Northeastern Naturalist 420 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 22001166 NORTHEASTERN NATURALIST 2V3(o3l). :2432,0 N–4o3. 33 A Fine-scale Examination of Larix laricina and Picea mariana Abundances along Abiotic Gradients in an Adirondack Peatland Celia Evans1,*, Robert DeSotle1, Chloe Mattilio1, Erik Yankowsky1, Andre-Anne Chenaille1, and Alexander Whiston1 Abstract - Wetland research has described changes in plant communities along environmental gradients, however, little is known about the relationship between fine-scale hydrologic and abiotic factors and the relative abundances of individual, co-occurring species. Larix laricina (Eastern Larch) and Picea mariana (Black Spruce) are the 2 dominant tree species in open boreal peatlands in the northeastern US. In order to describe abiotic gradients that correlate with species abundances at local spatial scales, we collected data on Eastern Larch and Black Spruce stem abundances, groundwater pH, conductivity, depth to water table, water temperature, dissolved oxygen, and canopy closure from 42 plots along 6 transects in an Adirondack wetland. We correlated stem abundances with each of the abiotic variables and then used regression to explain variation in stem abundances of the 2 species along those abiotic gradients. Percent canopy closure explained 56% of the variability in Eastern Larch stem abundance, and depth to groundwater was also positively correlated with number of Eastern Larch stems. These 2 abiotic conditions covaried; thus, the best model to explain variability in Eastern Larch stem abundance included only canopy closure. Black Spruce stem abundance was significantly lower in plots with higher water temperatures (R2 = 0.31). In a multiple-regression model, depth to the water table explained an additional 6% of the variance and substantially reduced Mallows’ Cp. Eastern Larch and Black Spruce appear to establish along different abiotic gradients at the scale of tens of meters within this study wetland. Although light levels, as mediated by canopy closure, would be predicted to influence the establishment of Eastern Larch based on its silvics, the strong negative relationship between Black Spruce stem abundance and water temperature has not been previously reported. Sampling other peatlands will allow us to determine the universality of these patterns and to better understand which environmental gradients operate at local spatial scales to structure patterns of tree distribution within peatlands. Introduction Peatlands are some of the most extreme plant environments in the northeastern US (Girardin et al. 2001). Belowground conditions, such as high water-tables that result in anoxia, low pH, and lack of available nutrients, create abiotic environments that require plants inhabiting them to have a suite of unique physiological traits (Islam and McDonald 2005, Zhang et al. 2013). Plant community structure in peatlands strongly depends on a variety of abiotic gradients that may or may not include nutrient levels (Girardin et al. 2001, Jean and Bouchard 1993, Jeglum and He 1995, 1Paul Smith’s College, School of Natural Resource Management and Ecology, 7777 State Routes 86 and 30, Paul Smiths, NY 12970. *Corresponding author - cevans@paulsmiths.edu. Manuscript Editor: Glen Motzkin Northeastern Naturalist Vol. 23, No.3 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 421 Whitehouse and Bayley 2005, Zhang et al. 2013). Although Jean and Bouchard (1993) found no correlation between wetland community structure and nutrient variables, Girardin et al. (2001) reported that the plant community they studied was structured along complex axes of pH, nitrate and cation concentration, and depth of Sphagnum sp. (sphagnum mosses). Jeglum and He (1995) also found that wetland community structure was determined by environmental factors including nitrogen content and levels of base cations. Differences in the outcomes of these studies are likely due to the steepness of abiotic gradients across their study areas, which may correspond with the spatial scale at which the research was conducted. The dominant tree species in many oligotrophic sphagnum bogs are Larix laricina (Du Roi) K. Koch (Eastern Larch) and Picea mariana (Mill.) (Black Spruce). Both of these coniferous species occur in 67%–99% wetlands; they are facultative wetland-indicators (http://www.plants.usda.gov). Eastern Larch and Black Spruce produce adventitious roots and reproduce some stems by layering (Begin and Filion 1999), an adaptation that creates an advantage for plants growing in systems with a high water-table or in riparian zones (Veverica et al. 2012). However, they share few other life-history characteristics. Eastern Larch is deciduous and shade intolerant, with consistently high leafnitrogen concentrations (Gower and Richards 