Forest Ecology and Management 256 (2008) 827–836

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Farms, fires, and forestry: Disturbance legacies in the soils of the Northwest Wisconsin (USA) Sand Plain Emilie B. Grossmann *, David J. Mladenoff University of Wisconsin-Madison, 1630 Linden Dr., Madison, WI 53706, United States

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 August 2007 Received in revised form 20 May 2008 Accepted 23 May 2008

We studied the long-term effects of disturbance within the Northwest Wisconsin (USA) Sand Plain (NWSP), an ecoregion that is characterized by very sandy soil and an active disturbance history that includes fire, agriculture and industrial forestry, largely clearcut logging of jack pine (Pinus banksiana) and aspen (Populus spp.). Open ‘‘barrens’’ communities on this landscape were formerly maintained by fire, and are a high conservation priority. Hill’s Oak (Quercus ellipsoidalis) can also dominate forest canopies, while blueberry (Vaccinium angustifolium), and sweetfern (Comptonia peregrina) are common shrub species. We structured a field sampling design with a spatial-temporal database built from historic airphotos (1938 and 1997) and fire records to examine whether soil organic matter and nutrients vary with disturbance history in the nonforest habitats of the sand plain. We sampled soils along 83 transects, randomly stratified among five sampled classes: (1) nonforest-farming history; (2) nonforest-fire history; (3) nonforest-clearcut only history; (4) evergreen forest of jack pine and red pine (P. resinosa); and (5) deciduous forest of Hill’s oak and aspen. Logging of the original forest took place in the late 1800s– early 1900s. The farms were abandoned between 1938 and 1960, and the most recent fire occurred in 1977. Thus, the duration of the agricultural legacy is approximately 45–65 years while observed fire effects have lasted for 26 years. We observed strong agricultural legacies, including high P and low OM, N and Ca. One possible explanation for the N legacy is that it is tied to soil OM accretion which may be driven by plant growth. We detected no difference in mean values for any of the soil properties between soils from nonforested areas within the Five-Mile fire and soils from nonforested areas with a clearcut-only history. We did observe a fire effect in high variance for soil P. This could have resulted from variations in fire severity and ash convection and deposition. Forest soils generally had lower pH than the nonforest soils, and the deciduous forest soils had the lowest pH and also very low Ca. We also observed high within-transect coefficient of variation for Ca in the forest soils. We conclude that agriculture is a qualitatively different disturbance-type than fire or clearcutting, that disturbance legacies tend to be most persistent with geologically stable elements, such as P, and that management and conservation planning within the NWSP would benefit from site-specific agricultural history, as well as attention to Ca. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Northwest Wisconsin Sand Plain Disturbance legacy Nitrogen Organic matter Phosphorus Potassium Calcium Magnesium pH Variance Scale

1. Introduction Disturbances can leave long-term imprints or legacies that persist for decades in ecosystems (Goodale and Aber, 2001; Foster et al., 2003). These may be detected in the form of altered

* Corresponding author at: Oregon State University, Department of Forest Science, Forestry Sciences Laboratory, 3200 Jefferson Way, Corvallis, OR 97331, United States. Tel.: +1 541 750 7361. E-mail address: [email protected] (E.B. Grossmann). 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.05.048

landscape structure (Mladenoff et al., 1993; Foster et al., 1998), vegetation (Motzkin and Foster, 1998) and soil properties (Compton et al., 1998; Lynham et al., 1998; Latty et al., 2004). Because there are feedbacks between vegetation and soil (Wedin and Tilman, 1990; Binkley and Giardina, 1998), it is vital to understand disturbance legacies in soil to plan for conservation (Foster et al., 2003) and sustainable forest management (Attiwill, 1994; Chapin et al., 1996). The Northwest Wisconsin Sand Plain (NWSP) is a region where conservation concerns for jack pine (Pinus banksiana) barrens (an endangered, predominately nonforested ecosystem) intersect with

