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a Weyerhaeuser Co., 2730 Pacific Blvd. Albany, OR 97322
b Weyerhaeuser Co., 505 N. Pearl, Centralia, WA 98531
c USDA Forest Service, Pacific Northwest Research Station, Olympia Forestry Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512
d ENSR International, 9521 Willows Rd. NE, Redmond, WA 98052
* Corresponding author (adrian.ares{at}weyerhaeuser.com)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AWC, available water content • Db, bulk density • DBH, stem diameter at 1.3 m above ground • DC, disturbance class • MV, macropore volume • rp, pore radius • SVOL, volume index • TH, total height • 
P, pressure difference across an air–water interface at equilibrium • 
, volumetric soil water content • r, residual volumetric soil water content • s, volumetric soil water content at saturation • 
, soil water potential • 
, surface tension of water
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Recent studies have revealed a large degree of site-specificity both in soil and tree growth responses to soil compaction (Brais, 2001; Gomez et al., 2002b; Smith, 2003). These research results have promoted new insights on the effects of soil compaction on forest productivity in different soil types, and demonstrated the importance of having site-specific soil-quality assessments in forests. Moreover, there is still limited information on the relationships between soil compaction/disturbance and tree responses. Also, compaction is not the only type of soil disturbance resulting from harvesting activities. Soil mixing, puddling, and rutting can cause disruption of water flow and other effects. Topsoil can also be displaced if stumps are bladed to make access for skidding or forwarding equipment. It is therefore important to characterize disturbance so that compaction is differentiated from detrimental impacts on soils such as topsoil removal or disrupted soil drainage (Miller et al., 1989; Heninger et al., 1997). Generalizations about negative effects of harvest-related soil disturbance on tree growth may be in error because these impacts depend on their type and severity, and on soil properties and climatic conditions (Heninger et al., 2002).
These issues are relevant for the Pacific Northwest, a highly productive timber production region of the USA where heavy equipment is commonly used for site preparation and timber harvest. The region contains large areas of forest soils with inherently high organic C content, and relatively low soil bulk density that apparently mitigate some of the impacts on forest site productivity from intensive management. Supporting this view, 8-yr stem diameter and height growth, and survival of Douglas-fir on Inceptisols in coastal Washington were similar on non-tilled and tilled skid trails, and on nontrafficked areas in spite of the fact that bulk density on skid trails was 41 to 52% greater than in non-trail areas after logging (Miller et al., 1996). For sites with less soil organic matter, greater soil clay content, and more severe drought stress than in coastal Washington, impacts of soil compaction on early growth of Douglas-fir were negative but effects on annual growth rates were temporary. Thus, Douglas-fir height growth on non-tilled and tilled skid trails on Ultisols in Oregon Cascades averaged 24% less on non-tilled skid trails than on tilled trails in Year 4 after planting but differences in annual growth decreased to 6% in Year 7, and by age 8 to 10 year growth rates were similar (Heninger et al., 2002). Ten years after planting trees in 15-cm deep skid-trail ruts were 10% shorter on average with 29% less volume than those on undisturbed areas (Heninger et al., 2002).
Many studies in the Pacific Northwest and elsewhere have assessed tree growth and soil response in logged sites using the "after-the-fact" retrospective approach described by Powers (1989), which may not allow to ascertain the original type, degree, and extent of disturbance. In addition, tree growth may have been unknowingly and differentially affected by plant competition, disease, herbivory and other factors. Tree growth impacts are also sometimes measured for short periods of time and these data are then incorrectly used to project long-term effects. For these reasons, the retrospective approach was not used in our research.
In this study, we examine harvesting effects on soil properties and tree growth on a highly productive site intensively managed for Douglas-fir production in coastal Washington. This is a long-term, replicated experiment in which imposed disturbance reflects field operational conditions as closely as possible, and the confounding effects of competition to trees by vegetation, big game browse, and unknown site history have been removed. In addition, the experimental site has homogenous soil and topographic conditions, and relatively low levels of tree root-rot diseases such as Armillaria or Phellinus. This study is an affiliate installation to the U.S. Forest Service Long-Term Soil Productivity research program that encompasses numerous sites across the USA and Canada (Powers and Fiddler, 1997).
