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Ground-dwelling Beetle Responses to Long-term Precipitation Alterations in a Hardwood Forest
Ray S. Williams, Bryan S. Marbert, Melany C. Fisk, and Paul J. Hanson

Southeastern Naturalist, Volume 13, Issue 1 (2014): 138–155

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Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 138 2014 SOUTHEASTERN NATURALIST 13(1):138–155 Ground-dwelling Beetle Responses to Long-term Precipitation Alterations in a Hardwood Forest Ray S. Williams1,*, Bryan S. Marbert1,2, Melany C. Fisk1,3, and Paul J. Hanson4 Abstract - It is widely predicted that regional precipitation patterns may be altered due to climate change, and these changes may affect areas with extensive forests. Therefore, studies investigating the role of this climate driver on forest floor fauna are timely. We examined the impact of precipitation alteration over 13 years on Coleoptera (specifically Family Carabidae) communities in a temperate forest by testing the effects of dry (33% precipitation interception), ambient (control), and wet (33% precipitation addition) treatments. We collected insects in pitfall traps and quantified forest-floor physical and chemical parameters. Beetle abundance and Carabidae tribe richness were significantly reduced in dry plots. Community similarity was substantially higher between wet and ambient plots compared to dry plots due to the substantial reduction of three dominant carabid tribes. Litter mass increased overall, litter nitrogen decreased, and carbon:nitrogen ratio (C:N) and total phenolics increased in the dry-plot Oi horizon. Beetle abundance and tribe richness were positively related to soil moisture, and beetle abundance was negatively related to litter mass. Microarthropod abundance was highest in the dry treatment. This study provides evidence that shifting precipitation patterns predicted with climate change could alter important ground-fauna communities in extensive ecosystems such as temperate forests. Introduction Climate is a primary factor shaping the geographic distribution of biota (Coope and Wilkins 1994). Thus, human-induced alterations in climate elements such as precipitation patterns may affect the diversity of biota in terrestrial ecosystems on broad scales. During the past century, the burning of fossil fuels has substantially increased atmospheric carbon dioxide (IPCC 2007), with consequences that include rising global mean temperature and changes in precipitation patterns in broad regions of the planet (IPCC 2007). Changes to the global hydrologic cycle have the potential to affect plant productivity, biogeochemical cycling, and water resource availability in many ecosystems, including forests (Hanson and O’Hara 2003). Specific effects on biodiversity remain largely uncertain and should be explored as part of comprehensive efforts to predict impacts of changing precipitation patterns on terrestrial ecosystems (Weltzin et al. 2003). Potential changes in community composition after long-term exposure to precipitation alterations may be an especially relevant aspect of diversity to examine 1Department of Biology, PO Box 32027, Appalachian State University, Boone, NC 28608. 2Current address - Department of Sciences, Health and Physical Education, Randolph Community College, 629 Industrial Park Avenue, Asheboro, NC 27205. 3Current address - Department of Zoology, 212 Pearson Hall, Miami University, Oxford, OH 45056. 4Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN, 37831. *Corresponding author - willmsrs@appstate.edu. Manuscript Editor: Wade Worthen Southeastern Naturalist 139 R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 in relation to climate. Walther (2007) argued that species’ reactions to factors such as drought are part of a complex cascade of reciprocal responses and feedback processes that could influence other biota and ecosystem functions. Furthermore, some animal taxa have been recognized as bioindicators and may be useful in studies to monitor and detect changes in the environment (Bohac 1999, Rainio and Niemela 2003). For example, studies indicate that the length of drought in forests affects the recovery of key soil arthropods (Lindberg and Bengston 2006), and climate changeinduced drought affects insect outbreak species (Negrón et al. 2009) and possibly invasive insect species dynamics (see Dale et al. 2001). Precipitation reduction in forests can alter the diversity of top invertebrate predators such as spiders (Cramer 2003), which suggests that a close examination of trophic responses to environmental change is warranted. Understanding precipitation alteration in ecosystems will clearly benefit from more studies focusing on changes in community composition and biodiversity. Ground beetles (Order Coleoptera:Family Carabidae) are an ideal taxon to use in studies that examine the effects of precipitation change