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forest management. It has a complex disturbance history that combines fire (Vogl, 1970; Lynch et al., 2006), historic agriculture in some locations (ca. 1900–1940) (Gough, 1997) and industrial forestry (Rowlands, 1949). Each of these disturbance types may affect soils in ways that direct and constrain future conservation and forest management. Within this context, we examine the links between site history and soils by comparing areas with different disturbance histories: (1) clearcut forestry only (nearly all areas have been clearcut at least once), (2) agricultural fields that were abandoned 45–65 years ago, and (3) one running crown fire that occurred 26 years ago. Our study emphasizes the soils of the nonforested areas, but we include forest soils for comparison. Although there are many parallels between the effects of clearcuts, fires and farming on soils, the balance of nutrient losses and subsequent gains varies by element and disturbance-type. Thus, variations in site history can yield different soil environments, generating patterns that may persist for decades. 1.1. Clearcutting Northern Wisconsin was completely clearcut from the mid 1800s–early 1900s (Gough, 1997; Schulte et al., 2007; Rhemtulla et al., 2007), and clearcutting is common throughout the sand plain today (Radeloff et al., 2000a). We use the clearcuts as a point of comparison for the legacies of agricultural and fire disturbances. Clearcuts have a variety of effects on soil. Soil carbon is lost due to faster decomposition and curtailed inputs but re-growing vegetation can rebuild soil organic matter over time (Covington, 1981; Kim et al., 1996). N can be lost in new clearcuts due to nitrate leaching (Likens et al., 1978; Matson and Vitousek, 1981; Burns and Murdoch, 2005) and the sandy soils of the NWSP may be prone to leaching (Hole, 1976). Re-sprouting shrubs and grasses can diminish leaching (Likens et al., 1978), but site preparation methods vary in their impacts on shrubby vegetation (Haeussler et al., 2002), and so leaching losses may vary among recent harvests. In the NWSP, trenching, scarification and herbicides are often used to establish new pine stands (personal observation). In boreal jack pine forests, N can be recovered via symbiotic Nfixation associated with sweetfern (Comptonia peregrina) (Lynham et al., 1998), a shrub that is also common in the NWSP (Grossmann, 2006). Whole tree harvesting can result in losses to ecosystem P (Yanai, 1998). In NWSP soils, P content varies naturally with the apatite content of the soil parent material (Hole, 1976). Phosphorus losses may be persistent since weathering can take centuries to millennia (Crews et al., 1995). Vegetation removal can also result in net losses of base cations and an associated drop in pH (Likens et al., 1970; Johnson et al., 1991, 1988). However, if foliage and slash are left on site, mineral soil Ca can rise, along with soil pH (Pietika¨inen and Fritze, 1995; Simard et al., 2001). 1.2. Fire The direct effects of wildfire on soil are generally similar to, but often stronger than those of clearcutting (Simard et al., 2001); although less woody biomass is removed by fire than by clearcutting (Tinker and Knight, 2000). Soil organic carbon is lost during fire when soil temperatures reach 100 8C, and then decomposition accelerates after the fire, and C inputs are reduced (DeBano et al., 1998). These loses can persist for 17–50 years (Parker et al., 2001), and recovery is influenced by recolonizing vegetation (Neary et al., 1999). Soil N is lost to fire first via volatilization and later via leaching (Vitousek and Melillo, 1979; DeBano et al., 1998; Choromanska and DeLuca, 2002), but can be recovered with N-fixation (Lynham et al., 1998). N-fixing plants are important components of many fire-prone native plant commu-

