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Characterization and classification of vernal pool vegetation, soil, and amphibians of pictured Rocks National Lakeshore.


Woodland vernal pools are geographically isolated, seasonally flooded bodies of water that are surrounded by forested uplands (Tiner, 2003). Though these pools are relatively small in terms of the overall geographic area they encompass, their distribution, species diversity, and fluctuating hydroperiod make them disproportionally important on the landscape (Hunter, 2008). Their temporary nature, filled with water during spring or fall and dry during summer or droughts, ensures fish are absent from these pools, making them ideal and required breeding habitat for a variety of woodland amphibian and invertebrate species (Colburn, 2004; Calhoun and deMaynadier, 2008). The geographic isolation of vernal pools caters to species with life history strategies that incorporate aquatic and terrestrial components such as mole salamanders (Ambystomatids) and wood frogs (Lithobates sylvaticus) (Williams, 1996; Zedler, 2003; Batzer et al., 2004; Colburn, 2004). Isolation also makes vernal pools important sources of food and water for upland terrestrial species including bats, reptiles, small mammals, and birds (Paton, 2005; Francl, 2008).

Though geographic isolation is a characteristic that makes vernal pools valuable in the landscape, it also inhibits the protection of vernal pools. Because of their classification as isolated wetlands, vernal pools do not have federal protection from the Clean Water Act (Downing et al., 2003), leaving few laws to protect these temporary wetlands (Preisser et al., 2000). Vernal pools are threatened directly by human development (Mahoney and Klemens, 2008) and indirectly by invasive species, climate change (Brooks, 2009), habitat fragmentation, groundwater extraction, and water contamination (Carrino-Kyker and Swanson, 2007).

A critical and poorly understood component of vernal pool ecology and conservation is the effect of hydroperiod on vernal pool ecosystems (Brooks, 2004; Brooks and Hayashi, 2002). Amphibians are particularly sensitive to hydroperiod as they require standing water to remain long enough for eggs to develop and larvae to metamorphose but need dry conditions during some portion of the season to prevent colonization of pools by predators (Boone et al., 2006; Brooks, 2004). Depth or pool morphology are often used as predictors of hydroperiod, however the relationship between pool size, depth, area and/or volume, and hydroperiod is weak, inconsistent, and difficult to model because of the volume of data required (Brooks and Hayashi, 2002; Calhoun et al., 2003; Boone et al., 2006). Local weather conditions, precipitation (Brooks, 2004; Gamble and Mitsch, 2009), and ground-water exchange (Brooks and Hayashi, 2002) seem to be critical factors affecting hydroperiod. Gamble and Mitsch (2009) suggested soil differences may help explain the variation in hydroperiod between human created and natural pools, but this relationship between soil and hydroperiod has not been well studied in vernal pool ecosystems. Though hydroperiod prediction is difficult, it may be among the most important factors to understand and determine wetland function (Gamble and Mitsch, 2009); especially as climate change affects vernal pool hydroperiods in the future (Brooks, 2009).

Another major obstacle in vernal pool conservation is that the variability in vernal pool physical and hydrologic characteristics makes it difficult to determine which vernal pools are most important to conserve (Colburn, 2004). For example hydroperiod and canopy closure are two of the most important aspects of vernal pools for amphibian breeding (Baton, 2005), yet there is no commonly used classification system for vernal pools that would allow managers to rapidly prioritize vernal pool conservation across the landscape. There have been attempts to develop a classification system, for example Rheinhardt and Hollands (2008) suggested five broad hydrogeomorphic categories to help determine regional pool characteristics, but not individual pool variability within regions. Colburn (2004) proposed a hydrologic classification system, though documenting hydrologic regime often requires repeated visits to a pool. Colburn (2004) also suggested a vegetative classification system that might be useful for predicting hydroperiod. Finally, Calhoun et al. (2003) suggested using amphibian indicators, specifically number of egg masses per pool and number of indicator species using pools, to determine "significant" vernal pools to conserve. However, documenting amphibian breeding requires intensive field surveys to determine which species are there and when peak breeding occurs. Other researchers have suggested that combining characteristics, such as using pool depth in combination with the type and amount of vegetation may better predict hydroperiod and pool function (Eagen and Paton, 2004).