The objectives of this investigation were to (i) assess soil disturbance after ground and cable-based logging; (ii) determine the impacts of soil disturbance and compaction on soil properties and tree growth, and the effectiveness of tillage in maintaining or enhancing site productivity; and (iii) widen the knowledge base for refining site preparation and harvesting standards for soil disturbance and best management guidelines for intensively managed Douglas-fir sites.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Fall River study was installed, except for vegetation control treatments, from April to July of 1999 to examine the effects of ground-based harvesting, organic matter retention, vegetation control and fertilization on soil characteristics and growth of Douglas-fir. The whole study contains 12 treatments replicated four times in a randomized complete block design. Treatment plots are 30 m by 85 m (0.25 ha), with an internal 15 m by 70 m (0.10 ha) measurement plot. In this paper, we report harvesting effects on soil quality and tree growth for three treatments: (i) bole-only removal with no soil compaction, (ii) bole-only removal with soil compaction, and (iii) bole-only removal with soil compaction plus tillage. The bole-only removal harvest followed conventional merchantability standards of 3-m log length and 8- to 13-cm small-end top diameter. Logging slash was scattered uniformly across each plot during the log-forwarding operation.
All harvested trees were directionally hand-felled between May and July of 1999 so that all tree tops remained within the plot. In non-compacted plots, logs were cable-removed with a CAT 330L (Caterpillar, Peoria, IL) two-drum shovel yarder, and a CAT tail-hold tractor to minimize site disturbance. In compacted plots, trees were yarded in May 1999 with a CAT 330L shovel with 70-cm wide pads. The soil water content at time of yarding was near field capacity. Eight equally spaced traffic lanes were flagged in each plot to be trafficked to make soil disturbance comparable across blocks. Every other lane was trafficked twice to more closely simulate traffic patterns that operationally occur when both a feller-buncher and a shovel yarder are used during harvesting. Thorough soil tillage to the 60-cm depth on tilled plots was accomplished on the areas trafficked by the shovel with a small CAT 322BL excavator shovel fitted with a PSM bucket with two 70-cm-long tillage tines.
Plots were planted with 1 + 1 Douglas-fir seedlings on a 2.5 by 2.5 m spacing (1600 trees ha–1) in March 2000. Seedlings were produced from a mixed seed lot of 23 first-generation half-sib families. Only seedlings between 5 and 10 mm in caliper, and 35 and 51 cm in height were shovel planted. Planters were instructed to plant seedlings within 30 cm of pin-flags placed on a 2.5 m by 2.5 m spacing grid so trees would be planted in the full range of microsites represented in each treatment and each plot would have the same planting density. The study area was fenced to eliminate deer and elk browsing. All treatments included weed control using ground-applied herbicides in the four growing seasons after planting to eliminate the confounding effects that soil disturbance and compaction can have on vegetation communities and competition pressure (Dyck and Cole, 1994).
We followed a quantitative approach to assess soil disturbance that visually characterizes five classes developed by W. Scott (for details, see Heninger et al., 2002). We encountered three soil disturbance conditions: Class 0 (DC0)–no soil disturbance because the soil has not been subjected to vehicular traffic, Class 1 (DC1)–soil compaction by vehicular traffic without puddling of soil structure or rutting, and Class 2 (DC2)–churning and mixing of the topsoil with the forest floor and slash with severe alteration of topsoil structure. The subsoil may or may not be compacted. Additionally, we included Class 6 (DC6)–presence of berms of soil and slash resulting from the turning and churning motion of the shovel tracks. There were no disturbance Class 3 (churning and mixing into the B horizon), disturbance Class 4 (topsoil displacement or removal), or disturbance Class 5 (drainage blocked or impeded).
