Forest, Range & Wildland Soils
Tree Encroachment Impacts Carbon Dynamics in a Sand Prairie in Wisconsin B. C. Scharenbroch* Dep. of Research The Morton Arboretum 4100 Illinois Route 53 Lisle, IL 60532-1293
M. L. Flores-Mangual
Dep. of Biology Pennsylvania State Univ. 208 Mueller Lab. University Park, PA 16802-3303
B. Lepore
Dep. of Natural Resources Ball State Univ. 2000 W. University Ave. Muncie, IN 47306-1022
J. G. Bockheim B. Lowery
Dep. of Soil Science Univ. of Wisconsin 1525 Observatory Dr. Madison, WI 53706-1299
A sandy prairie remnant in the Lower Wisconsin River Valley, encroachment areas within the prairie, and an adjacent red pine (Pinus resinosa Aiton) plantation were studied to determine the influence of woody cover on C dynamics. Field transects, aerial imagery, and a geographic information system were used to quantify encroachment from 1979 to 2002. A linear encroachment model predicted 100% encroachment of the 6.0-ha prairie in 50 yr. Four field plots in each of pine, prairie, and encroachment areas were sampled and soils collected (0–18, 18–38, and 38–75 cm) in 2004 and 2008. Total ecosystem C was greater in pine (126.6 Mg C ha−1) and encroachment areas (71.8 Mg C ha−1) than prairie (48.3 Mg C ha−1). In the 0 to 38 cm, coarse particulate organic matter (POM) (4.1, 6.3, and 7.5 Mg C ha−1) and the POM C/N ratio (15.1, 16.2, and 20.2) increased with woody encroachment (prairie, encroachment areas, and pine, respectively). Changes in POM suggest more organic inputs and slower decomposition, but due to minimal protection of C within aggregates, increased total soil C was not observed with woody advancement (46.4–47.2 Mg C ha−1). Microbial biomass (0–38 cm) was greatest in encroachment areas, followed by prairie, and then pine (108, 84, and 51 kg N ha−1, respectively), probably a result of more favorable microclimate and substrate at the ecotone boundary. Potential N mineralization (0.6, 2.8, and 4.8 kg N ha−1 d−1), extractable NH4+ (28, 33, and 57 kg ha−1), and Bray-1 P (380, 402, and 541 kg ha−1) (0–38 cm) increased with woody cover, and increased nutrient availability could lead to a greater aboveground C sink through increased tree growth. Abbreviations: cPOM, coarse particulate organic matter; DBH, diameter at breast height; DON, dissolved organic nitrogen; fPOM, fine particulate organic matter; MBC, microbial biomass carbon; POM, particulate organic matter; scSOM, silt- and clay-associated soil organic matter; SOC, soil organic carbon; SOM, soil organic matter; TEC, total ecosystem carbon.
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oody plant encroachment is a widespread and significant alteration to grasslands (Archer, 1995; Gill and Burke, 1999). Grasslands store more than 30% of global soil C and cover approximately 40% of terrestrial surface (Schlesinger et al., 1990; Schlesinger, 1997). Woody encroachment into grasslands and savannas in the United States has been estimated to result in the sequestration of 0.10 to 0.13 Pg C yr−1, or 20 to 40% of the C-sink strength (Tilman et al., 2000; Houghton et al., 1999, 2000; Pacala et al., 2001). Relatively little is known about the soil C dynamics associated with tree encroachment in dry sand prairies (e.g., Anderson, 1987; Quideau and Bockheim, 1996, 1997). Dry sand prairie is a unique landscape found in Iowa, Illinois, Indiana, Michigan, Minnesota, Wisconsin, and southern Ontario (Quideau and Bockheim, 1996; Kost, 2004). In 1960, sandy prairie soils covered approximately 1.65 million ha (0.8%) of the north-central United States (Bidwell, 1960). Land use change, fire suppression, vehicle traffic, invasive plants, and woody plant encroachment are major threats to dry sand prairies (Thompson, 1970; Wisconsin Department of Natural Resources, 1995; Packard and Mutel, 1997; Kost, 2004). Sand prairie soils have low soil C contents (Quideau and Bockheim, 1996), thus