nities (Newland and DeLuca, 2000; Johnson et al., 2005) and often increase after both fire and clearcutting (Hendrickson, 1986; Lynham et al., 1998). Fire’s interactions with the nitrogen cycle can also generate spatial heterogeneity in soil N pools (Smithwick et al., 2005). Phosphorus can be lost via volatilization and ash convection during very intense fires (DeBano et al., 1998; Certini, 2005). It is less susceptible to leaching than N because its mineral forms bind tightly to soil minerals, but as with clearcutting, recovery is a slow process (Newman, 1995; DeBano et al., 1998). Fire impacts soil pH via base cations. Severe fires in boreal jack pine forests can reduce forest floor base cations and mineral soil K, lowering pH (Brais et al., 2000), but mineral soil pH and Ca concentrations more commonly rise after fire due to ash deposition (DeBano et al., 1998; Simard et al., 2001). 1.3. Farming Agriculture’s effects on soils share some characteristics with clearcutting and fire, often including initial organic carbon and nitrogen losses due to faster decomposition or leaching (Schimel, 1986; Guo and Gifford, 2002; Murty et al., 2002; Houghton and Goodale, 2004), generally followed by gradual recovery (Prince et al., 1938; Inouye et al., 1987; Johnston et al., 1996; Hooker and Compton, 2003, but see Brye et al., 2002 and Falkengren-Grerup et al., 2006). The processes driving these losses and gains are similarly shaped by colonizing vegetation (Compton et al., 1998; Knops and Tilman, 2000) and soil texture (Bauer and Black, 1981; Campbell and Souster, 1982; Burke et al., 1995; Richter et al., 1999). In the NWSP, vegetation recovery (and hence organic matter and N recovery) may be slowed, due to the coarse texture and low water availability of the native soils. However, agricultural disturbance differs from fire and clearcutting in three important ways. Plowing (in cropped fields), soil amendments, and crop removal all have additional effects on the soil environment. Plowing removes re-sprouting shrubs (Motzkin et al., 1999), which can slow organic matter and nutrient recovery. It also compacts soil, which can confound measurements of organic matter and nutrient losses in plowed soils (Murty et al., 2002). Soil amendments can offset nitrogen losses (Compton and Boone, 2000), and elevate soil P, which may remain high for decades (Koerner et al., 1997; Compton and Boone, 2000; Falkengren-Grerup et al., 2006) to centuries (Verheyen et al., 1999, Dupouey et al., 2002) after agricultural abandonment. Agricultural practices may interact with base cations, and pH in a variety of ways. Acidity may rise with base cation uptake as roots exude protons to balance the charge (van Breemen et al., 1983). Lime additions tend to reduce acidity (Brady and Weil, 1996) while inorganic fertilizers (only in use after 1950) raise acidity (Barak et al., 1997). Because we do not know the soil amendment history of the sand plain farms, it is difficult to predict the likely effects that agriculture had on base cations and soil pH. The spatial distribution of soil nutrients are also altered through farming (Sauer and Meek, 2003), and those patterns can persist for decades (Fraterrigo et al., 2005). 1.4. Forests Forest and nonforest soils differ in a variety of ways. Three major effects of forest growth on soil properties include changes in soil C (Guo and Gifford, 2002), nutrient pool depletion (Attiwill and Adams, 1993; Binkley and Giardina, 1998) and soil acidification (Jobba´gy and Jackson, 2003). Carbon can accumulate in mineral soils with a transition from crop to plantation or secondary forest but landcover transitions toward pine plantations often result in mineral soil carbon losses

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Fig. 1. Three study blocks were located to overlap the Five-Mile fire which burned in the Northwest Wisconsin Sand Plain on May 30, 1977. Within the blocks, we sampled five classes, including two forest types and nonforested areas with three different disturbance histories. Sample transects were randomly stratified within the five classes. The number of transects within each class are listed in parentheses within the legend.