because they have a high species diversity and are relatively easy to identify taxonomically, are ubiquitous, easy to sample and sensitive to minor habitat modifications (Desender 1996, Niemela et al. 1996, Rykken et al. 1997). Ground beetles constitute a substantial fraction of the ground-dwelling fauna in temperate forest (Magura 2002). Moreover, these beetles are known to quickly colonize areas where suitable habitat becomes available (Elias 1991), and sensitive species are likely to respond to habitat changes resulting from precipitation alteration by dispersing to favorable environments. Carabidae are affected by both moisture availability (Antvogel and Bonn 2001, Maudsley et al. 2002, Rykken et al. 1997, Yi and Moldenke 2005) and the physical environment (Magura et al. 2004). Hence, precipitation could influence ground beetle community structure through ecosystem-level feedbacks between water availability and biotic processes that alter essential microhabitat for invertebrate inhabitants of the forest floor (Chikoski et al. 2006, Johnson et al. 2002, Taylor and Wolters 2005). Examples of such feedbacks include moisture effects on decomposition and hence forest-floor structure, and moisture effects on microinvertebrates that can influence decomposition and are also prey species for macroinvertebrates (Johnston 2000). The soil environment varies dramatically across the landscapes of the southern Appalachians, and soil properties that have developed in relation to topographic position are likely to influence ground-dwelling faunal communities. Moving from upper to lower slope positions, moisture and nutrient availability generally increase as soils become more finely textured and drainage is slower (Boerner 2006, Day and Monk 1974). As a consequence, the response of soil moisture to periods of drought in southern Appalachian forests depends strongly on topographic features of the landscape (Yeakley et al. 1998). These dynamics illustrate the importance of landscape position to soil processes driven by moisture, and these processes can influence landscape patterns of other characteristics important to ground fauna on small scales, such as soil carbon (Bolstad and Vose 2001). Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 140 This investigation was part of a larger experiment that examined forest responses to precipitation change over 13 years and provided a unique opportunity to investigate precipitation-induced effects on important ground fauna in an intact forest subjected to long-term precipitation alteration. Some researchers have predicted that the eastern US will experience overall moderate increases in precipitation as the planet warms (IPCC 2007). The data generated from this experiment shed light on the likely effects of two extremes in patterns of precipitation—increased precipitation and drought—on an extensive forest ecosystem. To better understand the interrelationships between climate alteration, habitat, and ground-dwelling arthropod community parameters, we sampled arthropods over a single growing season. The study design allowed us to quantify effects of precipitation manipulation on communities of Coleoptera (beetles), with a special emphasis on ground beetles (Family Carabidae). We addressed several questions in our study: (1) does altered precipitation influence the most prevalent beetle taxa and ground beetle communities in temperate forests of the Southern Appalachians? (2) are the responses of ground beetles to precipitation related to chemical or physical changes in the forest floor? and (3) do ground beetle responses relate to changes in potential prey as influenced by treatments? We hypothesized that precipitation alteration would result in community-level effects on dominant taxa such as Carabidae, as groups (e.g., tribes) within this large beetle family would shift to preferred habitats. We further hypothesized that changes in structural and chemical characteristics of the litter due to treatment would cause insect responses. Field Site Description This study was part of a large-scale experiment initiated in 1993 to examine the long-term effects of precipitation alteration on ecosystem processes in an intact temperate forest. The throughfall displacement experiment (TDE) was located on a south-facing slope in Walker Branch Watershed, part of the US Department of Energy’s (DOE) National Environmental Research Park near Oak Ridge, TN. Walker Branch is a temperate-zone forest with a mean annual precipitation of 1358 mm and an average temperature of 14.2 °C (Hanson et al. 2003). The area was chosen because of its uniform slope, consistent soils, and reasonably uniform distribution of vegetation. Quercus alba L. (White Oak), Quercus prinus Willd. (Chestnut Oak), and Acer rubrum L. (Red Maple) were the dominant tree species in the TDE (Hanson et al 2003). The overall experimental design at the TDE is described by Hanson