Soil Sci. Soc. Am. J. 74:2010 Published online 12 Feb. 2010 doi:10.2136/sssaj2009.0223 Received 15 June 2009. *Corresponding author (
[email protected]). © Soil Science Society of America, 677 S. Segoe Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
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landscape changes will probably have immediate and dramatic impacts on C turnover. Commonly cited consequences of woody encroachment include decreased plant diversity (Hoch and Briggs, 1999) and increased productivity and nutrient storage in woody biomass (Gill and Burke, 1999; Norris et al., 2001; Knapp et al., 2008). The direction and magnitude of responses in soil properties to encroachment have been less consistent but impacts have been reported on soil climate (Smith and Johnson, 2004), soil nutrient availability (McPherson et al., 1993; Archer, 1995), and soil microbial activity and biomass (Hibbard et al., 2001; Liao and Boutton, 2008). Soil C storage has been found to be unaffected (McCarron et al., 2003; Smith and Johnson, 2003), to decrease ( Jackson et al., 2002), and to increase (Mordelet et al., 1993; Archer et al., 2001; Jessup et al., 2003; Liao et al., 2006b; Filley et al., 2008) with woody encroachment. Although the response of total soil C to encroachment seems inconsistent, many have found labile soil C pools to increase with increased woody cover (Quideau and Bockheim, 1996; Billings, 2006; Liao et al., 2006a,b; Filley et al., 2008). Soil organic matter (SOM) is a heterogeneous pool of C compounds with varying degrees of C sequestration potential and soil residence times (Buyanovsky et al., 1994; Lal et al., 2001). Physical SOM fractionation identifies size or density separates with differing turnover rates, sensitive to land use and land cover changes (Tiessen and Stewart, 1983; Elliot, 1986; Elliot and Cambardella, 1991; Jastrow, 1996; Christensen, 2001; Chefetz et al., 2002; Six et al., 2002; Conant et al., 2003; Gregorich et al., 2006). Billings (2006) compared acid hydrolysis, density, and size fractionation techniques, finding the size technique to be the most effective method to differentiate SOM between grassland and adjacent forest. Greater nutrient mineralization rates are often associated with newer, less protected, macroaggregate fractions (i.e., POM, 2.0–0.053 mm), while older silt- and clay-associated SOM (scSOM, 50% of the projected ground surface area. The remnant prairie is a dry grassland community dominated by grasses (Andropogon scoparius Michx., Andropogon gerardii Vitman, Carex pensylvanica Lam., Schizachyrium scoparium (Michx.) Nash, Koeleria spp., and Panicum virgatum L.) and drought-adapted fungi, lichens, and mosses. Trees in encroachment areas included oak (Quercus velutina Lam., Q. macrocarpa Michx., and Q. alba L.) and red, white (P. strobus L.), and jack pine (P. banksiana Lamb.). The remnant prairie is surrounded on the east and west sides by plantations of red pine and white pine. The effective rooting depths of the prairie and pine plantation were measured at 71 and 64 cm, respectively (Quideau and Bockheim, 1997). Leaf area indices for the prairie and pine were 0.3 and 5.6 m2 m−2 (Quideau and Bockheim, 1997). Soils from the experimental area have formed in sandy outwash deposited on nearly level terraces along the Wisconsin River during the late Wisconsin glaciation (?14,000 yr BPE). The soils are classified as sandy, mixed, mesic Entic Hapludolls, Sparta series, which are prairie soils with thick (45-cm), dark A horizons. Groundwater depths range from 2.5 to 3 m. The C horizons have a sandy texture (99.6–99.7% sand, 0.1% silt, 0.1–0.2% clay), a medium-acid pH (5.6–5.9), bulk density of 1.52 to 1.54 g cm−3, and low exchangeable bases (0.10–0.11 cmolc kg−1) (Quideau and Bockheim, 1997). The climate is continental with a mean annual air temperature of 7.4°C and mean annual precipitation of 790 mm (Quideau and Bockheim, 1997). Average January and July temperatures are −8.8 and 22°C, respectively.
