Full text: Mesures physiques et signatures en télédétection

G.A. Blackburn 
Department of Geography. King's College London, Strand, London. WC2R 2LS 
E.J. Milton 
Department of Geography, University of Southampton, Southampton, S09 5NH. 
The directional spectral reflectance of deciduous forests changes markedly throughout the year as a 
consequence of the phenology of the canopy and understorey and changes in the spatial arrangement and 
density of scene components. This paper presents the results of a 1-year experiment using a tower-mounted 
spectroradiometer to measure the seasonal changes in the nadir reflectance properties of ash and beech 
canopies. Seasonal variations in mean reflectance in visible wavelengths were found to be inversely related to 
percentage cover, whilst variations in near inffa-red reflectance were directly related to percentage cover 
although for both wavelength regions changes in reflectance were not always exactly synchronous with changes 
in percentage cover. A strong rectilinear relationship was observed between NDV1 and percentage cover when 
data from both species were combined. The shape of the seasonal NDV1 profile revealed differences between 
the two species which was ascribed to phonological differences between ash and beech canopies. The position of 
the wavelength of maximum slope in the region 680-750nm, was found to shift between two domains 
depending on the particular association of scene elements presented to the sensor as canopy cover changed. 
KEY WORDS: Field spectroscopy, woodlands, phenology, canopy cover, NDV1 
Forests and woodlands cover approximately 40% of the global land surface and are responsible for around 70% 
of the annual net global carbon accumulation (Lieth, 1975). There is considerable interest in the use of remote 
sensing to estimate variables affecting the rate of operation of forest ecosystem processes, such as 
evapotranspiration, photosynthesis and nutrient cycling (e g. Running et al., 1989), and those affecting the state 
of the forest ecosystem such as leaf area index (LAI) and foliar chemistry (Peterson et al. 1987; Spanner 1989; 
Wessman et al. 1988). Most attention is directed to the spectral variation of forest canopies and allied to this it 
is widely recognised that spatial variability within forest stands and the directional reflectance properties of 
stands are both important factors in determining the way in which functional models developed at the leaf scale 
may be transferred to the canopy scale (Wessman et al., 1991). The timing of acquisition of remotely sensed 
data is also important in relation both to the phenology of the canopy and that of the understorey, although this 
has not received the same attention to date. 
This paper reports a field-based experiment designed to investigate the seasonal variations in the reflectance 
properties of a deciduous woodland canopy . A number of previous studies (e g. Boyer, 1988) have examined 
temporal changes in the reflectance of individual components of deciduous tree canopies, however the objective 
of this project was to expand on such studies by concentrating on the canopy as a whole. The aims of this study 
were four-fold: (a) To record the spectral reflectance properties of two deciduous woodland canopies at regular 
intervals throughout a year; (b) To measure the seasonal variations in percentage canopy cover at each site; (c) 
To determine the effect of the dominant tree species at each site upon the reflectance characteristics and canopy 
cover variations; and (d) To investigate the relationship between the reflectance properties (broad band 
response, NDV1, and wavelength of the 'red edge') and canopy cover, at each site.

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