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Published in Soil Sci. Soc. Am. J. 68:263-271 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-7—FOREST & RANGE SOILS

Role of Vegetation in Mitigating Soil Quality Impacted by Forest Harvesting

T. W. Listera, J. A. Burger*,a and S. C. Pattersonb

a Dep. of Forestry (0324), Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061
b Forest Science Lab., Westvaco Corp., 180 Westvaco Road, Summerville, SC 29483

* Corresponding author (jaburger{at}vt.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The initial growth response of loblolly pine (Pinus taeda L.) to competition control is well documented. However, the benefits of competing vegetation to soil quality have not been thoroughly investigated. A study was conducted: (i) to evaluate the effectiveness of bedding site preparation in recovering soil processes and productivity following disturbance; and (ii) to determine if different levels of vegetation control have differential effects on soil processes that aid in the recovery and maintenance of soil productivity. Study plots were established on a wet pine flat on South Carolina's Coastal Plain. Treatments included a range of three disturbance classes (undisturbed, compression tracked, and churned), two site preparation treatments (flat planted and bedded), and a gradient of vegetation control (no vegetation control, operational level weed control, and complete weed control). Compacted soils generally increased in bulk density (14%) and decreased in macroporosity (25%) and hydraulic conductivity (69%) compared with undisturbed treatments. Churning had no significant net effect on the soil physical properties measured; however, it did increase net N mineralization by over 100% on flat-planted treatment plots. Bedding fully ameliorated the effects of soil compaction based on the physical properties measured. Trends suggest some improvements in soil quality with increasing levels of non-crop vegetation biomass; however, during 2 yr of operational vegetation control, the beneficial effects of the non-crop vegetation were marginal.

Abbreviations: SMBC, soil microbial biomass carbon • TDR, time domain reflectometry


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CONCERN over the effect of harvesting disturbance on sustained forest and soil productivity has raised questions about the role of roots and soil flora and fauna in natural soil amelioration. Competitive non-crop vegetation is commonly controlled during site preparation in intensively managed plantation forests because of the growth response of southern pine seedlings to weed control; however, the benefits of a diverse herbaceous cover following disturbance and the role non-crop vegetation plays in mitigating soil quality are not well known.

Harvesting operations on poorly drained sites in the lower Coastal Plain can cause severe soil disturbance, including compaction, rutting, and churning (Aust et al., 1993, Gent et al., 1983). Soil compaction generally results in an increase in bulk density due to the consolidation of soil particles as air-filled voids are reduced. This results in increases in bulk density and soil strength, and decreases in macroporosity and saturated hydraulic conductivity (Greacen and Sands, 1980). Consequently, crop tree growth may be reduced due to poor soil aeration and a restricted, unfavorable rooting environment.

When trafficking occurs under saturated conditions and pores are water-filled, soil may flow, resulting in rutting and churning of surface soil layers (Aust et al., 1993). The characteristics of this churning and puddling are a destruction of natural soil structure, stability, and aggregation (Koenigs, 1963). The soil becomes uniform and massive, which may limit crop tree growth and reduce long-term site productivity.

Many silvicultural treatments, including bedding, ripping, and disking, are imposed to mitigate the effects of harvesting disturbance; however, these efforts to restore favorable soil properties may be short-lived or may be counterproductive (Greacen and Sands, 1980). In poorly drained lower Coastal Plain sites, the benefits of bedding to crop tree survival and growth are well documented (McKee and Shoulders, 1974; McKee, 1989). Bedding creates an elevated, well-drained, and aerated rooting zone for early pine growth. The effectiveness of bedding in restoring the soil physical environment for crop tree growth following disturbance, however, is not well known.

Natural processes such as weathering, the shrinking and swelling of 2:1 clays (McGowan et al., 1983; Sarmah et al., 1996), soil biological activity (Oades, 1993), and the active probing and sloughing of plant roots (Larson and Allmaras, 1971; Perfect et al., 1990), also help to restore soil properties in the lower Coastal Plain following disturbance. Of the natural recovery processes in this area, we hypothesized that root exploitation and the increased biological activity associated with the rhizosphere are the most important. The influence of the soil biota in the creation and stabilization of soil aggregates is well documented. Especially in soil where organic matter is a major binding agent, plant roots, and soil fauna contribute greatly to the formation and stability of soil aggregates (Jastrow and Miller, 1991). Structural pores are created and soil strength is reduced by the active growth and sloughing of roots. Mycorrhizal hyphae and root mucigels have been observed to enmesh soil particles and contribute to the formation and stabilization of macroaggregates (Jastrow and Miller, 1991). The presence of a dense and diverse belowground root system promotes soil biological processes including decomposition and N mineralization. Symbiotic plant-microbe associations may improve the soil environment and increase crop-species productivity.