(Guo and Gifford, 2002). For geologically derived elements (e.g., Ca, Mg, K and P), forest growth can deplete mineral soil supplies if uptake and leaching losses exceed weathering rates (Richter et al., 1994). Soil acidification associated with Ca depletion has been shown below aspen (Populus sp.) in northern Minnesota (Perala and Alban, 1982), below cork oak (Q. suber) in New Zealand (Noble et al., 1999), and below loblolly pine (P. taeda) in South Carolina (Richter et al., 1994). 1.5. Research question Our research addresses the question: ‘‘Does the average concentration and variability of soil organic matter, N, P, K, Ca, Mg and pH vary with disturbance history in the nonforested habitats of the NWSP?’’ We hypothesized that soils in nonforested habitats that were burned in 1977 would contain less N and organic matter but more Ca, Mg, P and K than nonforests with a clearcut-only history, and that fire would have generated higher variance for the measured soil variables. We also hypothesized that nonforested areas with a history of farming would have less organic matter, N, Mg and Ca, but more extractable P when compared with nonforested soils with a clearcut-only history, and that farming would also tend to decrease variability in measured soil properties. Finally, we expected that the differences among the nonforested areas with varying disturbance histories would be of similar magnitude to the differences between the soils beneath evergreen and deciduous forests. 2. Methods We addressed this research question by analyzing soil samples that were collected within the NWSP at locations that had experienced different histories. Sampling sites were located

according to information gained from historic airphotos (1938), historic fire records and recent airphotos (1997). 2.1. Study area The NWSP (approximately 928W, 468N) (Fig. 1) is a sandy glacial outwash plain that dates from the retreat of the Laurentide ice sheet at the end of the Wisconsin glaciation, about 10,000 years ago (Clayton, 1984). The sandy parent material has undergone minimal weathering, and the soils can be classified as Entic Haplorthods. They are extremely drought-prone and nutrient-poor because of the sandy parent material and the active fire history of the region. In particular, they have low N and organic matter and highly variable P (Hole, 1976). The regional climate is continental with cold, dry winters and warm, wet summers. Despite the abundance of summer precipitation, drought conditions are common during the growing season because of the coarse soil texture (Hole, 1976). The native vegetation includes P. banksiana and P. resinosa forests and savannas, oak woodlands and open pine barrens (Pregitzer and Saunders, 1999). We focused on the soils underlying nonforest vegetation (i.e., potential pine barrens) because of conservation concerns. Pine barrens are globally rare and in decline due to fire suppression (Givnish, 1995). We sampled the soils underlying forests as a point of comparison for the patterns observed within the nonforested areas. This work was paired with a vegetation study describing disturbance legacies in vegetation (Grossmann, 2006). Like the rest of northern Wisconsin, the forested portions of the sand plain were clearcut near the turn of the 20th century (Rowlands, 1949). Afterwards, it experienced a brief history of farming in limited areas (ca. 1900–1940) (Gough, 1997). Now, production-oriented forestry and recreation are the primary land

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Table 1 Soil variables and analysis methods used Variable

Method of measurement

Reference

Total nitrogen Total organic matter (OM) Total phosphorus pH Ca Mg K

Kjeldahl Loss on ignition Bray’s pH meter in buffered soil solution Atomic absorption Atomic absorption Atomic absorption

Bremner (1965) Combs and Nathan (1998) Bray and Kurtz (1945) McLean (1982) Thomas, 1982

All nutrient concentrations are reported as milligrams per kilogram (mg/kg), with the exception of organic matter which is reported as a percentage.

uses. Wildfire suppression began near the turn of the century (Luening, 1928) and became effective in approximately 1930. However, in years of extreme drought, large intense wildfires sometimes still occur, including the Five-Mile fire that burned 5400 ha in 1977.

3. Results and discussion Our data consistently and distinctly showed agricultural legacies within the soil in a variety of dimensions, but we observed few differences between the soils of the nonforest-fire class and the nonforest-clearcut class.

