Table 2. Area of soil disturbed in areas A and B
443
Percentage of area disturbed
No disturbance Light/medium disturbance Heavy disturbance
(Stratum 0) (Strata 1+2) (Strata 3+4)
Area A
No restriction on machine travel 20 63 17
Area B
Machine restricted to designated trails 77 3 20
New Zealand is 30-35 years. A search will be
made among stands currently approaching maturity
for stands which were photographed (aerially) at
the time of their establishment. The soil
disturbance will be mapped on the old photographs
using the strata in Table 1 and then the skid
trails will be relocated in the field. Trees
growing on the skid trails will be measured for
volume. The results should indicate the nature
of the long term effect of soil disturbance on
tree growth.
4 REDUCING SOIL DAMAGE
One way of reducing soil disturbance is by good
planning, for which aerial photography can be an
invaluable asset. This was demonstrated recently
at an FRI trial on the heavy clay soils of Ngaumu
State Forest (W. Blundell, unpubl. data). A
gently sloping logging coupe was halved into
similar areas A and B. In area A, the operator
of the logging machine (FMC 220) followed his
normal practice of driving to each log, hooking
it up, and dragging it out along the most
suitable route. In area B aerial photographs
taken before logging were used to design a
network of skid trails. The FMC 220 was confined
to these trails and the operator was required to
pull the winch rope out to each log. After the
entire coupe had been logged, more aerial
photographs were taken to determine the area of
soil disturbed in A and B (see Table 2). The
results showed that a markedly greater area of
soil was disturbed in area A compared with that
in area B. For regions which are susceptible
to erosion, this difference will probably be an
important consideration. As far as tree growth
is concerned, however, it will be some years
before the trials show whether the area of soil
disturbance is a significant factor for the
Ngaumu clay soils.
5 SUMMARY
The amount of soil disturbance which takes place
during harvesting operations is substantial.
Aerial photographs are proving to be invaluable
assets for classifying and mapping this
disturbance and useful aids in designing trials
to determine the effects of soil disturbance on
tree growth. As the results from these trials
come to hand, aerial photography will continue to
provide a valuable role in reducing soil
disturbance to a minimum through good planning
and wise choice of machinery.
REFERENCES
Dyrness, C.T. 1965. Soil surface condition
following tractor and high-lead logging in the
Oregon Cascades. Journal of Forestry 63:
272-275.
Murphy, G. 1982. Soil damage associated with
production thinning. New Zealand Journal of
Forestry Science 12(2): 281-292.
Murphy, G. 1984. A survey of soil disturbance
caused by harvesting machinery in New Zealand
plantation forests. FRI Bulletin No. 69.
Steinbrenner, E.C. & S.P. Gessel 1955. Effect of
tractor logging on physical properties of some
forest soils in southwestern Washington. Soil
Sci. Soc. Am. Proc. 19: 372-376.