Non-crop vegetation in pine plantation systems is commonly controlled with herbicides due to the positive response of pines to competition control (Morris et al. 1993; USDA Forest Service, 1984; Miller et al., 1991; Cain, 1991). The consequences of herbaceous vegetation removal for soil recovery and soil and forest productivity, however, have not been thoroughly explored.

Accordingly, the objectives of this study were: (i) to determine if soil disturbance during harvesting operations affects soil properties important for plant growth and productivity; (ii) to evaluate the effectiveness of bedding site preparation in recovering soil processes and productivity following disturbance; and (iii) to determine if different levels of vegetation control have differential effects on soil processes which aid in the recovery and maintenance of soil productivity.


   MATERIAL AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description and Research Design
The research site was established in a wet pine flat, intensively managed for loblolly pine production and located on the Coastal Plain of South Carolina, in Colleton County. The poorly drained soils on the study site, which were derived from marine and fluvial deposits, are dominated by two soil types, Argent loam (fine, mixed, active, thermic Typic Endoaqualfs) and Santee loam (fine, mixed, active, thermic Typic Argiaquolls) (Stuck, 1982). The average annual rainfall in this warm temperate region is 132 cm, with 82 cm falling between March and October. The average temperature between March and October is 31°C and between November and February is 18°C (Stuck, 1982).

The experiment was a 2 x 3 factoral split plot within a randomized complete block design, where blocks were chosen based on differences in drainage ditches and soil types (Fig. 1) . Factorial treatments included three levels of harvesting disturbance (none, compression track, and churn) and two types of site preparation (none or flat planted, and bedded). Each disturbance/site preparation combination was split into three 3.05 by 6.10 m plots, which received different levels of herbaceous weed control (none, operational control, and complete control). The soil disturbance gradient was achieved as a result of operational scale harvesting under both dry and wet conditions in the fall of 1993 and spring of 1994. Following clearcutting, soil disturbance was classified as described by Preston (1996). On average, 77% of the wet harvested area was disturbed, with 22% showing compression tracks, 31% shallow ruts, 20% deep ruts, and 4% churning. The dry-harvest operation caused only soil compaction, which covered 8% of the total dry-harvested area leaving the remaining land visually undisturbed Preston, 1996). In 1995, site preparation treatments were installed (see Kelting et al. [1999] for a detailed description of silvicultural methods and equipment used) and the sites were planted with 1-yr-old loblolly pine bare-root seedlings from genetically improved seed stock.



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  		Fig. 1. Schematic of research plots showing soil disturbance, site preparation and vegetation treatments for the 2 x 3 factorial with split randomized complete block study design, Colleton Co., SC.

 
The vegetation control treatment gradient was composed of three levels: the vegetated control, operational control, and complete vegetation control. The vegetated control subplots were protected from the initial herbicide application using plastic tarps. Before seedling planting, operational subplots were sprayed aerially with 280 g of Oust (75% sulfometuron-methyl) and 35 g of Escort (60% metsulfuron-methyl) per hectare. Vegetation-free subplots were treated as needed to maintain complete non-crop weed control throughout the first two growing seasons. A low-volume directed spray of 5% (by volume) Accord (41.5% glyphosate; Monsanto Co., St. Louis, MO) plus surfactant was applied with a backpack sprayer as needed.

Field and Laboratory Methods
To characterize soil processes within vegetation, disturbance class, and site preparation treatments, a series of in situ measurements was taken bimonthly for 1 yr beginning in June 1997.

Volumetric soil moisture, N mineralization, and aerated soil volume data were collected from each treatment combination (Fig. 2) . Time Domain Reflectometry (TDR) was used to measure the soil volumetric moisture content (TRASE System, Soil Moisture Equipment Corp., Goleta, CA). A set of TDR wave-guide rods was vertically inserted into the upper 30 cm of soil at each sample point. In addition, the volume of aerated soil was determined by measuring the depth to a reduced zone on a buried Fe rod (Carnell and Anderson, 1986; Bridgham et al., 1991).



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  		Fig. 2. Schematic of a soil process sampling point showing measurements and soil sampling depths.