Quantification of Encroachment in the Prairie Remnant Two approaches were used to quantify encroachment in the remnant prairie. The first utilized a temporal sequence of spatial data from the USDA Farm Service Agency aerial compliance imagery (Iowa County, WI, 2004). These aerial photos were taken annually during the growing season from a small aircraft for measuring crop coverage for tax and conservation assessments. The slides were scanned using a Kodak slide scanner and saved as TIFF images. A digital orthophoto of Iowa County was obtained and ArcMap was used to georeference the scanned TIFF files and convert them to GeoTiff files (ArcGIS version
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9, ESRI, Redlands, CA). Once georeferenced, the area of encroachment within the prairie remnant for 6 yr (1979, 1983, 1987, 1991, 1999, and 2002) was estimated by visual inspection of the image. Shapefiles were created by drawing borders around encroached areas, and the areas of the shapefiles were calculated. The total encroached area as a percentage of the prairie remnant was calculated. The second approach to quantify encroachment used field measurements along random transects in the remnant prairie. On 4 and 25 Apr. 2005, six transects in random azimuths from randomly selected points were measured in the prairie remnant. Transect lengths ranged from 27 to 120 m and the transects ran until they crossed the remnant prairie boundary, up to 120-m maximum. The species of all trees within the 1.0-m-wide transect were identified. Tree diameters were measured at 1.37 m (diameter at breast height, DBH). Any stem 94% sand). Soil samples (50 g) were shaken for 15 h with sodium hexametaphosphate [(NaPO3)6] and then passed through a series of nested sieves. The organic matter collected on the 250- and 53-µm sieves was considered cPOM and fPOM, respectively. Silt- and clay-associated SOM passed through the 53-µm sieve and was determined by scSOM = total SOM – fPOM − cPOM. The SOM fractions were analyzed for C and N contents using automated dry combustion. Samples from 2004 were analyzed on a LECO CN-2000 (LECO Corp., St. Joseph, MI) and an Elementar Vario EL III (Elementar, Hanau, Germany) was used for the 2008 samples. Values of C and N fractions were expressed as megagrams N or C per hectare or as the C/N ratio of those fractions. The sum of tree, understory, standing dead wood, down dead wood, forest floor, and soil organic C (SOC) was total ecosystem C (TEC). The following analyses were performed on the 2008 soils only. Soil subsamples were weighed at field moisture content, dried at 105°C, and then reweighed to determine the gravimetric soil moisture content (%). Soil pH and electrical conductivity (µS cm−1) were measured using 1:1 soil/deionized water pastes (Model Orion 5-Star, Thermo Fisher Scientific, Waltham, MA). Soil subsamples were extracted 1 mol L−1 NH4OAc (pH 7.0) and exchangeable K+, Ca2+, Mg2+, and Na+ (mg kg−1) were determined with atomic adsorption spectroscopy (Model A5000, PerkinElmer, Waltham, MA) (Schollenberger and Simon, 1945). Soil P was determined with the Bray-1 extraction and analyzed colorimetrically at 882 nm on a spectrophotometer (Model UV mini 1240, Shidmadzu, Kyoto, Japan) (Olsen and Sommers, 1982). The fumigation–extraction method (Brookes et al., 1985; Cabrera and Beare, 1993) was used to determine microbial biomass C (MBC) and N. Soil subsamples were fumigated with ethanol-free chloroform for 5 d, extracted with 0.5 mol L−1 K2SO4, and total extractable N was reduced to NH4+ with persulfate and Devarda’s alloy for absorbance readings at 650 nm (Model ELx 800, Biotek Instruments, Winooski, VT) (Sims et al., 1995). Microbial biomass N was the difference in N between the fumigated and unfumigated samples, using an extraction efficiency factor of KEN = 0.54 ( Joergensen and Mueller, 1996). Colorimetric N analyses were also used for determination of dissolved organic N (DON), NO3−, and NH4+ in unfumigated samples (Sims et al., 1995). Dissolved organic C in fumigated and unfumigated extracts was determined by persulfate digestion (Horwath and Paul, 1994). Soil microbial biomass and nutrient concentrations were converted to kilograms per hectare using bulk density and sample depth. Potential C and N mineralization was measured using 20-d soil incubations in the dark, at 25°C, and with soils adjusted to 40% water content. Carbon dioxide in 0.25 mol L−1 NaOH traps was precipitated with 0.5 mol L−1 BaCl2, followed by 0.25 mol L−1 HCl (standardized) titration to a phenolphthalein endpoint (Anderson and Domsch, 1978; Parkin et al., 1996). Concentrations of NH4+ and NO3− in incubated soils and unincubated soils were determined colorimetrically as previously described. Potential N mineralization was determined by subtracting the total inorganic N (NH4+ and NO3−) in base extracts from the extracts of the incubated soils. The metabolic quotient qCO2 (g CO2 h−1 kg−1/g MBC kg−1) and MBC/SOC were computed for each sample using potential C mineralization, MBC, and SOC.
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Statistical Analysis Statistical analyses were conducted using SAS JMP 7.0 software (SAS Institute, Cary, NC). Normality assumptions were met according to the Shiparo–Wilk W goodness-of-fit test, and data transformations were not necessary. Significance of the main effects of prairie, pine, and encroachment areas and interactions with depth were analyzed using ANOVA. Temporal changes and time × treatment interactions between 2004 and 2008 were analyzed using repeated measure ANOVA. Mean separations were performed with Tukey’s honestly significant difference test. Linear correlations were performed to identify relationships among the variables in our data set. All significant differences were determined assuming a 95% confidence level.