1990), which allows relatively higher rates of photosynthesis per needle area than is observed in most evergreen trees (Girardin et al. 2001, Islam and Macdonald 2005, Matyssek 1986, Sullivan et al. 2006, Zhang et al. 2013). The characteristic of deciduousness is, in itself, unusual in boreal wetlands, because evergreen leaves are considered advantageous in nutrientpoor, harsh environments where needles can take several seasons to decompose and release nutrients back into the system (Gower and Richards 1990). Experimental studies have shown that Eastern Larch growth increases in response to nutrient additions and that subsequent reduction in nutrient availability can significantly inhibit growth (Islam and MacDonald 2005). Eastern Larch also translocates an unusually high proportion of leaf nitrogen back into the tree before senescence each autumn (Gower and Richards 1990, Killingbeck 1996, Matyssek 1986). Black Spruce, a shade-tolerant evergreen, invests resources in production of a waxy cuticle and long-lived leaves, which allows for superior water and nutrient retention (Islam and Macdonald 2005). Black Spruce growth in wetlands has also been shown to respond positively, albeit modestly, to nitrogen additions, though this species is able to maintain growth rates in nutrient-poor conditions, which allows trees to weather seasonal nutrient-declines (Islam and Macdonald 2005). Existing studies suggest that these very different physiological strategies result in similar above-ground productivity for Eastern Larch and evergreen conifers in harsh growing environments (Gower and Richards 1990, Matyssek 1986). We hypothesized that unique sets of abiotic factors would be correlated with the relative abundances of Eastern Larch and Black Spruce within a peatland, due to the differences in their silvics and the possibility that niche separation may occur for species coexisting in an environment with scarce resources. We examined univariate correlations and multiple-regression models for Eastern Larch and Black Northeastern Naturalist 422 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 Vol. 23, No. 3 Spruce abundances across a fine-scale gradient of abiotic factors in an Adirondack peatland. In particular, we were interested in identifying the potential influence of groundwater pH, water temperature, depth to ground water, dissolved oxygen, canopy cover, and organic and inorganic nitrogen concentrations on the relative abundances of these 2 co-occurring species at the scale of tens of meters. Outcomes of this research add to the body of information about environmental gradients that structure wetland communities, and shed light on how the relative importance of particular gradients may change depending on spatial scale. Field-site Description We conducted our study in a wetland complex located in the Paul Smith’s College Visitor’s Interpretive Center (VIC) in the town of Paul Smiths, Franklin County, in the Adirondack Park of New York (44.4372°N, 74.2528°W). The wetland, known as Heron Marsh, is 1.2 ha in size. The peatland includes a coniferous border consisting of Abies balsamea (L.) Mill. (Balsam Fir), Black Spruce, Thuja occidentalis L. (Northern White Cedar), and Eastern Larch and grades into an open sphagnum-bog, with typical wetland graminoids (Carex spp. and Scirpus spp. [sedges], Juncus [rushes], and Glyceria spp. [manna grasses]), and shrubs including Chamaedaphne calyculata (L.) Moench (Leatherleaf), Rhododendron groenlandicum (Oeder) Kron and Judd (Labrador Tea ), and Ilex mucronata (L.) Powell (Mountain Holly). Eastern Larch and Black Spruce are the dominant trees. The center of the peatland is a narrow, open marsh with flowing water and Typha latifolia L. (Broadleaf Cattail). A boardwalk crosses Heron Marsh (Fig. 1). Figure 1. Photograph of Heron Marsh (October 2015) taken looking into the peatland from the point at which the coniferous swamp transitions into a more-open peatland. Transects 1–3 were located approximately parallel to and on the left of the boardwalk, and plots 4–6 parallel and on the right. Northeastern Naturalist Vol. 23, No.3 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 423 Methods Study design During 2014 and 2015, we established 6 parallel, 105-m transects 30 m apart and perpendicular to the wetland edge; 3 transects were established in 2014 and 3 in 2015. We also set up seven 5-m–radius (78 m2 ) plots at 15-m intervals along each transect (n = 42 plots). We placed the 3rd plot of each transect on the outer edge of the border between the coniferous wetland and the open bog, then set the remaining 6 plots on that transect relative to it: 2 plots in the coniferous wetland edge and 4 more in the open bog. This design was intended to ensure that our samples captured any differences in the abiotic variables of interest along the