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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.