2.2. Field sampling 3.1. Average soil properties We located three study blocks (4 km  4 km) partly overlapping the Five-Mile fire, and placed eighty-three 150 m transects, randomly stratified among five sample classes within each block (illustrated in Fig. 1). We identified the locations meeting the criteria for our sample classes with a GIS (ESRI, 2002) that incorporated information on 1938 farms (identified from historic airphotos), an outline of the Five-Mile fire (historic fire records, Wisconsin Department of Natural Resources) and current forest cover (1998 airphotos). We do not have records of the type of farming that occurred within our ‘‘nonforest-farm’’ class due to poor resolution in the 1938 airphotos. Along each transect, we collected five soil samples at 30 m intervals. Each sample was a composite of the top 5 cm of the mineral soil within five soil cores (2 cm diameter). All samples were analyzed for a variety of properties (Table 1) at the University of Wisconsin soil and plant analysis lab in Madison, WI. All nutrient concentrations are reported here as milligrams per kilogram (mg/ kg), with the exception of organic matter which is reported as a percentage. When organic matter was compared or modeled with other nutrient concentrations, both were reported as percentages. Although agricultural practices can alter soil bulk density, confounding legacies in nutrient concentrations (Compton et al., 1998), we report only concentrations here because we did not directly measure bulk density, and expect little change in this parameter due to the coarse, sandy soils. 2.3. Data analysis We analyzed average values for the five soil samples within each transect as well as average elemental ratios (i.e., N:P, OM:N and OM:P, Ca:Mg, pH:OM, pH:base cations and base cations:OM). We compared response variables among the sample classes, with generalized least squares models (analogous to ANOVA), incorporating spatial autocorrelation into the error-term when necessary (r module nlme: Pinheiro et al., 2005). We also applied power analysis to determine the minimum difference that we could detect with 95% power. We assessed variability in the data at two scales. We assessed among-transect variability, with Levene’s test for homogeneity of variance (Schultz, 1985), and assessed within-transect variability by modeling the coefficient of variation within each transect as a response variable for generalized least squares models as described above. All analyses were carried out within the R software package, an open-source environment that is closely related to S+ (R Core development team 2005).

For all of the soil variables measured, the nonforest-clearcut and nonforest-fire classes were statistically similar. We list the sizes of the differences that we were able to detect with 95% statistical power in (Table 2). Organic matter and N concentrations were lower within the nonforest-farm class than the other classes (Fig. 2a and b). Phosphorus concentrations were higher within the nonforest-farm class (45 mg/kg) than in the other classes (25– 29 mg/kg) (Fig. 2c). Calcium was lower in the deciduous forest soils (244 mg/kg) and the nonforest-farm soils (257 mg/kg) than soils in the other classes (380–460 mg/kg) (Fig. 2e). Forest soils were more acidic than nonforest soils, and deciduous forest soils (pH 4.6) were more acidic than both evergreen forest soils (pH 4.8) and nonforest soils (pH 5.0) (Fig. 3). Potassium concentrations were slightly lower in the evergreen forest soils (47 mg/kg) than the soils of the other classes (50–61 mg/kg) (Fig. 2d). Magnesium was higher in the clearcut and fire classes (57 and 55 mg/kg respectively) than the other classes (35–44 mg/kg) (Fig. 2f). The similarities we observed between the nonforest-clearcut class and nonforest-fire class with respect to organic matter and N were inconsistent with other studies of fire effects on soil C and N (e.g., DeLuca and Zouhar, 2000), probably because our observations came 26 years after the Five-Mile fire. The average P concentration was also similar between fire and clearcuts. Because fire did not deplete soil P on average, it is possible that N-fixation has restored N lost to combustion during the fire. In boreal jack pine forests, soil N gradually rebuilds from N-fixation associated with sweetfern (Lynham et al., 1998), a shrub that is widespread within the NWSP, especially in the nonforest-fire class (Grossmann, 2006). The Nfixing guild can contribute to ecosystem resilience with respect to N in native fire-prone ecosystems (DeBano et al., 1998; Newland and DeLuca, 2000). Table 2 Minimum detectable differences between the nonforest-clearcut and nonforest-fire classes, with 95% statistical power Detectable difference OM N P K Ca Mg pH

0.7 441 18 10 161 17 0.23

% mg/kg mg/kg mg/kg mg/kg mg/kg log[H+]

Note: instrumental detection limits were not accounted for in this calculation.

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Fig. 2. Means and standard errors of soil nutrient concentrations among the five sample classes. Error bars represent one standard error, based on generalized least squares models. We modeled average transect values for each nutrient. p-Values are derived from the generalized least squares models.