In August 1999, soil bulk density samples were taken in all plots before tillage with a 31.2-mm diam. punch-tube volumetric sampler (Clements Associates, Newton, IA) at the 0- to 10-, 10- to 20-, and 20- to 30-cm depths in 13 sampling spots randomly located on each disturbance class in the plots. Class 0 disturbance was sampled in non-compacted plots while DC1 and DC2 were sampled within compacted plots. Paired comparisons of bulk density values obtained with this sampler and with an 808-cm3 hammer-driven sampler did not differ (P = 0.65) in a complementary field test (Ares and Terry, unpublished data, 2004). Samples with significant amounts of sound or decayed wood material were discarded and replaced with new samples. Samples were then oven-dried at 105°C for determinations of bulk density, and soil gravimetric and volumetric water content. No coarse fragments were present.
In August 2000, we coded soil disturbance conditions within the compacted plots at the point where each measurement tree was planted using the classes described above. Where the code was DC2, the rut depth from the original soil level was recorded. Around every other measurement tree, we recorded the percentage of each disturbance class in a tree-centered 2.5 by 2.5 m square area. We calculated the percentage of ground area and the tree frequency in each soil disturbance class.
Soil strength was measured in August 2001, the driest period of the year, in eight locations per plot with a cone penetromer (DELMI, Shafter, CA) having a 30° angle cone tip of 2.02 cm in diameter and shaft length adjustable to about 90 cm. We sampled at this time because soil strength values were likely around the maximum values for the year. Additional penetrometer readings were planned to be taken during the soil drying stage—early to late summer—the following year if results indicated that soil strength values were limiting root growth. Eight to twelve penetrometer reading measurements were taken at 2-cm intervals to the 60-cm depth on each non-compacted, compacted, and compacted and tilled plots. Sample points within non-compacted plots were randomly located within each quartile of the tree measurement plots. In compacted plots, four readings were taken in each DC1 and DC2 disturbance classes while in tilled plots we recorded soil strength in the tilled harvest equipment trails. Readings within compacted and tilled plots were taken as close as possible to a randomly located point within the tree measurement plot with the desired soil disturbance condition. Cone index values were read using a scale template aid over the penetrometer cards. High-spike values created when roots or buried wood were hit by the penetrometer were noted on the cards in the field and those measurements were discarded from the dataset.
To determine soil porosity and water retention curves, we randomly collected non-disturbed soil cores on non-compacted, compacted, and compacted and tilled plots with 68.7-cm3 cylinder rings centered at 5- and 15-cm depths to characterize the 0- to 10- and 10- to 20-cm depths, respectively. In compacted, and compacted and tilled plots, sampling was restricted to traffic lanes with DC2 soil disturbance. Four cores per depth were taken in all plots of two blocks in 2001, and six cores per depth were obtained in the remaining two blocks in 2002. Samples at the two depths were taken at different sampling points and therefore were independent. The samples were located halfway to the north between a randomly selected tree and the next tree (or to the east, south, or west if the north position was occupied by a stump or other non-representative condition). Soil water retention curves and particle density were determined at the Soil Physics Laboratory of Oregon State University. Total porosity was determined by both the gravimetric method with water saturation (Flint and Flint, 2002), and the particle density method (Black and Hartge, 1984) in 2001, but only by the first method in 2002. Correlation between data from these two methods in 2001 was used to estimate total porosity by the particle density method in 2002. Total porosity reported in the paper is based on the particle density method estimates because during saturation there may have been some soil sample swelling causing a slight bias in the total porosity estimate.