The variability of vernal pool types as well as the lack of federal protection makes parks and other conservation areas especially critical for protection of vernal pools. In fact Calhoun et al. (2003) suggested it may be wise to consider all vernal pools in protected areas such as parks or preserved lands as "significant vernal pools" (i.e., critical to conserve) because the connection between vernal pools and surrounding high quality uplands is usually intact in these areas. Whereas woodland vernal pools have been relatively well-studied in the northeastern United States, there is comparatively less information on vernal pools in the Midwest and Michigan in particular.

The goal of our study was to provide quantitative and qualitative evaluation of vernal pool abiotic and biotic characteristics within Pictured Rocks National Lakeshore (PIRO), located along Lake Superior in the Upper Peninsula of Michigan (Fig. 1). A few studies in PIRO, which have included vernal pool habitat, indicate that some pools support fish-intolerant species of fairy shrimp (Eubranchipus) and amphibians (Casper, 2005) and are used by large mammals such as black bears (Ursus americanus) (DeBruyn, 1997). However, detailed information on pool characteristics and use by amphibians was lacking. Therefore, our objectives were to provide park managers with information about pool characteristics and classify vernal pools across the landscape of PIRO to determine which pools warrant the greatest conservation effort.



PIRO (46[degrees] 33'44"N, 86[degrees] 18'45"W) is a 29,637 ha area extending 68 km along the southern shore of Lake Superior (Fig. 1). The dominant forest types are northern hardwoods, upland conifers, and conifer swamps; peatlands are also significantly represented in the area (Albert, 1995). From 2005 through 2010, average Jan. and Feb. maximum and minimum temperatures were -3.5 and -10.3 C, and average Jul. and Aug. maximum and minimum temperatures were 23.1 and 13.7 C. Average annual precipitation for 2005-2009 was 904 mm with approximately 43% of that falling as snow.

PIRO is comprised of two zones with differing management priorities: the shoreline zone (13,731 ha) that is owned in fee by the National Park Service (NPS) with preservation as a priority and the inland buffer zone (IBZ; 15,907 ha) that allows economic utilization of renewable natural resources such as timber production. The IBZ consists largely of lands owned by a commercial timber corporation (45%), the State of Michigan (35%), and private citizens (15%). Slightly less than 5% of the IBZ is owned by NPS.

The vernal pools, within PIRO selected for this study, were a subset of those identified via aerial photography and field checked during a collaborative and concurrent study by Previant and Nagel (2014). Using a stratified design (Resh et al., 2013), we randomly selected 21 vernal pools (total pools available = 49; Fig. 1) at which to assess physical and biological characteristics during spring and summer of 2010. These 21 pools were located [greater than or equal to] 152 m away from inland lakes, rivers, or streams and [greater than or equal to] 30.5 m away from Lake Superior (Previant and Nagel, 2014). Of the 21 sampled vernal pools, 16 were in the shoreline zone and five were in the IBZ.


The maximum area of each vernal pool was estimated during the 2010 field season. Vernal pool perimeters were defined as an abrupt change in the herbaceous layer or by a micro-topography break. Vernal pool characteristics (e.g., water depth, irregular boundaries, and vegetation density) dictated which one of the following three methods was used to determine vernal pool maximum area. The first method required establishing a center point of each pool and measuring to the pool's perimeter at cardinal and sub-cardinal directions using a Suunto Kb-14 Compass (Vantaa, Finland) and Haglof Vertex III Hypsometer (Laangsele, Sweden). This provided eight triangles that were then combined to estimate the total area of the vernal pool. The second method required walking the perimeter of a vernal pool using a DeLorme PN-30 GPS (WAAS enabled; Yarmouth, ME) unit. In ArcMap 9.3 (Redland, CA), a polygon for each vernal pool was created from the waypoints to estimate area, while the third method used May 2004 leaf-off aerial photography (1:12,000). The pool perimeter was digitized in ArcGIS 9.3, creating a polygon to determine the area.


We measured soil pH (Kelway soil pH meter; Wycoff, NJ) at nine locations within the dried pool area and then averaged those readings for a final soil pH value. When visited pools were inundated (n = 4), water pH was measured from a single location using an YSI 63 handheld pH meter (Yellow Springs, OH). A soil auger (5.08 cm diameter) was used to collect the top 20 cm of soil from the center or wettest part of the vernal pool. The soil sample was sealed in a plastic bag, transported from the field, and immediately frozen and stored at -15 C until analysis for carbon (C) and nitrogen (N). In the lab soil samples were dried at 60 C, sieved (2mm opening) to remove rocks and large organic debris, finely ground with a ball mill (SPEX 8000M; Metuchen, NJ), and weighed into tins for C and N elemental analysis (Fisons NA 1500; Milan, Italy).