RESULTS
Tree Encroachment in the Remnant Prairie
Tree density in the remnant prairie averaged 4839 stems ha−1 (Table 1). The mean DBH was 8.8 cm and mean caliper of seedlings was 13.6 mm (Table 1). The average tree height was 3.2 m and the average age was 11.1 yr (Table 1). The majority (73%) of trees in the encroached area of the remnant prairie occurred in the 6- to 15-yr age class. The average number of trees on the four 196-m2 encroachment plots was 43, with a mean density of 2207 stems ha−1 (Table 2). Tree density was less on the encroachment plots because seedlings and saplings were not quantified on the 196-m2 plots, but they were on the six transects. The mean tree diameter on the encroachment plots was 8.7 cm (Table 2). Pine plots had on average 17 trees (867 trees ha−1), with an average DBH of 21.8 cm (Table 2), compared with the encroachment plots with 43 trees and a mean DBH of 8.7 cm (Table 2). Encroachment plots were 90% red, white, and jack pine, with the remaining 10% as Quercus velutina, Juniperus virginiana L., and Prunus serotina Ehrh. Of the 6.0-ha original prairie, the tree-encroached area was 19% (1.14 ha) in 1979 and 59% (3.54 ha) in 2002 (Fig. 2). The greatest increase in encroachment occurred in the period 1987 to 1999 (Fig. 2). A linear model (y = 2.0117x + 0.4641; R2 = 0.9072; P = 0.0033), provided the best fit to the temporal sequence of encroachment in the remnant prairie. According to the linear model, the 6.0-ha remnant prairie was 0% encroached in 1973 and will be 100% forest in 2023.
Microclimate Mean monthly air temperatures did not differ between pine and prairie environments (mean of 6.5 and a range from −7.7°C in January to 22.1°C in July). Solar radiation was greater (P ≤ 0.0001) in the prairie than the pine forest throughout the year (annual mean in the prairie of 137 W m−2, and 34 W m−2 in the pine). Mean monthly soil temperatures were greater (P ≤ 0.0500) in the remnant prairie than the pine plantation during the growing season (April–August), but during the dormant season (October–February), soil temperatures under the pine were greater (P ≤ 0.0500) than under the prairie (Fig. 3). Soil moisture contents were greater (P ≤ 0.0500) in the prairie than the pine in all months but December (Fig. 3). Soil moisture determined SSSAJ: Volume 74: Number 3 • May–June 2010
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2 1 0 0 4 1 1 2 3 0 0 2 0 1
>25 yr 21–25 yr 16–20 yr
—————— no. in class —————— 11 5 2.0 1 11 2 0 0 6 4 6 2 0 0 0 0 9 1 0 0 0 16 0 2 6 5 1 1 — yr — 11.2 6.5 11.3 11.5 13.7 5.6 0.0 0.0 16.6 16.9 14.1 5.0 11.1 7.6 –m– 3.6 3.2 2.4 3.6 4.5 3.6 0.0 0.0 3.9 5.6 4.5 1.9 3.2 3.0 - mm 12.4 5.9 17.1 7.6 17.2 4.9 0.0 0.0 15.2 7.7 19.5 6.8 13.6 5.5
Age
10–15 yr 6–10 yr 0–5 yr
Height Caliper‡
Mean SD Mean SD Mean SD SD
DBH†
Mean
Density Length Azimuth Longitude Latitude
— cm — stems ha−1 1 43°10.745 N 89°56.385 W 5,932 5.7 4.1 2 43°10.811 N 89°56.389 W 10,370 9.1 18.7 3 43°10.880 N 89°56.390 W 3,315 10.2 7.4 4 43°10.764 N 89°56.318 W 0 0.0 0.0 5 43°10.925 N 89°56.370 W 2,655 21.5 11.3 6 43°10.771 N 89°56.377 W 6,764 6.6 6.0 Mean 4,839 8.8 7.9 † Diameter at breast height at 1.37 m from ground level was measured for all trees ≥ 3 m tall. ‡ Stem caliper at 0.15 m from ground level was measured for all trees F
Trees, no. plot−1 Density, trees ha−1
17.0 ± 1.4 a† 0.0 ± 0.0 a 43.3 ± 8.8 a 18.01 0.0007*** 867.4 ± 72.2 b 0.0 ± 0.0 b 2206.6 ± 447.9 a 18.01 0.0007*** Diameter at breast height, cm 21.8 ± 0.7 a 0.0 ± 0.0 c 8.7 ± 0.7 b 347.75
