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Increased Springtail Abundance in a Garlic Mustard-Invaded Forest
Anne B. Alerding and Roy M. Hunter

Northeastern Naturalist, Volume 20, Issue 2 (2013): 275–288

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2013 NORTHEASTERN NATURALIST 20(2):275–288 Increased Springtail Abundance in a Garlic Mustard-Invaded Forest Anne B. Alerding1,* and Roy M. Hunter1,2 Abstract - Alliaria petiolata (Garlic Mustard) decreases diversity of native plants and introduces novel litter inputs to forests, but whether Garlic Mustard’s presence influences detritivores is unclear. Our goal was to determine if Garlic Mustard alters springtail (detritivore) densities during early invasion. We obtained epigeal (litter, humus, and soil) cores from invaded and uninvaded areas in a Pinus strobus (White Pine) forest and used high-gradient dynamic sampling to extract springtails. Invaded areas contained nearly three times more springtails than uninvaded areas, attributable to two springtail morphospecies from Collembola families Tomoceridae and Entomobryidae. Higher pH in invaded epigeal samples correlated with increased springtail abundance. pH alkalization also correlated with increased proportion of juvenile rosettes in mixed stands. Our results suggest a possible role for pH modulation of springtail abundance in response to Garlic Mustard invasion. Introduction Non-native (invasive) plants are well known for their disruptive biotic impacts on ecosystems. As invaders increase in abundance, measurable declines in density and diversity of native plants occur concomitantly with physical restructuring of plant biomass that alters niche utilization by animal inhabitants (Baiser et al. 2008, Bultman and DeWitt 2008, Pearson 2009). Invasive plants are often chemically distinct from native plants, as their tissues contain novel chemical toxins that deter biomass loss to herbivores (Callaway and Ridenour 2004). The resulting increased consumption of native plants by herbivores hastens biomass conversion to invasive plant biomass (Carvalheiro et al. 2010, Eschtruth and Battles 2009, Schaffner et al. 2011), which then enters the food web as litter. While it might be predicted that chemically distinct invasive plant litter would also have slower rates of consumption by detritivores (animals, fungi, and bacteria), surprisingly, invasive litter frequently shows accelerated rates of decomposition (Allison and Vitousek 2004, Knight et al. 2007, Liao et al. 2008, Standish et al. 2004). This apparent dichotomy of lower consumption of living invasive biomass but enhanced turnover of invasive litter suggests that native herbivores and detritivores can respond differently to plant invasion (Kappes et al. 2007, Levin et al. 2006). It is currently unknown if invasive litter inputs cause trophic shifts that favor detritivore food webs in forests. In the northeastern United States, one of the most successful temperate forest invaders is the herbaceous biennial Alliaria petiolata (M. Bieb.) Cavara & Grande (Garlic Mustard; Rodgers et al. 1Department of Biology, Virginia Military Institute, 203 Maury-Brooke Hall, Lexington, VA 24450. 2Current address - Virginia Department of Health, 600 Bedford Avenue, Bedford, VA 24523. *Corresponding author - 276 Northeastern Naturalist Vol. 20, No. 2 2008a). Garlic Mustard grows in a patchy distribution in forest understories, where it reaches densities as high as 350 plants per m2 (Stinson et al. 2007, Winterer et al. 2005). Garlic Mustard living biomass is mainly avoided by native herbivores because its tissues are defended by chemical toxins: cyanide, glucosinolates, and cyanopropenyl glycosides (Barto et al. 2010, Cipollini and Gruner 2007, Evans and Landis 2007, Freeland and Janzen 1974, Renwick 2002). Thus, the bulk of Garlic Mustard biomass enters the forest floor as litter, and forests invaded by Garlic Mustard show accelerated rates of decomposition (Rodgers et al. 2008b). A mechanism for enhanced decomposition is unknown but may involve increased activity of detritivores living in the forest understory. The objective of our research is to measure the detritivore response to Garlic Mustard invasion in a Pinus strobus (L.) (White Pine) forest. Collembola (springtails) is an important group of detritivores in White Pine forests. Springtails facilitate decomposition by breaking litter into smaller pieces as they feed on both the plant litter and the microflora (bacteria and fungi), thereby increasing litter surface