Table 1. Strata descriptions for classifying soil disturbance on aerial photographs. 
442 
Stratum Description 
0 Undisturbed areas 
There is no evidence of soil disturbance either from log pulling or machinery travelling over 
the area. The litter layer is present in an undisturbed state, and there is no soil 
compaction. 
1 Lightly disturbed areas 
Lightly disturbed areas are created when machinery and/or logs move over the ground once or 
twice only. The litter layer is still present in this stratum, although it may be disturbed 
slightly and, in sloping country, it may have been moved a short distance by rain. Soil 
compaction is low. 
2 Minor skid trails and moderately disturbed areas 
As the soil disturbance increases, clear skid trails form. On minor skid trails, litter and 
vegetation are still present, usually mixed with the topsoil. The strip down the centre of 
the trail still has litter present. Stratum 2 also consists of areas between the minor skid 
trails where the litter layer has been removed by log pulling or other means. Soil 
compaction is moderate. 
3 Most major skid trails 
As the minor skid trails meet and soil disturbance increases further, major skid trails form. 
The litter has gone completely, revealing the topsoil, which gives the stratum its colour. 
Subsoil may sometimes be seen mixed with the topsoil. Soil compaction is usually high. 
4 Landings and some major skid trails 
The topsoil and litter layer have been completely removed, revealing the subsoil, which gives 
the stratum its colour. The subsoil has been penetrated to a significant degree and is 
usually severely compacted. 
3 RELATIONSHIP BETWEEN SOIL DISTURBANCE AND TREE 
GROWTH 
When a forest manager is confronted with soil 
disturbance, his main concern is whether it is 
detrimental to tree growth. To study this 
problem, FRI has established several trials of 
various designs throughout New Zealand. While it 
will be a few years before these trials are 
completed and firm conclusions can be drawn, 
there are some interesting preliminary results. 
For example, in a trial situated on a sandy 
soil in Esk Forest there was little difference in 
height between trees in strata 0, 1, 2, and 3 
(Figure 1, based on 25 nine-tree plots per 
stratum). There was a 19% reduction in height 
for trees in stratum 4. If this trend has a long 
term nature, then for this soil type the five 
soil disturbance strata can be reduced to two: 
1. landings and major skid trails where the 
subsoil is visible and ground heavily compacted, 
and 
2. all other areas. 
This reduced classification would make soil 
disturbance mapping much simpler and easier where 
there is a distinct colour difference between the 
subsoil and the other soil horizons. Tests have 
shown that in this situation, panchromatic 
photography can also be used. Because this film 
type is normally used by mapping agencies in 
New Zealand, photographs are readily available 
for most areas. 
The existence of panchromatic coverage of many 
New Zealand forests means that we can address the 
all important question, "If soil disturbance 
reduces tree growth, how long does the effect 
last?" The oldest of FRI's soil disturbance 
growth trials is described by G. Murphy (unpubl. 
data). This trial, situated on Tairua Forest 
MEAN TREE 
HEIGHT 
+ 
STANDARD 
ERROR 
(CM) 
SOIL DISTURBANCE STRATUM 
Figure 1. Relationship between tree height and 
soil disturbance for Pinus radiata age 2 years. 
clay soil, has demonstrated that heavy soil 
disturbance (strata 3 and 4) can have a marked 
effect on radiata pine growth even seven years 
after the disturbance has occurred. 
It is intended to use old aerial photographs to 
extend observations beyond the seven year period 
to a full rotation, which for radiata pine in