upland– wetland gradient. Field-data collection Stem-abundance data. We conducted stem counts for all plots along transects 1–3 from September through November 2014 and along transects 4–6 from September through October 2015. We counted only clearly established seedlings; thus, we were not concerned about the single growing season that separated our 2 sampling periods. We recorded the number of Black Spruce and Eastern Larch seedlings, saplings, and trees in each plot. Seedlings were classified as <1.35 m tall, saplings were ≥1.35 m tall with a diameter of <5 cm at 1.35 m, and we considered a stem to be a tree if it was >5 cm diameter at 1.35 m. We observed layering in both species; stems that emerged separately from the sphagnum were counted as separate individuals because, as they develop roots, the layers are no longer dependent on the parent plant (Begin and Filion 1999). Abiotic conditions. We installed a 0.5-m piezometer in each plot in transects 1–5 by removing a peat/soil core with an auger and inserting a 6.35-cm–diameter PVC pipe. In transect 6, we removed the peat cores and collected data from the holes without inserting a PVC pipe. We placed each piezometer on the hummock closest to the plot center and buried it flush with the sphagnum surface in each plot. The following abiotic data were collected in each plot along the gradient from coniferous wetland edge to open bog mat: water temperature, dissolved oxygen (DO% saturation), water pH and conductivity, and depth from the sphagnum surface to the water table. In July 2015, we used a spherical densiometer to estimate canopy closure at the height of the growing season. We averaged measurements made at a height of 1.35 m in all 4 cardinal directions from the plot center. Data from spherical densiometers are best interpreted as relative estimates of canopy closure. We measured pH, conductivity, temperature, dissolved oxygen, and depth to groundwater in September 2015. Temperature and dissolved oxygen were measured in the field using a dissolved oxygen meter (YSI 550A, Xylem, Inc., Yellow Springs, OH). We measured the depth to groundwater (cm) by inserting a rod to reach the water and measuring the distance from the top of the sphagnum to groundwater. We collected water samples in clean Nalgene vials and measured pH and specific conductivity at 25 °C in the laboratory on the day of collection. We measured nitrogen concentrations in a subset of plots in 2014 (n = 21) but found negligible and highly variable Northeastern Naturalist 424 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 Vol. 23, No. 3 concentrations of ammonium and nitrate in the water below the sphagnum. Therefore, we removed this variable from our sampling in 2015. Within- versus among-plot variability 2015. In order to evaluate the ecological significance of relationships among plots, it was critical to determine that withinplot variability in abiotic conditions was not significant. In early October 2015, we resampled a subset of 12 plots from transects 1–5 and across plots 1–7. We collected 4 sub-samples in each of the 12 plots. Three new cores were made 2.5 m from the center of the plot, separated by 120°; data were collected and analyzed as reported above. We used general linear models and ran 2-factor ANOVAs (plot, subsample) for conductivity, pH, dissolved oxygen, depth to groundwater, and water temperature. Dissolved oxygen was significantly different within and among plots (P = 0.002 and P = 0.04, respectively). Conductivity and pH were not significantly different at either spatial scale. Depth to groundwater and water temperature were not different within plots (P = 0.58, and P = 0.38, respectively), but were significantly different among plots (P = 0.005, and P < 0.001, respectively). Thus, we determined that for these variables, the single measurement at plot center was representative of the plot. We did not statistically analyze percent canopy-cover variability in each plot. Spherical densiometer measures integrate a large area of the overhead canopy and we averaged measures in the 4 cardinal directions at the plot level. Subsampling would have resulted in overlap. Plot-level data analysis We used Pearson correlation to examine relationships between total stems/plot of each species and each of the single abiotic variables we measured. Of the abiotic variables we measured, only percent canopy closure and depth to the water table covaried; thus, we used a regression approach to examine models that might best explain variability in stem abundances along abiotic gradients. Regression statistics can be used appropriately to examine the strength of descriptive relationships (Shmueli 2010); however, we do not mean to imply causation from the results of this field study. We did not differentiate between life stages (seedling, sapling, mature