The change of spectral signatures of 
beech and spruce by forest damage 
W. Kirchhof & H. Hoffmann 
DLR - German Aerospace Research Establishment, 
Institute for Optoelectronics, D-8031 Oberpfaffenhofen, 
F.R. Germany 
Abstract 
In the Federal Republic of Germany 
multispectral scanner techniques were 
developed for forest damage 
classification and mapping. To 
improve our understanding of spectral 
signature changes by forest damage 
and to optimize damage identification 
and separation high resolution 
spectroradiometer measurements of 
tree components were performed in the 
laboratory, simulating scanner data 
acquisition. The measurements of 
healthy and damaged beech and spruce 
branches proved the influence of 
primary effects of damage on spectral 
reflectance in the 0.49 - 2.50 pm 
region. In the visible discolouration 
influences largely the course of 
spectral signatures, causes red shift 
of the reflexion peak and a variation 
of shape at the beginning to the NIR 
plateau. In the IR reflectance 
decreases with increasing damage 
level for beech. For spruce branches 
the NIR plateau level reduces also 
with increasing damage. For 
wavelengths greater 1,4 pm the curves 
raise with higher damage level. 
Keywords: Forest damage, spectral 
signature, beech, spruce, 
remote sensing, classifi 
cation 
0 Introduction 
During the last decade in the Federal 
Republik of Germany and adjacent 
central European countries severe 
forest damage has been observed. This 
damage comprises large regions and 
varies largely. It seems neither 
caused by natural hazards like storms 
or climatic anomalies or of "classic" 
biotic or abiotic origin. 
Damage is optically visible by the 
loss of leaves or needles, in many 
cases by their discolouration (yello 
wing) , the thinning of crowns and 
anomalous branching. 
Damage symptoms become at first 
visible in the crown section. Damage 
measuring systems, therefore should 
preferably gather data with the view 
at the top of trees. Aerial photo 
graphs or multispectral scanner data 
from airborne or satellite plattforms 
fulfill these requirements. 
At the beginning of forest damage 
mainly fir and spruce trees were 
affected, Hermann et al., 1988, but 
since 1983 steep damage increase for 
deciduous trees was recorded, Land 
auer et al., 1989. Following the 
results of the 1987 forest damage 
statistics for the Federal Requblic 
of Germany 65,7 percent of beech and 
64.5 percent of oak trees, Anon., 
1987, were already damaged. 39 
percent of forests are deciduous, 61 
percent coniferous forests, total 
damage amounts to 50,2 percent in 
1984 52,3 in 1987 and 52,4 in 1988. 
Forest damage demanded inventories of 
its distribution and change over 
large areas in support of further 
studies, to find the reasons for the 
disease. Therefore, in 1987- 1989 the 
German Ministry for Research and 
Technology (BMFT), Bonn, supported a 
cooperative research project for the 
development of methods for the 
detection, classification and mapping 
of forest damage in larger areas of 
Germany by the use of remotely sensed 
multispectral data. 
The main research objective was the 
development and to some extent the 
verification of new multispectral 
scanner techniques including image 
processing. This approach required a 
new understanding of the information 
content of multispectral forest data 
in respect to damage classes of the 
same species. Five university 
institutes, two private firms and the 
German Aerospace Research Establish 
ment (DLR) took part in the project. 
The test areas were selected for 
forest damage investigations on fir, 
spruce, pine, beech and oak trees and 
stands. A description of the project 
and results obtained is given in 
Landauer et al., 1989. 
At the beginning of the project 
forest damage inventories were based 
on interpretation of CIR - aerial 
photographs. By the use of standard 
evalution methods and a new forest 
damage classification key comparable 
results could be reached for sample 
areas in different regions and at 
variable data acquisition times. The 
main information about damage or 
disease was derived from colour, but 
this source was varying rather widely 
i.e. with film type, development 
process and flight mission. It was 
one of the major results of a 
specialist group for photo 
interpretation of forest damage, 
summarized by Kenneweg, 1989, that' a 
general interpretation key, 
applicable for different regions and 
flight times is possible and 
structural elements of tree crowns 
and their reproduction in aerial 
photographs rather than colours are 
used as the main source of 
information for identifying different 
damage classes". in ’ addition 
discolouration by forest damage 
can