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Figure 4. Seasonal variations in NDVI and canopy cover. 
Figure 5. Linear regression between NDVI and canopy cover, both sites combined. 
Previous studies have related the NDVI, and similar combinations of visible and near-infrared reflectance, to 
biophysical properties of vegetation canopies, notably leaf area index (LAI) (Curran, 1983, Jackson, 1983, 
Perry and Lautenschlager, 1984). Howev er, Tucker and Sellers (1986) used a simple radiative-transfer model to 
show that the NDVI is more closely related to instantaneous physiological rates associated with plant canopies, 
namely gross primary productivity and évapotranspiration, rather than any state associated with the vegetation. 
The results of the present experiment indicate that temporal fluctuations in the NDVI are related to the 
percentage of canopy cover, and NDVI remains high until the phase of rapid leaf abscission, despite a large 
increase in the number of dead and senescent leaves in the canopy. 
3.3 Seasonal variation in the position of the maximum slope ("red edge") 
During the leafless period, the error associated with measurement of the "red edge" position is far greater as the 
reflectance spectra do not exhibit distinct "red edge" features (refer to Figures 1 and 2). For such spectra, the 
feature identified using the derivative method is merely the position of maximum slope rather than a "red edge" 
in the conventional sense. We therefore prefer to use the term "position of maximum slope" (in the region 680- 
750nm) rather than the "red edge". 
Figure 6 illustrates that both sites show a very similar behaviour in terms of the overall seasonal fluctuations of 
the position of maximum slope, with rapid shifts between two extremes of around 700 and 725nm, however the 
timing of these shifts varies between the sites. This figure also shows the temporal relationships between the 
position of maximum slope and canopy cover. At the ash site there appears to be a threshold canopy cover of 
around 55%, above which the red edge is at its longer wavelength domain. On both occasions that the red edge