Dust Production Mechanisms - Mineral Dust: A Key Player in the Earth System - page 103

arrangement of the vegetation (MacKinnon et al. 2004 ). For example, in the
Chihuahuan Desert, measurements or erosion fluxes over surfaces with different
vegetation type and density have shown that the highest fluxes were measured
over surfaces with very dense mesquite vegetation, but arranged as linear “streets”
parallel to the wind direction of the highest winds (Gillette and Pitchford 2004 ).
Such vegetated surfaces exhibit high saltation thresholds and consequently low dust
emission frequencies. But under extremely high wind conditions, source regions
with high erosion thresholds can be responsible for extremely intense dust events
(Laurent et al. 2008 ). Grasslands and shrublands represent significant fractions of
the semi-arid areas and can be subject to large seasonal or interannual variability
of the vegetation cover and thus erosion thresholds. Such surfaces may also be the
most impacted by climate change. Better constrains on the parameterisation of the
saltation thresholds over vegetated surfaces are needed that account for the specific
arrangement of the vegetation that allow wind erosion to occur even over densely
vegetated surfaces and that account for the specific properties (porosity, flexibility)
of the different vegetation types in arid and semi-arid areas (herbaceous vegetation,
shrubs, etc.).

5.3.4 Other Factors

Crusts are thin soil surface layers more compact and harder than the material
directly beneath, produced by physical or biological binding agents (e.g. Valentin
and Bresson 1992 ). They are often formed following a thorough wetting by
precipitation. Surface crusts may be thick and hard and protective of the surface
from almost all winds. However, crusts may also be thin, weak and destroyed by
winds that are occasionally experienced in many environments. Observations of
crust formation and destruction on Owens Lake (California, USA) show extreme
differences in the amount of crusted sediment from year to year and from season to
season (Gillette et al. 2001 ). Additionally, biological crusts can form that provide
sufficient surface aggregation to protect the surface from wind erosion (Belnap and
Gillette 1997 ). Physical properties of crusts appear to be strongly influenced by
soil texture and soil composition. For natural crusts found in desert soils in the
Southwestern USA, Gillette et al. ( 1982 ) observed that silt and clay sediments
form thicker and stronger crusts than predominantly sandy sediments. For clay-
rich soils, Breuninger et al. ( 1989 ) observed that increased organic matter content
correlates with lack of crusting and breakage of the clay into increasingly smaller
peds (i.e. soil aggregates), which are vulnerable to wind erosion. For many of these
broken clay crusts, however, the pieces are large and hard enough to resist erosion
(Gillette et al. 1982 ). Surface crusting can be one of the main effects of precipitation
on dust emissions. Indeed, a fully crusted surface provides cover for possibly
more vulnerable soil beneath. Partially or fully crusted surfaces may modify soil
sediment supply and limit erosion fluxes (Lopez 1998 ). On the contrary, any process
that acts to break crusts or prevent their formation will increase the potential for