 
Nitrogen mineralization was determined using a standard buried bag technique (Eno, 1960). A portion of a composite loose soil sample from the surface 30 cm at each point was air-dried to a workable moisture content, sieved (2 mm mesh) and analyzed for available inorganic N (N03–N and NH4–N). The other portion was buried for two months in a sealed polyethylene bag 5 cm beneath the soil surface, before the available N extraction. Inorganic soil N was extracted from approximately 5 g (dry mass) of soil using 100 mL of 2 M KCl. After shaking for 1 h, samples were filtered (Whatman No. 1, Whatman Ltd., Maidstone, UK) and filtrate was analyzed colorometrically using a Technicon Autoanalyzer II (Technicon, 1973). Data were converted to kilograms per hectare using bulk density measurements.

Four core samples (5 cm long, 4.8 cm diameter) were collected from the mineral soil between a depth of 5 and 10 cm in each treatment plot in June 1997 and 1998. For each year, two cores were used to determine bulk density (Blake and Hartge, 1986), total, macro-, and microporosity (Danielson and Sutherland, 1986), saturated hydraulic conductivity (Ksat) (Klute and Dirksen, 1986), and air permeability (Corey, 1986). Macroporosity is equivalent to non-capillary or aeration porosity, the pore space evacuated with a tension table with 50-cm water column (0.005 MPa). Microporosity is equivalent to capillary porosity, the pore volume remaining full of water under tension exerted by a 50-cm water column (Kohnke, 1968). The remaining two cores were air-dried and broken on planes of weakness. Aggregates >1 and <2 mm were analyzed for the percentage of stable aggregates using a standard wet sieve method (Kemper and Rosenau, 1986).

During the April and June 1998 sampling periods, a composite loose soil sample was collected and analyzed for soil microbial biomass C (SMBC) using a chloroform fumigation and C extraction procedure (Vance et al., 1987; Wu et al., 1990). For SMBC determination, fresh soil was sieved (0.6-cm mesh) and thoroughly mixed. Duplicate subsamples of approximately 25 g (dry weight basis) of soil were weighed in separate 50-mL beakers and placed in a vacuum desiccator along with a beaker of 25 mL of ethanol-free chloroform (CHCl3) and 100 mL of deionized water. A vacuum was drawn, allowing the chloroform to boil for 5 min. The vacuum was slowly released and then drawn three more times for 5 min. The desiccator was left under vacuum pressure in a darkened fume hood for 24 h. Fumigated soils and duplicate samples of unfumigated soil were then extracted with 100 mL of 0.5M K2SO4 and filtered (Whatman No. 42, Whatman Ltd., Maidstone, UK) before organic C analysis by ultraviolet-persulfate oxidation using a Dohrman DC 80 automatic analyzer. Soil microbial biomass C was calculated using the following equation:

where Kec is the rate constant (the fraction of SMBC extracted), in this case 0.39. (Sparling and West, 1988).

To assess the effect of the soil microenvironment on soil biological activity, the degree and characteristics of wood decomposition were evaluated. Two strips (2 x 10 x 0.32 cm) of loblolly pine wood were arranged within a flat mesh (0.32-cm hole openings) bag. Duplicate bags were buried at each treatment point, such that the top of each strip was approximately 5 cm below the soil surface. One decomposition bag was co-located with the bimonthly soil sampling and the other was buried in an area representative of the treatment plot. To bury the decomposition bags with minimal soil and vegetation disturbance, a flat spade was driven into the ground at approximately a 30° angle with the soil surface, and was lifted gently to allow room to insert the bag. After insertion, the upper soil block was packed down slightly to ensure good soil-wood contact. Decomposition bags were buried for 1 yr in July 1998. Initial weight of dry (65°C) wood strips were compared with an ash corrected (muffle furnace 500°C) post-harvest dry weight for percentage of weight loss determination.

In the fall of 1997 and 1998, measurements of height and base diameter were recorded for crop pines falling within each treatment plot. Non-crop vegetation aboveground biomass was sampled every 3 mo for 1 yr, starting in May 1997. All aboveground vegetation within a 30.5 x 305 cm strip-transect, oriented lengthwise and reaching halfway across each vegetation subplot, was collected for dry mass determination. The sampling period with the largest biomass was considered the peak production period, and the foliage from this sampling period was used as an estimate of aboveground net primary production.

