa count data did not contain excess zeros, thus, there was no need to accommodate zero inflation (Sileshi 2008). Plot was considered as a random factor. Treatment (burned and 2011 M.W. Hutchins, B.C. Reynolds, and S.P. Patch 493 unburned) and aspects (north and south) were between-plot factors; sampling date and the interaction between sampling date and treatment were within-plot factors. For each microarthropod taxon, plots of the residual versus fitted values were consistent with homoskedastic residuals. There was no evidence of nonnormality in the residuals for any taxon (all Shapiro-Wilks P-values > 0.10). Because the natural log transformed counts were used, inferences were made for the geometric mean counts instead of the mean counts. Least square means and standard errors were used for comparison purposes of dates and any significant interactions using the family significance level of 0.05 with Tukey adjustment for pair-wise comparisons. The resulting 95% confidence intervals for the means of the log transformed counts were reverse transformed to obtain corresponding confidence intervals for geometric mean counts. These confidence intervals for geometric mean counts had individual confidence levels of 95%. Results A total of 6685 microarthropods were extracted from our soil cores, including 810 prostigmatid mites (10% of total), 3259 immature oribatids (50% of total), 1830 mature oribatids (28% of total), and 786 Collembola (12% of total). The abundance of immature oribatids were mainly attributed to the increased abundance one year after the burn in summer 2006 (Table 1). All four microarthropod taxa (prostigmatids, immature oribatids, mature oribatids, and Collembola) were most abundant in the summer of 2006, one year following the burn. Prostigmatid populations differed by date and had a significant treatment*date interaction (F = 5.42, P = 0.0063). Prostigmata abundance was lower at the burned treatment site in 2005 compared to 2004, but returned to pre-burn levels in 2006 (Fig. 1). The treatment*date interaction for Prostigmata presumably indicates a significant effect from the burn. Within the oribatids, both immature and mature groups were significantly more abundant by date with no treatment interaction; thus, fluctuations in populations were similar in plots at both burned and unburned sites. Immature oribatids were significantly more abundant in summer 2006 than summer 2004 Table 1. Abundance of immature oribatids, mature oribatids, and Collembola. Numbers are geometric means (GM) of all burned and unburned samples and represent 95% confidence intervals (CI) for geometric mean counts. For each taxon, means with the same letter are not significantly different. Pre-burn Four month post-burn 17 month post-burn (Summer 2004) (Summer 2005) (Summer 2006) Avg. # / soil Avg. # / soil Avg. # / soil Taxon core (GM) 95% CI core (GM) 95% CI core (GM) 95% CI Immature oribatids 7.72 (B) 5.05–11.56 4.36 (B) 2.74–6.68 36.48 (A) 25.15–52.71 Mature oribatids 12.39 (A) 9.08–16.78 3.92 (B) 2.72–5.51 15.23 (A) 11.27–20.47 Collembola 3.80 (B) 2.76–5.13 3.11 (B) 2.23–4.24 7.29 (A) 5.51–9.56 494 Southeastern Naturalist Vol. 10, No. 3 Figure 2. Collembola at burned treatment and unburned control sites. Bars are means combining 2004, 2005, and 2006 data at each site and represent individual 95% confidence intervals for geometric mean counts. Bars with the same letter are not significantly different. Figure 1. Effect of prescribed burning on Prostigmata. Bars are means at the burned treatment and unburned control sites and represent individual 95% confidence intervals for geometric mean counts. Bars with the same letter are not significantly different. 2011 M.W. Hutchins, B.C. Reynolds, and S.P. Patch 495 or summer 2005 (F = 35.56, P < 0.0001; Table 1). However, unlike the immatures, mature oribatid abundance decreased from 2004 to 2005, and then increased from 2005 to 2006 (F = 23.35, P < 0.0001; Table 1). The abundances of mature oribatids were similar in 2004 and 2006. Collembola abundance differed between treatment and control sites with no interaction with date or aspect (F = 7.46, P = 0.0097). Collembola also differed by date with no interaction with treatment or aspect (F = 9.56, P = 0.0002). Collembola were more abundant at the treatment site, regardless of year, pre and post-burn (Fig. 2). Among sampling years, Collembola were also more abundant in summer 2006 than in summer 2004 and summer 2005 (Table 1). Aspect was not significant for any microarthropod group. Discussion The low-intensity prescribed fire had negative effects on the abundance of prostigmatid mites four months after the burn in 2005, but their populations returned to pre-burn levels 17 months post-burn. While the populations of each microarthropod taxon fluctuated over the sampling period, oribatids and Collembola showed no effects from the burn. Barratt et al. (2006) and Seastedt (1984) found Prostigmata to be unaffected by fire, but others found lasting negative effects of prescribed burning on microarthropod taxa including Prostigmata (Barratt et al. 2006, Berch et al. 2007, Cole et al. 2008, Dress and Boerner 2004). Also, Coleman and Rieske (2006) examined impacts of prescribed burning on ground-dwelling and leaf-litter arthropods in a southeastern forest and found differential effects among the two groups. While leaf-litter arthropods were negatively affected for two years following a prescribed fire, ground-dwelling arthropods were apparently unaffected (Coleman and Rieske 2006). Negative short-term effects of fire on microarthropods have been shown to depend on the intensity and frequency of burning (Barratt et al. 2006, Dress and Boerner 2004). Fire disturbs nutrient cycling and biotic components of the ecosystem, but also provides sources of nutrients and resources to the soil food web in the form of charcoal and burned vegetation. This reciprocal effect of fire disturbance results in direct and indirect impacts to soil microarthropods (Hart et al. 2005). Fire directly affects microarthropods by causing mortality due to temperature or combustion. Temperature thresholds, related to intensity and duration of fire, have been demonstrated as direct effects to soil microarthopods (Haimi et al. 2000, Malmstrom 2008). While some microarthropods dwell deeply enough in the soil horizon to be insulated from even high-severity fire (Coleman and Rieske 2006, DeBano et al. 1998), the samples from our study were not deep enough to include these organisms. Indirect effects of fire on microarthropods include changes in habitat (resulting in increased vulnerability to environmental variables) and resources (such as food sources) (Barratt et al. 2006, Brand 2002, Malmstrom 2008). 