tree) in this analysis because the great majority of Eastern Larch and Black Spruce stems were seedlings (Eastern Larch = 1693 of 1815 stems, Black Spruce = 2697 of 3087 stems). Data were analyzed in MiniTab (version 17; www.minitab.com). Table 1. Descriptive statistics for measured hydrological and water-chemistry variables in Heron Marsh peatland, in Paul Smiths, NY. Data were collected in September 2015. Variable n Mean SD Median Min Max pH 42 5.4 0.6 5.4 4.3 6.3 Conductivity 42 114.7 93.0 83.3 20.2 436.0 DO (% saturation) 42 8.7 10.4 5.1 0.8 63.4 H20 temperature. (°C) 42 15.1 1.7 15.0 12.0 19.4 Depth to groundwater (cm) 42 42.3 8.5 42.0 20.0 58.5 Proportion canopy closure 42 0.28 0.30 0.16 0.0 0.79 Northeastern Naturalist Vol. 23, No.3 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 425 Results Overview of belowground abiotic habitat characteristics Table 1 summarizes measured hydrological and chemical variables from Heron Marsh. Groundwater pH in the plots was acidic (range = 4.3–6.3, median = 5.4). Conductivity (range = 20–436 uS/cm, median = 83.3 uS/cm) and dissolved oxygen (range = 0.8–63.4% saturation, median = 5.1% saturation) were highly variable. Depth to the groundwater ranged from 20 cm to 58.5 cm, water temperatures within the study area ranged from 12 °C to 19.4 °C, and canopy closure ranged from 0% to 79%. Summary of tree community structure results Stem abundances of the 2 focal species were not related (r = 0.001, P = 0.995). The number of stems per plot ranged from 0 to 145 (mean = 4, median = 40) for Eastern Larch and 0 to 220 (mean = 74, median = 67) for Black Spruce. Across all plots, most stems were seedlings (93% for Eastern Larch and 87% for Black Spruce). Figure 1 provides a visual impression of the structure, with a scattered overstory of Black Spruce and Eastern Larch becoming less dense further out into the peatland. Relationships between stem abundance and abiotic variables Biological responses to abiotic resources and conditions are rarely linear across a very wide gradient. However, in this analysis, we found that within the range of our belowground abiotic conditions, linear relationships were appropriate and explained a surprising amount of the variability in stem abundances. The exception to this was the relationship between Eastern Larch stems and percent canopy closure. A 2nd-order polynomial increased R2 by 5%, and the relationship was clearly nonlinear. Because our study was intended to describe relationships, we chose to use the polynomial statistic to describe this univariate relationship. Table 2 displays Pearson correlation coefficients between Eastern Larch and Black Spruce stem abundances and each of the measured abiotic variables. Eastern Larch was strongly negatively correlated with percent canopy closure (r = -0.75, P < 0.001), and significantly positively correlated with the depth to groundwater (r = 0.35, P = 0.02). Black Spruce stem abundance was strongly negatively correlated with water temperature (r = -0.56, P < 0.001). The next-strongest correlation was depth to groundwater; however, this univariate relationship was not statistically significant (r = -0.23, P = 0.15). Table 2. Pearson correlation coefficients for total stem abundance and individual abiotic variables in the Heron Marsh study site (n = 42 plots). Pearson correlation coefficients are shown with P values in parentheses. * indicates P < 0.100. Independent variable Eastern Larch Black Spruce pH -0.09 (0.58) 0.05 (0.77) Conductivity (uS/cm) -0.12 (0.45) 0.05 (0.77) DO (% saturation) 0.08 (0.61) -0.01 (0.93) H2O temperature (°C) 0.01 (0.95) -0.56 (less than 0.001)* Depth to groundwater 0.35 (0.02)* -0.23 (0.15) Proportion canopy cover -0.75 ( less than 0.001)* -0.01 (0.97) Northeastern Naturalist 426 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 Vol. 23, No. 3 To determine whether we could use a regression approach to examine possible multivariate models to describe variance in the stem abundances, we developed a correlation matrix of abiotic variables (Table 3). Only percent canopy closure and depth to groundwater were significantly negatively correlated and thus could not be included together in multiple-regression models. We chose to examine all possible models (Best subsets, Minitab 17) to determine if variability was best explained by a single abiotic gradient, or if adding additional variables significantly improved a model’s predictive power. In the case of Eastern Larch, percent canopy closure described 55% of the variability in stem abundance (linear model). A 2nd-order polynomial fit to the relationship between stems and canopy closure increased r2 to 0.61 (Fig. 2). When depth to groundwater was regressed with Eastern Larch abundance, it described 12% of the variability (Fig. 3). However, canopy closure and depth to