61 Forest Community Structure Differs, but not Ecosystem Processes, 25 Years after Eastern Hemlock Removal in an Accidental Experiment Jenna M. Zukswert1,2,*, Jesse Bellemare1, Amy L. Rhodes3, Theo Sweezy3, Meredith Gallogly1, Stephanie Acevedo1, and Rebecca S. Taylor1 Abstract - The spread of Adelges tsugae (Hemlock Woolly Adelgid) directly threatens the survival of Tsuga canadensis (Eastern Hemlock) and has also triggered pre-emptive and salvage logging. In this study, we took advantage of a 25-year-old accidental experiment involving Eastern Hemlock removal by logging at Smith College’s MacLeish Field Station, in western Massachusetts, to investigate how microclimate, ecosystem processes, and forest-floor animal communities might change in the decades following Eastern Hemlock loss. On average, mean understory light levels in summer were 68% higher under young Black Birch (Betula lenta) canopies as compared to adjacent mature Eastern Hemlock forest. Mean daily air temperature, relative humidity, soil temperature, and organic-layer moisture content were similar between young Black Birch and mature Eastern Hemlock plots, although some of these factors were significantly more variable in the former. The soil organic horizon was significantly thicker in Eastern Hemlock plots, but net nitrification rates did not differ substantially between young Black Birch and mature Eastern Hemlock forest plots. We detected significantly greater densities of microarthropods (e.g., mites, collembolans) in the forest floors of Eastern Hemlock plots, possibly linked to the thicker organic horizon. Our results indicate substantial changes in forest structure and microarthropod communities with Eastern Hemlock removal, but little evidence of large changes in key ecosystem processes, like nitrogen cycling. Other sites that represent similar accidental experiments with Eastern Hemlock removal due to past human disturbance likely exist and should be studied before intact reference stands are lost to Hemlock Woolly Adelgid or preemptive salvage logging. Introduction The decline of Tsuga canadensis (L.) Carriere (Eastern Hemlock, hereafter Hemlock) in eastern North America due to the spread of the exotic pest Adelges tsugae Annand (Hemlock Woolly Adelgid [HWA]) is expected to significantly influence ecosystem processes and forest communities due to Hemlock’s role as a foundation tree species (Ellison et al. 2005). Accidentally introduced to the United States in the 1950s, HWA is an invasive insect that feeds on xylem ray parenchyma cells of Hemlocks, often causing tree mortality within as few as 4 years (McClure 1991, Young et al. 1995). Beyond the direct threat of HWA, the spread 1Department of Biological Sciences, Smith College, Northampton, MA 01063. 2Faculty of Forestry, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. 3Department of Geosciences, Smith College, Northampton, MA 01063. *Corresponding author - j.zukswert@alumni.ubc.ca. Manuscript Editor: Roland de Gouvenain Forest Impacts and Ecosystem Effects of the Hemlock Woolly Adelgid in the Eastern US 2014 Southeastern Naturalist 13(Special Issue 6):61–87 Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 62 Vol. 13, Special Issue 6 of this invasive insect has also led to pre-emptive and salvage logging efforts (Foster and Orwig 2006, Kizlinski et al. 2002), despite Hemlock’s limited value as a lumber species. Whether due to HWA-induced mortality, pre-emptive cuts, or salvage logging, the community structure and function of the Hemlock-dominated forests in eastern North America is predicted to change dramatically with the loss of this important foundation species in coming decades (Ellison et al. 2005, Orwig and Foster 1998). In areas where Hemlock trees have died from HWA infestation or have been logged, deciduous tree species rather than other conifer species (Orwig and Foster 1998) have typically emerged in their place. The tree species most commonly seen replacing Hemlock in the northeastern United States is Betula lenta L. (Black Birch; Orwig and Foster 1998). In the long term, forest succession resulting from the loss of Hemlocks is predicted to result in mature mixed deciduous forests, and Hemlocks are unlikely to re-colonize these forests for many decades, if ever (Ellison et al. 2005, Orwig and Foster 1998). This transition from evergreen, conifer-dominated ecosystems to deciduous, angiosperm-dominated ecosystems is expected to produce significant changes in associated plant and animal communities and in key biogeochemical processes (Cobb 2010, Ellison et al. 2005, Orwig and Foster 1998). As a foundation species, Hemlock strongly influences forest community structure by creating a unique understory environment that is often cooler, darker, moister, and characterized by more acidic soils than typically seen in deciduous forests (Ellison et al. 2005). The changes in understory microclimate accompanying Hemlock loss as well as changes in leaf-litter inputs, forest-floor composition, and decomposition could alter net rates of nitrogen transformation in soil, stimulating increases in net nitrogen mineralization and nitrification (Cobb 2010, Jenkins et al. 1999, Orwig et al. 2008, 2013). Beyond changes in ecosystem processes, changes are also predicted for plant and animal communities in forests that experience Hemlock loss, including shifts in the composition, abundance, or diversity of bird (Tingley et al. 2002), arthropod (Rohr et al. 2009), salamander (Mathewson 2009), and bryophyte assemblages (Cleavitt et al. 2008). Numerous studies have been launched in the past 10–15 years to examine the ecological changes that might result from the loss of Hemlock due to HWA or logging (Kizlinski et al. 2002, Orwig and Foster 1998). The majority of these studies have been observational or descriptive, involving longitudinal studies of sites succumbing to HWA (e.g., Cleavitt et al. 2008, Eschtruth et al. 2013), or employing space-for-time substitutions along regional gradients of HWA infestation (e.g., Jenkins et al. 1999, Orwig and Foster 1998, Orwig et al. 2008). These studies have provided valuable insights into the environmental changes occurring in the wake of Hemlock loss; however, some longitudinal studies could be confounded by interannual variability in climate, and space-for-time substitutions might be influenced by underlying differences in soils, geology, and physiography among sites. To address some of these limitations, experimental manipulations have recently been implemented in which intact Hemlock control stands are compared to nearby plots in which the species has either been girdled to simulate slow death Southeastern Naturalist 63 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 by HWA or harvested to simulate salvage or pre-emptive logging (e.g., Ellison et al. 2010, Knoepp et al. 2011). For example, large-scale removal experiments have been launched at the Coweeta Hydrologic Laboratory in North Carolina and at Harvard Forest in Massachusetts within the past decade. Ideally, these experiments allow side-by-side comparison of intact Eastern Hemlock stands with forested areas where removal of the species has triggered replacement by deciduous tree species like Black Birch (e.g., Orwig et al. 2013). Although these planned experimental manipulations are already providing important new insight into the early stages of forest succession following Hemlock removal, their relatively recent initiation might confound the specific effects of Hemlock loss with general