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ASSESSMENT OF DIEBACK
In late 1972, at the request of the Norfolk Island Administration,
the Australian Forestry and Timber Bureau undertook to investigate and
report on the occurrence of the dieback and to suggest a course of action
for the administration.
The assessment incorporated two main activities -
(i) an examination of aerial photographs of the island, and
(ii) a site inspection to study the symptoms and associated
land use practices.
In addition to the information gathered in these activities, the
Bureau had access to the large number of reports concerning vegetation,
geology, soils, water resources, land use, forestry potential, development
potential, etc. which have resulted from numerous studies carried out
over the years. Many historic ground photographs, some more than 70
years old, were also available.
Two missions of aerial photography existed at the time, a black and
white 1:12 500 scale cover taken in 1944 and a colour 1:4000 scale cover
taken in 1968. The early photography was of little value in looking for
dieback symptoms because of scale and film type, however, it was useful in
providing a statement of current land use patterns and development on the
island. The 1968 colour photography was obtained for use in the prep-
aration of a detailed vegetation map of the potential national park site.
Benson (1974) provided details of the mission which resulted in a set of
photographs of outstanding colour fidelity and resolution. The 180
photographs covering the island were exposed in a Zeiss RMK camera fitted
with a 650 mm focal length lens. The format was 230 x 230 mm.
Because of the large scale of the photographs, individual pine trees
were clearly visible with crown images ranging from 1 to 3 mm in diameter.
The dieback symptoms mentioned earlier were clearly recognisable since the
dead branches were considerably lighter in colour than the healthy branches
and they contrasted sharply with the green understorey or ground vegetation.
These symptoms were readily separable from the appearance of healthy trees
carrying infestations of the bearded moss, Usnea.
The crowns of all mature trees were examined and boundaries of
consistent areas were outlined according to the following severity of
dieback scale:
H - healthy - no dieback symptoms visible
L - lightly affected - less than 50$ of mature crowns
showing some signs of ill health
S - severely affected - more than 50$ of mature trees
dead or having dead portions,
or with very thin crowns carrying
considerable quantities of bearded
moss.