496 Southeastern Naturalist Vol. 10, No. 3 Most studies attribute the effects of fire on forest-dwelling arthropods to the the amount and quality of leaf litter remaining after a fire-disturbance event (Coleman and Rieske 2006, Donegan et al. 2001, Dress and Boerner 2004). Generally, Prostigmata, Oribatida, and Collembola are positively correlated with organic matter (Peterson and Luxton 1982, Vreeken-Bruijs et al. 1997), although one study has found exceptions to this assumption (Hasegawa 2001). Organic matter in the litter layer influences microarthropod abundance by creating nutrient pools for soil microarthropod food sources, such as the microbial community (Neary et al. 1999). Considering that most Prostigmata are predators and depend on prey (Coleman and Crossley 2004), perhaps our observation of their decrease in 2005 was the bottom-up effect of a decrease in their prey (other microinvertebrates) that rely on microbial activity. Oribatids were not affected by the burn, but they have been reported to be suppressed by a wide range of disturbances due to their slow reproduction rates (Cancela da Fonseca and Sarkar 1998, Cole at al. 2008, Crossley et al. 1992, Lindberg and Bengtsson 2005). We found different responses between the immature and mature oribatids over the three years of the experiment. Immature oribatid abundance was relatively low in 2004 and 2005, with a large increase in 2006, seeming to indicate fluctuation between years. The numbers of mature oribatids significantly fluctuated year to year during the sampling period, but again, changes in abundance were apparently unrelated to the burn as fluctuations occurred at both sites. Collembola significantly increased from 2005 to 2006 at both the burned treatment site and unburned control site. Collembola were also more abundant at the burned treatment site compared to the control including pre-burn, with no interaction with year, perhaps reflecting site differences, such as land-use history and current canopy cover. The higher predominance of pine at the burn site also may have attributed to site differences. Collembola have been shown to respond positively to disturbance events (Coleman et al. 2004). Other studies have found that Collembola respond negatively to fire (Berch et al. 2007, Brand 2002, Collett 1998). However, these studies tended to examine effects of fire on species richness and diversity, which we did not address. While our study did not measure nitrogen in the litter-layer, the presence of increased soil-litter nitrogen following the burn may have contributed to the stability of oribatids and Collembola. Nitrogen is often the most limiting nutrient in ecosystems, but soil nitrogen often increases directly after a low-intensity fire (DeBano et al. 1998, Elliot et al. 2004). Nitrogen is known to stimulate the growth of fungi, an important food source for Oribatida and Collembola (Vreeken-Buijs et al. 1997). Perhaps an initial increase of nitrogen following the burn gave a temporary boost to Oribatida and Collembola populations in the burned treatment site, superseding any negative effects of organic matter or litter loss. Because there was only one fire disturbance, our findings depend on the assumption that the disturbance location and control location had the same 2011 M.W. Hutchins, B.C. Reynolds, and S.P. Patch 497 properties relative to the effects of fire on soil microarthropods. While the preburn data helps support the sampling of the disturbance event, the effects, or lack of effects, found in this study could be partially due to differences in site location. Ideally, a study would have several small fire disturbances created in random locations, which would allow one to adjust tests of disturbance effects for variation in the implementation of creating the fire disturbance and the properties of the locations of fire disturbance, but as with many disturbance studies, that was not possible in this setting. Conclusion Our results assist in understanding the effects of single low-intensity burning in southeastern pine-hardwood forests. Our findings suggest Prostigmata abundance was significantly decreased by the low-intensity prescribed burn, but recovered one year following the burn. This short-term negative response by the Prostigmata was most likely due to temporary loss of habitat and resources. The other microarthropod taxa—immature oribatids, mature oribatids, and Collembola—were apparently unaffected by the prescribed burn, which probably demonstrates their opportunistic feeding behavior. The results from this study also indicate the great extent to which environmental variables affect the abundance of microarthropod populations. Our study demonstrates the need to further understand the underlying controls on microarthropod abundance and the links among organic matter, microbial activity, and taxa of soil microarthropods. Because soil microarthropods are indicators of soil quality and serve important roles in the soil food web, our results should assist resource managers in understanding some ecological implications of prescribed burning in the southeastern US. Our findings add to the literature suggesting that soil systems in the Southeast are highly adaptable to fire disturbances. Acknowledgments We would like to give a special thanks to Irene Rossell, Ernest C. 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