Similar to work by Brooks and Hayashi (2002), we developed a relative hydroperiod index based on the presence or absence of standing water during periodic (3 to 5) visits over two field seasons (2009/2010). We assigned a hydric value at each visit: "0" if dry, "0.5" if saturated, or "1" if standing water was present. The mean hydric value of each pool was calculated by adding the hydric values from all visits and dividing by the number of visits.


Vegetation data were collected from all pools (n = 21) during Jun. and Jul. 2010. Vegetation composition within each homogenous vegetation stand was analyzed using the releve method to acquire a complete species list from each stand (Mueller-Dombois and Ellenberg, 1974). Releve data were collected from a 3x3 m plot within a homogenous vegetation stand within the vernal pool boundaries, and the identity and cover class of each vascular plant species were estimated. Cover class was later converted to percent cover (PC) using a scale of 1 (0-1 PC), 2 (1-5 PC), 3 (5-25 PC), 4 (25-50 PC), 5 (50-75 PC), and 6 (75-100 PC). Each releve plot was then marked using a handheld GPS unit. Taxonomy of the USDA Plant Database was followed (USDA, NRCS 2011).


Terrestrial amphibians were sampled during May 2010 using area-based visual encounter surveys (Harding, 1997; Dodd, 2009). We chose a 25 m transect around the perimeter of each vernal pool in an area where the density of woody debris was highest. Two surveyors walked side by side along the length of the transect surveying a swath 5 m in width (total area =125 [m.sup.2]). Within the survey area all woody debris was turned over and the number and species of amphibians present were recorded. Woody debris was then returned to its original position. Other amphibians encountered outside the survey area during data collection at each vernal pool were recorded separately. Amphibian density was calculated as number of amphibians per 125 [m.sup.2].


Differences in vernal pool maximum water surface area and soil C concentration among vegetation communities were evaluated using one-way ANOVA. Due to violations in equal variance assumptions, the nonparametric Kruskal-Wallis test for ANOVA model significance was used with Dwass-Steel-Critchlow-Fligner post-hoc test of all pairwise comparisons (SYSTAT, 2009).

Vegetation was classified using agglomerative cluster analysis with Sorensen distance measure and flexible beta linkages method with (5 = -0.25, using PC-ORD 5.0 (McCune and Mefford, 2006). Indicator species analysis was used to prune the dendrogram and optimize the number of clusters (Chimner et al., 2010). We averaged P-values across all species for each cluster level using Monte Carlo Analysis. The cluster level with the lowest average P-value was used as the optimal level.

Both average density and average species richness of amphibians were compared between vernal pools using (-tests (SYSTAT, 2009). Statistical comparisons were not made among all vernal pool types due to low sample sizes. Equality of variance was evaluated before conducting t-tests using F-tests.



Many of the pools that were visited in 2010 were dry due to the warm and dry conditions during spring 2010 and several winters of lower than average snowfall. Spring 2010 (Mar., Apr., and May) was the warmest and driest on record (1912-2010; data provide by NOAA's National Climatic Data Center (NCDC) for Munising; The average daily temperature for Mar.-May was 6.9 C, a full 3 C warmer than average. Total precipitation for Mar.-May was 6.35 cm, which is only one-third of the long-term average of 18.33 cm. Mar. of 2010 was the driest on record with less than 1 cm of precipitation, with Apr. and May the 10th driest on record with ~3 cm per month of precipitation.

Vernal pool maximum water surface area ranged between 0.001 and 0.567 ha, with an average of 0.124 ha (Table 1). Soil pH for the pools averaged 5.9 with a range of 5.0-6.5, and water pH for the four pools with standing water ranged from 3.9 to 6.3, with an average of 5.0. Soil C and N concentrations averaged 13.5% and 0.7%, respectively, and ranged between 0.5 to 46.2% and <0.1 to 2.2%, respectively (Table 1). Soil C concentration was correlated with relative hydroperiod index (Fig. 2A). Pools that had standing water or saturated soil during the majority of visits had greater soil carbon; whereas pools that tended to be dry when visited had lower soil carbon. There was no correlation between vernal pool maximum water surface area and hydroperiod index (Fig. 2b).


