- A 1.5 ha mixed conifer stand in the Blue Mountains of Oregon
was intensively examined to assess the impact of logging disturbance
on soils and herbaceous vegetation. Sampling was conducted six
years after much of the timber overstory was removed in a
shelterwood cut and yarded by crawler tractor. A year after harvest
slash was machine-piled and burned. Skid trails and other areas of
disturbance were seeded with a mixture of perennial grasses.
Logging impact was divided into five "disturbance classes'*
as treatments: general, berm, slash fire ring, skid trail, and
undisturbed. Analysis of variance was used to test treatment
differences. Characteristics of skid trails were analyzed
separately from those of non-skid trail areas.
Soil compaction was evident on skid trails from substantially
lower rates of infiltration and saturated hydraulic conductivity
(SHC), and a higher average soil strength, relative to undisturbed
areas. Skid trail compaction probably resulted from mixing of denser subsoils with the low-density volcanic ash soils at the
surface, and from compression by tractor activity. Bulk density of
skid trails was not statistically different from that of undisturbed
areas. Soil densities of all classes of disturbance were generally
lower than 0.9 g/cm³. Non-skid trail areas were not compacted.
Soil water conductance rates markedly differed among
disturbance classes, but were considerably higher than storm
intensities projected for the region. High sample variability of
both infiltration and SHC rates made interpreation of statistical
comparisons among treatments difficult.
Some watershed protection values were diminished on the
disturbed areas. Litter cover of skid trails, general disturbance
areas, and berms was 27, 45, and 68 percent lower, respectively,
than that of undisturbed areas. Depth of litter decreased
correspondingly among the disturbed treatments. Litter was nearly
absent on the fire rings. Another agent in soil stabilization,
belowground biomass, was two-thirds less on skid trails, in
comparision with undisturbed areas.
Amounts of soil organic matter in the surface 3 cm were
significantly lower for the four types of ground disturbance,
relative to undisturbed areas; however, organic matter was higher at
some subsurface levels of berms, fire rings, and skid trails. Bulk
density of non-skid trail soils was inversely correlated with
organic matter content.
There were no statistically significant differences among
treatments for soil water retained at four potentials, for coarse mineral fragment content, or for fine earth particle size
Aboveground vegetation production of the seeded skid trails was
66 percent greater than that of undisturbed areas. Understory plant
cover of skid trails and undisturbed areas was equivalent.
Correlation between aboveground herbaceous production and soil
strength at the 5-10 cm depth of skid trails was not statistically