groundwater were significantly correlated and therefore could not be included in a multiple-regression model. Thus, no additional variables improved the explanatory strength of the model. For Black Spruce stems, water temperature alone described 31% of the variability in abundance (Fig. 4). In an examination of all possible models to explain variance in Black Spruce stem abundance, adding depth to groundwater increased the descriptive power of the model by approximately 6%, decreased model variance, and resulted in a substantially lower Mallows’ Cp value (data not shown). A small Mallows’ Cp value indicates precision in the estimation of regression coefficients. Figure 5 shows residuals of the first relationship plotted against depth to groundwater, as an approximation of the contribution of this second variable to the relationship. Of all model choices, Mallows’ Cp was lowest for the model that included both water temperature and depth to groundwater (data not shown), and so we concluded this was the best model. Discussion The goal of this study was to begin to describe the distribution of Eastern Larch and Black Spruce along abiotic gradients in an oligotrophic peatland. Microtopography in peatlands can lead to high spatial-variability in soil moisture, nutrients, and chemistry, resulting in strong gradients even at fine scales (Islam and MacDonald 2005, Macrae et al. 2013). Several research projects have examined community structure and species diversity in wetland systems in northeastern North Table 3. Pearson Correlation coefficients among independent hydrological and chemical variables in Heron Marsh peatland, Paul Smiths, NY. (n = 42). * indicates P ≤ 0.05. DO Groundwater Groundwater Conductance Variables (% saturation) temp. (°C) depth (cm) pH (uS/cm) H20 temperature (°C) -0.088 Groundwater depth (cm) -0.072 -0.049 pH 0.030 -0.174 -0.053 Conductivity (uS/cm) -0.164 -0.183 0.000 0.267 Percent canopy closure -0.056 0.067 -0.424* 0.108 -0.029 Northeastern Naturalist Vol. 23, No.3 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 427 Figure 2. Eastern Larch (Larix laricina) stem abundance versus proportion canopy closure in Heron Marsh peatland in Paul Smiths, NY (n = 42). Data were collected in September 2015 from seven 5-m–radius plots along 6 transects extending into the peatland from the edge of a coniferous swamp. Fit is a 2nd-order polynomial. Figure 3. Eastern Larch (Larix laricina) stem abundance versus depth to groundwater in Heron Marsh peatland in Paul Smiths, NY (n = 42). Data were collected in September 2015 from seven 5-m-radius plots along 6 transects extending into the peatland from the edge of a coniferous swamp. Northeastern Naturalist 428 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 Vol. 23, No. 3 Figure 4. Black Spruce (Picea mariana) stem abundance versus temperature of groundwater below the surface of the sphagnum in Heron Marsh peatland in Paul Smiths, NY (n = 42). Data were collected in September 2015 from seven 5-m–radius plots along 3 transects extending into the peatland from the edge of a coniferous swamp. Figure 5. Residual variance from the relationship between Black Spruce stem abundance and water temperature, plotted against the depth to groundwater (2nd variable in a multipleregression model). Data were collected in September 2015 from seven 5-m–radius plots along 3 transects extending into the peatland from the edge of a coniferous swamp. Northeastern Naturalist Vol. 23, No.3 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 429 America that include the focal species in our study. All of the authors of these studies (Davis and Anderson 2001, Girardin et al. 2001, Glaser et al. 1990, Jeglum and He 1995, Walker and Johnstone 2014) stated the importance of environmental factors in structuring wetland communities, but none attempted to delineate possible drivers of individual species abundances at this fine spatial scale. We hypothesized that under the stressful growing conditions at our site, Eastern Larch and Black Spruce—2 species with different life histories and physiologies—would occupy different niches and thus be associated with different aspects of hydrology and chemistry. We found strong and different relationships between abiotic conditions and the abundance of Eastern Larch and Black Spruce stems. The finding that abiotic gradients influence spatial patterns in wetland communities is generally consistent with Jeglum and He (1995) who reported that 81% of variation in wetland-plant distribution was explained by environmental variables. Not surprisingly, light level, as mediated by canopy closure, was the strongest single correlate with Eastern Larch stem abundance in our study, followed by a less strong yet significant positive relationship with depth to the ground water. Walters and Reich (2000) studied