1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Modelling Wind Erosion and Dust Emission on Vegetated Surfaces 139
Figure 7.1 The Jornada ‘scraped site’ shown here in a 1999 low-altitude airborne visible/infrared
spectroradiometer (AVIRIS) image with resolution of ∼5 m. This site proves that
wind erosion can affect ecosystem stability in this ecosystem. It has experienced an average
deflation rate of 1.8 cm yr –1 (Gillette and Chen, 2001) and plant-available N and P have been
reduced by 89% and 78%, respectively (Okin et al., 2001b). No vegetation has regrown on
the scraped site itself. The vegetation community downwind of the site has changed from a
grassland to a shrubland due to burial, abrasion and leaf stripping from saltating particles from
the scraped site itself (Okin et al., 2001a) and plant-available N and P in surface soils in the
downwind area have been reduced by 82% and 62%, respectively (Okin et al., 2001b)
in the topmost soil layer is inhibited. The seedbank may also be depleted in the winnowed
surface soils (A. van Rooyen, personal communication). Thus perennial grasses are killed,
new vegetation growth is suppressed and shrubs, where established, gain a competitive
edge. Creation of an erosive discontinuity in a mixed shrub/grass ecosystem accelerates the
conversion of grassland to shrubland.
7.1.2 Where Does the Dust Come from?
Despite the importance of desert dust, it is often unclear in detail where it is produced and
what role humans play in mediating its production. Sensors such as TOMS can be used to
directly observe dust in the atmosphere using methods such as the TOMS aerosol index
(e.g. Prospero, 1999), but current technologies do not allow unique identification of the loci
due to their coarse resolution and, in the case of TOMS, because of problems in observing
the lower levels of the atmosphere. Because of this, controversy remains about the extent to
which land use contributes to the atmospheric mineral dust. Recent work by Prospero et al.
(2002) and Ginoux et al. (2001) suggests that the overwhelming majority of desert dust
comes from closed basins in arid areas related to now-dry or ephemeral lakes. They argue
further that humans do not significantly perturb the dust cycle, a conclusion supported by