We identified 115 vascular plant species at the 21 vernal pools. Vegetation composition varied greatly with wetness of the site, microtopographic features, and canopy openness. Cluster analysis grouped the vegetation into five distinct community types (Table 2), which resulted in an information retention of about 25%, with an overall percent chaining of 6.09%.

Average species richness for vascular plants varied between 21 and 59 per community type (Table 2). The two forested communities (red maple and sugar maple) had the highest species richness. Increased species richness in forested communities could be due to the greater number of microtopography features. The forested communities, especially red maple, had vegetation growing on downed logs in the pools and on drier hummocks.


A total of 17 vernal pools were surveyed for amphibians during May of 2010 (Table 1). Amphibians were encountered during our area-based survey at seven vernal pools (41% of total pools sampled for amphibians) and were noted outside of our survey at an additional four pools (65% of total pools sampled for amphibians) (Table 1). Average species richness across all 17 pools was 0.5 during the survey and 0.8 including all survey and non-survey sightings. Average density of amphibians was 0.65/125 [m.sup.2] or 0.005/[m.sup.2].


We created a classification system for PIRO vernal pools based first on pool depression status and canopy closure and then on vegetation and soil carbon. Depression characteristics described whether the pool was one depression versus many interconnected depressions and whether the canopy was closed or open. Classic pools had open canopies and a single depression; complex pools had closed canopies and small interconnected depressions (Fig. 3). The classification system resulted in five distinct types: classic sedge pools, classic mudflat/graminoid pools, classic grass pools, red maple complexes and sugar maple complexes (Table 3). Soil C concentration was significantly different among pool types (P [less than or equal to] 0.01, df = 4, Fig. 4). Classic sedge pools had the greatest soil C concentration (31.0%) followed by red maple complexes (12.7%). Both of these types had high enough soil C concentrations to be classified as organic soils (Soil Survey Staff, 1999). At the other extreme, classic grass pools had less than 1% soil C. There were no significant differences among vernal pool types based on maximum water surface area, though the vernal pool complexes tended to have larger maximum water surface areas relative to the classic pools (P = 0.86, df = 4, Fig. 5).

Average amphibian density tended to be higher at classic vernal pools compared to vernal pool complexes, however this difference was not statistically significant (t = 2.31, P = 0.11, df = 8, Fig. 6). Similarly, when all amphibian sightings were taken into account, amphibians were found at a higher proportion of classic vernal pools compared to vernal pool complexes (Fig. 7). Amphibians were observed during surveys most frequently at classic sedge pools (80% of pools had amphibians), followed by the classic mudflat/graminoid pools (50% had amphibians), and the red maple complex pools (33% had amphibians). When all encounters and sightings of amphibians were combined (survey and nonsurvey), the trend was similar (Fig. 7).

Despite our small sample size, average amphibian species richness was significantly higher at classic vernal pools compared to vernal pool complexes (t = 2.13, P = 0.04, df = 15, Fig. 8). Wood frogs (n = 10), one leopard frog (Lithobatespipiens) and one Eastern newt (Notophthalmus viridescens) were found only in classic vernal pools; whereas, red-backed salamanders (n = 4) and American toads (Anaxyrus americanus, n = 3), the only two species found in vernal pool complexes, were found at both pool depression types. The American toad was the only amphibian found in the sugar maple complex.


One of the major challenges for conservation of vernal pools is the inherent variability among vernal pool ecosystems and the inability to prioritize individual pools for conservation (Preisser et al., 2000; Calhoun et al., 2003; Brooks, 2004; Colburn, 2004; Rheinhardt and Hollands, 2008). To attempt to resolve this difficulty, we created a vernal pool classification system for woodland vernal pools. Though our classification was developed in the upper Great Lakes region encompassing PIRO, we believe the variables used can be easily adapted to other regions, specifically, woodland vernal pools throughout glaciated northeastern North America (Calhoun and deMaynardier, 2008; Rheinhardt and Hollands, 2008). Our classification system initially separates vernal pools by depression characteristics (classic vs. complex; Colburn, 2004; Rheinhardt and Hollands, 2008) and then focuses on the dominant vegetation community and soil carbon in each depression type. While other vernal pool classification systems have been suggested based on single variables such as hydrogeomorphic features (Rheinhardt and Hollands, 2008), hydroperiod (Colburn, 2004), vegetation (Colburn, 2004), and amphibian presence (Calhoun et al., 2003), our classification system uses multiple variables that can be rapidly assessed by managers with minimal visits required to vernal pools.