the influence of seed mass, light, and nitrogen availability and the interactions among these variables on physiology of a suite of evergreen and deciduous tree species. Eastern Larch was the only one of the 10 species studied for which the relative growth-rate (RGR) ranking changed from near the lowest to highest when grown in very low versus high levels of light. Hydrologic variability and/or connections among microforms (i.e., hummocks) have been shown to influence spatial patterning of wetland vegetation (Glaser et al. 1990, Macrae et al. 2013). We observed, but did not quantify, that Eastern Larch and, to a lesser extent, Black Spruce were typically located on hummocks in our plots. Distance to the water table was significantly positively correlated with Eastern Larch stem abundance and, though not significant as a single variable, was also included in the model that best predicted Black Spruce abundance. These relationships were in opposite directions: positive for Eastern Larch and negative for Black Spruce. Lieffers and Rothwell (1987) reported that fine-root biomass and growth rates of both Eastern Larch and Black Spruce were positively correlated with greater depth to groundwater. This result for Black Spruce seems inconsistent with the negative relationship we report; however, our metric of stem abundance only relates to establishment of individuals, not growth. Additionally, the spatial scale of that study was much larger than the one we employed, and the authors reported differences across 4 forested fens with different water-table levels, rather than within-site variability in depth to water table, as in the present study. In laboratory studies, Eastern Larch seed germination was reduced by flooding, and submergence of roots for over a week was detrimental to young seedlings (Duncan 1954). Our results corroborate this pattern in that Eastern Larch abundance was greater with increased depth to the water table. However, Duncan (1954) found that when substrate moisture was lower than 10–33% by volume, drought caused high mortality of young Eastern Larch seedlings. Other studies have shown that Northeastern Naturalist 430 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 Vol. 23, No. 3 water-table depth and fluctuation in depth strongly influence the structure of wetland plant communities (Laitinen and Rehell 2008). The strong negative relationship between water temperature and Black Spruce stem abundance that we observed has not previously been reported in the literature. However, the negative relationship between Black Spruce growth and warmer air temperatures has been shown across large spatial scales. The prevalent hypothesis for growth reductions seen in some boreal forest tree species, including Black Spruce, is that warmer temperatures decrease moisture availability and cause drought stress (Walker and Johnstone 2014). Juday and Alix (2012) found a strong negative relationship between air temperature and growth of Picea glauca (Moench) Voss (White Spruce) in interior Alaska. Walker and Johnstone (2014) also reported a negative association between air temperature and Black Spruce radial growth across a large area in Alaska and the Yukon. In that study, contrary to the expectation that the negative effect of warmer air temperatures would be explained by the resulting drought stress at higher, drier landscape positions, they found no such relationship. There was no pattern across landscape position in the Alaska data, and there was a trend for the greatest reductions in radial growth to be associated with warmer air temperatures at generally cooler, wetter landscape locations in the Yukon, suggesting the possibility of a different cause of reduced growth. Although we did not measure air temperature, patterns observed in the present study suggest the possibility that negative growth responses of Black Spruce to warmer air temperatures observed by Walker and Johnstone (2014) may have been more directly a function of higher water temperatures. Increased water temperatures can cause elevated rates of root respiration (Burton and Pregitzer 2003). For Black Spruce, a slow-growing, shade-tolerant species, increased root respiration could divert valuable carbon away from growth or other physiological functions. In future research at this site, we will examine height and radial growth of trees to determine if a reduction in radial growth, shown by Walker and Johnstone (2014) at the landscape scale, correlates with water-temperature gradients at the local scale. We also note that future research should examine whether water temperature and groundwater-depth gradients covary with any other hydrologic factors not measured in this study, such as cation concentrations, that might be associated with groundwater upwellings, and thus, influence fine-scale correlations