Data were analyzed using analysis of variance (ANOVA) with SAS PROC MIXED procedures (SAS Institute, Inc., 1985). Statistical differences were calculated using Tukey's mean separation (p ≤ 0.10). A one-way ANOVA was used to test soil disturbance effects for each of the two site preparation treatments (flat-planted and bedded). When no significant interactions were found between soil disturbance and site preparation, the main effect of site preparation was presented as a pooled, main-effect value across soil disturbance and vegetation treatments. A single averaged value of the 1997 and 1998 soil core measurements (bulk density, porosity, Ksat, air permeability) and aggregate stability was used to analyze all treatment effects because no significant differences were found between years. The results from two sample periods for SMBC data were also averaged before statistical analysis.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Disturbance Effect
To evaluate the effect of soil disturbance without the confounding effects and added soil disturbance associated with bedding, disturbance treatments were evaluated in flat-planted plots only in this section. Compacted soil samples in compression tracks tended to have lower aeration porosity and lower saturated hydraulic conductivity compared with undisturbed soil; however, these differences were not significant (Table 1). High spatial variability and inadequate sample replication may explain why statistically significant relationships were not found in the data presented here.


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  		Table 1. Disturbance and site preparation effect on soil properties. Flat planted and bedded treatments analyzed separately, then pooled when no significant interactions were found between site preparation and soil disturbance treatments. (Unlike letters within treatments represent significant differences at p < 0.1.)

 
Several recent studies show that harvesting operations resulting in compression tracks cause soil compaction as indicated by increases in bulk density and soil strength (Gent et al., 1983; Incerti et al., 1987; Shetron et al., 1988; Aust et al., 1995). Results reported by Aust et al. (1995) are typical; they found 20% increases in bulk density and 80 and 20% decreases in average soil macroporosity and saturated hydraulic conductivity, respectively, following harvesting disturbance on wet pine flats that had been compacted. These results are somewhat consistent with those found in this study; bulk density increased by 4%, and macroporosity and saturated hydraulic conductivity decreased by 25 and 68%, respectively, suggesting that adequate soil aeration may be compromised by the compaction disturbance.

In this study, percentage of water-stable aggregates and a stability index were determined to test for treatment effects on soil structure. When well-aggregated soils are subjected to compactive forces, soil structure changes as aggregates are crushed and soil particles fill pore spaces (Lull, 1959). These changes may be reflected in the stability of soil aggregation and structure. A stability index based on the work of Reeve (1953) and modified by Whelan et al. (1995) was determined by comparing the relative permeabilities of soil to water and air. Based on this theory, a soil with poor structure would be less permeable to water (a ratio of air/water permeability less than unity) due to the instability of the soil when subjected to the wetting and slaking action of water. In this study, compacted soil aggregates were 64% water stable compared with 77% for undisturbed soil aggregates (Table 1), but again, in this highly heterogeneous field environment, these average values across blocks were not significantly different.

In spite of severe soil mixing, churning, and rearrangement of surface soils by logging machinery, measured soil physical properties in the churned treatment were not different from the undisturbed (non-trafficked) treatment. These findings differ from the results of most previous studies where churning caused soils to become puddled, generally resulting in increased bulk density, and decreased macroporosity and hydraulic conductivity (Koenigs, 1963; Bodman and Rubin, 1948; Aust et al., 1995). However, on this site, large amounts of litter, slash, and other coarse woody debris were incorporated into the surface 30 cm of the soil profile. The soil was kneaded and churned, but net changes in hydraulic properties, aeration and soil density were not observed due to the presence of fine and coarse organic debris.

Biological activity, measured by rate of wood decomposition and microbial biomass, tended to increase with disturbance (Table 1). Soil churning caused the rate of N mineralization to double from 47 to 101 kg ha–1 yr–1. This higher level of biological activity soon after harvesting is consistent with the literature (e.g., Burger and Pritchett, 1988). Slash that was mixed into the soil profile on the churned sites is more accessible because of its proximity to mineral soil, and populations of soil organisms may grow in response to a larger substrate pool. Accelerated decomposition and mineralization may not, however, be desirable in young forest systems if the increased supply of nutrients greatly exceeds the seedling demand, and nutrient pools for future use are diminished by leaching (Burger and Pritchett, 1984; Vitousek and Matson, 1985).

Neither soil compaction nor soil churning had a negative effect on 2-yr pine growth and aboveground net primary production (ANPP) (Fig. 3) . The trend of increasing production with increasing soil disturbance in flat-planted sites suggests that planted pines and herbaceous pioneer species may temporarily benefit from disturbance-induced increases in microtopography and nutrient supply. These results differ from some results in the literature, which report negative effects of increasing disturbance on plant production (Skinner et al., 1989). However, it remains to be seen how these disturbances will affect tree growth with time as the trees are forced to exploit larger soil volumes.