and Wullschleger (2003). Briefly, the site consisted of 3 adjacent 80-m x 80-m treatment areas (dry, ambient, and wet) positioned at the upper divide of the watershed to avoid lateral flow into the site from an upslope position. An elevation map developed for the site shows gradual relief from the upslope to the downslope position, with an overall change in elevation of 21 m from top to bottom, on average (Hanson et al. 2003). Researchers manipulated hydrologic inputs with a network of 2000 sub-canopy troughs (0.3 m x 5 m) that diverted an estimated 33% of available precipitation from the dry to the wet treatment. The ambient treatment, with no precipitation Southeastern Naturalist 141 R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 alteration, served as a control. Each treatment area was divided into one hundred 8-m x8-m plots. Soil-moisture measurements began one year pre-treatment. During the seven years of precipitation manipulation, researchers observed significant differences between treatments; wet plots had higher soil moisture compared to ambient and dry plots except during the dormant season and under extreme drought (Hanson et al. 2003). These data clearly demonstrate a developing difference in soil moisture levels between dry plots and those receiving normal or augmented water as a growing season progresses. Methods For this experiment, we randomly selected 10 of the 100 plots in each treatment area for sampling arthropods and abiotic parameters. Plots used in the study were free of towers, cages, or other equipment that could alter forest-floor conditions. We maintained a large buffer between treatments by only interior plots within treatment. Of the 10 plots in each treatment, we sampled 5 plots in upper slope positions and 5 plots at lower slope positions to account for any differences caused by landscape position. Biotic measurements In 2005, we used to pitfall traps to collect ground-dwelling arthropods during 3 sampling periods. Traps consisted of 250-ml Nalgene bottles with a 10-cm diameter funnel inserted at the top and situated so that the funnel mouth was flush with the soil surface; each bottle contained 200 ml of a 50% ethanol solution for preservation of specimens. Although this method of collecting ground-dwelling beetles likely measures activity rather than absolute numbers or density, we believe that differences in abundance and diversity were still reflected in our captures. To simplify the data presentation, we use the term abundance, rather than terms such as activity density, etc., with respect to beetles. We initiated arthropod collections on 11 May, 16 July, and 21 September 2005, and traps remained open for 5 days and nights for each collection. We placed 5 traps at 1-m intervals along a linear transect in the center of each plot running across the slope. At the conclusion of each sample period, we combined the contents of the 5 traps within a plot into one composite sample, for a total of 10 samples per treatment per sample date. We sorted contents of traps into broad taxonomic categories: Aranae (Spiders), Opiliones (Harvestmen), Orthoptera (Grasshoppers), Formicidae (Ants), Coleoptera (Beetles), Chilipoda/Diplopoda (Millipedes/Centipedes), and other, which consisted of the Orders Collembola and Hymenoptera. We classified beetles to the family level, and further identified ground beetles (Family Carabidae) to tribe following Arnett and Thomas (2001). Although multiple invertebrate taxa that serve as bioindicators may be necessary to fully assess environmental change (see Riggins et al. 2009), for the purpose of this study, we focused our analysis on Coleoptera because of their substantially greater overall abundance compared to other arthropods and because the sampling method we used was more suited to this group, reducing the problem of sampling artifacts. In addition, unlike the situation for other arthropods, previous studies Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 142 have described specific habitat requirements for the Family Carabidae and their data provided a basis for comparison that would not have been available for other taxa (see Cameron and Leather 2012, Desender 1996, Rykken et al. 1997). Further, Carabidae response to biotic and abiotic factors at different spatial scales provides a potentially useful framework for looking at effects of environmental change (Koivula 2011). The use of tribes provided a manageable and ecologically relevant level of classification based on known characteristics of many groups. Previous work provides evidence that supra-species groupings are appropriate when univariate analyses are used to detect large habitat-fragmentation and landscape-level effects, but less robust on finer scales and with multivariate analyses (see Grimbacher et al. 2007). Additionally, taxonomy at a level such as Family can provide indications of environmental change in