disturbance effects and also limit the information available on later successional stages. Unfortunately, the rapid spread of HWA also threatens to compromise intact Hemlock control stands before these experiments have had a chance to fully simulate the decades following Hemlock loss. To obtain a longer-term view of the effects of Hemlock removal while still retaining many of the desirable aspects of an experimental manipulation, the present study at Smith College’s MacLeish Field Station in Whately, MA takes advantage of a 25-year-old accidental experiment involving partial removal of Hemlock from a Hemlock-northern hardwoods forest by small-scale logging in the late 1980s. This logging activity created a patchwork of forest stands, with young Black Birch-dominated stands embedded within a more mature Hemlock-dominated forest matrix. This study system allows for documentation of a later stage in the successional trajectory following Hemlock removal, while still minimizing differences in underlying environmental conditions through close spatial proximity of Hemlock-dominated areas and young Black Birch-dominated patches generated by logging. The objective of this study was to collect baseline ecological data from forest stands at Smith College’s MacLeish Field Station in order to investigate the longer-term effects of Hemlock removal on a subset of ecosystem processes and forest-floor community components. In particular, we collected microclimatic and biogeochemical data to observe how understory environmental conditions and forest-floor ecosystem processes, particularly nitrogen cycling, differed between mature Hemlock-dominated forest and young Black Birch-dominated forest ≈25 years after Hemlock removal. We predicted that Hemlock stands would be cooler and darker than young Black Birch stands, and hypothesized that soils beneath the young Black Birch canopy would have thinner organic horizons and exhibit higher net nitrogen transformation rates than soils beneath the Hemlock canopy (cf. Finzi et al. 1998a, b; Lovett et al. 2004). We also investigated forest-floor animal communities to test for differences in the abundance of a key functional group involved in decomposition: forest-floor microarthropods (e.g., mites and collembolans; Coleman et al. 2004). We hypothesized that these forest-floor mesofauna would be more abundant under Black Birch canopies because the higher nutrient content and lower C:N of Black Birch leaf litter (e.g., Cobb 2010) might be expected to stimulate more productive detritivore communities. Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 64 Vol. 13, Special Issue 6 Field-Site Description The study site for this project was located at Smith College’s 98-hectare MacLeish Field Station in Whately, MA (42°27'N, 72°40'W), in the foothills of the Berkshire Plateau (270 m elevation). Soils at the field station are predominantly inceptisols, specifically classified as extremely rocky loam of the Westminster Series (Mott and Fuller 1967), and tend to be acidic (pH ≈4.1–4.7; Zukswert 2013). The glacial till-derived soil is shallow, well drained, and typically has a persistent organic horizon. The mineral subsoil extends ≈45 cm until reaching Devonian gray mica schist and quartz bedrock of the Waits River Formation, a bedrock type that extends through the foothills of the Berkshire Plateau in Massachusetts into Vermont and includes occasional beds of impure calcitic marble (USGS 2013, Willard 1956, Zen et al. 1983). The natural vegetation of the study area is northern hardwoods-Hemlock-Pinus strobus L. (Eastern White Pine) forest (Westveld 1956). At the MacLeish Field Station, forests are dominated by Hemlock, Black Birch, and Quercus rubra (L.) (Red Oak); canopy trees range from ≈90 to 110 years in age (J. Bellemare, unpubl. data). The land was cleared for farming during the late 18th and early to mid-19th centuries (Crafts and Temple 1899), but then largely abandoned to secondary forest succession in the late 19th to early 20th centuries, as has occurred with many upland areas in southern and central New England (Bellemare et al. 2002, Foster et al. 1998). The forests at the MacLeish Field Station appear to have experienced occasional selective cutting in the early to mid-20th century while under private ownership, with a more substantial, 36-acre commercial cut in 1988 managed by Smith College. In the latter operation, many large Red Oak trees were selectively cut and small patches of forest (≈20 × 20 m) were clear-cut; the rationale for the small intensive cuts is not known because the supervising forester is now deceased. About 94,540 board feet were cut in 1988, with Hemlock representing 35.8% of the total board feet removed (Davies 1988). Black Birch has regenerated vigorously and now forms dense stands of 20–25 year-old trees in the gaps created by this logging operation, surrounded by a matrix forest of mature Hemlock, Red Oak, and Black Birch (Fig. 1). Methods Plot establishment To investigate changes in ecosystem processes and forest-floor communities associated with Hemlock removal, we established a series of research plots situated in adjacent patches of mature Hemlock-dominated forest and young Black Birch stands generated by the 1988 logging operation. Specifically, in 2010, we delineated 4 adjacent pairs of 10 × 15 m young Black Birch and mature Hemlock forest plots, as well as 3 additional Hemlock forest plots (n = 11 plots total, Appendix A). The plot size of 10 x 15 m was selected to reduce edge effects in the ≈20 x 20 m Black Birch gap areas. In the context of the accidental experiment initiated by 1988 logging, we presume that the forest vegetation prior to this cut Southeastern Naturalist 65 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 Figure 1. One of the young Black Birch forest plots situated within an Eastern Hemlockdominated forest at the MacLeish Field Station in Whately, MA. These former logging gaps measure ≈20 × 20 m, but our study plots within them measure 10 × 15 m. Wooden boards on the forest floor are being used as artificial cover objects in an ongoing study of forestfloor animal populations. Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 66 Vol. 13, Special Issue 6 was relatively homogenous across the entire study site (i.e., similar to the mature Hemlock-Red Oak-Black Birch matrix forest surrounding the young Black Birchdominated gaps today) and that any differences currently observed in ecosystem processes and forest-floor ecology between the two forest types trace to their divergent histories and differing canopy compositions since 1988. The presence of Hemlock stumps in the young Black Birch plots confirms that these areas supported Hemlock prior to logging. In our baseline tree survey in Fall 2012, the 7 Hemlock plots investigated averaged 51.7 m2 ha-1 total basal area and 1489 tree stems per hectare, with Hemlock trees constituting 56% of total basal area, on average (Fig. 2). The young Black Birch plots averaged 14.0 m2 ha-1 total basal area and 7822 tree stems per hectare, with Black Birch trees constituting 84% of the basal area, on average (Fig. 2). We also delineated one mature deciduous forest plot near the study area (≈50 m away), dominated by mature Black Birch of similar age to the mature Hemlock forest, to serve as a proxy for the type of forest into which the young Black Birch stands might develop in ≈50–100 years (Fig. 2). Black Birch has the potential to dominate secondary forests in this area for many decades (e.g., Bellemare