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  		Fig. 3. Disturbance effect on early pine productivity and non-crop biomass. (Within site preparation, different lower case letters represent significant differences; within disturbance class, different capital letters represent differences at p < 0.1.)

 
Bedding Effect
Bedding improves the appearance of severely disturbed clearcut sites; however, the effectiveness of bedding in restoring soil conditions and soil productivity has been questioned (Dulohery et al., 1996), especially in areas where soil disturbance extends below surface soil horizons (Gent et al., 1983). In this study, bedding generally improved the soil physical properties of compacted and churned soil, and based on the properties we measured, soils appear to be fully ameliorated. When data from all disturbance classes were pooled, bedding reduced bulk density values by 17% and total porosity and macroporosity were increased by 19 and 24%, respectively (Table 1). There was significantly more SMBC on bedded sites compared with flat-planted sites, and although not statistically significant, aggregate stability, aeration depth, and the degree of wood decomposition were greater with bedding. Overall soil biological activity is higher due to the mixing of surface soil and the creation of a more aerated soil environment.

Bedding on undisturbed and compacted treatments significantly increased pine volume; however, no increase was observed on the bedded churned sites (Fig. 3). After bedding, the soil physical properties between undisturbed and churned sites were similar. Although not significant due to high variation in this operational field setting, SMBC and pine wood weight loss were 25 and 50% less, respectively, on the churned sites. These results are consistent with those of Dulohery et al. (1996), who found that gross soil biological activity was suppressed by harvest damage and not restored by bedding under similar conditions. Churned areas on non-bedded sites have the advantage of being slightly elevated above undisturbed and compacted areas. In wet flats, subtle changes in elevation greatly influence seedling survival and growth due to characteristically high water tables and poor soil aeration. If bedding and churning both serve to create a more aerated rooting environment, then it is logical that further improvements in seedling productivity would not be observed when churned areas are bedded. In fact, pine productivity was lower on bedded-churned areas than on bedded-undisturbed and compression-tracked sites. Bed quality may have been reduced by severe soil disturbance.

Vegetation Effect
As expected, pine growth increased significantly with increasing vegetation control (Fig. 4A) . These results are consistent with the findings of other studies (USDA Forest Service, 1984; Cain, 1991; Miller et al., 1991). There were, however, only small gains in pine production achieved with the operational level of control because, on average, operational levels of vegetation control decreased non-crop vegetation biomass by only 17% (Fig. 4B). Pine response between fully vegetated and operationally controlled plots was significant when sites were bedded, due to an interaction between site preparation and vegetation treatments (Fig. 4A). Complete control of herbaceous vegetation on bedded sites caused a dramatic increase in pine volume. Bedding and vegetation control acted synergistically to double and more than quadruple the average pine volume on the operational and total vegetation control plots, respectively. Controlling non-crop vegetation gives planted pines a competitive advantage, but this early pine response must be weighed against suppressing the growth of vegetation that may enhance soil quality and provide other ecosystem services.



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  		Fig. 4. (A) Pine volume response to non-crop biomass. (Within site preparation, different lower case letters represent significant differences; within vegetation level (biomass), different capital letters denote significant differences at p < 0.1.) (B) Seasonal dynamics of non-crop vegetation showing peak production in August. (Different letters depict significant differences among vegetation within each season at p < 0.1.)

 
Soil quality may be described as the ability of a soil to fulfill certain functions. One such function is maintaining tree productivity, and key attributes of this function include: promoting root growth, providing for optimum gas exchange and promoting soil biological activity (Kelting et al. 1999). We hypothesized that increasing amounts of herbaceous vegetation could have a positive influence on these soil quality attributes. To provide perspective on the effect of different levels of vegetation control on soil quality, data are presented in the context of established sufficiency curves, where a value of one denotes sufficiency for the given soil property (Fig. 5) .



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  		Fig. 5. Sufficiency curves for vegetation treatment effect on the (A) soil rooting environment, (B and C) aeration, and (D) soil biological activity. (No significant treatment effects were found for any of the soil quality attributes.)