The key depression characteristics defining our classic and complex descriptions are: (1) depression dominance, a single depression versus small interconnected depressions (Brooks and Hayashi, 2002; Leibowitz and Brooks, 2008; Rheinhardt and Hollands, 2008) and (2) closed versus open canopy, a critical aspect for amphibians (Patton, 2005). Rheinhardt and Hollands (2008) suggested hydrogeomorphic status be the top level of any hierarchical classification of vernal pools due to its importance in determining pool function. Whether a vernal pool depression is single or multiple can yield insight into hydroperiod (Brooks and Hayashi, 2002), disturbance frequency (Rheinhardt and Hollands, 2008), frequency of changes in canopy structure, and amount of coarse woody debris (deMaynadier and Houlanhan, 2008). The relationship between canopy cover and amphibian use of vernal pools is complicated as different species prefer different degrees of canopy closure and this preference varies by time of year (Egan and Patton, 2004; Baldwin et al., 2006). The important conservation aspect is to ensure that a suite of vernal pools with varying degrees of canopy closure are available to amphibians throughout the breeding and summer seasons (Kerraker and Gibbs, 2009).

We found the density and species richness of amphibians in PIRO tended to be highest at classic vernal pool types compared to vernal pool complexes (Fig. 6, 7). However, due to the timing of our sampling effort and the unusual dryness of the year we sampled, we were unable to capture breeding effort of amphibians at these pools, therefore we are limited in the conclusions we can draw about amphibian use of the pools in our study. The only evidence of amphibian breeding we observed was in six classic pools (five sedge and one grass). In two sedge pools we observed egg masses with amphibian larvae (though true standing water was absent, pools had recently dried) and in the other three pools (two sedge and one grass) we observed recently metamorphosed wood frogs that presumably emerged from each vernal pool (as there were no other water sources nearby). Though this is only anecdotal evidence, compared to the other pools we sampled, these six classic pools may have had a more consistent hydroperiod during this extremely dry spring, a characteristic needed to ensure larvae have time to develop to metamorphosis (Berven and Grudzien, 1990; Semlitsch, 2008). Though our observation of more amphibians at classic pools may warrant further study, these data must be viewed with caution because of our single sampling effort during an unusually dry year and lack of breeding season data for other vernal pool obligates (notably Ambystomatid species). Furthermore, as Calhoun et al. (2003) cautioned, simple depressional status should not be used to define vernal pool significance. Consequently, we have combined depressional status with vegetation composition to capture additional variation in vernal pool hydroperiod and soil conditions.

The vegetation component of our classification system is based on the dominant community present in the vernal pool. Vegetation communities are important in that they integrate aspects of canopy cover, pool morphology, soil characteristics and hydroperiod (Colburn, 2004). For example, Palik et al. (2007) found that shorter hydroperiods were associated with more upland plant functional groups, while longer hydroperiods were associated with wetland plant functional groups. We found amphibians were most common in classic sedge pools (Fig. 6). In fact this pool type was the site with the highest amphibian species richness (pool 280), including our single observation of the aquatic adult phase of an Eastern newt (Notophthalmus viridescens). Classic sedge pools had the highest proportion of obligate wetland vegetation and the highest soil C concentration (Table 3), indicating they were more reliably wet. This is similar to the findings of Egan and Paton (2004), who suggested woody and persistent emergent vegetation are characteristics of pools with intermediate hydroperiods and ideal vegetation substrates for amphibian egg deposition. Vegetation community is a useful assessment tool because it is relatively easy to assess both in terms of sampling methodology and time. For example there is no need to sample the vernal pool only during the brief spring wet season; the vegetation community will be present through most of the year (Cutko and Rawinski, 2008).

