Abstract:
This study evaluated the effect of soil compaction on the growth
of natural regeneration on volcanic ash-influenced soils in the
southern Washington Cascades. Growth of 9 to 18 year-old sapling-.
sized Ponderosa pine (Pinus ponderosa Laws.) was studied on sites
ranging from 915 to 1006 m elevation in an area selectively logged in
1959, and for 10 to 13 year-old lodgepole pine (Pinus contorta
Dougl.) established following a group selection harvest on a 1342 in
elevation site logged in 1967. Soils ranged from loam to sandy loam
texture. Height, diameter, and volume growth were measured for trees
growing under an array of disturbance conditions to determine the
influence of soil compaction on their size and growth rates.
A number of soil, vegetation, and site variables were studied to
determine possible cause and effect relationships with growth parameters.
Bulk density of the surface 30.5 cm was measured within the
lateral rooting zone to provide an index of compaction. Trees were
destructively sampled to obtain a detailed record of their
development, and to adjust for differences in age as a result of
variable establishment delays.
Average bulk density increases of 15.4 and 27.5 percent relative
to adjacent undisturbed soil were found for skid trails in the
ponderosa pine and lodgepole pine study areas, respectively. The
effect of soil displacement overshadowed any possible relationship
between bulk density and growth for lodgepole pine. The strong
correlation of growth with organic matter content indicates that
removal of nutrient-rich surface soil during logging and slash disposal
operations may significantly affect site productivity, particularly
for poorly developed skeletal soils.
Regression analysis showed that several growth parameters for
ponderosa p1ne were strongly associated with the increase in bulk
density despite additional significant relationships with tree age,
site index, and overstory cover. Reductions in total growth of 4.8,
7.7 and 20.4 percent were predicted for height, diameter, and stem
volume of 14 year-old skid trail regeneration based on the mean bulk
density increase. Evaluation of current growth increment was effective
in adjusting for differences in tree age. Predicted average
reductions in height, diameter, and volume growth of 7.1, 11.8, and
18.9 percent were estimated for young ponderosa pine based on the
last five year period. Projected impacts from regression analysis
represent conservative estimates, since the mean density increase
used is the prediction model included measurements for sample trees
growing in soil with bulk densities comparable to undisturbed levels.
When the ponderosa pine sample was stratified into low and high
impact groups based on bulk density increases, differences in the
shape of height-age and diameter-age curves were apparent. A significant
decrease in the rate of growth was noted for trees growing
under highly disturbed conditions. Projected effects of compaction
on site productivity throughout the rotation are difficult to assess,
but measurable reductions in young tree growth coupled with frequent
stand entries and the slow rate of natural soil recovery provide a
basis for concern for long-term impacts.
