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The Switchback Trail. The aim of this study was thus to investigate whether skidding would result in more adverse soil physical properties dry bulk density, total porosity, macroporosity, and microporosity in switchbacks compared to straight skid trail segments. Specifically, we hypothesized that physical soil properties following skidding would:.

The study area is located at an elevation of approximately m above sea level with a northerly aspect. The average annual rainfall recorded at the closest national weather station about 20 km from the research site is mm. The maximum mean monthly rainfall of mm usually occurs in October, while the minimum rainfall of 25 mm occurs in August.

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The average depth of soil to the bedrock is 70 cm. The soil had not been driven on before the experiment. At the study site, a combination of group selection and single-tree selection silvicultural treatments was applied. In Hyrcanian forests, harvesting and silviculture operations are most common in autumn and winter, while extraction of logs is usually completed in spring and summer.

Harvesting and processing are typically performed by hand-felling using chainsaws. Highly mechanized systems, such as harvesters and feller-bunchers, are not used because most of the mature hardwood forests are located on mountainous sites with steep slopes or in lowlands on clay soils sensitive to machine disturbance. For this study, a skid trail containing multiple switchbacks, of varying curvature, that encompassed a range of longitudinal slope steepness but no lateral slope was delineated.

Najafi et al. The deflection angle of a curve was measured from a single measuring location positioned at the point beyond the curve where the two extrapolated straight skid trail segments would have intersected Fig. The deflection angle was determined by measuring the azimuths of the two lines with a compass and computing the differences between the two azimuths. Four switchback positions were sampled: one plot was established in the middle apex of each curve central plot, CP , one each 3 m away on either side of the CP first plots, FP , one each 3 m further out from the FP second plots, SP , and one each 15 m further out from the SP third plots, TP.

In addition, two plots were placed on straight segments of the skid trail on either side of the switchback, 30—50 m away from the SP. Shown are the plots located at different positions in the switchback: central plot CP at the apex of the curve; first plots FP each 3 m on either side of the CP ; second plots SP each 3 m further out from the FP ; third plots TP each 15 m further out from the SP.

The full design included 2 additional plots on straight segments of the trail SST located on additional 30—50 m beyond the TP not shown. Within each plot, three soil samples each were taken from a depth of 0—10 cm along two parallel transects, including within the left and right wheel tracks and between the tracks.

The three soil samples on each transect were taken at 0—10 cm soil depth from the middle of the left wheel track, between the wheel tracks, and the middle of the right wheel track. Finally, a total of 28 soil samples were taken from undisturbed control locations, randomly dispersed throughout the study area but at least 50—60 m ca. Soil samples were collected with a soil hammer and rings diameter 5 cm, length 10 cm and immediately put in polyethylene bags and labeled.

Samples were averaged to the plot-level for analysis i. The SPSS software version Average values of all four soil physical properties differed significantly between switchbacks and straight segments of the skid trail and depended strongly on the degree of curvature Table 1. On average, D b 1.

Switchback Trail Hiking Trail, Port Angeles, Washington

Table 1 Analysis of variance p values of the effects of switchback curvature, slope gradient, and their interaction on the soil physical attributes dry bulk density, total porosity, macroporosity, and microporosity on a skid trail after three passes with a Timberjack C skidder. Regardless of curvature class, the effects of skidding on soil physical properties differed strongly and significantly by position within the switchback, with the greatest effects at the apex of the curve Table 2.

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However, even the SP and TP were significantly different for all four soil properties in narrow curves on gentle slopes. In general, differences in average soil physical properties among positions along the switchback were more pronounced on gentle than steep slopes. Regardless of switchback position or curvature class, D b was consistently greater, and TP and MP consistently lower, on steeper than gentle slopes; MIP did not differ consistently by slope class.

MIP was not consistently lower in wide compared to narrow curves, however. In this study, we are among the first to document the adverse effects of frequent switchback curves that characterize skid trails traversing steep slopes in mountainous forests with uneven topography Cavalli and Grigolato Whereas adverse effects on soil physical properties by ground-based timber harvesting with heavy machinery has been amply documented on skid trails e.

As expected, increased soil bulk density compaction translated into reduced macroporosity and total porosity and increased microporosity, which is a commonly observed response in the pore size distribution following skidding Greacen and Sands In this initial investigation of the effects of harvest traffic in switchbacks, we did not formally examine the specific causes that may be responsible for more adverse soil physical conditions along the curve.

Nonetheless, as increased soil bulk density from harvest traffic is primarily a function of universal physical forces Pacejka , the most plausible causes may be hypothesized for future exploration.


