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Proceedings of the Symposium on Global and Environmental Monitoring

Spectral response from thinning cuttings
measured by multitemporal satellitedata.
HSkan Olsson
Dept, of Biometry and Forest Management, Remote Sensing Lab.,
Swedish University of Agricultural Sciences,S-901 83 UmeS.
Phone +46 90 166596, telefax +46 90 165925.
Spectral mean values for thinning cuttings and seed tree stands were measured
before and after cutting, using a time series of six digital Landsat TM
acquisitions. The studied area, 30 km by 40 km, was situated in boreal forest
at 65° N,20° E. The type of forest and degree of cutting were surveyed in
field using objective sampling methods. Dominant species were Scots Pine
(Pinus Silvestris), Norway Spruce (Picea Abies) and Birch (Betula Spp.).
Linear regression functions for prediction of the DN values at the
following year were computed for each of the first five Landsat acquisitions.
This was done by using a systematic sample of pixels taken under a forest
mask. Differences between the actually measured stand mean values and the
predicted were obtained for the thinned stands. Thinning caused a minor
increase in spectral radiance in the visual channels TM 1, TM 2 and TM 3, and
a more significant increase in the middle infrared channels TM 5 and TM 7. A
decrease in radiance was observed for the near infrared TM 4. Between 19 %
and 51 % of the basal area was cut in the thinnings. The variations in
spectral response among the thinnings could hardly be explained as a function
of thinning grade. The most important factor found was the remaining
proportion pine forest after cutting. The change in proportion of deciduous
forest had a strong influence on TM 4. By multispectral discriminant analysis
81 % of the thinnings was detected. The spectral radiance starts to increase
when more than 40 % of the basal area is cut. This is shown by measurements
from five seed tree stands. The study is of basic interest for the
development of standwise change detection in a GIS environment. The results
are also of interest for the evaluation of Remote Sensing for the Swedish
National Forest Inventory.
KEYWORDS: Landsat TM, change-detection, thinning-cuttings.
The life cycle of a forest stand in Sweden is 80-
100 years. One to three thinning cuttings are
normally carried out before the final cutting. The
aim of the thinnings is to give space for the
most valuable stems to grow to timber, the
thinnings will also give pulp wood for sale.
On the National level the forest resources in
Sweden are monitored by the National Forest
Inventory (NFI). The NFI covers yearly the whole
of Sweden in a ground based sample plot design
(Ranneby et. al. 1987). At present the possi
bilities to incorporate satellite data analysis in
the sampling design of the NFI are investigated
(Thomas and Ranneby 1990). One of the questions
raised is then how reliably thinning cuttings can
be detected by satellite data analysis.
The present study is part of a larger project at
the Remote Sensing Laboratory in Umea. The overall
aim of this project is to develop general methods
for standwise change detection in a GIS
environment. The most important features to
detect with this technique are probably forest
damages (Olsson 1989, 1990). The spectral changes
to be detected are equally small in the cases of
both thinnings and damages. The development of
procedures for calibration and discrimination
might therefore be carried out on thinnings, for
future applications on damages.
Earlier work on detection of thinning cuttings in
Scandinavian forests has been carried out in
Finland (Häme 1986). Häme found a mean increase in
TM 3 and a decrease in TM 4. He considered
thinnings as difficult to detect.
In this paper the first results from a Swedish
study are presented. It is investigated how
spectrally separable thinning cuttings are from
unchanged forest, using a multitemporal analysis.
The spectral response in different channels as a
function of thinning grade, type of forest and
changes in tree species combinations are studied.
It is also of interest to find linear combinations
of the satellite channels that are effective in
discriminating newly thinned stands from the
unthinned stands.
The approach to change-detection used in this
study is calibration with regression functions. It
has the advantages that no absolute calibration of
the data is necessary, the regressors used can
also be extended. The approach has successfully
been tested by the author (Olsson 1990), as well
as by others (Sing 1989) .
Presented at the ISPRS commission VII Mid-Term Symposiom,
Victoria, Canada. September 17-21 1990.