et al. 2002). Figure 2. Mean basal area (m2 ha-1 ± SE) and stem density (x = # trees ha-1) of Black Birch, Eastern Hemlock and other trees species in the young Black Birch, mature Eastern Hemlock, and mature deciduous forest types. Southeastern Naturalist 67 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 Microclimate characterization In order to investigate microclimatic differences among Black Birch and Hemlock plots, we measured understory environmental conditions for a 30-day period from 16 July through 15 August 2010. This time period was selected to provide a snapshot of understory conditions in mid-summer when influences of Hemlock in the canopy might be most important for moderating understory conditions. We installed HOBO Micro Station data loggers (Onset Computer Corporation, Bourne, MA) in 2 pairs of adjacent Hemlock and Black Birch plots (n = 4 data loggers total). Each data logger had an air temperature and relative humidity smart-sensor probe (model #S-THB-M00x) and a 12-bit temperature smart-sensor probe (model #S-TMB-M0XX) inserted below the soil organic horizon in the first 1–2 cm of the mineral soil, and a photosynthetically active radiation (PAR) smart-sensor probe (model #S-LIA-M003). The PAR smart sensors were situated on a horizontal brace ≈75 cm above the forest floor, and the air temperature and relative humidity probe was situated ≈50 cm above the forest floor inside a solar radiation shield (model #RS3). The data loggers collected light and microclimate observations at 1-minute intervals; by using a short sampling interval, we sought to capture data on the understory light environment, which often exhibits rapid fluctuations due to sunflecks (Neufeld and Young 2003). Based on the microclimate and light data collected, daily mean values and standard deviations were calculated for each data logger for daytime (6:00 to 18:00 EST) air temperature, relative humidity, soil temperature, and PAR levels (μmol m-2 s-1 in the 400–700 nm wavelength range). We calculated the percent (%) difference in these mean values for each 12-hour daytime sampling period for each pair of adjacent plots as the mean Black Birch plot value minus the mean Hemlock plot value, divided by the Black Birch mean, multiplied by 100 (positive scores represent higher mean values in the young Black Birch plot). To provide a measure of microclimate variability in the two habitats, we also compared daily standard deviation (SD) values for each metric for each pair of plots, and calculated the % difference in SD between adjacent Black Birch and Hemlock plots. For PAR, air temperature, and soil temperature, we calculated mean % differences and standard deviations based on the differences observed in the 2 pairs of plots (i.e., n = 2 data logger pairs); for relative humidity, a sensor in 1 station malfunctioned, so we present data from a single pair of stations (i.e., n = 1 data logger pair). To evaluate the significance of microclimatic differences between the two forest types, we conducted one-sample t-tests to test the null hypothesis that the mean of the 30 daily mean % difference values for each microclimate factor was zero (i.e., environmental conditions did not systematically vary by forest type). Similarly, we used one-sample t-tests to test the null hypothesis that the variability in microclimate, estimated by % differences in standard deviation, did not differ systematically between the two habitat types. We conducted our analyses using the mosaic package in R (version 2.15.2). Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 68 Vol. 13, Special Issue 6 Litterfall characterization To characterize and compare leaf-litter inputs to the forest floor of the mature Hemlock and young Black Birch plots, we collected leaf litterfall from July 2012 through June 2013 in 4 of the plots (n = 2 Hemlock, n = 2 Black Birch; Appendix A). We collected litter in rectangular laundry baskets (0.55 × 0.39 m) lined with nylon tulle mesh and held in place with landscape staples. Starting on 5 July 2012, we randomly placed 5 collection baskets in each of the 4 plots (n = 20 collectors at start); collection continued until 25 June 2013. We lost samples from December 2012 due to heavy accumulations of snow and ice that tore the mesh in many of the baskets. In addition, animals overturned 2 baskets in the Hemlock plots in Fall 2012; therefore, we excluded them from the analysis, resulting in a total of 8 Hemlock baskets and 10 young Black Birch baskets for analysis. In total, the collections represent 11 of 12 months for 2012–2013. We retrieved leaf litter from the baskets several times during the year, dried it at 70 °C for >48 h in a laboratory oven prior to sorting it by genus and type of litter (e.g., leaf, twig, seed), and weighed it to the nearest 0.01 g. We summed raw leaflitter data (i.e., dry mass per species per basket) over the study period, converted it to g m-2, and analyzed it with two-sample t-tests contrasting the two forest types; we also calculated % composition by tree species. Organic horizon characterization and N cycling In June 2013, we estimated organic horizon depth for each plot by extracting 10 randomly placed soil cores per plot and measuring the combined depth of the Oe and Oa portions of the O horizon. Additionally, on 20 June and 15 July 2013, we randomly collected ten ≈5 g samples from the Oe and Oa in a subset of 3 young Black Birch and 6 adjacent Hemlock plots (n = 90 samples total) and estimated % moisture content by measuring field-moist and oven-dry mass after >48 h at 75 °C. We analyzed data on organic-layer depth and moisture content with nested ANOVA in R, testing for significant forest-type ef fects. We investigated net nitrogen mineralization (hereafter net Nmin) and nitrification rates in one pair of adjacent mature Hemlock and young Black Birch plots and in the nearby mature deciduous forest plot using an incubated-core method modified from Orwig et al. (2008) and Robertson et al. (1999). We subdivided each plot into seven 2 m × 2 m subplots; during each sampling period, we obtained two 5-cmdiameter × 20–30-cm-long soil cores from each subplot using a soil core sampler with a sliding hammer (AMS, Inc., American Falls, ID). We immediately brought back one soil core (initial core) to the laboratory in an ice-chilled cooler to be analyzed for baseline N levels, and we placed a second core (incubated core), obtained adjacent to the first core (within 0.5 m), back into the ground in a PVC sleeve. The incubated core was capped at the base to prevent net Nmin products from leaching out with downward flow of soil water, and the PVC sleeve was loosely capped at the top to maintain airflow but prevent rainfall from entering the core (Robertson et al. 1999). We separated the organic (O + A) and mineral horizons (B) of each of the 7 soil cores, sieved the material to 2 mm, and then composited and homogenized Southeastern Naturalist 69 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 the samples to generate 1 organic-soil sample and 1 mineral-soil sample per plot per sampling period. We performed this process for the initial soil cores and then followed the same protocol 3 or more weeks later for the incubated soil cores in order to estimate daily rates of net Nmin over the incubation period. We performed soil incubations during May 2011–July 2013 with occasional breaks, e.g., during inactive periods in winter (Appendix B). We determined the amount of exchangeable NO3 - and NH4 + in organic and mineral