Assessing forest condition
 |
Long-term
sequences of aerial
photographs can be
combined with
elevation data to
identify vegetation
boundaries
and changes in vegetation
density over time |
Dieback disease caused by phytphthora species is
a major threat to the ecology and conservation of susceptible plant communities of
southwestern Australia. During the relatively short time since its introduction, infection
by phytphthora species has irreversibly destroyed the structure and diversity of
many plant communities. Because no cure has been found for the disease, large area of
trees, shrubs and smaller plants are laid to waste.
A reliable monitoring mechanism which can detect and quantify changes in vegetation is
important for understanding and managing the dieback problem. The existing archive of
aerial photographs is a resource which can be used for this purpose. With careful
processing, sequences of aerial photographs can be analysed to give a representation of
environmental changes that have occurred through a significant time frame. The resulting
information can be used to update digital data bases on the spread of dieback.
In a study of an area of jarrah forest, a combination of historical aerial photography
and elevation data was used to examine changes in the forest over the period. The changes
may result from various causes. This study looked at the impacts of vegetation change
attributed to dieback disease. The results show that the combination of historical aerial
photography and elevation data may be used to identify vegetation boundary change due to
dieback impacts.
Example study : Changes in jarrah forest 1972-1993
A sequence of historical aerial photographs was carefully processed to produce a
multi-date image. Selected aerial photographs were digitally scanned. Abutting photos were
then joined to form a mosaic for each date.
Differential rectification was used for each of the mosaics to rectify to a common
base. Illumination correction and image calibration were performed, so that changes in
different areas and times can be compared quantitatively.
Multi-date image (1972:blue, 1985:green, 1993:red) combined with
elevation shows changes in forest condition
The colours in the above map indicate the location, timing and severity of changes over
a twenty-year period. Blue shows areas where vegetation cover increased early in the
period, such as pine plantations planted in the 1970's. Red areas indicate loss of
vegetation since1985, either from clearing or gradual change such as forest disease
movement. Yellow areas have shown gradual decreases in vegetation density, primarily
indicating selective logging or forest disease.
The digital elevation model is incorporated to display the higher regions within the
landscape in lighter colours. Contour lines are overlaid to aid the visual interpretation
of the likely causes of change. The colour variations can be measured to determine rates
of disease spread.
This work was produced by Graeme Behn ,
WA Conservation and Land Management.
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