The similarities between the nonforest-fire and nonforestclearcut classes with respect to all of the other variables suggest that the sand plain ecosystem may be resilient to fire and clearcutting disturbances, or at least that these two disturbances act in similar ways. Given its long-term history of fire (Hotchkiss et al., 2007; Lynch et al., 2006; Radeloff et al., 2000b; Niemuth and Boyce, 1998), resilience to fire seems likely. A large portion of the vegetation exhibits adaptations to fire disturbance, such as the ability to re-sprout, symbiotic nitrogen fixation and serotinous cones (e.g., oaks, sweetfern and jack pine respectively). Therefore, vegetation cover often rebounds quickly after canopy removal (Niemuth and Boyce, 1998). Quick recovery of the re-sprouting shrubs could help to limit leaching and erosion losses and speed organic matter and nutrient recovery. Contrasting with the minimal effect of fire, the soils beneath former farms differed from those that had experienced fire or

clearcutting. Although agriculture often causes significant erosion (Pimentel and Kounang, 1998), the flat topography and very sandy, well-drained soils present in the NWSP suggest that erosion from agriculture may have been relatively limited here. The low organic matter suggests that the farms were probably plowed at least once, rather than only pastured. Whatever the practice, the former farms are now associated with the most nutrient-poor soils in the sand plain, except for with respect to P. Most farming during the early 1900s relied on organic fertilizers, such as manure, which can raise soil P dramatically (Sharpley et al., 2004). If local farmers used synthetic fertilizers when they became cheaply available in the 1950s, this effect would only have been accentuated. Because it is less prone to leaching losses than mineral N, P additions are more likely to persist for long periods (Newman, 1995). Phosphorus legacies have even been detected from Roman land use >1750 years ago in France (Dupouey et al., 2002), and so a persistent P legacy is unsurprising here even though the Roman gardens may have been in use for longer than the farms of the NWSP. Although there were significant base cation losses in the former farms there was no associated drop in pH. For most of the measured soil properties, the evergreen and deciduous forests were similar. The greatest differences between the two forest types emerged in pH and Ca. Aspen is known to sequester large amounts of Ca within its bole bark (Ruark and Bockheim, 1988), and H+ ions exuded by roots during Ca uptake by aspen can lower soil pH (Perala and Alban, 1982). Additionally, Noble et al. (1999) showed more acid soils beneath cork oak than below radiata pine (P. radiata). At the same time, they observed depletion of mineral soil Ca below the oaks, but increased Ca concentrations in oak litter. 3.2. Ratios

Fig. 3. Means and standard errors of soil pH among the five sample classes. Error bars represent one standard error, based on a generalized least squares model. We modeled average transect values for pH. The p-Value is also derived from the generalized least squares model.

3.2.1. Organic matter, N and P The organic matter to N (OM:N) ratio was consistent across all of the sample classes (Fig. 4a), while OM:P and N:P were lower within the soils of the nonforest-farming sample class (Fig. 4b and