 
Root growth has been found to be restricted in fine silty soils having bulk densities above 1.4 g cm–3 (Pierce et al., 1983). The average bulk density values for all our vegetation treatments fell within this threshold for sufficient root growth (Fig. 5A); however, there was a trend of increasing soil bulk density with increasing vegetation control. Soil structure and stability are additional indicators of the quality of the soil environment for root growth, although sufficiency curves have not been established. On flat-planted plots, the percentage of water-stable aggregates increased with decreasing levels of vegetation control (Fig. 6) . This relationship did not hold on the bedded sites, where differences among vegetation treatments were not significant. Depending on the species of vegetation present, plant roots may or may not improve soil structure and stability; however, cultural activities such as bedding generally decrease soil aggregate stability (Lynch, 1984). Miller and Jastrow (1990) concluded that nearly all belowground biota contribute to the formation and stabilization of soil aggregates, and root activity is especially important in soils such as those at our study site where organic matter is a major binding agent.



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  		Fig. 6. Interaction between vegetation treatment and site preparation. (Within site preparation, differing lower case letters represent significant differences at p < 0.1; within vegetation treatment level, differing capital letters denote significance.)

 
Soils in the total vegetation control treatment had slightly lower average values for aeration porosity; however, in all plots, aeration porosity was ≥10%, which is considered sufficient (Fig. 5B). Overall, aeration depth was inadequate and herbaceous vegetation improved it slightly. Total porosity was also 4% higher on the vegetated plots than on the total vegetation control treatment (Table 2). In the total vegetation control treatment, there were probably fewer root channels, which typically provide a connective matrix of soil pores for improved drainage and aeration. Saturated hydraulic conductivity, however, appears to be unaffected by the differing vegetation treatment levels, suggesting that the presence of non-crop vegetation did not significantly affect soil permeability (Table 2).


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  		Table 2. Vegetation treatment effect on soil properties. (Treatments that differ significantly [p < 0.1] are followed by dissimilar letters.)

 
A curve developed by Kelting et al. (1999) was used to assess the importance of differences in the sufficiency of biological activity for pine growth. The model was created based on the work of Skopp et al. (1990), who studied the relationship between soil moisture and total porosity and found that soil biological activity was maximized at a moisture content of 60% of total porosity. No statistical differences or consistent trends were found among our vegetation treatments based on the soil water/porosity sufficiency function (Fig. 5D); however, consistent trends were found for other indicators of soil biological activity. Soil microbial biomass C was 15% lower on the total vegetation control treatment plots than on the vegetated treatment plots (Table 2), which may offer support for our hypothesis that the presence of roots promotes soil biological activity. The results from a study conducted by Busse et al. (1996) where understory vegetation was suppressed in a ponderosa pine forest also showed higher SMBC when vegetation was present.

The trend in wood decomposition rate lends some support to our hypothesis that soil biological activity is reduced as a result of controlling vegetation (Table 2); however, the coefficients of variation for the three variables were all >50% and treatment differences were not significant. Bimonthly measurements of net N mineralization were also extremely variable within vegetation treatments. The trend suggests that greater N immobilization occurs with more microbial biomass.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Soil disturbance during harvesting operations had relatively small effects on soil quality. Compaction disturbance may compromise adequate soil aeration as indicated by increases in bulk density and decreases in macroporosity and hydraulic conductivity. Soil churning, however, did not degrade the measured soil physical environment, largely because slash and litter were incorporated into surface soil horizons. Disturbance tended to enhance soil biological activity as measured by decomposition rates, microbial biomass, and N mineralization. Accordingly, a trend of increasing pine and non-crop vegetation production was observed with increasing disturbance, suggesting that plants may temporarily benefit from disturbance-induced improvements in microtopography and nutrient supply. Therefore, this may be positive in the short term, but long-term effects of accelerated soil biological activity are uncertain. Bedding fully ameliorated the effects of soil compaction based on the physical properties measured; however, bed quality may be compromised on churned sites as indicated by lower pine productivity on the churned bedded treatment. When soil churning occurs, modifications in bedding methods may be necessary.

Trends suggest some minor improvements in soil quality with increasing levels of non-crop vegetation biomass; however, during 2 yr of operational vegetation control, the beneficial effects of the non-crop vegetation were marginal. Vegetated treatment plots were generally more aerated, had lower bulk densities and higher aggregate stabilities. The most dramatic improvement in plant growth was seen when the effects of bedding and vegetation control were combined; pine growth increased by nearly 800%. Future studies should explore the potential for the combination of bedding and vegetation control to increase N leaching below the rooting zone.


   ACKNOWLEDGMENTS
 
The authors thank Mead-Westvaco Corporation and the National Council on Air and Stream Improvement, Inc. for their support of this research.

Received for publication February 13, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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