Wandoo Recovery Group, overall a sight improvement has been
observed in the wandoo crowns (Whitford et al 2010).
Whether any increase in cover identified in the imagery is due to
wandoo crowns improving needs to be validated in the field.
These locations can be identified in the trend imagery displays.
4.5 Is the recovery to previous canopy cover?
The spectral image index utilised represents increasing cover
with lower index values. Therefore by subtracting the highest
cover value in image sequence from the 2010 index cover value,
an image may be produced showing the difference between the
current cover and the highest cover. Pixel values of zero
indicate that 2010 is the year of highest cover, and positive
values the difference. A frequency histogram of the differences
may be produced. The histogram shows that the majority of
values are in the range 0 to 30 with an average of 12.58. Using
information from field sites observed by the authors an index
value of 12.58 can be related to a projective foliage cover of
approximately 7%. 2010 is the year of highest cover for 1.5%
of the total area.
The cover in wandoo woodland is generally less than 40%
(Hewett and Underwood 1963). Therefore a difference on
average of 7% in projected foliage cover between the 2010
cover and the highest year of cover is considerable. The canopy
shape of trees recovering from WCD is of a different form to the
healthy tree. Originally the tree would have a wide canopy with
leaves at the ends of branches and in recovery this changes to
clumps of leaves forming along the main branches, these are
called epicorms. Due to this contraction of the crown during
WCD and initial recovery, there will probably be a delay in
canopy recovery being observable in a 25m Landsat pixel.
4.6 Issues
Fire impacts will influence the trends and may mask declines
and recovery attributable to other causes. Due to the scale at
which fire history data has been collected it is not feasible to
mask these areas, but where declining trends are identified, any
fires occurring in the epoch could be attributed to refine the
analysis.
Climate and seasonality will affect the cover observed in
summer. The drying trend of the last ten years may be causing a
loss of cover, and whether is or isn’t related to WCD is
unknown. Mercer (2008) did find a trend of decreasing crown
rating with decreasing rainfall as field sites moved west to east,
but no relationship with average annual rainfall.
The image sequence has not been terrain corrected. Although
the terrain is not rugged, other work in the south west Western
Australian forests has shown that where there is a difference in
time of year of a few months subtleties in the cover index
images are affected. Repeating the analysis with terrain
corrected imagery would give greater certainty that the
differences observed were real on ground changes.
Cloud masking will affect the trend calculation especially the
four date trends, the areas affected are minimal and no areas
have less than three dates from which to calculate a trend.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B3, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
5. CONCLUSION
Further investigation could look at other ancillary data such as
position in landscape. This attribute was found to have a
significant correlation with WCD in Mercer 2008. Statistical
analysis of significance of differences between years and epochs
would direct the interrogation of the results to the most
important changes.
This method of interrogating an appropriately processed time
series of Landsat imagery has proven useful in extending the
understanding crown declines in the native forests in Western
Australia. The time periods of canopy loss identified by the
analysis correlate well with previous surveys and observations.
Of the field sites visited in 2011 that were wandoo woodland all
showed signs of previous crown decline and varying levels of
recovery or lack of. Future field sites to confirm decline impact
and recovery can be identified from the trend imagery.
6. REFERENCES
Brown, P., and Tippett, J. 1986. The role of biotic agents in the
rural decline of Eucalyptus wandoo Blakely in South-West
Western Australia. Report prepared by CALM Science Division,
Como, Western Australia.
Caccetta, P. A., Campbell, N. A., Evans, F. H., Furby, S. L.,
Kiiveri, H. T. and Wallace, J. F. 2000. Mapping and monitoring
land use and condition change in the South-West of Western
Australia using remote sensing and other data, Proceedings of
the Europa 2000 Conference, Barcelona.
Capill, L. G. 1984. Wandoo Woodland Conservation. A
proposal for a system of ecological reserves. Campaign to
Save Native Forests (W.A.).
Curry, P., Zdunic, K., Wallace, J. and Law, J. 2008. Landsat
monitoring of woodland regeneration in degraded mulga
rangelands: implications of arid landscaped managed for carbon
sequestration, In: Proceedings 14" Australian Remote Sensing
and Photogrammetry Conference, Darwin, Australia.
Draper, N. and Smith, H., 1980. Applied Regression Analysis,
Second Edition, Wiley: New York, pg. 266.
Furby, S., Zhu, M., Wu, X. and Wallace J.F. 2008. Vegetation
Trends 1990-2008 South West Agricultural Region of Western
Australia, Land Monitor II Project, CSIRO MIS, Available at:
http://www landmonitor.wa.gov.au/reports/landmon, II/LM2008
i. VegTrend, 1990. 2008.pdf
Garkaklis M. and Behn, G. 2009. Assessment of Eucalyptus
Wandoo (Wandoo) and other tree canopy decline using Landsat
Trend Analysis, Report to Department of Environment and
Convservation, Western Australia, pg 15.
Hewett, P.N. and Underwood, R.J. 1963. The wandoo forest.
Department of Conservation and Land Management, Woodvale,
Western Australia.
Land Monitor, 2012. Land Monitor A project of the Western
Australian Salinity Action Plan supported by Natural Heritage
Trust, http://www landmonitor wa.gov.au, [21 Mar 2012].
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