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c). OM:P was consistent among the other four sample classes (Fig. 4b), but there was suggestive evidence that the soils affected by the Five-Mile fire have a slightly higher N:P ratio than the unburned soils. However, we lack the statistical power to confirm the trend (Fig. 4c). The consistency of OM:N that we observed contrasts with the work of Parker et al. (2001) who found low C:N ratios within soils affected by fire. Given that C:N often correlates with N-mineralization rates (Brady and Weil, 1996), it is possible that the FiveMile fire had no long-term impact on plant-available N. However, other factors that we did not measure could be important determinants of N availability such as litter C:N ratio, microbial community composition (Pietika¨inen and Fritze, 1995) and the presence of phenolic secondary chemicals (Wardle et al., 1998). Further investigation into the relationships between N-fixers, soil P and soil N would be useful for clarifying how N-fixers can contribute to ecosystem resilience in the NWSP. N-accumulation within the soils of the NWSP could be limited by N-fixation rates, or organic matter accumulation. The second explanation (OM limits N-accumulation) is supported by the consistent OM:N ratio among all of the sample classes, as well as a tight correlation between N and OM within our data (R2 = 0.75, Fig. 5). 3.2.2. Base cations and pH The ratios contrasting the base cations (Ca, Mg and K, referred to collectively as BC) with pH and organic matter varied among the sample classes (Fig. 6). The pH:OM ratio was highest within the nonforest-farm class. The BC:OM and Ca:Mg ratios were lowest within the deciduous forests. The pH:BC ratio was highest within the deciduous forests and nonforest-farm classes. The high pH:BC ratio that we observed is consistent with the observations of (Perala and Alban, 1982; Noble et al., 1999). The relatively low Ca:Mg ratio below the deciduous forests suggests that leaching of base cations probably does not explain the Ca losses in this case, as leaching should deplete each cation equally. The low pH that shapes the pH:BC ratio may relate to humus chemistry (Binkley and Richter, 1987). Further study may be warranted to assess whether Ca could be a limiting element in these soils. A different pattern emerges within the nonforest-farm class. Given the depletion of Ca within the mineral soil, we expected but did not find an associated drop in pH (Figs. 2 and 3). Because Ca is not depleted relative to Mg within the farmed soils, as it is in the deciduous forests (Fig. 6d), base cation leaching may be a more likely explanation for the low Ca than plant uptake in this case. The unusual properties of the formerly farmed soils may help to account for the unusual components of their vegetation. For example, oval milkweed (Asclepias ovalifolia, which is listed as threatened in Wisconsin) is significantly more common within the former farms (Grossmann, 2006). Other factors, such as alteration

Fig. 5. There is a strong relationship between organic matter and nitrogen, as shown by this simple plot, and linear regression model (described at the top of the plot). The horizontal lines within the graph originate from the instrumental detection limits for nitrogen. One outlier was deleted from this analysis.

of the soil seed and bud bank from farming activities might also account for the unique aspects of the plant communities of the former farms. Regardless of the reason, the unique soils of the abandoned fields within the NWSP support a component of the native flora, and their value should be acknowledged in regional conservation plans. 3.3. Variability For six of the seven soil variables, Levene’s test suggested that among-transect variance was constant among the five sample classes. Only P showed differences in among-transect variance among the sample classes (Levene’s test p = 0.044, Fig. 7). Pairwise comparisons among the sample classes showed that amongtransect variance for P was higher within the Five-Mile fire than it was within the deciduous forests (p = 0.008) and the clearcuts (p = 0.024). Additionally, the deciduous forests had lower amongtransect variance in soil P than the nonforest-farm class (p = 0.054). It is surprising that among-transect variability for most of the soil properties was consistent among the sample classes. Fraterrigo et al. (2005) showed that variability in soil nutrients can be a powerful tool for detecting agricultural legacies in southern

Fig. 4. Means and standard errors of elemental ratios for N, P and organic matter among the five sample classes. Error bars represent one standard error, based on generalized least squares models. Ratios were generated from average transect values for each nutrient. p-Values are derived from the generalized least squares models.

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Fig. 6. Means and standard errors for ratios among pH, Ca, Mg, and the base cations among the five sample classes. Error bars represent one standard error, based on generalized least squares models. Ratios were generated from average transect values for each nutrient. p-Values are derived from the generalized least squares models.