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Greater effects of skidding in switchbacks than on straight segments of the skid trail, greater effects in narrow than wider curves, and non-constant effects on soil physical properties across different positions within the switchback are largely due to:. As physical forces are particularly pronounced in narrow curves Pacejka , soils in skid trails with narrow curves are more susceptible to greater compaction than in wide curves.

As skidders slow to enter and pass through a curve, engine vibration increases, thus compacting the top soil more in curves than on skidder trail segments that are traversed at higher speeds Horn et al. In contrast to paved highway roads that are engineered to have a more or less constant radius throughout the curve for a constant centrifugal force that enhances the safety even at higher speeds Slimi et al.

Finally, although soil bulk density was greater and macroporosity was smaller on steep slopes than on gentle slopes on straight segments of the skid trail, as is typically the case e. Greater soil loss in narrower curves on gentler slopes Thawornwong could compensate for the soil damage incurred by increased steepness, explaining the lack of a slope effect at the apex of narrow curves, and may again reflect differential skidder speeds: due to the lower traction of machinery on steeper slopes, skidder speeds on steep slopes may already be reduced compared to more gentle slopes Najafi et al.

Regardless of the specific causes of soil damage in switchbacks, our results indicate that, after as few as three equipment passes, the apex of the switchback curve may be particularly vulnerable to deteriorating soil conditions following skidding, especially given that soil conditions on both slope gradients came close to the critical threshold values shown to be detrimental to future plant growth and long-term site productivity.

In the central plots, particularly in narrow curves, bulk densities fell only slightly short of the critical threshold between 1. Thus, on a portion of the site, a loss of future site productivity has already been achieved after only three skidder passes, which must be regarded as a low traffic volume considering that skid trails are designed to concentrate as much equipment traffic on as small an area as possible e. Although we did not investigate the effects of greater traffic frequencies in this study, given that the majority of adverse effects on soil physical properties typically occurs in the first three passes of a vehicle, with modest additional impact with subsequent passes Hatchell et al.


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However, further studies will be needed to assess whether the observed effects of curvature and switchback position remain constant or diminish as increasing traffic volume increases soil damage toward critical threshold values on ever larger proportions on the trail cf. Kozlowski Either way, differential soil damage at various positions along skid trails containing switchbacks indicates that soil damage assessed on straight segments of skid trails may severely underestimate the actual soil damage in mountainous areas that include curves.

The results of this study reiterate recommendations for careful planning of skid trail networks that includes confining skidder traffic to as few designated skid trails as possible, maintaining slope gradients of skid trails that are as low as possible, and keeping skid trails as straight as possible.

How to properly hike a trail with a switchback

This can minimize the proportion of total stand area adversely impacted by traffic, reduce damage to the residual stand and disturbances to the forest floor, and decrease skidder travel time e. Given greater soil bulk density and reduced porosity in narrower compared to wider curves following skidder traffic, particular attention must be paid to the placement of these curves for a more optimal routing of machinery.

In the current study, we compared the effect of skidding on dry bulk density, total porosity, macroporosity, and microporosity in the straight segments of the skid trail and in various positions within switchbacks of differing trail curvature deflection angle on different slope gradients. We have concluded that adverse skidding effects on switchbacks are substantially greater than on straight segments of the skid trail. Despite the limitations of this initial study, which investigated soil physical properties on only one soil type Cambisol after a fixed number of passes with one particular harvesting machine, the universality of physical forces that induce soil damage on skid trails Pacejka suggests that some of the observed results may be broadly generalizable to other soils, forest types, climates, years, and equipment.

If switchback curves are necessary to provide access to the entire cutting area, these generalizations include that:.

In conclusion, our results indicate that simple extrapolation of soil-machine interactions from straight segments of skid trails to the entire skid trail may not be prudent in mountainous areas, where trails include numerous switchback curves. This may particularly be the case given that only three passes of harvesting equipment i. The subsequent implication for the layout of skid trail systems in mountainous terrains is that optimal routing of the extracting ground-based machinery may require that particular attention be paid to the placement of skid trail curves.

In addition to skid trail planning, primary timber extraction planning needs to consider the type of skid trail i. Further research is required, however, to verify the specific causes of adverse effects of skidding in switchbacks on physical soil properties, to confirm these effects across a broader range of soil conditions, and determine how these effects are modified by increased traffic intensity. Ampoorter, E. Brady, N. Cambi, M. Cavalli, R. Dey, D. In: Rice, J.

Texas Switchback

Marie, Ontario, Canada, 53— Dykstra, D. Ezzati, S.

Part D: Trans. Silva Fen. Farrakh Nawaz, M.