with differences in water temperature. Also, because Black Spruce reproduces by layering, particularly on nutrient-poor sites (Stanek 1975), future studies should explore whether these gradients influence layering, which would affect local stem abundance. Studies have linked high rates of warming in northern high-latitude ecosystems (IPCC 2013) to changes in the hydrology of boreal wetland communities and to the possible reduction in tree regeneration and growth (Girardin et al. 2001, Juday and Alix 2012, Walker and Johnstone 2014). With projected rising temperatures, some boreal species, particularly plants, will likely have to tolerate temperatures above their optimal ranges (Jenkins 2010). These temperature changes should have the most immediate effect on boreal species at the southern extent of their distributions. Northeastern Naturalist Vol. 23, No.3 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 431 Additionally, bogs are particularly vulnerable to climate change because their very existence is dependent on a balance between precipitation and evapotranspiration (Moore et al. 1997). Predicting the effects of environmental change on wetland communities begins with an understanding of how individual species will likely respond with respect to establishment and growth across gradients in hydrology, temperature, and water quality (Burkett and Kusler 2000). Conclusions As predicted, abundances of Eastern Larch and Black Spruce in Heron Marsh wetland were correlated with different components of the belowground habitat. Eastern Larch was most abundant in areas of high-light (lower canopy-closure), and abundance covaried with locations where depth to the water table was greater. Black Spruce was most abundant where water temperatures were lowest and the groundwater was closer to the surface of the sphagnum. The questions of if and when nutrient availability plays a role in structuring peatland plant communities remains a fruitful area for research, but for the low-nutrient conditions and small spatial scale of our study, light and hydrologic gradients appear to be much more important. Understanding the responses of individual species to environmental gradients is the first step to predicting changes in plant community structure in a changing climate (Locky et al. 2005). Eastern Larch and Black Spruce are in the southern portions of their geographic distributions in Adirondack wetlands. Individuals at the southern edges of their species’ geographic distributions, and thus, often at the edge of their zones of tolerance for some suite of abiotic conditions (i.e., temperature), are likely to respond to changes in abiotic variables more readily than those nearer the center of their species’ range. The hydrology and chemistry of boreal peatlands are changing in response to increased temperatures, altered precipitation regimes, and variability in these factors due to climate change (Bridgham et al. 1994, Walker and Johnstone 2014). Increasing our understanding of species responses to fine-scale variation in abiotic gradients in other peatlands will allow us to determine if these patterns are universal, and will add to the empirical basis for models that seek to understand how environmental changes affect peatland communities. Acknowledgments We thank Paul Smith’s College and the National Science Foundation S-STEM grant, which supports the education and research of several students each year, including the coauthors of this paper. We are also grateful to Paul Smith’s College for a Faculty Research Grant to C. Evans. We acknowledge the Adirondack Watershed Institute staff at Paul Smith’s College for the use of equipment and laboratory facilities. We thank Dan Kelting and Corey Laxson, the editorial staff at the Northeastern Naturalist, and 3 outside reviewers who provided valuable feedback, allowing us to improve the manuscript. Literature Cited Begin, C., and L. Filion. 1999. Black Spruce (Picea mariana) architecture. Canadian Journal of Botany 77:664–672. Northeastern Naturalist 432 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 Vol. 23, No. 3 Bridgham, S., J. Pastor, J. Janssens, C. Chapin, and T. Malterer. 1996. Multiple limiting gradients in peatlands: A call for a new paradigm. Wetlands 16(1):45–65. Burkett, V., and J. Kusler. 2000. Climate change: Potential impacts and interactions in wetlands of the United States. Journal of the American Water Resources Association 36:313–320. Burton, A.J., and K. Pregitzer. 2003. Field measurements of root respiration indicate little to no seasonal temperature acclimation for Sugar Maple and Red Pine. Tree Physiology 23:273–280. Davis, R.B., and D.S. Anderson. 2001. Classification and distribution of freshwater peatlands in Maine. Northeastern Naturalist 8:1–50. Duncan, D.P. 1954. A study of some of the factors affecting the natural regeneration of Tamarack (Larix laricina) in Minnesota. Ecology 35: 498–521. Girardin, M.