area for further decomposition (Seastedt 1984). Springtails are also unaffected by glucosinolate chemical defenses (Kabouw et al. 2010), such as those produced by Garlic Mustard. Springtail responses to Garlic Mustard invasion are poorly characterized, the only information coming from a study focused on carabid beetles. Dávalos and Blossey (2004) measured carabid beetle prey abundance in two forests by sampling soil beneath the coverboards and litter beside the coverboards on one occasion during a 6-month study. One forest showed no change, but the other forest revealed an increase in springtail abundance (64.6%). To improve accuracy of springtail abundance estimates in our study, we used an epigeal sampling device to obtain litter, humus, and soil (together) from each sampling location. We additionally conducted a temporal analysis of springtail abundance over a 3-week period in early summer, when both phases of Garlic Mustard (juveniles and adults) co-occurred in the forest. We hypothesize that abundance of springtails, a vital detritivore in this White Pine forest, will increase in response to Garlic Mustard invasion. Invasive plants not only affect biotic interactions in ecosystems, but their presence impacts the abiotic environment. Abiotic variables such as temperature, pH, and moisture influence decomposition rates through their effects on detritivores and microflora (Aerts 1997). In a meta-analysis conducted from 199 articles, it was recently demonstrated that ecosystems experiencing plant invasion show enhanced nutrient mineralization and increased levels of macronutrients (P, N) that facilitate greater productivity of invasive plants (Vilà et al. 2011). While soil moisture is generally unaltered during plant invasions, pH levels resulting from interactions between water, soil, humus, litter, and the living biota show an overall decrease, or acidification (Vilà et al. 2011). Garlic Mustard invasion causes similar increases in nutrient mineralization in invaded soils, but the pH response differs from the generalized (meta-analysis) response reported by Vilà et al. (2011). Garlic Mustard has been shown to alkalize soils in both field and laboratory experiments (Anderson and Kelley 1995, deHart and Strand 2012, Rodgers et al. 2008b). Because springtails show high reproductive rates at pH 2013 A.B. Alerding and R.M. Hunter 277 5–6 (Hopkin 1997), a pH range that is characteristic of Garlic Mustard-invaded soils (Meekins and McCarthy 2001), we investigated pH change as a possible mechanism for enhanced springtail success during Garlic Mustard invasion. We hypothesized that springtail abundance would correlate with pH levels known to maximize their fitness. In this study, epigeal cores were randomly sampled from a White Pine forest experiencing early Garlic Mustard invasion. Garlic Mustard density surrounding the cores was assessed, and samples were measured for pH, moisture, litter composition, and springtail abundance. We present an exploratory analysis of abiotic factors and developmental features of Garlic Mustard and their possible roles in influencing springtail abundance during invasion. Field-Site Description This study was conducted in a mature White Pine forest adjacent to Washington and Lee University in Lexington, VA. from June into July 2010. The 60-year-old White Pine forest is undergoing secondary succession to a mixed deciduous canopy dominated by Fraxinus americana L. (American Ash) and Platanus occidentalis L. (American Sycamore) in the overstory and Lindera benzoin L. (Spicebush), Microstegium vimineum (Trin.) (Japanese Stiltgrass), and Garlic Mustard in the understory. At the time of the study, Garlic Mustard invasion was in its early stages, with seed propagules spreading southward into the forest from an adjacent section to the north that was separated from our field site by a walking trail. Methods Sampling sites We established two adjacent 20-m x 20-m sites, one invaded by Garlic Mustard and one uninvaded, with similar canopy openness, understory plant community structure, epigeal composition, slope, and aspect. A few Garlic Mustard first-year juveniles were found in the uninvaded site during the course of the study, consistent with the southerly movement of Garlic Mustard into this forest. Garlic Mustard plants occurred in a patchy distribution within the invaded site. Garlic Mustard densities in this forest (15–78 plants per m2) reflect low levels of early-stage invasion, or between 5–20% of the density attained in a highly invaded forest (Stinson et al. 2007). Abiotic measurements and epigeal samples were collected over a three-week period, beginning in June 2010, when juveniles and adults were fully grown and siliques were beginning to release seeds. Epigeal sampling Epigeal samples taken included the O horizon (plant litter and humus) and the first few centimeters of the A horizon (inorganic matter). Temporal changes in abiotic (moisture and pH) and biotic epigeal variables (litter composition and springtail abundance) were monitored from randomly collected epigeal cores that were sampled at weekly intervals between June 28 and July 13 (a once per week for three weeks sampling protocol). Each 20-m x 20-m field site was subdivided 278 Northeastern Naturalist Vol. 20, No. 2 into four equally sized quadrats. For each sampling date during the temporal sampling period, five points were randomly selected from each quadrat using a random number table and X, Y coordinates. One point was selected from each of the four quadrats per field site, with the fifth randomly selected from any of the four quadrats. Because Garlic Mustard cover was patchy, sampling locations were chosen in the invaded quadrats when they contained Garlic Mustard within a 1-m2 perimeter of the selected X, Y sampling points. Samples were collected using a 10-cm-diameter hole cutter (foot extractor with outside edge scalloped blade, Par Aide Products, Lino Lakes, MN). Two 5-cm deep cores were removed at each sampling point: one to measure springtail abundance and the other for epigeal composition, pH, and moisture. Samples were transferred from the hole cutter to sealed plastic bags and stored at 4 °C until analysis, within 6 hours for springtail abundance and within 24 h for abiotic measures. Depth of epigeal litter was measured with a ruler inserted through the litter. Garlic Mustard density at each sampling point was assessed as the number of juveniles and adults within 1 m2 surrounding each sampling point. Epigeal characteristics (moisture, pH, and composition) Reserved cores were sorted to remove living plant matter and mixed by hand to achieve an even mixture of humus, litter, and clay soil. Moisture was estimated in fresh epigeal mixtures using the gravimetric method of Gardner (1986). pH was measured in air-dried epigeal mixtures according to McLean (1982) with the following modifications. The amount of dd H2O added to 2 g epigeal samples prior to pH measurement was adjusted to compensate for increased water absorption by needle-leaf litter (10 mL) and less absorption by clay or humus (4 mL). Composition of needle litter, humus, and clay in the epigeal mixture was determined as relative abundance (%, v/v) in each sample. Light intensity and air temperature Light intensity just above the Garlic Mustard adult canopy was measured at each sampling point in μmol.m-2.s-1 photosynthetically active radiation with a Field Scout Light Sensor (Spectrum Technologies, Inc., Plainfield, IL). Air temperature was measured with a Kestrel 4500 Weather Meter (, Sylvan Lake, MI). Epigeal springtail abundance A high-gradient dynamic extraction method was used to estimate springtail abundance in epigeal core samples. Methods were modified from Winter and Behan-Pelletier (2007) as described below. Two sets of five Tullgren funnel temperature extractors processed the 10 springtail samples that we collected each week. Each funnel contained an 11-cm-long by 8.8-cm-wide section of black ABS pipe attached to an ABS pi278pe coupler fitted with a grid panel containing 4-mm holes. The funnel was attached to 6.6 cm of drain pipe, and the extractor was completed by inserting a plastic cup (11.5 cm by 9.3 cm) beneath the drain pipe. Heat was applied to the top of the funnel using a vanity light fixture containing five light sockets. Fixtures were attached to 8.8-cm x 13.2-cm x 2.2-cm wooden boards to 2013 A.B. Alerding and R.M. Hunter 279 suspend each light bank 8 mm above the top edge of the funnels. Light sockets were fitted with 60-W bulbs and rheostats to generate precise heat gradients. Epigeal samples were placed upside-down inside the black pipe of the funnel on the evening of each sampling date, after the previous week’s samples were processed. Collection fluid (100 mL of 70 % v/v ethanol) was placed into each cup to preserve animals as they moved into the collection cups from the epigeal samples. In our field site, springtails represented over 90% of the number of animals collected by this method. Mites (Order Oribatidae) represented the remaining ≈10% of animals. Extraction temperatures began at 20 °C on the first day and were increased by 5 °C per day to a maximum of 40 °C. On the seventh day, the extraction was terminated by removing springtails from collection cups using vacuum infiltration with Whatman #1 filter paper. Springtails were stored in 20-ml glass scintillation vials and counted using a dissecting microscope and fine forceps. Density of springtails was calculated by converting the number per core sample to number per m2 of epigeal surface area. Springtails were identified to family using two online taxonomic keys accessed in December 2012 (Christiansen and Janssens 2010, Christiansen et al. 2012). Preserved morphospecies were photographed using a Nikon SMZ 1500 dissecting microscope with Nikon Sight DS-Fi2 digital camera and NIS-Elements 4.00.07 software (Nikon Instruments, Inc., Melville, NY). Voucher specimens for morphospecies shown in Figure 1 are curated at the Smithsonian Institution (National Museum of Natural History, Department of Entomology, Washington, DC, reference number 2062591). Statistical analyses Minitab Statistical Software (v. 16) was used for all statistical analyses. We used ANOVA (general linear model) to investigate effects of temporal sampling time and presence/absence of Garlic Mustard on springtail abundance. One-factor ANOVA was used to investigate differences of abiotic factors (light intensity, air temperature, epigeal moisture, pH, composition, and litter depth) between the field sites. Pairwise comparisons of means were assessed with Tukey tests. Linear regressions were used to explore relationships between epigeal pH, moisture, Garlic Mustard density, and springtail abundance. Additionally, we investigated the relationship between epigeal pH (Rodgers et al. 2008b) and % juvenile rosettes (Rodgers et al. 2008b) in northern hardwood-conifer forests by linear regression. Results Epigeal characteristics of invaded and uninvaded field sites Relative amounts of the three components of epigeal core samples (clay, humus, and litter) were similar between invaded and uninvaded sites (Table 1). Depth of leaf litter was also statistically similar between the sites. Nearly 3 times more light reached the forest floor in the uninvaded site (P = 0.049). Even with higher light levels in the uninvaded plot, air temperature was slightly cooler (P = 0.04), possibly related to evapotranspiration of plants photosynthesizing under higher light conditions. At the ground level, epigeal moisture was significantly lower 280 Northeastern Naturalist Vol. 20, No. 2 (54%) in the invaded site. The most significant difference between the sites was enhanced epigeal alkalinity in the invaded site. While both sites were slightly acidic (pH 4.2 versus 5.5), the 30% higher alkalinity in the invaded site (P < 0.001) indicates a lower concentration of hydrogen ions (by 105 mM) in epigeal substrates. Relationships between epigeal characteristics and springtail abundance Five springtail morphospecies representing two families, Tomoceridae (morphs T1, T2) and Entomobryidae (morphs E1, E2, E3), were detected in this White Pine forest (Fig. 1). With the exception of morphospecies E3, which was present only in Garlic Mustard sites, all other morphospecies were present in equal relative proportions in both invaded and uninvaded sites. The dominant morphospecies was T1 (on average 52% of collected individuals per epigeal Figure 1. Springtail morphospecies from family Tomoceridae (T1, T2) and Entomobryidae (E1, E2, E3) present in epigeal samples from the White Pine forest (Scale bar in each photo represents 100 μm). Table 1. Abiotic characteristics of two field sites established in a White Pine forest understory. Plots were either uninvaded or invaded by Garlic Mustard. Numbers represent the mean ± SE of ten sampling points. Statistical results from one-factor ANOVA are presented as the F statistic and P-value for each variable. Uninvaded plot Invaded plot Abiotic variable Mean (SE) Mean (SE) F statistic P-value Light intensity (μmol.m-2.s-1) 126.8 (34.3) 45.9 (17.2) 4.45 0.049 Air temperature (˚C) 29.6 (0.2) 30.2 (0.2) 4.90 0.040 Epigeal moisture (%) 51.0 (10.8) 22.5 (1.2) 6.89 0.017 Epigeal pH 4.2 (0.1) 5.5 (0.1) 97.49 less than 0.001 Humus (% of epigeal core) 38.0 (6.3) 27.5 (4.4) 1.86 0.189 Clay (% of epigeal core) 26.5 (8.8) 38.5 (7.0) 1.14 0.301 Litter (% of epigeal core) 35.5 (8.7) 34.0 (9.6) 0.01 0.909 Litter depth (cm) 34.2 (3.2) 27.4 (2.9) 2.42 0.137 2013 A.B. Alerding and R.M. Hunter 281 