Tsioras Petros A., PhD.

The objective of this study was to compare forest soil bulk density values obtained through
conventional sampling methods such as the volumetric ring (VR: diameter 5 cm, length 10 cm)
and paraffin sealed clod (PSC), with a variation of the VR, where rectangular boxes (RB) of
four different dimensions were used. Sampling transects were established on a machine operating
trail located in a beech (Fagus orientalis Lipsky) stand in Northern Iran. At each
transect, three soil samples were collected at three different locations. Samples from different
methods were spaced by a 50 cm distance to avoid direct interactions. The soil class of our
study area was Combisols according to the WRB classification with a clay texture. Soil bulk
density differed significantly between the three sampling methods. The lowest values were
obtained with the RB (average 1.25 g cm-3), followed by the VR (average 1.40 g cm-3), and
lastly the PSC (average 1.52 g cm-3). The values obtained with four variations of the RB
method ranged from 1.22 to 1.28 g cm-3 and were not found significantly different. When soil
bulk density was calculated after the removal of the weight and volume of roots included in
the samples, the values were determined to be higher than before but with the same range of
magnitude. The lowest coefficient of variation was found for RB4 (CV=2.3%), while the highest
values were observed for VR and RB1 (CV=5.7%).

Effective skid-trail design requires a solid understanding of vehicle-soil interactions, yet virtually no data exist on the effects of harvest traffic on soils in the switchback curves common in mountainous terrain. We contrast for the first time 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. Treatment plots with three replications included combinations of two classes of curvature (narrow = high deflection angle, 60–70°; wide = low deflection angle, 110–130°) and two categories of slope gradient (gentle = ≤20%; steep = >20%). The Cambisol soil was sampled in control and trafficked areas both before and after three passes with a rubber-tired skidder. After only three passes, significant effects were seen for dry soil bulk density (+), total porosity (–), macroporosity (–), and microporosity (+), with steady trends from undisturbed controls to straight segments to wide curves to narrow curves. Soil damage increased gradually and consistently toward the apex of the curve, particularly in narrow curves on gentle slopes. Our results establish that curvature and switchback position are important factors affecting soil compaction in ground skidding. The strong observed effects of even low harvest traffic volume on soil physical properties in curves indicate that the degree of soil compaction in skid trails may be underestimated in areas with numerous switchbacks, the placement of which within a skid trail system may require careful consideration on mountainous terrain.