-P., J. Tardif, and Y. Bergeron. 2001. Gradient analysis of Larix laricinadominated wetands in Canada’s southeastern boreal forest. Canadian Journal of Botany 79:444–456. Glaser, P.H., J.A. Janssens, and D.I. Siegel. 1990. The response of vegetation to chemical and hydrological gradients in the Lost River Peatland, Northern Minnesota. Journal of Ecology 78:1021–1048. Gower, S.T., and J.H. Richards. 1990. Larches: Deciduous conifers in an evergreen world. Bioscience 40:818–826. Intergovernmental Panel on Climate Change (IPCC). 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the 5th assessment report of the Intergovernmental Panel on Climate Change. T.F. Stocker, D. Qin, G.K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P.M. Midgley (Eds.). Cambridge University Press, Cambridge, UK. 1535 pp Islam, M.A., and S.E. Macdonald. 2005. Effects of variable nitrogen fertilization on growth, gas exchange, and biomass partitioning in Black Spruce and Tamarack seedlings. Canadian Journal of Botany 83:1574–1580. Jean, M., and A. Bouchard. 1996. Tree-ring analysis of wetlands of the upper St. Lawrence River, Québec: Response to hydrology and climate. Canadian Journal of Forest Research 26:482–491. Jeglum, J.K., and F. He. 1995. Pattern and vegetation: Environment relationships in a boreal forested wetland in northeastern Ontario. Canadian Journal of Botany 73:629–637. Jenkins, J. 2010. Climate Change in the Adirondacks: The Path to Sustainability. Cornell University Press, Ithaca, NY. 200 pp. Juday, G.P., and C. Alix. 2012. Consistent negative temperature-sensitivity and positive influence of precipitation on growth of floodplain Picea glauca in Interior Alaska. Canadian Journal of Forestry Research 42:561–573. Killingbeck, K.T. 1996. Nutrients in senesced leaves: Keys to the search for potential resorption and resorption proficiency. Ecology 77:1716–1727. Laitinen, J., and S. Rehell. 2008. Community and species responses to water-level fluctuation with reference to soil layers in different habitats of mid-boreal mire complexes. Plant Ecology 194:17–36. Lieffers, V.J., and R.L. Rothwell. 1987. Rooting of peatland Black Spruce and Tamarack in relation to depth of water table. Canadian Journal of Botany 65:817–821. Locky, D.A., S.E. Bayley, and D.H. Vitt. 2005. The vegetational ecology of Black Spruce swamps, fens, and bogs in boreal Manitoba, Canada. Wetlands 25:564–582. Northeastern Naturalist Vol. 23, No.3 C. Evans, R. DeSotle, and C. Mattilio, E. Yankowsky, A.-A. Chenaille, and A. Whiston 2016 433 Macrae, M.L., K.J. Devito, M. Strack, and J.M. Waddington. 2013. Effect of watertable drawdown on peatland nutrient-dynamics: Implications for climate change. Biogoechemistry 112:661–676. Matyssek, R. 1986. Carbon, water, and nitrogen relations in evergreen and deciduous conifers. Tree Physiology 2:177–187. Moore, M.V., M.L. Pace, J.R. Mather, P.S. Murdoch, R.W. Howarth, C.L. Folt, C.Y. Chen, H.F. Hemond, P.A. Flebbe, and C.T. Driscoll. 1997. Potential effects of climate change on freshwater ecosystems of the New England/Mid-Atlantic Region. Hydrological Processes 11:925–947. Shmueli, G. 2010. To explain or to predict? Statistical Science 25:289–310. Stanek, W. 1975. The role of layerings in Black Spruce forests on peatlands in the Clay Belt of northern Ontario. Pp. 242–249, In Proceedings of the Black Spruce Symposium. Canadian Forestry Service, Great Lakes Forest Research Centre, Sault Ste. Marie, ON, Canada. Sullivan, T.J., I.J. Fernandez, A.T. Herlihy, C.T. Driscoll, T.C. McDonnell, and N.A. Nowicki. 2006. Acid–base characteristics of soils in the Adirondack mountains, New York. Soil Science Society of America 70:141–152. Veverica, T.J., E.S. Kane, and E.S. Kasischke. 2012. Tamarack and Black Spruce adventitious- root patterns are similar in their ability to estimate organic-layer depths in northern temperate forests. Canadian Journal of Soil Science 92:799–802. Walker, X., and J.F. Johnstone. 2014. Widespread negative correlations between Black Spruce growth and temperature across topographic moisture gradients in the boreal forest. Environmental Research Letters 9:1–9. Walters, M.B., and P.B. Reich. 2000. Seed size, nitrogen supply, and growth rate affect tree-seedling survival in deep shade. Ecology 81:1887–1901. Whitehouse, H.E., and S.E. Bayley. 2005. Vegetation patterns and biodiversity of peatland plant communities surrounding mid-boreal wetland ponds in Alberta, Canada. Canadian Journal of Botany 83:621–637. Zhang, W., M. Calvo-Polanco, Z.C. Chen, and J.J. Zwiazek. 2013. Growth and physiological responses of Trembling Aspen (Populous tremuloides), White Spruce (Picea glauca), and Tamarack (Larix laricina) seedlings to root-zone pH. Plant Soil 373:775–786.