Pore-size distribution was determined by the water-desorption method (Danielson and Sutherland, 1986). The effective pore diameter dividing water-filled and drained pores was calculated from the following function:
 | [1] |
P is the pressure difference across an air–water interface at equilibrium (Pa), is the surface tension of water at ambient temperature (J m–2), and rp is the pore radius (m). We calculated macroporosity as the difference in volumetric water content between saturation and –10 kPa, where effective pore diameter equals 58 µm (Soil Science Society of America, 1997). Saturation was assumed to be when total pore space, as calculated from the particle density method, was filled with water. Pores larger than 58 µm would be air-filled at –10-kPa tension and defined as macro and mesopores (Soil Science Society of America, 1997). Gravimetric and volumetric water contents were determined after equilibrating the soil with water at tensions of –6, –10, –80, –200, and –1500 kPa. Water retention was determined with a ceramic-plate system at tensions from –6 to –200 kPa. After equilibrating at each tension on the ceramic plate, the soil cores were weighted and transferred to a plate with the next greater tension. After being subjected to the –200-kPa tension, cores were weighed, dried at 105°C, and then reweighed. For the –1500-kPa tension, a membrane pressure system was used.
We estimated available water content (AWC) for volumetric water contents between –6 and –1500 kPa, and between –10 and –1500 kPa. We also calculated AWC between –10 and –200 kPa, a range that may better reflect water availability affecting tree growth than that for wider tension intervals. Volumetric soil water content ( in %) was related to soil water tension ( in kPa) by using a three-parameter equation (Van Genuchten, 1980):
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s = soil volumetric water content at saturation (%), and 
and n are equation parameters. The parameter is proportional to the inverse of at the midpoint between s and the residual volumetric water content (r), while n indicates the steepness of the water-release curve (Hodnett and Tomasella, 2002). The r term was excluded from the equation because the data indicated that soil volumetric water content in compacted and non-compacted soil converged at large negative values of . In the field, soil water content was measured during the growing season in 2001 and 2002 with a Hydrosense CS620 probe (Campbell Scientific Inc., Logan, UT) that gives integrative measures for the 0- to 20-cm depth. Five measurements per plot in 2001 and eight in 2002 were taken approximately monthly from May 2001 to October 2002 on the buffer zone of each plot as part of a related experiment on woody biomass decomposition. A locally developed calibration function was used to convert Hydrosense readings to volumetric soil water values.
Instantaneous measurements of soil temperature were taken monthly between 0700 and 1100 h from April 2000 to July 2002 at the 20-cm depth in compacted and noncompacted plots with ELE International moisture-temperature cells (Soiltest, WoodDale, IL) calibrated for temperature accuracy. Soil temperature was also recorded continuously and averaged hourly during July 2002 at 5-cm depth on shovel traffic lanes and adjacent non-disturbed areas with iButton digital temperature loggers (Maxim/Dallas Semiconductor, Dallas, TX).
Trees within measurement plots were measured immediately after planting and yearly after the first four growing seasons. Measurements included total height (TH), stem basal diameter (BD) measured at a permanently marked location 15 cm above ground level in growing seasons 1 through 3, and stem diameter at 1.3 m above ground (DBH) in growing seasons 2, 3, and 4. Height measurements were done with a telescopic pole and stem diameters were measured with a diameter tape. A stem volume index (SVOL) was calculated as (BD)2 TH.
Harvesting effects on bulk density and soil strength were analyzed as a mixed model of repeated measures data with soil disturbance and soil depth as fixed effects, and block as a random effect (Littell et al., 1996). In mixed models, the overall error associated with the model is allocated properly to the error term and the random block factor, and allows for accurate calculation of probability values to draw inferences on soil disturbance, depth, and soil disturbance x depth effects. In addition, a repeated measures analysis was appropriate because measurements of bulk density and soil strength were done at different depths at the same sampling point, and, therefore, sampling errors were not independent. Measurements taken at adjacent depths are expected to be more correlated than measurements taken some distance apart. The covariance structures associated with the within-subject factor (i.e., depth) were selected by choosing those with the lowest value for the Bayesian Criterion and Akaike's Information Criterion. The first-order autoregressive heterogeneous covariance structure provided the lowest values for both criteria.