sample), followed by E1 (32 %), E2 (8 %), T2 (5%), and E3 (3 %). Total springtail abundance (of all morphospecies combined) was higher in the invaded forest understory. We detected between 2–3 times more springtails in the epigeal cores of the invaded forest understory site (F = 10.65, P = 0.003; Fig. 2). This finding is likely related to significant increases in abundance of two springtail morphospecies in invaded sites, T1 (F = 5.79, P = 0.029) and E2 (F = 5.13, P = 0.039). Morphospecies E1 and T2 were found in equal abundance in invaded and uninvaded sites (F = 0.20, P = 0.659 and F = 0.21, P = 0.65, respectively). Sampling date had no effect on total springtail densities (F = 1.88, P = 0.174), as elevated springtail abundance was noted in Garlic Mustard sampling sites on three occasions during the months of June and July. However, when considered separately, we noted that entomobryid densities as a whole were significantly greater at the second sampling date (F = 6.02, P = 0.027), due to greater abundance of morphospecies E1. Exploratory regressions were performed to examine abiotic factors that differed between the two sites (Table 1) and their relationships with total springtail abundance. Three of the variables that differed between the plots showed no significant relationship with springtails (light: F = 0.015, P = 0.904; air temperature: Figure 2. Total springtails present in epigeal samples from White Pine understory sites either invaded or uninvaded by Garlic Mustard. Each point represents the mean springtail density (#.m-2) ± SE of five cores randomly sampled from each plot. Springtail density responses to site location and sampling dates were determined by ANOVA (general linear model). 282 Northeastern Naturalist Vol. 20, No. 2 F = 2.10, P = 0.166; soil moisture: F = 2.69, P = 0.120). The fourth variable, epigeal pH, positively related with total springtail abundance (F = 5.31, P = 0.0349; Fig. 3), a relationship that was only found for the dominant morphospecies, T1 (F = 6.71, P = 0.02). Relationships between epigeal characteristics and Garlic Mustard abundance Invaded sites had on average 33 ± 4.6 SE individuals per m2, with juveniles representing 50.1% of the total Garlic Mustard plants counted on the three sampling dates shown in Figure 2. However, the percentage of juveniles varied from 0–87% of the total individuals when epigeal cores were considered separately. To explore possible relationships between Garlic Mustard abundance and epigeal pH, we performed regressions for the two sampling dates when abiotic factors were most similar. For these regressions, July 13 results were not included because the first rainfall to occur in five weeks took place on July 12, decreasing soil pH to the same level as uninvaded sites (data not shown). Neither the density of juveniles (F = 0.045, P = 0.836) nor adults (F = 0.300, P = 0.593) had significant relationships with epigeal pH. However, when juvenile density was expressed as a percentage of total Garlic Mustard plants, almost 80% of the variation in epigeal pH correlated with increased presence of juvenile rosettes (F = 22.082; P = 0.002; Fig. 4). Using soil pH and % juvenile rosette values reported separately in Figure 4 and Table 1, respectively, of Rodgers et al. (2008b), we Figure 3. Relationship between epigeal pH and springtail density in a White Pine forest understory. pH and springtails were sampled at 18 random points on two sampling dates that occurred after weeks with no rainfall. Relationship between epigeal pH and springtail density was investigated with regression analysis. 2013 A.B. Alerding and R.M. Hunter 283 found a similar positive relationship between juvenile abundance and soil pH. Data from the Rodgers et al. (2008b) study, as re-analyzed and presented in Figure 5 in this study, show that for at least five other forests in the northeastern US, increasing proportion of juveniles in mixed patches of Garlic Mustard is associated with greater soil alkalinity (F = 12.362; P = 0.039). Discussion Increased springtail abundance We found springtail density to be nearly 3 times greater in the epigeal layer of an invaded forest, a result that was consistently measured on three consecutive sampling dates. These results support our hypothesis that springtail abundance increases in response to Garlic Mustard invasion. Our finding that morphospecies from two springtail families showed idiosyncratic responses to invasion (T1 and E2 showing elevations, T2 and E1 remaining unchanged), while E3 was present in