soil horizons after preparing soil extracts using 0.02M strontium chloride (SrCl2), which has been shown by Li et al. (2006) to be as effective as the more commonly used 1M KCl and 0.01M CaCl2 extractant solutions for both acidic and calcareous soils. The less concentrated 0.02M SrCl2 solutions enabled analysis of NO3 - and NH4 + by ion chromatography (Dionex ICS Model 3000, Dionex Corporation, Sunnyvale, CA). Using the ion chromatograph, we measured NO3 - with an AS19 column with an isocratic hydroxide eluent, and by using both conductivity and ultraviolet (UV) detection, which eliminates peak interference between NO3 - and Cl-. The limit of quantification for NO3 - was 0.09 mg L-1 for conductivity detection and 0.12 mg L-1 for UV detection, and the mean standard deviation was ± 0.096 mg L-1. We measured concentrations of NH4 + using a CS12 cation column with an isocratic methane sulfonic acid eluent and a conductivity detector. The limit of quantification for NH4 + was 0.03 mg L-1, and the mean standard deviation was ± 0.103 mg L-1. We prepared 3 SrCl2 replicates for each composite sample by mixing ≈10 g of field-moist soil with 100 mL of SrCl2, and concentrations of NO3 - and NH4 + were corrected for mass of the soil sample, soil water, and bulk density of the <2 mm fraction using an equation described by Robertson et al. (1999); we reported concentrations as kilograms of nitrogen per hectare (Appendix C). We averaged results from all 3 replicates for each composite sample. We determined net Nmin and net nitrification rates using the corrected concentrations of NO3 - and NH4 + in equations from Robertson et al. (1999), in which the time was measured in days (Appendix C). We focused our analysis on nitrogen cycling rates in the organic horizon because we hypothesized that tree-species effects on nitrogen cycling due to differences in leaf-litter chemistry would be most evident in these horizons (Binkley and Fisher 2013) and our initial analyses indicated that the vast majority of N cycling in the mor-type humus soils of our study system took place in the O and thin A horizons, as opposed to deeper in the mineral soil. We measured soil moisture of composite samples to standardize N concentrations by dry soil mass, following Jarrell et al. (1999), for the 3 plots where nitrogen cycling was monitored. We dried a subsample of the sieved, composite soil sample in an oven at 105 °C for at least 48 h. In 2013, we determined the bulk density of the <2-mm fraction organic soil horizons using a pin-block method (Table 1; Bailey et al. 2005), which minimized compaction of soils during collection. Forest-floor mesofauna We surveyed densities of forest-floor mesofauna in 3 mature Hemlock plots and 3 adjacent young Black Birch plots (Appendix A). We extracted Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 70 Vol. 13, Special Issue 6 microarthropods, including Acari (mites), Collembola (collembolans), and Pseudoscorpionida (pseudoscorpions), from 60 organic-horizon samples collected from the 6 plots over a two-day period in July 2012 (10 samples per plot). Each sample included ≈25 g of field-moist organic horizon material composed of well-rotted leaf litter and homogenous rotted organic material from the Oa and Oe components of the soil organic horizon. We placed these samples into sixty 12-cm-diameter Berlese-type funnel extractors (Small Berlese Funnel Trap #2845, Bioquip Products, Inc., Rancho Dominguez, CA) with a 1 mm × 2 mm wire-mesh insert to prevent fine-textured organic material from falling into the collection vials at the base of the funnel. The organic horizon material was left to air-dry in the lab for 7 days, during which time microarthropods retreated downward into collection vials containing 70% isopropyl alcohol. We examined the collected microarthopods under an Olympus SZX61 zoom stereomicroscope and assigned them to broad taxonomic categories based on morphology, e.g., mites, collembolans, and pseudoscorpions. We then oven-dried the organic-material samples from the Berlese traps at 70 °C for >48 h to determine dry mass. We analyzed mesofauna density data (# individuals per gram dry mass of organic material) with nested ANOVA testing for plot and forest-type effects. Results Microclimate characterization Over the 30-day observation period in July–August 2010, light levels in the forest understory were significantly higher under young Black Birch canopies (mean = 36.7 μmol m-2 s-1 ± 3.0 SE) than under nearby mature Hemlock canopies (mean = 8.51 μmol m-2 s-1 ± 0.4 SE; mean difference = 68%, t30 = 65.7, P < 0.0001; Fig. 3). Variability in PAR light levels, as estimated by SD, was also significantly higher in young Black Birch plots (mean = 66% greater, t30 = 24.6, P < 0.0001; Fig. 3), likely indicating a higher incidence of sun flecks in the Black Birch u nderstory. Air temperature did not differ substantially between the two habitat types. Over the course of the 30-day sampling period, the mean daytime air temperature in all 4 plots was 22.8 °C (± 0.3 SE). Although air temperatures tended to be marginally higher in the young Black Birch plots (mean = 0.4% difference), this slight difference fell below the accuracy range of the air temperature probes. However, variability in air temperature, as estimated by SD per day, was significantly higher (mean = 4% greater) in young Black Birch plots (t30 = 11.7, P < 0.0001; Fig. 3). Table 1. Mean (and standard deviation) bulk density of the organic soil horizons in the young Black Birch, mature Hemlock, and mature deciduous forest plots used for soil chemistry analyses at the MacLeish Field Station, Whately, MA. Bulk density of organic horizons was determined with a pin-block method (Bailey et al. 2005), using only the <2 mm fraction for the final calculation of bulk density. Forest type Horizon Bulk density (g soil cm-3) Black Birch Organic 0.11 ± 0.02 Eastern Hemlock Organic 0.17 ± 0.02 Mature deciduous Organic 0.39 ± 0.03 Southeastern Naturalist 71 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 Figure 3. Mean percent difference in the mean (left column) and standard deviation (right column) of light intensity (photosynthetically active radiation; a and b), air temperature (c and d), and soil temperature (e and f) in two pairs of adjacent young Black Birch and mature Eastern Hemlock plots over 30 days from 16 July through 15 August 2010. Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 72 Vol. 13, Special Issue 6 Relative humidity did not differ substantially between the one young Black Birch and mature Hemlock plot pair where it was measured, dif fering at most by ≈3%. Daytime soil temperature in the plots, as measured in the mineral soil at the base of organic horizon, averaged 18.7 °C ± 0.2 SE across the 4 plots and tended to be slightly, but significantly, higher under young Black Birch canopies than under mature Hemlock canopies (mean = 2% greater; t30 = 18.5, P < 0.0001; Fig. 3). The variability in soil temperature, as estimated by SD, was substantially and significantly higher in the young Black Birch plots (mean 36% greater, median = 33.2%) with a strong positive skew toward disproportionately higher values in young Black Birch plots. After natural log transformation, these data yielded a highly significant difference in soil temperature variability between the two forest types (t30 = 68.9, P < 0.0001). Soil-moisture levels in O horizon material did not vary significantly between young Black Birch and mature Hemlock plots on either of the two