Résumé
Dense vegetation bands aligned parallel to contour levels and alternating
at regular intervals with relatively barren interbands have been reported
at the margins of all tropical deserts. Since their discovery in the
1950s, it has been suspected that these vegetation bands migrate upslope,
forming a space-time cyclic pattern. Evidence to date has been relatively
sparse and indirect, and observations have remained conflicting.
Unequivocal photographic evidence of upslope migration (a few dm yr-1) is
provided here for three independent dryland areas exhibiting periodic
banded pattern the U.S. northeastern Chihuahuan Desert (1), the Somalian
Haud (2), and the Mediterranean steppes of eastern Morocco (3). Migration
speeds, averaged through time and space using Fourier cross-spectral
analysis, are shown to be directly proportional to pattern scale
(wavelength). An aerial photography sequence of (1) showed that migration
was not continuous but intermittent in response to fluctuating weather
regimes. The rates at which bands expanded upslope and contracted
downslope were better predicted by the change in annual rainfall than by
its average level. However, migration of banded pattern cannot be
considered as systematic because observations are reported from three
other banded systems located in the Somalian Haud, central Australia, and
western New South Wales in which migration was undetectable at the
available image resolution. In each of the six sites under study, the
modal value of band orientation axes was verified to be approximately
orthogonal to the steepest slope. Our results underscore the importance of
taking both the spatial structure and the past climate sequence into
account for understanding vegetation dynamics in arid to semi-arid
ecosystems. In addition, we show how Fourier spectral analysis applied to
historical series of optical images can serve to quantify landscape
dynamics at a decadal time scale.