Vegetation can also indicate potential habitat for species that use vernal pools later in the summer. For example only two amphibians (both American toads) were observed in sugar maple complexes and classic grass pools, our driest and lowest soil C sites (Table 3). American toads typically tolerate drier conditions than other anurans, although standing water is required for breeding (Harding, 1997). Though sugar maple complexes were the driest sites, the abundance of leaf litter may be beneficial to this species as they often spend daylight hours buried under leaf litter, absorbing moisture from soil when it is available (Forester et al., 2006). Because the vernal pools we studied were located at least 152 m from any other inland water source (Previant and Nagel, 2014) we suspect these animals were responding to the lingering moisture as a result of the vernal pool, not other hydrological features. Though the temptation is to focus conservation efforts on the wettest vernal pools that may be most successful for amphibian breeding, it should be pointed out that even the drier vernal pool complexes are relevant to amphibians. Vernal pool complexes have vegetation components that are likely important to amphibians that spend their adulthood in drier, upland sites because the moisture retained in the soil as vernal pools dry, provides ideal amphibian habitat.

Though many studies have focused on pool physical characteristics as predictors of hydroperiod, we did not include these elements in our classification system. Hydroperiod is particularly difficult to predict as it may fluctuate frequently and rapidly because of annual precipitation variation (Brooks, 2004) and the often remote location of pools makes repeat visits to measure water depths time consuming and expensive. For example Boone et al. (2006) successfully predicted vernal pool hydroperiod using a classic water-balance model with detailed depth information about each pool; however, the authors concluded that modelling hydroperiod regionally would be impractical using this method because of the large volume of specific water depth data that is needed for each pool. Brooks and Hayashi (2002) found pools with higher maximum volumes, larger surface areas, or greater depths were more likely to have surface water when visited, but that relationship became highly variable with decreased maximum volume, surface area, or depth. We did not find a relationship between pool maximum water surface area and relative hydroperiod index at PIRO (Fig. 2b). This may be because only five of the pools were larger than 0.1 ha, the lower limit for the relationship found by Brooks and Hayashi (2002). Brooks and Hayashi (2002) suggested smaller pools and shallower pools would likely have high variability in hydroperiod due to other factors such as precipitation, evapotranspiration, and ground water exchange.

Though vernal pool maximum water surface area did not predict hydroperiod, our data suggested quantifying soil C concentration might be a way to assess hydroperiod, even when the pools are dry (Fig. 2a). Low soil C concentrations (roughly < 4%) predicted a low hydroperiod index, indicating drier soil conditions. When soil C concentrations were high (roughly > 4%), the predicted hydroperiod index ranged from saturated to standing water conditions, indicating higher variability in predicted conditions, but always wetter. Soil C generally increases with increasing soil moisture or longer hydroperiod due to increased productivity, decreased decomposition, or both (Bernal and Mitsch, 2008; Chimner et al., 2011). Based on these findings, soil C may be a good metric for rapid assessment of vernal pool hydroperiod that warrants further investigation. It is particularly significant that a relationship was detectable during the driest spring on record, underscoring the potential of this tool for evaluation of hydroperiod despite conditions at the time of sampling.

Our limited data showed the vernal pools with higher soil C concentrations tended to be the sites where we found amphibians. If potential amphibian habitat could be assessed with a soil C sample, which requires only one visit, and simple laboratory analysis, a manager would have a relatively easy method of focusing vernal pool preservation on those pools with the highest potential of providing amphibian habitat without the need of monitoring all vernal pools for amphibians. The relationship between soil C and vernal pool hydroperiod shows promise as a predictor of amphibian occurrence, but this idea warrants further study.

One complicating factor in our analysis is that amphibian breeding might not have been as successful in 2010 in PIRO compared to years with more snowpack and/or early spring precipitation. Although vernal pools were examined for amphibians during May, usually a wet month, 13 of the 18 pools we visited had no standing water. Muddy soil at many sites, as well as evidence of empty caddis fly cases and fingernail clam shells, indicated that most sites were inundated at some point earlier in the spring. We observed egg masses and live embryos in muddy soil in two pools, indicating that these pools had recently dried. Evidently, eggs at some pools did not have sufficient time to develop into juveniles before pools dried. Although some amphibians may not have survived from eggs to juveniles, evidence of recently metamorphosed wood frogs was observed at four pools, indicating some reproduction took place. Though we sampled during an unusually dry year, this suggests that the relationships that we did find may be even stronger during years with more typical moisture conditions.