The arc-sine square-root transformation was used for data in percent. A mixed-model approach was also used to test for soil disturbance-effects on tree size and growth in Years 0 to 4. Comparison of treatment means was made using one degree of freedom orthogonal contrasts. Procedure MIXED in SAS 8.2 (SAS Institute, 1999) that estimates variance components using restricted maximum likelihood methods was used for the statistical analyses. Coefficients of the van Genuchten equation (Van Genuchten, 1980) were calculated using procedure NLIN in SAS. An = 0.05 was used in all statistical analyses for determining significance.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Soil bulk density increased with both soil disturbance (P < 0.001) and depth (P < 0.001), but the soil disturbance x depth interaction was not significant (P = 0.080) (Table 2). Relative to the non-disturbed soil condition, soil bulk density at the 0- to 10-cm depth increased 23.2% for DC1 and 37.5% for DC2 in compacted plots. At depths of 10 to 20 and 20 to 30 cm, the increases in soil bulk density were similar for DC1 and DC2; 18.5 and 19.1% for DC1, and 27.7 and 26.5% for DC2, respectively. Weighted by the ground area affected by soil disturbance, bulk density in the 0- to 30-cm soil depth for ground-harvested plots was 27% greater than for non-compacted plots.
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Soil water retention curves indicated that compacted soil consistently retained more water at –10 to –200 kPa water potentials than non-compacted, or compacted and tilled soil (P < 0.05) (Fig. 4)
. The Van Genuchten function explained 97% or more of the variation in volumetric soil water content (Table 5). Saturated volumetric water content was significantly higher for non-compacted soil than for compacted soil at both the 0- to 10-cm and 10- to 20-cm depths (P < 0.05). The parameter did not differ between compacted and non-compacted soil probably because of the wide intervals of confidence for in non-compacted soils. The slope of the water retention curve indicated by the n parameter was somewhat steeper in compacted than in non-compacted soil at the 10- to 20-cm depth (Table 5, Fig. 4).
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Douglas-fir Mortality and Growth
At planting, trees had similar TH (P = 0.65) and BD (P = 0.24). Tree mortality in Year 4 was 3.4, 4.5, and 4.8% for compacted, non-compacted and, compacted and tillage treatments, respectively, and did not significantly differ among treatments. There were no significant differences in mean TH, BD, and SVOL for trees growing on non-compacted, compacted, and compacted and tilled treatments at Years 1 to 4 (Table 6). In Year 3, mean DBH was greater for trees in compacted, and compacted and tilled treatments than that for non-compacted. Within compacted plots, mean TH, BD, DBH, and SVOL were in general similar for trees growing on DC0, DC1, DC2, and DC6 except in Year 3 when trees growing in DC1 compacted areas had greater BD than those growing on DC0 non-compacted areas (Table 7). In year 3, SVOL in the compacted treatment was also greater for trees growing on DC1 and DC2 areas within the compacted treatment than those growing on non-compacted DC0 soil.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Spodosols of Vancouver Island, BC were differently affected by increasing passes of both a rubber-tired skidder with low ground pressure tires and a grapple skidder with conventional tires and chains, but disturbance depended more on soil water content at trafficking time than on other soil characteristics (Senyk and Craigdallie, 1997). In their study, a gravelly loamy sand soil of the Honeymoon series had high water content at time of trafficking and significant puddling occurred. In contrast, significant rutting but not puddling was observed in a silt loam to loam soil of the Snuggery series that was relatively dry when trafficked. Bulk density of the Honeymoon series soil of Vancouver Island increased to 1.4 Mg m–3 after 30 to 40 equipment turns but total soil porosity did not drop below 50% (Senyk and Craigdallie, 1997).
