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Mesures physiques et signatures en télédétection

1 LERTS (CNES/CNRS UM C00010), 18 av. E Belin, bpi 2801.
F - 31055 Toulouse Cedex - France
2 CESR (CNRS/UPS), 9 av. du colonel Roche. BP 4346,
F - 31029 Toulouse Cedex - France
Satellite data, in various wavelengths, have already been used successfully to monitor spatial pattern of terrestrial
vegetation and its temporal variations from the local to the global scale. Their ability to validate and generalize
some ecological models is a very exciting topic in the context of global change, because it suggests that these
models could be used in a predictive mode with more confidence. They must be able to describe as
mechanistically as possible the various exchanges between the soil, the biosphere and die atmosphere, as well as
the internal biological processes, which drive growth, canopy development, and senescence. They also should
describe the behaviors of the different functional types in response to variations in die environmental conditions,
in order to better understand species competition and shifts of ecotanes. Difficulties appear because the
knowledge of the various processes is sometimes weak, and many empirical rules cannot be applied outside of
their experimental context. This paper discusses the perpectives expected from die strategy called: “assimilation
of satellite data”. Tte coupling of a functional model and a radiative transfer model leads to a complete modeling
of the satellite signal which than can be compared to the observations. This approach allows the recalibration of
the values of some parameters (or some initial conditions) not directly available from remote sensing
measurements, and their spatialization. A better description of the various processes involved in the model is then
KEY-WORDS: Functional model. Seasonal dynamics. Ecosystem dynamics. Satellite data assimilation.
Ecosystem functioning - Ecosystem dynamics.
One definition of the vegetation functioning of an ecosystem classically refers to die various exchanges (energy,
mass, momentum) between the canopy and die atmosphere, and between the canopy and die soil. Typical time
scales are very different, depending on the processes: hours or days for the photosynthesis, while for the soil
carbon release, it can be monthes until centuries. At a given time, the state of the vegetation is described by
variables like the biomass of the various components, the Leaf Are Index (LAI), the phenological stage... Due to
internal development and to the environmental forcing, the state and the activity of the vegetation are changing in
time, resulting from various exchanges mechanisms and biological processes. The states succession leads to
seasonal and interannual dynamics. Processes like foliage establishment and senescence produce a seasonal
dynamics. Processes like growth and mortality produce a seasonal dynamics for annual grasslands, and an
interannual dynamics for forests, far instance. In that last case, we can observe or not an annual periodicity
depending on the deciduous or evergreen characteristics of the canopy. The natural climate variability, as well as
the larger term trend in the environmental conditions (whether human-induced or not) have different impacts on
the functioning and on die seasonal and intra- annual behavior of the various species. As a consequence, the
different levels of responses to these changes of each functional type within an ecosystem lead to mechanisms
like species competition, and provide eventual shifts of the boundaries between ecosystems. These phenomena
are described in what is called “ecosystems dynamics”, and involve processes which drive reproduction and seed
dispersion. Typical time scales are between 10 and 1000 years.
In the above description, the term “dynamics” has two different meanings The dynamics
behavior which decribes the changes in the state of a canopy over a few years has only a temporal dimension,