invertebrates inhabiting soil (Riggins et al. 2009), though groups below this level with considerable variation in trophic response could prove problematic. We feel that our analysis and hypothesis regarding habitat preferences supports the use of supra-species groupings. We calculated the community level parameters of richness and evenness using both dominant beetle families (see below for a more detailed description) and ground beetle tribes. Here. richness (R) is the number of families or tribes contained in a sample. We calculated evenness as E = H'/logeS, where H' is the Shannon diversity index and S is the number of families or tribes within the sample (formula following Magurran 2004). Finally, for Carabidae, we calculated the Sørensen similarity index to compare tribe similarity: S = A/ (B + C) x 200, where letters A, B and C represent the minimum number of individuals in tribes within treatments (formula following van Tongeren 1995). We sampled microarthropods using 5-cm-diameter x 4-cm-depth cores at the same time as pitfall trap collections. The 4-cm depth included Oe and Oa horizons and a small amount of mineral soil. We collected 5 cores per plot and combined them into one sample per plot (total 30 samples/collection date). We kept samples at 4 °C for no more than 24 h until we extracted the organisms using the methods of Crossley and Blair (1991) with 10-cm-diameter x 21.5-cm-high PVC extractor tubes with a mesh screen on the bottom. We fitted each extractor with a 5-watt bulb light and we controlled intensity with a rheostat. We continued microarthropod extraction for 7 days, increasing the intensity of light (i.e., heat) with each consecutive day. We quantified organisms in the Order Acari (mites) and expressed the counts as number per m2. Abiotic measurements We quantified the mass of the O horizons in summer (June 21) and fall (September 22), 2005. We refer to the sum of the 3 O horizons (Oi, Oe and Oa) as forest floor, and the un-decomposed Oi layer as litter. We sampled the Oi and Oe horizons by cutting around a 15-cm x 15-cm wooden square and excavating to the bottom of each horizon. We collected the Oa horizon in cores (5-cmdiam to the surface of the mineral soil) removed from the center of the excavated squares. In summer, we collected 4 samples from each plot for a total of 40 samples in each horizon/treatment. We took our samples from the corner of the plot to Southeastern Naturalist 143 R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 minimize disturbance to ground-dwelling fauna. In the fall, we collected only 2 samples from the Oi and Oe horizon (20 samples/treatment), and we did not sample the Oa horizon. For our data analysis, we used soil moisture (expressed as % v/v) values TDE data archive (http://tde.ornl.gov/tdedata.html). We averaged biweekly measurements throughout 2005 using a time domain reflectometer (TDR) technique at the 0–35 cm soil layer for each treatment (Hanson et al. 2003). Average soil moisture was 21.2 (wet), 20.3 (ambient) and 17.7 (dry). We recorded surface temperature using iButton continuous data loggers (Maxim Integrated Products, San Jose, CA) positioned in the center of each plot directly beneath the Oi layer of the organic horizon. We programmed data loggers to record temperature at 4-h intervals from 9 May–28 September 2005. We downloaded temperature data using iButton TMEX Application software and calculated weekly minimum, maximum, and mean temperatures We measured total carbon (C), total nitrogen (N), and carbon:nitrogen ratio (C:N) for the Oi, Oe, and Oa horizons using samples collected in May 2005. We pooled dried samples within each plot and ground each one in a coffee mill and then in a ball mill until the material had a talcum powder consistency. We sent 1 sample per plot (total 10 per treatment) from each of the Oi, Oe, and Oa horizons to the University of Georgia Institute of Ecology Stable Isotope Laboratory (Athens, GA) for aqnalysis of total C and N using the Micro-Dumas combustion technique. We analyzed total phenolic content in the same Oi, Oe and Oa samples following the Folin-Ciocalteu (FC) reagent technique of Singleton and Rossi (1965) and expressed the values as percent tannic acid equivalents (%TAE). Statistical analyses We tested effects of treatment on dominant beetle family and Carabid tribe community parameters (i.e., abundance, richness, and evenness) and mite abundance using one-way ANOVA (Proc GLM, SAS 9.1, SAS Institute, Cary, NC) with all samples combined (hereafter, the cumulative dataset). We log transformed the data to achieve normality. We present untransformed data where appropriate. We used Tukey’s honestly significant difference (HSD) test for selected insect communityparameter post hoc pair-wise comparisons of treatment means. We used Proc GLM to test effects of treatment on average forest-floor mass and moisture; N (%); C:N ratio; total phenolic content