Soil and Hillslope (Eco)Hydrology - Environmental Modelling: Finding Simplicity in Complexity - page 162

10.2 Tyger, tyger ...

Tyger tyger, burning bright,
In the forests of the night;
What immortal hand or eye,
Could frame thy fearful symmetry?
( The Tyger , William Blake, Songs of Experience , 1794).

An hour or so spent on Google Images and Google
Earth will reveal clear patterns in the vegetation in many
areas of the Earth. Perhaps the most remarkable pat-
terns are the so-called tiger bush from the drylands of
Africa, Australia, and north America (see Figure 10.1).
That such striped patterns are found in different conti-
nents and are common across a range of dryland settings
might give the lie to the uniqueness problem (see Section
10.1 and Section 10.5). In other words they might sug-
gest that similar processes operate across these drylands
regardless of variation in local factors such as soil and
rock type. However, as Figure 10.2 shows, striped pat-
terns may be found in peatlands where the processes
involved in soil formation are certainly quite differ-
ent from those in drylands. Although soil formation
processes may differ quite markedly between different
ecosystems, their role in pattern formation may be quite
similar and this possibility is currently being researched
(e.g. Rietkerk et al ., 2004). It should also be remembered
that surface pattern cannot, in isolation, be used to infer
process ( cf . Grimm et al ., 2005) (see later this section and
Section 10.3).
The patterns shown in Figures 10.1 and 10.2 seem to
reveal the first attribute of CAS noted above: i.e., they
appear to be aggregations of similar fundamental units.
In the case of the peatland in Figure 10.2, two basic
units or system states may be recognized: 'water' and
'peat hummock' ( cf . Belyea and Baird, 2006; Baird et al .,
2009). When aggregated along the contours they form
linear pools and ridges. In the dryland examples, the states
are 'vegetated' and 'bare soil', or in some cases 'vegetated'
and 'sparsely vegetated' ( cf . Lefever and Lejeune, 1997).
In both drylands and peatlands the patterns that are
seen are often independent of any underlying geological
controls. Within drylands, banded vegetation occurs on a
wide range of soil and rock types, and the species of plant
forming the patterns vary from grasses to bushes to trees
(Lefever and Lejeune, 1997). The density and thickness
of stripes may depend in part on slope length, steepness,

Figure 10.1 Striped vegetation patterns in drylands. Upper
picture: near Menindee, New South Wales, Australia (paler
areas are bare ground) (see scale bar for size). Image obtained
from Google Earth (copyright 2010 MapData Sciences Pty Ltd,
PSMA; copyright 2010 Cnes/Spot Image). Lower picture: tiger
bush plateau in Niger. Image obtained from Wikimedia
Commons (2010) shows an area of 5 5kmfromtheUS

declassified Corona KH-4A National Intelligence
Reconnaissance System (taken 31st December 1965).

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and curvature but do not reflect any other geological
features; there is no obvious outside control (or designing
'hand') on the formation of the 'fearful symmetry' of
these systems. These patterns may also be long lived. In
peatlands, persistence over hundreds of years of units
such as hummocks and pools has been reported by, for
example, Barber (1981) and Kettridge et al . (2008). In
drylands, Dunkerley and Brown (1995) and Dunkerley