only invaded sites, shows the importance of lower taxonomic-level responses to plant invasion. Neither family- nor morphospecies-level responses to Garlic Mustard were considered by Dávalos and Blossey (2004), which could explain the different responses to invasion reported in their study. To our knowledge, our data Figure 4. Relationship between juvenile rosettes of Garlic Mustard and epigeal pH in an invaded White Pine forest. For each of the nine values obtained on 28 June 28 and 5 July 2010, epigeal samples were removed before counting the number of juveniles and adults within a 1-m2 area of the sample. Rosettes are expressed as a percentage of the total number of Garlic Mustard (GM) plants present for each epigeal sample. Relationship between percent juveniles and epigeal pH was investigated with regression analysis. 284 Northeastern Naturalist Vol. 20, No. 2 represent the first temporal, morphospecies-level analysis of springtail abundance in an invaded forest. Additionally, because springtails represent 90% of the total detritivore densities in this forest, our results suggest Garlic Mustard has a profound impact on the detritivore trophic level of the decomposer food web. Whether elevated springtail abundance directly influences higher trophic interactions remains to be determined. A recent study of food-web responses to Garlic Mustard by deHart and Strand (2012) used stable isotopes to infer predator dietary changes in a White Pine forest located a few kilometers from our forest. Interestingly, predators in the Formicidae and Opiliones families showed changes in stable isotope signatures, suggesting shifts in feeding preferences away from springtails and toward Noctuidae caterpillars that were feeding solely on Garlic Mustard inflorescences. Since springtail densities and predator feeding behaviors were not measured by deHart and Strand (2012), it is unknown if this feeding shift occurred as a direct response to altered abundance of Entomobryidae or Tomoceridae springtails or indirectly because caterpillars were more apparent (and easier to catch) on inflorescences. Moreover, we note that we followed published protocols to measure epigeal densities of springtails, while deHart and Strand (2012) collected arthropods non-randomly, using forceps. This difference in methodology may explain why deHart and Strand (2012) did not capture springtails in invaded sites. Figure 5. Relationship between juvenile rosettes of Garlic Mustard and soil pH in five invaded hardwood-conifer forests in Massachusetts and Connecticut. Data presented in this figure represent a novel analysis of results from Table 1 (% juvenile rosettes) and Figure 3 (epigeal pH in invaded sites) from Rodgers et al. (2008b). Relationship between percent juveniles and epigeal pH was investigated with regression analysis. 2013 A.B. Alerding and R.M. Hunter 285 Springtails and abiotic features of the epigeal environment In our study, epigeal pH correlated with nearly 25% of the variation in total springtails in this White Pine forest. Highest springtail densities were measured in Garlic Mustard-invaded sampling points, with an average value of pH 5.5, supporting our hypothesis that springtail abundance would be maximized at pH levels known to enhance springtail growth and reproduction (Hopkin 1997). At least two other invasive plant species have been shown to increase soil alkalization (Japanese Stiltgrass and Berberis thunbergii (DC) [Japanese Barberry]; Ehrenfeld et al. 2001, McGrath and Binkley 2009, Nuzzo et al. 2009). It is unknown if springtails respond similarly to soil alkalization in forests invaded by these invasives. We did discover that alkalization in our study was not additionally enhanced by the presence of Japanese Stiltgrass in our sampling sites (data not shown). A precise mechanism relating pH alkalization to increased springtail abundance is unknown, but one possibility is that higher pH levels in the epigeal layer of invaded forests enhance ventral tube functioning. The ventral tube is a characteristic structure protruding from the springtail abdomen that is involved in maintaining water homeostasis and in adhering springtails to smooth substrates through adhesive properties (Hopkin 1997). Proper functioning of the ventral tube is thought to require precise amounts of water and pH levels in the environment, with ranges varying among species (Hopkin 1997). In Tomoceridae, fluid uptake through the ventral tube is strongly inhibited in acidic environments (pH 2–3) and highest between