collection dates in June–July 2013 (nested ANOVA: forest-type effect P > 0.1 in both cases; plot effects were also non-significant). For example, in samples from 15 July, moisture content averaged 53.8% (± 1.4 SE) in young Black Birch plots and 54.3% (± 1.0 SE) in mature Hemlock plots. Litterfall characterization Over the July 2012 through June 2013 collection period (minus December 2012), non-woody leaf-litter inputs averaged 275.4 g m-2 (± 8.1 SE) across the 4 plots sampled. Total non-woody litter inputs were significantly greater in the Hemlock plots (mean = 300.8 g m-2 ± 10.7 SE) than in the young Black Birch plots (mean = 255.2 g m-2 ± 7.1 SE; t-test on natural-log transformed data: t17.5 = 4.4, P = 0.0004; Fig. 4). On average, Hemlock needles contributed 26.6% or 79.6 g m-2 (± 5.8 SE) of leaf litter in the Hemlock-dominated plots, which was significantly more than the 12.2% or 30.8 g m-2 (± 10.1 SE) of non-woody litter that Hemlock needles contributed in the young Black Birch plots (t-test on natural-log transformed mass data: t15.9 = 8.3, P < 0.0001; Fig. 4). Needles falling in the young Black Birch plots were due to drift into the plots from Hemlock trees in the adjacent mature forest. Black Birch leaf litter was collected in both forest types as well due, in part, to the presence of mature Black Birch trees in the canopy of the Hemlock plots. On average, Black Birch litter contributed 54.5% of total non-woody leaf litter or 138.9 g m-2 (± 6.6 SE) in the young Black Birch plots and 37.6% or 114.1 g m-2 (± 14.1 SE) in the Hemlock plots (mass data t-test: t13.1 = 0.9, P = 0.37; Fig. 4). Organic horizon characterization and N cycling In our June 2013 survey of forest-floor structure, we determined that the depth of the O horizon was significantly greater in Hemlock plots (mean = 4.1 cm ± 0.1 SE) compared to young Black Birch-dominated plots (mean = 2.5 cm ± 0.1 SE; nested ANOVA: forest-type effect F1 = 76.0, P < 0.0001). We also detected evidence of additional variation in O horizon depth among plots that was not explained by canopy type (nested ANOVA: plot effect F9 = 4.4, P < 0.0001). Although not included in the nested ANOVA, we also surveyed O horizon depth in the mature deciduous forest Southeastern Naturalist 73 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 plot dominated by mature Black Birch; in this plot, the O horizon was poorly developed, averaging 0.3 cm in depth (± 0.1 SE), and was absent at 4 of the 10 sample points, where leaf litter transitioned almost directly to the A horizon. Overall, net Nmin rates in the combined O and A soil horizons were consistently higher during spring and summer months than in fall and winter (Fig. 5). During the timeframe of our study (May 2011–July 2013), net Nmin rates in soils from the young Black Birch plot were consistently low (<0.25 kg N ha-1 day-1) across all 3 growing seasons (Fig. 5). Soils from the Hemlock plot showed similar net Nmin rates to those of the young Black Birch soils in 2011, but then increased substantially (up to 0.65 kg N ha-1 day-1) during Summer 2012 when the health of one canopy and 2 sapling Hemlocks declined in the plot. A similar pattern of higher net Nmin rates in the soils of the Hemlock plot than in the adjacent young Black Birch plot soils was apparent during the 2013 growing season as well (Fig. 5). Net Nmin rates of both the Black Birch and Hemlock soils were dominated by NH4 + production; in contrast, net nitrification ranged from -7 to 17 % of net Nmin in the Black Birch plots and -6 to 11% of net Nmin in the Hemlock plots throughout the study period. Soils from the mature deciduous plot showed the greatest net Nmin Figure 4. Comparison of mean (with SE) Black Birch, Eastern Hemlock, and total nonwoody l i t t e r mass (g) collected in young Black Birch forest plots (n = 10 collectors) and mature Eastern Hemlock forest plots (n = 8 collectors). Percent by weight of each litter type is also displayed. Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 74 Vol. 13, Special Issue 6 Figure 5. Comparison of net Nmin rates (kg N ha-1 day-1; (a), net nitrification rates (kg NO3 - ha-1 day-1; (b), and % nitrified (c) from May 2011 through July 2013 in the Black Birch plot (black circle) and the Hemlock plot (white square), and from May 2012 through July 2013 in the mature deciduous plot (black triangle) in organic soil horizons. The rate is graphed at the midpoint of the incubation period. Dashed lines represent periods during which no soil incubations were performed. Standard deviation bars are depicted; in some cases SD bars are obscured by the data. Southeastern Naturalist 75 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 rates of the 3 plots, up to 1.36 kg N ha-1 day-1, with 4 to 68% of net Nmin due to nitrification during the summer (Fig. 5). Forest-floor mesofauna Invertebrate mesofauna were present in relatively high numbers in the 60 forest-floor organic-material samples from 3 young Black Birch plots and 3 mature Hemlock plots: the overall mean density was 5.2 mesofauna individuals per gram dry mass (± 0.5 SE). The mesofauna community was comprised largely of microarthropods; we observed mites and collembolans in every sample and they accounted for the bulk of individuals, 49% and 45%, respectively, of 3580 total mesofauna specimens collected. Pseudoscorpions were another distinctive microarthropod group observed in some samples; however, they comprised only ≈2% of the total mesofauna individuals and were detected in fewer samples overall (57% of samples). The abundance distributions for both mites and collembolans were positively skewed; to better meet assumptions of normality, we transformed the mite-sample abundance data by first adding 1 to all per-gram values (to avoid fractional values <1) and then taking the natural log of this sum. We transformed the data for collembolans by converting their abundance to counts per 10 grams dry organic material, and then taking the natural log of this value. Following these Figure 6. Mean abundance (and SE) of three major microarthropod taxa—mites (Acari), collembolans (Collembola), and pseudoscorpions (Pseudoscorpionida)— per gram dry mass in forest-floor organic- matter samples from young Black Birch (slantedlined infill, n = 30 samples) and mature Eastern Hemlock plots (horizontal-lined infill, n = 30 samples). All three microarthropod groups showed either significantly (P < 0.001) higher abundances (mites, collembolans) or greater frequency (pseudoscorpions) in samples from mature Eastern Hemlock plots based on nested ANOVA forest-type effect or chi-square test, respectively. Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 76 Vol. 13, Special Issue 6 transformations, both variables met basic assumptions of normality (e.g., Shapiro- Wilk goodness-of-fit test), and we analyzed these data using ANOVA with plot effect nested within forest type. Mites were significantly more abundant in forest-floor organic material from the mature Hemlock plots (3.0 g ± 0.3 SE) as compared to young Black Birch plots (2.0 g ± 0.3 SE) (ANOVA: forest-type effect F1 = 12.6, P = 0.0008; Fig. 6). We also detected a significant plot effect (F4 = 5.9, P = 0.0005), indicating variation in mite density among plots that was not explained by the dichotomous forest-type factor. The density of collembolans followed a similar pattern to that of mites, with their numbers significantly greater in organic material collected from mature Hemlock plots (mean = 3.5 individuals per gram ± 0.6 SE) compared to young Black Birch plots (mean = 1.2 individuals per gram ± 0.2 SE; nested ANOVA: forest-type effect F1 = 28.9, P < 0.0001; Fig. 6). The plot effect within the overall ANOVA model was also significant for collembolans (F4 = 3.3, P = 0.0163). Although they occurred in much lower numbers than mites and collembolans, pseudoscorpions also showed a significant positive association with forest-floor organic material from the Hemlock plots (present in 80% of samples) compared to young Black Birch plots (33% of samples; chi-square statistic based on presence/absence: χ2 1 = 11.5, P = 0.0007; Fig. 6). Discussion In this study, selective logging of a Hemlock-dominated forest in the late 1980s yielded an accidental experiment in which the biogeochemical and ecological effects of Hemlock on the forest ecosystem can be evaluated a quarter century after its removal. The results provide new insight into the longer-term effects that the loss of this foundation tree species might have on ecosystem processes and ecological communities in the northeastern US. Our findings build on those of shorter-term, planned experimental manipulations in the eastern US (e.g., Ellison et al. 2010, Knoepp et al. 2011, Orwig et al. 2013), but also underscore the possibility that some ecosystem characteristics might be slower to change than predicted, suggesting a process of environmental change that may take many decades to unfold. Most importantly, our results suggest a complex and temporally varied pattern of ecological and ecosystem change associated with Hemlock removal, hinging primarily on shifts in forest community structure, forest-floor depth, and indications of lower forest-floor microarthropod mesofauna abundances. We did not, however, see clear evidence of major changes in a key ecosystem process, N cycling, when we compared mature Hemlock-dominated forest to adjacent young Black Birchdominated forest. Broadly, these findings suggest substantial shifts in ecological community structure with Hemlock removal, but less evidence of major alteration in ecosystem processes, even after more than two decades of Black Birch regeneration. Even so, our mature deciduous plot, which has been dominated by Black Birch for close to a century, showed substantially higher N-cycling rates and a soil structure characterized by significantly thinner O horizons, a pattern more consistent with expectations drawn from species characteristics (e.g., lower C:N of Black Southeastern Naturalist 77 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 Birch leaf litter; Cobb 2010). This finding indicates that, over multiple decades, ecosystem processes might gradually shift to conditions more characteristic of dominance by deciduous tree species; however, the approach to this new state may be slow and non-linear, accelerating only as persistent effects of Hemlock on soil conditions fade and inputs from mature Black Birch increase. We note several important caveats to the results of our study. First, it is possible that the spatial scale of the accidental experiment we investigated might have influenced the nature and magnitude of effects detected. In particular, our plots were of modest size (10 × 15 m) and situated close together in order to minimize underlying environmental differences in soils, slope, and aspect. However, this proximity resulted in Hemlock needles contributing to the litterfall of the young Black Birch plots (≈12% of leaf litter, vs. ≈27% in Hemlock plots) due to drift from mature Hemlock trees in the surrounding mature forest (Fig. 4). Although other important aspects of forest community structure and environment were clearly differentiated between the two forest types (e.g., O horizon depth, mesofaunal community), these needle inputs to the Black Birch plots might have subtly altered forest-floor conditions and reduced the magnitude of change detected between the Hemlock and young Black Birch plots. A second caveat is the time scale considered for our microclimate measurements: 30 days in Summer 2010. This is a short interval over which to contrast the two habitats considered; however, our goal was to provide a snapshot of differences in the understory environment during the warmest portion of the growing season. The modest differences we detected would certainly be amplified if we had considered the time period before deciduous tree leaf-out in spring, or after leaf-fall in autumn. Finally, due to the unplanned nature of our study site’s accidental experiment and the limited number of young Black Birch gaps created by the original logging event in the late 1980s, replication was low for some components of the project (Appendix A). In particular, because soil N-cycling analyses might be highly sensitive to disturbance and forest-floor trampling, we conducted this element of the research in a single set of young Black Birch, mature Hemlock, and mature deciduous forest plots, while the other, more disruptive, research activities associated with the project were concentrated in the remaining plots. The N-cycle analyses were based on multiple pooled samples per time period, and extended across 2–3 growing seasons (Appendix B), but they were not replicated across multiple plots per forest type. While we believe the trends detected are informative, and consistent with other research, they should be interpreted with caution. Microclimate and understory environment Prior studies have suggested changes in microclimate following Hemlock decline, including shifts toward increased air and soil temperatures, decreased humidity and soil moisture, and increased light in the forest understory (Ellison et al. 2005, Kizlinski et al. 2002, Knoepp et al. 2011, Lustenhouwer et al. 2012, Orwig et al. 2008). Our findings confirm some of these expectations while diverging from others. For example, we observed significantly greater light intensity in the young Black Birch plots (mean = 68% higher; Fig. 3), consistent with a thinner, more open Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 78 Vol. 13, Special Issue 6 deciduous canopy, but our data provide little evidence of meaningful changes in air temperature or relative humidity, at least during the 30-day monitoring period within the 2010 growing season. The latter result is likely a consequence of the small size of the Black Birch-dominated patches we studied, limiting the potential for air conditions to diverge given airflow through the forest. More strikingly, we detected significantly greater variability in some microclimatic factors in young Black Birch plots (Fig. 3). Changes in soil temperature and variability, which were significantly greater in the Black Birch plots, are likely of most relevance to forest-floor mesofauna and soil N-cycling. The higher temperatures and greater temperature variability are likely reflections of the increased light intensity on the forest floor of the young Black Birch stands relative to the light intensity in the Hemlock plots, a factor that would contribute to increased solar heating of the soil (Binkley and Fisher 2013). Although Orwig et al. (2008) did not detect changes in soil temperature during their 3-year study of HWAaffected stands in southern New England, they predicted that such changes would occur in the future; our results support that prediction. Total leaf-litter inputs differed significantly between the two forest types, with the mature Hemlock forest plots receiving ≈18% more material, of which slower-decomposing Hemlock needles with higher C:N (Cobb 