Geoscience Reference
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Figure 7.9 Pinnacled crust, Utah USA. The lens cap is 49 mm in diameter. Soil surfaces such as the one illustrated have very
marked relief, but are quite fragile and become readily degraded under pressure of people, vehicles or grazing stock.
on some crusts, the runoff water may well pass downs-
lope to other locations where infitrability is higher, and
there be absorbed as ecologically important runon water.
In other words, the importance and role of BSCs cannot
necessarily be evaluated simply by local analyses of water
balance on the crusted patches themselves. Thus the role
of BSCs (just as for surface seals) cannot be evaluated
without undertanding their landscape context and the co-
variation of other soil properties with landscape position.
For example, in mosaic vegetation communities, the dis-
tribution of BSCs is linked to the distribution of vascular
plants, and other features relevant to infiltrability, such as
surface stone and litter cover, may also be linked spatially.
The work of Issa et al. (1999) on patterned vegetation in
Niger provides an example. They showed that there was
systematic variation in the nature and extent of crusting
across the repeating pattern of contour-aligned vegetation
patches and intergroves, with maximal crust development
immediately upslope of grove margins. At such locations,
water ponds briefly during and after rain and sediments
are laid down. They typically exhibit multiple surface
features, including cyanobacterial colonisation of depo-
sitional, inorganic seals. More work needs to be done
on the overall, hillslope-scale role of these crusts. Issa
et al. (1999) maintained that the zone of maximal BSC
development just above the grove boundary constituted
a relatively moist environment and would foster progres-
sive upslope colonisation by vascular plants. However, the
infiltrability of the soils in such locations is typically at
overland flow is partitioned into the adjoining grove. Thus,
in fact, conditions appear to mitigate against the coloni-
sation of these sites by plants. The important point here is
that the crust is of pivotal importance within the patterned
plant community, both for the offsite impacts of the over-
land flow that is generated, as well as for the local effects
on soil moisture immediately below the crust. Other field
studies reinforce the need for context always to be con-
sidered when analysing the nature and role of crusts. Ram
and Yair (2007) examined crusts along a rainfall gradient
from 86 mm to 160 mm annual rainfall at a site near the
Egypt-Israel border. They found that the amount of soil
moisture available to support vascular plants was actually
less at the higher rainfall sites than at the more arid ones,
owing to the abstraction of water on to thicker biological
crusts there. At the drier sites, there was less abstraction
and overland runoff could also arise and locally be con-
centrated downslope to create relatively moist microsites
of advantage to vascular plants. These results are con-
sistent with those of Grishkan, Zaady and Nevo (2006),
who also studied soil crust organisms along a rainfall gra-
dient in Israel. They found that there was only a weak
influence of rainfall on the spatial variation in biological
crust properties, which instead were largely governed by
microenvironmental characteristics such as levels of soil
moisture and organic matter.
Studies of the behaviour and environments of BSCs
on north- and south-facing dune flanks in the Hallamish
dune field, Israel, further illustrate the effects of microcli-
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