from the Oi, Oe, and Oa horizons; and mean weekly temperature minima and maxima. For all analyses, replication was at the level of plot, and significance assigned at P ≤ 0.05. We report results where 0.10 ≥ P ≥ 0.05 as marginally significant. We fully acknowledge the un-replicated design of the larger TDE experiment and suggest that this approach is reasonable when such costly experimental field designs are undertaken (see Eberhardt and Thomas 1991) and where sufficient pre-treatment data exist to demonstrate that any observed treatment effects are not due to variations across sites before the experiment began. We examined variables potentially important to ground fauna, including dominant trees, soils, microclimate, slope, and patterns of soil moisture, prior to setting up the experiment (Hanson et al 2003). Forest-stand and understory species composition Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 144 and basal area were not different between sites (Hanson et al. 2001) and a comparison of archived data (http:tde.ornl.gov) demonstrates strong similarity in soil characteristics. Along with numerous other authors, we conclude that sufficient similarity existed between sites prior to the manipulation treatments to allow for treatment comparisons. Based primarily on an initial correlation analysis (Proc CORR; SAS) that examined relationships between fauna and physical features of the forest floor, we used linear regression (Proc REG) to analyze relationships between beetle and carabid community parameters and principal abiotic variables. We used the same approach for cumulative mite abundance. We also used partial least squares regression (PSLR), where all soil and litter variables (moisture, N, CN, %TAE, etc.) are simultaneously loaded into this permutation procedure to generate predicted values for carabid community parameters. A linear regression of actual versus predicted values provided a measure of how the abiotic variables related to beetle abundance, richness, and evenness. This procedure is gaining wide aceptance in the ecological literature and is seen as an appropriate alternative to more classical regression analyses (Carrascal et al. 2009). Results Forest-floor biota We collected a total of 3244 beetles from 29 families during 2005 at the TDE: 1296 from the wet treatment, 1045 from the ambient treatment, and 903 from the dry treatment. Seven families of beetles comprised greater than 96% of the abundance. Twenty families had 9 or fewer individuals in traps in 2005, and in our estimation, these may have represented random captures. For this reason, our initial analysis focused on the 7 dominant beetle families; Carabidae, Curculionidae, Nitulidae, Staphylinidae, Scolytidae, Scarabaeidae, and Chrysomelidae, for a total of 3120 beetles. There was no treatment bias towards the number of beetles in other families excluded from the analysis: wet = 44, ambient = 52 and dry = 55. Two families—Carabidae and Curculionidae—comprised approximately 57% of all dominant beetles collected (Table 1). There was considerable variation in percent abundance between precipitation treatments in the 7 dominant beetle families, with no treatment difference observed (Tables 1, 2). Family-level richness was marginally affected by treatment (Table 2), and richness was lowest in plots where water was intercepted (mean ± SE; wet: 6.5 ± 0.2; ambient: 6.8 0.1; dry: 6.2 ± 0.2). The higher percentage of the (Carabidae and Curculionidae) in the wet plots compared to ambient or dry plots (Table 1) likely contributed to a significant effect on evenness (Table 2), which was lowest in plots that received additional water (wet: 0.792 0.03; ambient: 0.837 ± 0.03; dry: 0.918 ± 0.1). We collected a total of 1070 Carabidae from 9 tribes (Table 1). Overall, carabid beetles were more prevalent in wet or ambient plots than in dry plots. The tribes Harpalini, Callistini, and Pterostichini made up approximately 82% of all Carabidae in our samples (Table 1). As a percentage of total abundance, beetles in the tribe Southeastern Naturalist 145 R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 Harpalini were much more prevalent in wet plots compared to dry plots; in the tribe Pterostichini ,the difference between wet and dry plots was less pronounced. The abundance-based Sørensen similarity index indicated a substantial effect of drought treatment on community composition at the tribe level. Communities in the wet and ambient treatments were 90% similar, those in the dry and wet treatments were 56% similar and those in the dry and ambient treatments were 69% similar. These community differences appeared to be due largely to treatment effects on the 3 dominant tribes. Responses of some other tribes to treatment probably contributed to differences in community similarity, though we observed lower abundances and less consistent differences between treatments (Table 1). Precipitation