Appalachia, observing higher variance in intensively used soils for some of their measured properties, and lower variance for others. However, we observed stronger agricultural legacies in nutrient pools than variance, and among-transect variance for each soil property was quite consistent among the three nonforest sample classes. The apparent discrepancy between our work and theirs may be an artifact of the different spatial scales of our sampling

designs. They addressed a finer spatial grain. Additionally, the naturally high variance of the sand plain soils with respect to soil P (Hole, 1976) may have obscured the broad-scale effect that farming history might have had on P variance. At the withintransect scale, one potential explanation for why we observed no agricultural impact on soil P variance is the possibility that soil amendments were applied evenly by farmers in a cropping system, rather than haphazardly by animals in a pasturing system. The only case where we actually detected a correlation between variance and disturbance history was for P, which showed higher among-transect variance within the Five-mile fire. This suggests that the fire was intense enough to either volatilize P directly, or to redistribute P via ash convection. At a fine spatial scale, within-transect coefficient of variation (CV) for Ca varied among the sample classes (Fig. 8). Calcium was

Fig. 7. Simple boxplot of raw data illustrates among-transect variability in soil phosphorus. The variance (Var) and sample-size (n) for each class is listed at the bottom of the figure. Classes with the same letters do not have statistically different variances (a < 0.05 from pairwise Levene’s tests).

Fig. 8. Means and standard errors for within-transect coefficients of variation for calcium among the five sample classes. Error bars represent one standard error, based on generalized least squares models. p-Values are derived from the generalized least squares models.

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the only soil property to show detectable patterns in variability by this measure. Although we did not detect agricultural legacies in the variability of soil properties, it is possible that the spacing of our soil samples was too sparse to detect land use legacies in the spatial patterning of nutrients. Fraterrigo et al. (2005) sampled at a finer spatial scale (minimum spacing = 1 m). Finer scale studies might show legacies in nutrient spatial patterns within the nonforest soils. Further study into the scale of spatial patterns would also be useful because those patterns may shape plant community composition and diversity as post-disturbance succession proceeds (Hutchings et al., 2003). 4. Conclusions  The soils with fire and clearcut-only histories had similar nutrient pools, but the burned soils had higher variance in phosphorus.  Forested soils had lower pH, and lower calcium concentrations than nonforested soils.  Soils with an agricultural history had low organic matter and nitrogen, and very high phosphorus. They also had low calcium, but not low pH. The similarities between the soils with fire and clearcut-only histories suggest that soil nutrient pools in the NWSP may be resilient to above-ground disturbances within a 26-year timeframe. However, it is important to remember that these results only apply directly to the 26-year effects of a single wildfire within the northwest sands of Wisconsin. That said, our finding is fairly consistent with the ecological literature on the long-term fire and clearcutting effects on forest soils (Reich et al., 2001). It is also important to remember that we did not measure the short-term effects of the disturbances, which may be important to the region’s native plant diversity (Niemuth and Boyce, 1998; DeLuca and Zouhar, 2000; Giardina and Rhoades, 2001; Wienk et al., 2004), and should not be discounted. However, the similarities we observed in the fire and clearcut-only soils support the assertion that clearcutting may be one useful component of a landscape-scale restoration plan for this particular ecosystem that was once characterized by an active fire regime (Radeloff et al., 2000a). A variety of deciduous tree species can sequester significant amounts of Ca in their biomass, temporarily removing it from the soil Ca pool (Perala and Alban, 1982; Wilson and Grigal, 1995; Noble et al., 1999). Concern has been voiced that Ca removals from forest harvests may threaten long-term site productivity (Federer et al., 1989; Johnson et al., 1997; Brais et al., 2000; Adams et al., 2000). The pattern we observed of low soil Ca below the deciduous forests, in conjunction with the literature on the topic suggests that accounting for Ca in long-term forest management plans for the NWSP would be wise. The major long-term effects of farming include pronounced and persistent losses in organic matter and N as well as gains in soil P. These findings are consistent with the growing body of research on agricultural legacies in soil. The strength of the farming effect that we observed within these soils is highlighted here by contrasting it with the more recent fire disturbance that we studied. Agriculture is a clearly novel disturbance to this ecosystem, generating soils with unique attributes, and therefore agricultural history should be accounted for while planning for future forest management and conservation of open barrens in the region. References Adams, M.B., Burger, J.A., Jenkins, A.B., Zelazny, L., 2000. Impact of harvesting and atmospheric pollution on nutrient depletion of eastern US hardwood forests. Forest Ecology and Management 138, 301–319.

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