In the face of threats to vernal pools, including climate change and human development (Mahoney and Klemens, 2008; Brooks, 2009), science-based recommendations are needed to advance conservation of woodland vernal pools (Calhoun and deMaynardier, 2008). We suggest our vernal pool classification system is a first step towards a method that will allow rapid assessment and inventory of vernal pools across a landscape at any time during the year. Though our method was designed in the upper Great Lakes Region, it can be adapted and applied to woodland vernal pools throughout glaciated northeastern North America (Calhoun and deMaynardier, 2008; Rheinhardt and Hollands, 2008). Many researchers have suggested conservation should focus on preserving a range of vernal pool hydroperiods rather than focusing on a specific vernal pool type (Calhoun et al., 2003; Petranka el al., 2007; Karraker and Gibbs, 2009). A strength of our classification system is that it will allow managers to assess the availability of a range of vernal pool types across a landscape that vary with respect to depressional status, vegetation community, canopy closure, soil carbon, and consequently, hydroperiod. Using our classification system, managers will be able to effectively maintain a suite of vernal pools across the landscape and hydroperiod spectrum that are important for all life stages of the variety of species that use vernal pools throughout the year (Colburn, 2004; Previant and Nagel, 2014).

Acknowledgments.--We thank L. Loope and B. Leutscher for their assistance with field work and logistics at PIRO, C. Olson for his contributions to the GIS portion of this project, and J. Marr for her vegetation sampling and photographic contributions to this project. We also thank the friendly road construction crew for creating safe passage to and from several vernal pool locations. We further thank B. Moraska Lafrancois for initiating this project. Additionally, much gratitude is extended to J. Elias, U. GafVert, B. Moraska Lafrancois, L. Kainttlainen, L. Loope, and three anonymous reviewers for their reviews and suggestions on earlier versions of the manuscript.

This research was supported with funding from the Great Lakes Restoration Initiative, Environmental Protection Agency Project Number 90, under Task Agreement J6320106203 of the Great Lakes-Northern Forest cooperative Ecosystem Studies Unit and with funding from the National Park Service under Task Agreement J6320096201 of the Great Lakes-Northem Forest Cooperative Ecosystem Studies Unit to Michigan Technological University. Both task agreements were under Cooperative Agreement CAH6000082000 between the National Park Service and the University of Minnesota.


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School of Forest Resources and Environmental Science, Michigan Technological University, Houghton 49931

(1) Corresponding author: Telephone: (906) 487-1139; FAX (906) 487-2915; e-mail:

TABLE 1.--Hydrologic, physical, and soil
characteristics of 21 subsampled vernal pools

            Maximum water
            surface area    Hydroperiod     Depression
Pool #          (ha)           Index      characteristics

122             0.001          0.88           complex
125              --            0.75           complex
147 (1)         0.300          0.88           complex
152 (1)         0.058          0.50           complex
153 (1)         0.059          0.50           complex
161 (1)         0.068          0.88           classic
165 (1)         0.037          0.63           classic
181 (1,2)       0.041          1.00           complex
199 (1,2)       0.082          0.75           classic
265 (2)         0.052          0.33           classic
280 (1)         0.209          0.75           classic
281 (1)         0.079          0.50           classic
290 (1)         0.129          0.50           classic
304 (3)         0.078          0.00           classic
305 (3)         0.072          0.00           classic
312 (3)         0.567          1.00           complex
347 (3)          --            0.25           complex
394             0.063          0.60           complex
415 (1,2)       0.092          0.25           classic
444              --            0.17           complex
445 (3)         0.243          0.40           complex

            Soil (Water)
Pool #           pH        Soil C (%)   Soil N (%)

122             6.5           28.7         1.3
125             5.2           1.3          0.1
147 (1)         6.2           8.3          0.6
152 (1)         5.8           4.2          0.2
153 (1)         5.8           4.8          0.2
161 (1)         5.9           38.5         1.9
165 (1)         5.8           13.9         0.8
181 (1,2)    6.0 (4.9)        28.0         1.5
199 (1,2)      (6.3)          5.9          0.4
265 (2)         6.3           3.8          0.2
280 (1)      5.3 (4.9)        23.3         1.5
281 (1)         5.6           33.1         2.0
290 (1)         5.9           46.2         2.2
304 (3)         6.3           0.5          0.0
305 (3)         6.1           0.7          0.1
312 (3)         5.0           29.8         1.5
347 (3)         6.3           2.7          0.2
394             6.0           14.7         1.1
415 (1,2)       6.0           0.7          0.1
444             6.3           2.9          0.2
445 (3)        (3.9)          3.8          0.2

