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Remote sensing for resources development and environmental management
Damen, M. C. J.

Table 1. Strata descriptions for classifying soil disturbance on aerial photographs.
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
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.
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,
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
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