2010) comprised a significantly greater percentage (27% vs. 12%; Fig. 4). This shift in leaf-litter quantity and quality likely drives the significantly greater depth of the soil O horizon documented in the mature Hemlock plots. Given the differences detected in microclimate, light, and O horizon depth, it was surprising that we did not detect evidence of greater % moisture content in O horizon soil samples from the mature Hemlock plots (nested ANOVA: foresttype effect P > 0.1). It is possible that such effects, though widely predicted in the literature, are fleeting and contingent on short-term weather conditions. Alternatively, the greater basal area of trees in the mature Hemlock plots might support higher levels of fine-root biomass, leading to depletion of moisture in the forest floor via transpiration, despite less potential for evaporation in the darker Hemlock stand understory. Net nitrogen mineralization in soils following Eastern Hemlock decline Prior research examining biogeochemical changes on short timescales directly following HWA infestation or pre-emptive logging of Hemlock has yielded important insights into the short-term ecosystem responses resulting from disturbance and tree species turnover. For example, Orwig et al. (2008) reported increases in net Nmin rates in Hemlock-dominated forest 3 years after HWA invasion, with net nitrification rates in HWA-infested forests as high as 50% of net Nmin. Kizlinski et al. (2002) also observed increased net nitrification rates in soils following HWA infestation and pre-emptive logging in Massachusetts and Connecticut, with rates in the harvested stands at least twice the rates documented in HWA-infested stands. Most recently, however, a 5-year experimental manipulation by Orwig et al. (2013) showed that the higher net Nmin rates and increased availability of both NH4 + and Southeastern Naturalist 79 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 NO3 - in Hemlock soils following HWA infestation or logging might be relatively short-lived, disturbance-related responses that tend to return to pre-disturbance levels in about 3 years. In our study, the small spatial scale and close proximity of young Black Birch forest plots to mature Hemlock forest minimized underlying differences in soil type, soil moisture conditions, and atmospheric N deposition, making tree-species composition and stand age the primary factors that could alter N cycling. However, we observed no difference in net Nmin rates between the organic horizons in the mature Hemlock plot and young Black Birch plot during the 2011 growing season, and nearly 0% net nitrification in both habitats from Summer 2011 through 2013. In fact, soils in the young Black Birch maintained these low net Nmin and nitrification rates throughout the 3 growing seasons. In contrast, the mature deciduous plot showed substantially higher net Nmin rates with greater net nitrification (ranging from 4 to 68% of net Nmin) than either the Hemlock or the young Black Birch plots (Fig. 5). Therefore, it appears that tree-species effects on N cycling in organic soil horizons might take considerable time to either dissipate (post-Hemlock) or to shift toward conditions more typical of a deciduous tree-dominated ecosystem. These patterns might reflect what could be expected on the nutrient-poor, acidic upland soils where Hemlock populations often occur in the northeastern US. Consequently, the large changes in ecosystem processes projected under some Hemlock decline scenarios (e.g., increased N cycling and leaching rates) might be restricted to a disturbance-related pulse of only the few years following HWA infestation or logging, with a much more gradual, multidecadal shift toward characteristics linked to deciduous forest development (Cobb 2010, Jenkins et al. 1999, Lovett et al. 2004, Orwig et al. 2013). Our results might also have some bearing on the long-standing question of tree-species effects on soil chemistry. Mueller et al. (2012) have highlighted stand age as an important covariate of species effects, yet Binkley and Fisher (2013) note that the length of time necessary for species effects on soil biogeochemistry to develop is currently an open question. Our findings support those of Orwig et al. (2013), whose work also failed to detect a persistent increase in net N transformation rates in soils following replacement of Hemlock by young Black Birch. They propose that alteration of net Nmin and nitrification rates might take centuries to occur and might be influenced by Hemlock coarse woody-debris and associated fungal communities (Orwig et al. 2013). Further suggesting a lag time between changes in ecosystem structure and function, particularly nutrient cycling, Hix and Barnes (1984) found that soil pH and nutrient concentrations did not significantly differ between Hemlock plots and plots in which Hemlock had been clear-cut 46 years before. At our study site, greater similarity in soil cation exchange capacity and pH of the young Black Birch plots to the mature Hemlock plots rather than the mature deciduous forest plot also suggests a substantial lag time (Zukswert 2013). As with other studies investigating impacts of HWA on N cycling (Jenkins et al. 1999, Kizlinski et al. 2002, Orwig et al. 2013), our results show evidence of Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 80 Vol. 13, Special Issue 6 short-term alterations in N cycling linked to tree stress and mortality. Specifically, in 2012, soils from the Hemlock plot showed a substantial increase in net Nmin rate due to increased ammonification, which continued into 2013. During this time, we also observed a sharp decline in the health of a subset of Hemlock trees within the plot where soil N cycling was monitored. We also began to observe Fiorinia externa Ferris (Elongate Hemlock Scale [EHS]) and HWA on these trees and nearby trees in Fall–Winter 2012. Although HWA and EHS have now been observed on other trees at the field station, we have not yet seen such rapid declines in Hemlock health elsewhere at the site. The death of 3 Hemlock trees in this plot accounted for higher needlefall from July through November 2012 (42.0 ± 3.06 SE g) than in the other two Hemlock plots (11.6 ± 3.66 SE g), which, with reduced N uptake by Hemlock trees, likely explains the observed increase in net NH4 + production after 2011. The similarity of our observations to those by Orwig et al. (2013) for soils beneath Hemlock trees that were girdled or logged suggest that this increase in net Nmin rate will likely be a short-term rather than permanent change in soil chemistry. Continued monitoring will inform the longer-term outcome at the MacLeish Field Station. Forest-floor mesofauna We predicted that mesofauna (e.g., mites, collembolans, pseudoscorpions) would be more abundant in the young Black Birch-dominated plots because we expected the forest-floor communities to show greater productivity due to the lower C:N and higher nutrient content typical of Black Birch leaf litter (e.g., Cobb 2010), which forms the basis for mesofaunal food webs (Coleman et al. 2004). Instead, we documented significantly greater densities of microarthropods in forest-floor material from mature Hemlock plots for all three major taxa identified (Fig. 6). These differences might be attributable to a few factors, including the greater depth of the organic horizon in Hemlock plots providing more habitat or a greater diversity of habitats to sustain larger mesofaunal populations. In addition, forest-floor