treatment clearly altered total beetle abundance and tribe richness within the Carabidae (Table 2). Beetle abundance was 61% lower in the dry treatment compared to the wet and ambient treatments (P = 0.05, Table 1, Fig. 1A), and tribe richness was lower in the dry than wet and ambient treatments (Table 1, Fig. 1B). Tribe-level evenness was not affected by treatment (Table 2, Fig. 1C). Table 1. Percent average abundance by treatment for the seven dominant beetle families and Tribes in Family Carabidae. Numbers calculated from cumulative abundance data. n Wet Ambient Dry Family Carabidae 1070 45.7 37.2 17.1 Curculionidae 716 53.8 20.8 25.4 Staphylinidae 465 30.8 29.2 40.0 Nitulidae 394 32.2 33.5 34.3 Scolytidae 279 26.2 38.0 35.8 Scarabaeidae 132 23.5 32.6 43.9 Chrysomelidae 64 25.0 60.9 14.1 All families (N) 3120 1061 1121 938 Carabidae Tribes Harpalini 409 55.3 36.9 7.8 Pterostichini 362 39.2 37.0 23.8 Callistini 75 45.3 44.0 10.7 Licinini 61 52.5 29.5 18.0 Galeritini 51 27.5 41.2 31.4 Cychrini 48 33.3 35.4 31.3 Cicindelid 26 23.1 46.2 30.8 Notiophilinini 22 50.0 27.3 22.7 Scaratini 16 68.8 12.5 18.8 All tribes (N) 1070 492 394 184 Table 2. ANOVA results for the effects of treatment on dominant beetles and Carabidae tribe community parameters. df = 2, 27; n = 30; * = P ≤ 0.05. Abundance Richness Evenness F P F P F P All beetles 2.07 0.145 3.16 0.060 6.52 0.005* Carabidae tribes 13.1 0.0001* 4.03 0.029* 1.24 0.306 Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 146 Mite abundance (mean ± SE) was affected by treatment (df = 2, 24; P = 0.024), with more mites found in the dry (228 ± 28) than wet (156 ± 35) or ambient (134 ± 13) plots. Figure 1. Carabidae community parameters (mean + SE) by treatment for cumulative (A) abundance, (B) tribe richness, and (C) tribe evenness with slope combined. * = significant treatment difference (P ≤ 0.05), Tukey’s HSD test. Southeastern Naturalist 147 R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 Forest-floor characteristics Intercepting water from the canopy resulted in total forest-floor mass in the dry treatment approximately 2 times greater than in the ambient, and 1.5 times greater than in the wet treatment (Table 3). All layers, with the exception of the Oe, were significantly affected by treatment, and forest-floor mass was highest in the dry plots (Table 3). Weekly average forest floor minimum, maximum, and mean temperatures were not affected by treatment or slope during any sampling period (data not shown). Effects of treatment on forest-floor litter chemistry—%N, C:N ratio and total phenolics—was dependent on the O horizon. We observed significant effects of treatment only in the Oi layer for nutrients, where %N was lower and C:N ratio higher in dry plots compared to the ambient and wet plots (Table 3). Total phenolics were greater in the Oi horizon in the dry plots, and we observed a marginally significant reduction in this measure in the Oa horizon (i.e., h umic layer). Regression analyses The principal predictors of beetle community parameters were soil moisture and mass of the forest floor. Carabid abundance (P = 0.007, R2 = 0.34) and tribe richness (P = 0.05, R2 = 0.13) were positively and significantly related to soil moisture (Fig. 2A, B). Tribe evenness was marginally negatively related to soil moisture (P = 0.08, R2 = 0.11; Fig. 2C). Carabid abundance was marginally negatively related to forest- Table 3. Mean, standard error (SE), F Ratio, P valueA, and N (Proc GLM) for organic horizon mass, %N and C:N ratio and phenolics (%TAE).* = P ≤ 0.05 Wet Ambient Dry Mean SE Mean SE Mean SE F P Mass (g m-2) Oi 354 17 296 12 399 25 7.63 0.002* Oe 495 28 537 21 542 31 1.06 0.361 Oa 2417 264 1326 265 3231 322 8.11 0.002* Total 2782 496 2026 276 4173 342 6.29 0.006* N (%) Oi 1.31 0.06 1.30 0.03 1.11 0.07 4.62 0.019* Oe 1.56 0.08 1.40 0.06 1.45 0.04 1.85 0.117 Oa 1.12 0.07 1.08 0.06 1.26 0.06 1.54 0.237 C:N ratio (mg mg-1) Oi 37.2 1.6 37.1 1.2 44.2 2.7 4.02 0.031* Oe 30.5 1.3 31.0 1.7 31.8 1.5 0.20 0.827 Oa 27.2 1.5 32.1 5.2 26.1 1.1 1.06 0.366 Phenolics (%TAE) Oi 10.9 0.6 11.0 0.6 13.3 0.9 3.71 0.038* Oe 6.51 0.4 6.36 0.6 6.60 0.4 0.06 0.942 Oa 14.0 1.7 8.28 1.2 12.6 1.9 2.76 0.087 AOi, Oe, mass, total mass, Oe N% and C:N, Oi and Oe %TAE: d. f. = 2, 27, n = 30. Oi N% and C:N: df = 2, 25; n = 28. Oa mass: df = 2, 24; n = 27. Oa N%, C:N and %TAE: df = 2, 21; n = 24. Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 148 floor mass (P = 0.08, R2 = 0.11, Fig. 3A), and tribe richness was unrelated to mass (P = 0.217, R2 = 0.05, Fig. 3B). Tribe evenness marginally increased (P = 0.07, R2 = 0.11, Fig. 3C) with higher forest-floor mass. Partial least squares regression found Figure 2. Relationship between soil moisture (% volume/volume ) and cumulative (A) Carabidae abundance (P = 0.007, R2 = 0.34), (B) tribe richness (P = 0.05, R2 = 0.13), and (C) tribe evenness (P = 0.08, R2 = 0.11). Figure 3. Relationship between forest floor mass (Oi + Oe + Oa) and cumulative (A) Carabidae abundance (P = 0.08, R2 = 0.11), (B) tribe richness (P = 0.217, R2 = 0.05), and tribe evenness (P = 0.07, R2 = 0.11). Southeastern Naturalist 149 R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 relationships between forest-floor structural and chemical parameters with carabid abundance (P = 0.001, R2 = 0.52), richness (P = 0.001, R2 = 0.40), and evenness (P = 0.030, R2 = 0.28) (Fig. 4). Figure 4. Relationship between all soil and litter abiotic variables on actual versus predicted (A) Carabidae abundance (P = 0.001, R2 = 0.53), (B) tribe richness (P = 0.001, R2 = 0.40), and tribe evenness (P = 0.003, R2 = 0.28) using partial least squares regression. Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 150 Mite abundance was related to key physical characteristics of the forest floor. Abundance decreased with increasing soil moisture (P = 0.006, R2 = 0.24) and was positively related to forest-floor mass (P = 0.007, R2 = 0.23). None of the carabid community parameters was significantly related to mite abundance (data not shown). Discussion Manipulating the amount of natural precipitation that reached the forest floor affected the distribution of dominant beetle families, with resultant changes in community measures in a prevalent beetle family. Intercepting precipitation reduced the abundance of beetles and decreased the richness of tribes within the Carabidae, which comprised a substantial component of the macroarthropod community. Community-level effects of reduced precipitation were clearly evident in the higher similarity between beetle communities of wet and ambient treatments compared to the dry treatment. Our data provides evidence that key community parameters within the Family Carabidae could change in response to reduced precipitation, and that colonization of preferred habitats created by climate change is likely for certain tribes in this large family of beetles. Important forest-floor characteristics such as soil moisture, litter mass, and chemistry changed due to alterations in precipitation, which in turn may have affected insect responses. These results contribute to a better understanding of the potential effects of altered precipitation in temperate forests in at least 2 important ways. First, the large scale (1.92 ha) of the precipitation experiment allowed us to examine effects on the habitat and arthropods across an extensive landscape. Second, the duration (13 years) of the larger experiment enabled us to study the long-term impact of altered precipitation in forests. Although we sampled in only a single year, we conclude that observed changes in the physical characteristics of the forest floor that affected beetles had accumulated over the many years of the larger TDE experiment, resulting in the observed effects on the beetle community over a much longer time than a brief sampling period. Our results suggest that ground beetle communities responded to precipitation changes relative to the amount of water that reached the forest floor. This finding has implications for regions that may experience future drought due to lower precipitation, or conversely, areas that become wetter due to shifting precipitation patterns. Two prevalent taxa of Family Carabidae responded to precipitation alterations, though in slightly different ways. The dominant Tribe Harpalini clearly shifted to wetter plots, whereas another common tribe, Pterostichini, was somewhat less responsive to dry conditions even though their abundance was higher in the more moist plots (Table 1). Combined with the data on other tribes and dominant beetle families, these findings suggest that important ground-dwelling arthropods will respond to precipitation changes in different ways. In addition to affects on species abundance, our data shows that tribe-level richness is negatively affected by dry conditions, and that a shift in preference for wetter habitat results in community- level shifts in the distribution of dominant taxa. In the only other study on the TDE experimental site that examined ground-dwelling fauna (spiders), Cramer Southeastern Naturalist 151 R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 (2003) found evidence of preferences by certain spider species for either the wet or dry plots. Overall, our observations on abundance, richness, and evenness of the carabid beetle community strongly supported our hypothesis that long-term alterations of forest-floor physical characteristics influence community parameters in an intact deciduous forest. Water availability in forest-floor soils could also have important effects on carabid communities and contribute to the lower abundance and tribe-level richness that we observed in plots where water was intercepted. Soil water-content was a key predictor of the carabid community in our study, which is consistent with other similar