sequences on burned areas, clearings etc. can be important for
prognosis about future growth and population development
(patterns). Second, boreal forests are often characterised by low
biomass levels, comparable to that of the young stands used in
this investigation. Therefore, ERS-1 SAR could be sensitive to
changes in these ecosystems.
Since this study also indicated a high sensitivity of C-band
backscatter to effects of temporally varying environmental con-
ditions, these effects may mask backscatter variations due to dif-
ferences of forest stand parameters. This is an indication that
complex methods of model inversion including both the complex
scattering mechanisms of forest stands and the effects of varia-
tions in environmental conditions must be developed.
A synergistic relationship between optical and microwave data
has been established. The combined data set provided a combi-
nation that allowed better differentiation of forest age classes, as
compared to results obtained with the individual sensors. A ratio
of optical SPOT/XS3 reflectance and ERS-1 backscatter showed
higher sensitivity to stand age due to the opposite optical reflec-
tance/radar reflectivity behaviour of forest canopies in IR and C-
band frequencies. Age differences, however, are not directly de-
tected. Rather, it is the different tree and stand characteristics
(e.g. foliar vigor, quantity and distribution, tree height, stem and
branch size, standing biomass and canopy structure) of differ-
ently aged stands which will result in different optical and mi-
crowave signatures.
In the future, additional efforts should be undertaken in order to
establish efficient strategies for monitoring forest conditions and
for retrieval of forest biophysical attributes using combinations
of a variety of spaceborne sensors. These include:
- in improving understanding of both radar and optical data (e.g.
the influence of topography on measured signals),
- in relating changes occurring in spaceborne SAR time series to
spaceborne optical monitoring data.
ACKNOWLEDGEMENTS
This research and application work was conducted at the Joint
Research Centre of the European Communities, Institute of Re-
mote Sensing Applications, Advanced Techniques Unit, in the
framework of the IFIT /International Forest Investigation Team).
The support of Dr. A.J. Sieber, head of IRSA/AT, was greatly
appreciated. The work was realised within two CEC contracts:
EARS-92-0004-FR, and a Visiting Scientist Contract. The geo-
coding software for ERS-1 SLC data was developed under CEC
contract for the JRC by Earth Observation Systems Ltd. (UK).
The authors want to acknowledge Dr. P. Vossen from
JRC/IRSA, for his useful recommendations about their research.
We also want to thank Professor D. Pelz, from Freiburg Univer-
sity, Germany, who provided us with the forestry GIS database.
Dr. C. Lavalle, from JRC/IRSA, provided the software for an-
tenna pattern and range spreading loss corrections.
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