Soil strength at Fall River also increased after vehicle trafficking to about 50-cm depth but it remained well below the critical threshold for tree root growth considered to be around 2000 to 3000 kPa depending on tree species and soil conditions (Sands et al., 1979; Greacen and Sands, 1980). In northern Idaho, compaction increased soil strength to more than 2500 kPa in a Fragixeralf developed from volcanic ash, but shoot growth of 1-yr-old Douglas-fir seedlings was not affected (Page-Dumroese et al., 1998). Soil strength of a Hapludand and a Paleohumult in coastal Washington reached maximums of about 2500 and 3200 kPa at the 10- to 20-cm depth corresponding to fine-soil bulk densities of about 0.7 and 0.8 Mg m–3 (Miller et al., 2001). In trafficked areas at Fall River, mean soil strength by depth increment never exceeded 1300 kPa. Tillage with an excavator restored soil strength to a condition similar to that initially but this practice is deemed unnecessary for this site.
Decreases in total porosity at Fall River of 10 and 13% from initial porosities of 0.68 and 0.69 m3 m–3 at the 0- to 10-cm, and 10- to 20-cm depths, respectively, were similar to the 9 and 14% reductions at the 0- to 15- and 15- to 30-cm depths from initial porosities of 0.68 and 0.66 m3 m–3 for a loam soil in Northern California (Gomez et al., 2002b). Macropore volume at –10 kPa in Fall River dropped 40 to 52% with compaction confirming that macropore volume is a more sensitive indicator of soil changes than total porosity.
Water retention and AWC were increased by soil compaction. Compacted soil retained more water than non-compacted soil likely because compression converted larger pores to smaller ones, especially in the upper part of the soil profile. In a study with a silty clay loam, compaction decreased macroporosity (pores > 60-µm diam.) by more than 50% but pores < 6-µm diam. were unaffected by compaction (Bullock et al., 1985). At the Vancouver Island, BC study (Senyk and Craigdallie, 1997), water retention increased in compacted soil leading to saturation and increased seedling mortality in areas where soil drainage was blocked on rutted skid trails. Standing water was never observed in traffic lanes on compacted plots during the winter rainy season at Fall River indicating that water infiltration was not severely limited.
Douglas-Fir Early Growth
Despite noticeable effects on soil physical properties from ground-based harvest, stem diameter, height and a volume index of Douglas-fir were not reduced by soil disturbance or compaction. Values for these growth indicators were even greater for trees on disturbed soil than for those on non-disturbed soil in Year 3. Clearly, bulk density, soil strength, and macroporosity did not reach levels in compacted areas that reduced tree growth. Including compacted soils, observed bulk density of 0.56 to 0.86 Mg m–3 at Fall River was within the 0.70 to 1.15 Mg m–3 range for which Douglas-fir seedling growth was not affected on the eastern Cascade Mountains of Washington State (Zabowski et al., 2000). In other studies, Douglas-fir root growth decreased or ceased at soil bulk densities from 0.9 Mg m–3 for clay loams to 1.8 Mg m–3 for sandy loams although shoot growth was not generally affected both in field (Forristall and Gessel, 1955) and pot experiments (Minore et al., 1969; Heilman, 1981; Singer, 1981).
The greater plant-water availability in compacted soil compared with the undisturbed soil suggests that increased soil water mediated the growth increase of Douglas-fir measured at Year 3 on compacted soil. Soil water, however, could have been retained at more negative tensions in compacted soil, and become less available than in non-compacted soil during portions of the growing season. This was difficult to evaluate in this study because measurements of field volumetric water content integrated for the 0- to 20-cm depth (including the litter layer) were taken monthly within the plot buffers while soil water retention curves were derived from samples collected within the measurement plots and centered at the 5- and 15-cm depths. Also, the soil water potential at which growth of Douglas-fir seedlings or saplings start to decline is uncertain, and so was the lapse of time during which compacted and non-compacted soil would have reached soil water status limiting for Douglas-fir growth. Photosynthesis rates of 2- to 3-month-old Douglas-fir seedlings slowly declined with soil water potential decreasing to –100 kPa and decreased sharply below that water potential (Zavitkovski and Ferrell, 1968). Growth of conifer seedlings was considered to cease at soil water potential of –500 kPa (Spittlehouse and Childs, 1990). Additional work needs to be conducted to confirm at what soil water potentials Douglas-fir growth begins to be limited in Coastal Washington soils.