studies (Luff et al. 1989, Maudsley et al. 2002, Rykken et al. 1997). Ground beetle abundance and tribe-level richness were positively related to soil moisture content, with each measure generally higher in the wet than dry treatment. The specific moisture requirements of certain species are known to influence carabid community dynamics (Maudsley et al. 2002, Rykken et al. 1997). Other studies determined that dry conditions reduce overall beetle abundance and specific Carabidae taxa (Kiovula et al. 1999, Yi and Moldenke 2005). The tribe Harpalini was more abundant in the wet than dry treatment, consistent with findings that the distribution of a principal genus in this tribe was constrained by moisture deficits (Noonan 1990). Our data shows that increased precipitation in a deciduous forest could benefit this moisture-preferring taxon. It is also likely that the indirect effects and feedbacks of soil water availability are contributing to the community patterns that we found, including the preference by the tribe Harpalini for the wet treatment. Though only marginally significant, ground beetle abundance was inversely related to the mass of the forest floor, suggesting that precipitation influences ground beetle communities through effects on the forest floor. Our results contrast with previous litter- or resource-addition studies, where arthropod communities responded positively to increased litter depth and associated changes in architecture (Bultman and Uetz 1984, Halaj and Wise 2002, Kiovula et al. 1999). In our study, reduced water availability likely contributed to the greater total forest-floor mass found in the dry treatment compared to the ambient and wet treatments. Consistent treatment effects on soil water content have been demonstrated in long-term data sets at the TDE (Hanson and Wullschleger 2003). This result has implications for important forest-floor processes, because the frequency and intensity of drying-rewetting cycles are known to affect microorganisms responsible for the vast majority of decomposition in natural systems (Fierer at al. 2003, Schimel et al. 1999). The finding of lower %N and higher C:N ratio in the litter layer of the dry treatment plots is consistent with reduced decay rates, because N is generally enriched relative to C as decomposition progresses (Taylor et al. 1989). Diverting water may also have influenced decomposition in the dry plots by increasing carbon-based phenolics measured as tannic acid equivalents, thus potentially affecting decay processes (Gallardo and Merino 1993, Taylor et al. 1989). We found higher %TAE in the un-decomposed Oi (i.e., litter) layer. It seems likely, based on the chemical analyses, that removing natural precipitation results not only in litter accumulation but also in the production of a more slowly decomposing, Southeastern Naturalist R.S. Williams, B.S. Marbert, M.C. Fisk, and P.J. Hanson 2014 Vol. 13, No. 1 152 lower-quality litter, each of which could negatively affect important arthropod taxa. When all abiotic parameters were considered, our study showed that changes in precipitation affected physical and chemical characteristics, which, in turn, may have affected the ground beetle community. One indirect way that precipitation can alter forest beetle communities is by affecting trophic-level interactions. Ground beetle responses to precipitation in this study were not directly related to changes in microarthropods, which are potentially important prey species in the forest floor. In contrast to ground beetles, the abundance of mites increased in the dry treatment and declined relative to soil moisture across treatments. One possible explanation is that the larger number of beetles in wet plots reduced the mite abundance by predation. It is also possible that litter accumulation due to drought creates a more favorable physical environment for mites (Hansen 2000). In addition to this effect on the physical environment, drier conditions could increase mite abundance if it simultaneously reduced predatory beetle abundance through negative effects of dry soil on beetle larvae (see Loreau 1987). Conclusions This study found that precipitation manipulation over 13 years in an intact forest altered dominant beetle families and in particular, ground beetle communities, possibly through effects on forest-floor mass, soil moisture, and soil chemistry, which altered the structure and quality of the beetle habitat. This work points to the need to further pursue specific effects on the distribution of dominant species in Carabidae relative to environmental change. Our study provides insight into the influence of precipitation alteration on abiotic and biotic components of the forest floor in temperate hardwood forest landscapes, and our reslts increase our understanding of terrestrial ecosystem responses to future climate change. 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