(1) Indicates amphibians were found at this pool

(2) Indicates pools located in IBZ as opposed to shoreline zone

(3) Pools were not sampled for amphibians

TABLE 2.--Average percent cover of the most common plant species
and average species richness by vegetation community type

                                    Mudflat/             Red    Sugar
Species                     Sedge   Graminoid   Grass   maple   maple

Species richness              35          21      38      59      54
Pools sampled (n)              5           2       2       8       4
Scutellaria lateriflora       41           0       9       5       0
Onoclea sensibilis            28           0       6       4      11
Ilex verticillata             21           0       0       0       0
Torreyochloa pallida          16           0       0       0       0
Thelypteris phegopteris       11           0       0       0       0
Carex crinita                 10           0       1       7       0
Alopecurus aequalis           10           0       0       0       0
Iris versicolor                8           8       4       1       0
Scirpus cyperinus              7          10       2       0       0
Osmunda regalis                5           2       1       2       0
Dryopleris carthusiana         1           0       0       7       0
Athyrium filix-femina          1           0       0       5       5
Myrica gale                    0          22       0       0       0
Lycopus unijlorus             18          17       2      10       1
Poa sp.                        0          17       i       0       0
Dulichium arundinaceum         3          12       0       0       0
Potamogeton sp.                0          12       0       0       0
Carex vesicaria                0           5      52       0       0
Calamagrostis canadensis       0          10      17       1       0
Acer rulrrum                   0           0       8      47       1
Dryopleris intermedia          4           0       i      15       5
Carex brunnescens              1           0       i      13       1
Carex intumescens              0           0       8       5       0
Viola sp.                      0           0       8       5       0
Trientalis borealis            0           0       8       4       0
Acer saccharum                 0           0       8       4      21
Betula alleghaniensis          0           0       2      10      22
Fagus grandifolia              0           0       2       5       8
Tsuga canadensis               0           0       0       9       0
Osmunda claytoniana            0           0       0       5       0
Carex tuckermanii              0           0       0       5       i
Matteuccia struthiopteris      0           0       0       2      86
Impaliens capensis             0           0       0       2      21
Fraxinus nigra                 0           0       0       0       9
Milium effusum                 0           0       0       0       9

TABLE 3.--Vernal pools in PIRO were classified into five types
based on depression characteristics, canopy closure and
vegetation community.

Vernal pool
classification          Description

A. Classic vernal       Open canopy, single basin, typically surrounded
pools                     by forested upland

A.1. Classic sedge      * Dominated by sedges and ferns
pool (Carex retrorsa/   * Dominant plant species were obligate wetland
Onoclea sensibilis)       plants
                        * Average soil C concentration was high
                          (31.0%), indicating that this was probably
                          the wettest pool type
                        * Average maximum water surface area (0.100 ha)
                          was in the middle of other vernal pool type
                          average areas

A.2. Classic            * Dominated by grasses, sedges and ferns
mudflat/graminoid       * Most plant species were facultative wet to
pool (Juncus effusus/     obligate wetlands plants
Scirpus cyperinus)      * Low to medium average soil C concentration
                          (4.9%), indicating mesic conditions
                        * Lowest average maximum water surface area
                          (0.067 ha)

A.3. Classic grass      * Dominated by grasses
pool (Calamagrostis     * Most plant species were facultative wetland
canadensis/Carex          plants
vesicaria)              * Lowest average soil C concentration (0.6%),
                          indicating that these pools were normally dry
                        * Second lowest average maximum water surface
                          area (0.085 ha)

B. Vernal pool          Closed canopy with many small, interconnecting
complexes                 pools

B.1. Red maple          * Dominated by vascular plants
complex (Acer rubrum/   * Most plant species were facultative wet to
Tsuga canadensis)         obligate wedands species
                        * Second highest soil C content (12.7%),
                          indicating that this is a wet community
                        * Second largest average maximum water surface
                          area (0.149 ha)

B.2. Sugar maple        * Dominated by vascular plants
complex (Acer           * Most plant species were upland to facultative
saccharum/Matteuccia      wet species
struthiopteris)         * Medium soil C content (7.1%), indicating that
                          this was a mesic upland forest soil
                        * Largest average maximum water surface area
                          (0.180 ha)
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Author:Schrank, Amy J.; Resh, Sigrid C.; Previant, Wilfred J.; Chimner, Rodney A.
Publication:The American Midland Naturalist
Date:Jul 1, 2015
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