Available water held between –10 and –500 kPa may be more useful to characterize water availability and relate it to tree growth than the routinely calculated AWC between –10 and –1500 kPa for soils that likely never reach a soil water potential of –1500 kPa or other largely negative potentials that reduce seedling survival. The potential beneficial effects of other factors on tree growth such as increased N uptake (Gomez et al., 2002b; Kranabetter and Sanborn, 2003) or CO2 concentrations (Conlin and van den Driessche, 1996) found in compacted soils cannot be ruled out as a potential beneficial effect on tree growth at Fall River. In fact, CO2 concentrations were markedly greater in compacted than in non-compacted soil in an ancillary study at Fall River (M. Jurgensen, personal communication, 2002).
The tree growth advantage on traffic lanes did not continue at age 4 most likely because tree roots are likely growing out of the compacted track areas and penetrating deeper into the soil profile. At Age 3, mean root lateral spread from the tree trunk was 111 cm for trees growing on the 70-cm wide equipment tracks at Fall River, and did not differ from trees on non-compacted soil (C. Harrington, personal communication. 2004). A similar effect may probably explain why height growth of trees growing on skid-trails and non-compacted areas became similar after trees reached about 1.4 m in height in western Oregon (Heninger et al., 2002).
Root growth of Douglas fir is considered limited at soil temperatures less than 10°C with optimum growth occurring at 20°C (Lopushinsky and Max, 1990). At Fall River, soil temperature was higher than 10°C from early May to early October, and did not differ between compacted and non-compacted areas.
Soil Quality Thresholds for Best Management Practices
Critical threshold values for a wide range of soils are difficult to estimate because data are rather limited (Powers et al., 1998). At Fall River, no negative effects on tree growth were observed although the area compacted was greater than 15% and the soil bulk density increase exceeded 20%. These values are the recommended admissible increases over non-disturbed conditions for Andisols in the Pacific Northwest (Powers et al., 1998). The suggested threshold values are too conservative for the Boistfort soil at the Fall River study site. The reductions in total porosity and macroporosity observed in this study were marginally greater than the >10 and >50% reductions considered detrimental to soil quality (Powers et al., 1998). On the other hand, macroporosity did not decrease below 14% when at least 10% of total soil volume is considered necessary for plant growth (Grable and Siemer, 1968).
Andisols have good physical properties such as low bulk density, high water-holding capacity, good tilth, and stable aggregation, and constitute an excellent rooting media (Kimble et al., 2000). At Fall River, these advantageous soil physical characteristics confer the site the capacity to sustain harvest impacts without decreasing tree growth. The unique properties of Andisols have been only partially considered in soil quality standards and guidelines for sustainable forest productivity. As more data become available, soil quality standards for Andisols should be refined to better differentiate them from other soil types.
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CONCLUSIONS |
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Although both soil bulk density and soil strength were increased, they did not reach levels that reduced Douglas-fir growth. There was some evidence in this study that compaction could actually be beneficial as early tree growth at age 3 was greater on compacted soil compared with compacted soil. Increased available water in the –10- to –200-kPa range on compacted traffic lanes may explain this increased growth.
Consequences from heavy machinery use such as increased soil strength and bulk density can be ameliorated by tillage but this practice is deemed unnecessary on the tested site. Limiting soil disturbance to Classes 1 and 2 appears to be a sound recommendation for Andisols of coastal Washington. The long-term nature of this study will address possible changes in soil characteristics and tree growth responses with time as well as their interactions with climatic conditions.
Received for publication October 7, 2004.
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