Abstract:
Although previous research in central Oregon has shown soil compaction
can lead to a decline in site productivity, the subject is not understood well enough
to predict the growth changes resulting from a given level of soil compaction. A
study was initiated to relate the basal area, height and volume periodic annual
increment (PAT) of residual, 70 to 80 year-old ponderosa pine (Pinus ponderosa
Laws.) trees to compacted soil conditions, as measured by soil strength.
This study was superimposed on the USDA Forest Service Long Term Site
Productivity Project research plots in Central Oregon and thus constrained by its
design. Soil strength and tree growth were measured on six of these plots. Three
plots had been thinned with a mechanical harvester and the stems removed with a
grapple skidder from the plots (Complete Removal). Three other plots were
thinned to similar stocking levels with the harvester, but the stems were left in place
to minimize disturbance (No Removal). No true control existed for these
installations as both the Complete Removal and No Removal plots were compacted
by the harvester. A recording penetrometer was used to determine soil strength
along systematically spaced grid points, to a depth of 24 inches. Each tree within
each plot was mapped and measured for total height, diameter at breast height, and
radial growth increment at diameter breast high (DBH).
The soil conditions around each residual tree were evaluated using 15, 30-,
and 45-foot radius plots. The penetrometer readings that fell within each of these
plots were averaged to represent the overall soil conditions affecting each tree. The
Complete Removal plots had significantly higher soil strength conditions than the
No Removal plots (p<0.05). The percent increases in average soil strength of the
Complete Removal plots over the No Removal plots were 39, 42 and 44 percent for
the 15-, 30- and 45-foot radius plots, respectively.
Potential associations between basal area, height and cubic volume PAT
growth rates and replication, treatment, soil strength and other covariates were
explored with general linear models. Soil strength was not a significant factor for
basal area PAT or for volume PAT at the 30- and 45-foot radius. Total height and
cubic volume PAl at the 15-foot radius declined significantly (p<0.05) with
increasing soil strength.
The volcanic ash soils did compact as a result of the low level of
mechanical thinning activity conducted on the study sites. Tree growth was
statistically associated with increased soil compaction. Lack of a true control
prevents full evaluation of the mechanical harvesting-related compaction; however,
skidding resulted in a measurable increase in soil compaction in the Complete
Removal plots. Forest management practices that lead to frequent entries appear
likely to compact these volcanic ash soils. Depending on logging patterns, large
areas could be impacted without careful planning It appears that compaction
effects are long-lasting and cumulative, thus the risk of reducing long-term site
productivity is a concern.