pH 5–6 (Jaeger and Eisenbeis 1984). Our demonstration that total springtail abundance (and in particular, Tomoceridae morphospecies T1) increases in this White Pine forest in correlation with epigeal alkalization is suggestive of enhanced functioning of the ventral tube as a mechanism facilitating springtail growth in Garlic Mustard invaded forests. Even though ventral tube functioning is reportedly sensitive to external moisture levels (Hopkin 1997), we found no significant relationship between epigeal moisture and Entomobryidae springtail abundance in this White Pine forest. Because levels of epigeal soil moisture in this forest (on average 53.9%) were similar to those reported for other temperate forests that support springtail populations (Salamon et al. 2008), it is likely that levels of soil moisture were sufficient to support springtail growth and reproduction in this forest. In addition, our finding that epigeal moisture was similar between invaded and uninvaded sites, consistent with its general non-responsiveness to plant invasion (Vilà et al. 2011), suggests that moisture influence on ventral tube functioning may play little, if any, role in springtail responses to Garlic Mustard invasion. Invasion pH and Garlic Mustard juveniles Garlic Mustard’s biennial life cycle has important consequences for its longterm establishment. Co-occurrence of juveniles and adults in mixed patches results in competition between the developmental stages, with most adults surviving to reproduce at the expense of juveniles (Winterer et al. 2005). Results from our study show a possible relationship between the proportionate increase 286 Northeastern Naturalist Vol. 20, No. 2 in juvenile rosettes in mixed patches and epigeal alkalization. Our results are corroborated through original re-analysis of data presented in Rodgers et al. (2008b) and thus may indicate a competitive mechanism of juveniles in mixed patches. A mechanism for plant-mediated alkalization is currently unknown, but it has been suggested that H+ uptake by Garlic Mustard roots may increase as a compensatory response to rapid uptake of NO3 - in invaded soils (Ehrenfeld et al. 2001, Hewins and Hyatt 2010). It would be of interest to measure H+ and NO3 - uptake in roots of juveniles and adults growing in mixed patches that differ in the proportions of juveniles and adults. Perhaps juveniles have evolved faster uptake rates of both ions as a mechanism to increase their survival in mixed patches. Conclusions Our research shows that Garlic Mustard invasion not only impacts plant communities, but its presence in forests also affects animals, specifically detritivores. Forests with well-established populations of Garlic Mustard show enhanced rates of decomposition, lower amounts of litter, and increased levels of macronutrients (Rodgers et al. 2008b), but mechanisms for increased decomposition remain undetermined. Since springtail feeding enhances both decomposition and nutrient mineralization (Seastedt and Crossley 1984), our correlative findings between epigeal alkalization and enhanced springtail abundance suggest a possible mechanism that should be examined in future studies. It would be interesting to explore the nature of the relationship between springtail abundance and increased rates of litter decomposition in invaded forests. Ultimately, by stimulating springtail population growth, Garlic Mustard may provide greater supply of food for detritivores that rely on energy inputs from the base of this understory food web. Acknowledgments Financial support was provided by the Virginia Military Institute Center for Undergraduate Research and the Jackson-Hope Fund. We thank Matthew Waalkes and Sarah Strand for assisting with field measurements, Leslie Joyce and Karen Fredenburg for ordering equipment and chemicals, Megan Newman for expedited interlibrary loans, Peppy Kesler for helping construct the Tullgren funnel apparatus, and two anonymous reviewers for providing constructive feedback. Wade Bell assisted with springtail identification, and Dick Rowe, Paul Moosman, and Emily Lilly provided critical readings of this manuscript. Literature Cited Aerts, R. 1997. Climate, leaf-litter chemistry, and leaf-litter decomposition in terrestrial ecosystems: A triangular relationship. Oikos 79:439–449. Allison, S.D., and P.M. Vitousek. 2004. Rapid nutrient cycling in leaf litter from invasive plants in Hawai’i. Oecologia 141:612–619. Anderson, R.C., and T.M. Kelley. 1995. 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