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.