micro-environmental conditions might be more stable under Hemlock (e.g., soil temperature, moisture), allowing mesofaunal populations to grow to larger size. Although we did not detect differences in organic horizon moisture content between the two forest types, we still suspect that the deeper organic horizon of Hemlock stands might provide pockets of moist conditions that could sustain microarthropod survival and activity during dry periods. In the case of collembolans and oribatid mites, which were a distinctive and abundant subset of the mites observed, there is evidence that fungal mycelia are a preferred food source (Coleman et al. 2004), so there is some possibility that differences in the distribution or density of fungal hyphae in the soil organic horizon might influence abundance patterns for these microarthropods. Conclusions and future directions Overall, our findings suggest that ecological changes in forest structure and community composition in the decades following Hemlock decline may be more substantial and persistent than shifts in ecosystem processes, such as N cycling. Southeastern Naturalist 81 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 Similar trends are becoming apparent in a planned experimental removal of Hemlock in central Massachusetts (Orwig et al. 2013). More effort is needed to document and understand the biodiversity linked to Hemlock forests, particularly for diverse and inconspicuous groups that are not yet well documented (e.g., arthropods; Rohr et al. 2009, Sackett et al. 2011). Finally, we suspect that accidental experiments involving Hemlock removal, like the one described here, are not uncommon in the forests of the northeastern United States, where selective cuts and small-scale logging operations abound. While these unplanned settings often have limitations that would not be present in a planned experiment, they might be the only way to evaluate the long-term, multidecadal consequences of Hemlock removal given the spread of HWA in the region. We encourage others to seek out such sites and take advantage of them for research projects while the sites are still available. Acknowledgments We thank current and former Smith College students A. McGillis, C. Dwyer, E. Maley, J. Ludden, J. Warren, K. Dymek, M. Jackson, S. Blanchett, and S. Mansen for participating in field and laboratory research, and two anonymous reviewers for providing valuable comments on the draft manuscript. Smith College’s Center for Aqueous Biogeochemical Research (CABR) is funded by the NSF-MRI Grant CHE-0722678. We thank M. Anderson in the CABR for assistance with instrumental analysis and protocols. Funding for this project was provided by the B. Elizabeth Horner Fund, the Tomlinson Memorial Fund, the Schultz Fund, the Mellon Foundation, and the Smith College Center for the Environment, Ecological Design, and Sustainability (CEEDS). Writing of this manuscript was supported by NSF ADVANCE Grants NSF-0620101 and NSF-0620087. 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Ratcliffe, P. Robinson, R.S. Stanley, N.L. Hatch, A.F. Shride, E.G.A. Weed, and D.R. Wones. 1983. Bedrock geologic map of Massachusetts. US Geological Survey, scale 1:250,000, Reston, VA. Zukswert, J.M. 2013. Effects of Eastern Hemlock removal on nutrient cycling and forest ecosystem processes at the MacLeish Station, Whately, MA. B.A. Dissertation. Smith College, Northampton, MA. 130 pp. Southeastern Naturalist 85 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 Appendix A. Forest type and data collected in each of the 12 plots sampled in this study. Microclimate variables measured include air temperature, soil temperature, light intensity in terms of photosynthetically active radiation (PAR), and relative humidity. Micro- O horizon Leaf- arthropod Soil Micro- litter diversity/ N Plot Forest type climate Depth Moisture composition abundance cycling 1 Eastern Hemlock 2013 2013 2012 2 Black Birch 2010 2013 2013 2012-2013 2012 3 Eastern Hemlock 2010 2013 2013 2012–2013 2012 4 Black Birch 2010 2013 2013 2012–2013 2012 5 Eastern Hemlock 2013 2013 2012–2013 2012 6 Eastern Hemlock 2013 2013 2012 7 Black Birch 2013 2013 2012 8 Eastern Hemlock 2010 2013 2013 2012 9 Eastern Hemlock 2013 2013 2012 10 Black Birch 2013 2012 (Jul–Nov) 2011–2013 11 Eastern Hemlock 2013 2012 (Jul–Nov) 2011–2013 12 Mature deciduous 2013 2011–2013 Southeastern Naturalist J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 86 Vol. 13, Special Issue 6 Appendix B. Dates and duration of soil incubations to measure net N transformation rates from 2011 through 2013. Plots 10 (B = Black Birch), 11 (H = Eastern Hemlock), and 12 (D = mature deciduous) were used for these analyses. Days Net Nminerization rate Plot(s) Start date End date incubated (kg N ha-1 day-1) Black Birch and Eastern Hemlock 9 May 2011 20 June 2011 42 B = 0.045, H = 0.125 Black Birch and Eastern Hemlock 27 June 2011 19 July 2011 22 B = 0.195, H = 0.242 Black Birch and Eastern Hemlock 25 July 2011 19 October 2011 86 B = 0.047, H = 0.091 Black Birch and Eastern Hemlock 10 December 2011 28 March 2012 109 B = 0.007, H = 0.004 Black Birch 6 April 2012 9 May 2012 33 B = 0.026 Eastern Hemlock 7 April 2012 9 May 2012 32 H = 0.075 Black Birch and Eastern Hemlock 16 May 2012 13 June 2012 28 B = 0.243, H = 0.419 Mature Deciduous 21 May 2012 19 June 2012 29 D = 1.315 Black Birch and Eastern Hemlock 20 June 2012 16 July 2012 26 B = 0.175, H = 0.651 Mature Deciduous 26 June 2012 23 July 2012 27 D = 1.363 Black Birch and Eastern Hemlock 24 July 2012 8 September 2012 46 B = 0.174, H = 0.596 Mature Deciduous 30 July 2012 10 September 2012 42 D = 0.687 Black Birch and Eastern Hemlock 21 September 2012 19 October 2012 28 B = 0.100, H = 0.217 Mature Deciduous 27 September 2012 22 October 2012 25 D = 0.217 Black Birch and Eastern Hemlock 23 April 2013 28 May 2013 35 B = 0.081, H = 0.099 Mature Deciduous 30 April 2013 28 May 2013 28 D= 0.299 Mature Deciduous 3 June 2013 24 June 2013 21 D = 0.806 Black Birch and Eastern Hemlock 4 June 2013 24 June 2013 20 B = 0.148, H = 0.253 Black Birch, Eastern Hemlock, and Mature Deciduous 26 June 2013 17 July 2013 21 B = 0.210, H = 0.382, D = 0.606 Southeastern Naturalist 87 J.M. Zukswert, J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S. Taylor 2014 Vol. 13, Special Issue 6 Appendix C. Equations obtained from Robertson et al. (1999) were used to calculate the corrected soil N concentrations, net N mineralization rates, an d net nitrification rates. Equation Variables and units Corrected soil N = (C × V × W -1) × (B × D × 10 -1) ● Corrected soil N (kg N ha -1) ● C = concentration of either NO3 - or NH4 + (mg L-1) ● V = extract volume (SrCl2) + volume of H2O in soil sample (mL) ● W = dry weight of soil determined by gravimetric analysis (g) ● B = bulk density (g cm-3) ● D = depth of soil core (cm) Nmineralized = [(NO3 - final + NH4 + final) – (NO3 - initial + NH4 + initial)] × t -1 ● Nmineralized = net Nmin rate (kg N ha-1 day-1) ● NO3 - final and NH4 + final = corrected concentrations of NO3 - and NH4 + in incubated cores (kg N ha-1) ● NO3 - initial and NH4 + initial = corrected concentrations of NO3 - and NH4 + in the initial cores (kg N ha-1) ● t = length of the incubation (days) Nnitrified = (NO3 - final – NO3 - initial) × t -1 ● Nnitrified = net nitrification rate (kg N ha-1 day-1) ● NO3 - final = corrected concentration of NO3 - in the incubated core (kg N ha-1) ● NO3 - initial = concentration of NO3 - in the initial core (kg N ha-1) ● t = length of incubation (days)