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Okin G.S. (2013) Linked Aeolian-Vegetation Systems. In: John F. Shroder (ed.) Treatise on Geomorphology, Volume 11, pp. 428-439. San Diego: Academic Press.
11.22 Linked Aeolian-Vegetation SystemsGS Okin, University of California, Los Angeles, CA, USA
r 2013 Elsevier Inc. All rights reserved.
11.22.1 Introduction 428
11.22.2 How Vegetation Impacts Sand Transport 429 11.22.3 How Aeolian Transport Impacts Soil and Vegetation 431 11.22.4 Feedbacks between Aeolian Transport and Vegetation 435 11.22.5 Managed Ecosystems 435 11.22.6 Summary 436 References 436
Ok
in
Ge
Ge
42
GlossaryAbrasion The process of damage to plant tissue by
windblown particles.
Aeolian transport Transport of materials by wind.
Bistable system A system that has two equilibrium states.
Bluff body A nonporous nonerodible element.
Cation exchange capacity The capacity of the soil for
cation exchange between the soil and the soil solution.
Clast A rock fragment.
Colloid A particle smaller than 1 mm in diameter.
Coppice dunes Vegetated sand mounts.
Coppicing The ability of a plant to resprout after burial or
death of aboveground parts.
Deflation The lowering of the soil surface due to erosion.
Fertile island An area of higher soil resource content,
usually surrounding a plant, compared to the soil resource
content in inter-plant areas.
Gap-size distribution The statistical distribution of the
size of unvegetated gaps between plants.
Geostatistical analyses Analysis of data that incorporates
sample location.
Horizontal aeolian flux The mass of material passing
through a unit distance of a plane normal to the ground
and the direction of the wind per unit time (i.e., g m�1 s�1).
Hysteresis A phenomenon whereby the state of a system
depends on the path that brought it to that state.
in, G.S., 2013. Linked aeolian-vegetation systems. In: Shroder, J. (Editor
Chief), Lancaster, N., Sherman, D.J., Baas, A.C.W. (Eds.), Treatise on
omorphology. Academic Press, San Diego, CA, vol. 11, Aeolian
omorphology, pp. 428–439.
Treatise on Geomor8
Internode length The length between two lateral
meristems on plants.
Managed ecosystem Rangeland and field agricultural
ecosystems.
Monte Carlo approach A computational approach that
relies on repeated random sampling to compute a result.
Nebkhas Vegetated sand mounts (arabic).
Nonerodible elements An object on the surface that is
not transported by wind (e.g., vegetation, large rocks, etc.).
Pedestaling Exposure of a plant’s roots through erosion.
Saltation The movement of particles in wind consisting of
repeated arcuate jumps off the surface.
Sandblasting Damage of a surface (here either of the soil
or a vegetation element) by windblown sediment.
Shear stress Stress applied parallel to the ground by the
wind.
Streets Elongated interdune areas oriented with the
prevailing wind that serve as erodible high-fetch areas and
conduits for sand transport.
Suspension Entrainment of particles into the air in such a
way that the particles cannot settle out and are transported
with the airstream.
Vertical flux The mass of material passing through a unit
area of a plane parallel to the ground (i.e., g m�2 s�1).
Winnowing Preferential removal of fine material.
Abstract
The interactions between aeolian and biotic processes are discussed, including the ability of vegetation to control aeolian
transport, the impact of aeolian transport on soils and vegetation (including soil nutrient content), feedbacks between
vegetation and aeolian transport, and aeolian transport in managed (agriculture and rangeland), vegetated systems.
11.22.1 Introduction
Aeolian process – the erosion, transport, and deposition of
sediments by wind – have significant interactions with biotic
processes. The most obvious of these interactions is the impact
of vegetation on the flow of air over the surface, and the sub-
sequent alteration of the erosive capacity of the wind. However,
aeolian processes can have considerable impacts on vegetation
and the soils that sustain terrestrial ecosystems. Sandblasting of
vegetation leads to decreased growth, as does the coating of
leaves by deposited dust. Burial by blown sand can kill vege-
tation or cause vegetation shifts by favoring species that adapt
well to burial. Winnowing of fines that occurs during erosion
Figure 2 The impact of vegetation and soil variability on predicted horizontal flux (QTot), as denoted by the coefficient of variation (CV) offractional cover and threshold shear velocity of the soil (u�ts). Reproduced from Okin, G.S., 2005. Dependence of wind erosion and dustemission on surface heterogeneity: stochastic modeling. Journal of Geophysical Research – Atmosphere 110, D11208, with permission fromAGU.
Figure 3 Patterns of shear stress experienced by the soil surface inthe Okin (2008) model of aeolian flux in vegetated areas. In thisimage, the wind direction is from left to right. Each circle representsa plant. Areas with the darkest shading experience the lowest shearstress and areas with the least shading experience the highest shearstress. Modified from Okin, G.S., 2008. A new model for winderosion in the presence of vegetation. Journal of GeophysicalResearch – Earth Surface 113, F02S10, with permission from AGU.
430 Linked Aeolian-Vegetation Systems
Author's personal copy
from this modeling exercise highlight the importance of
variation in surface shear stress, particularly as aeolian flux is
a nonlinear threshold-controlled process. Small parts of the
landscape in which the threshold was exceeded could result
in significant average flux from the landscape as a whole
(Figure 2). Despite this success, there is little practical use for
this model because it requires knowledge of the variability of
key vegetation parameters (cover, size, and shape) as well as
landscape-scale averages.
A recent model by Okin (2008) moves away from the
underlying problems of previous models in predicting aeolian
flux in vegetated systems by considering the shear stress ex-
perienced by the soil in the area immediately downwind of
vegetation. The Raupach (1992) model assumed that the shear
stress in the wake area behind plants was zero, and although
the author acknowledged that this is not actually the case, the
mathematical approach taken required this assumption. The
model of Okin (2008) allows variable shear stress experienced
by the soil surface downwind of plants, where locations
closest to the plant experience the lowest stress and the stress
increases asymptotically to unvegetated values as the distance
from an upwind plant increases (Figure 3).
A significant hurdle that any new model must overcome is
the explanation or reproduction of existing experimental
measurements. The Okin (2008) model of aeolian flux on
vegetated surfaces does this (Figure 4). More importantly,
however, the model predicts aeolian flux at relatively high
amounts of vegetation cover. This is consistent with field
measurements (e.g., Lancaster and Baas, 1998; Li et al., 2007).
Lancaster and Baas (1998) reported significant horizontal flux
(40.1 g m�2 s�1) with high lateral cover (B0.2) in some
of the larger storms they observed. Though horizontal fluxes
at high lateral cover were observed to be up to three orders
of magnitude lower than in unvegetated areas during the
same event, they are nonetheless nonzero. The Raupach et al.
(1993) model typically predicts no flux for these cases because
it predicts high threshold wind velocities (B160 cm s�1 for the
data of Lancaster and Baas), whereas the Okin (2008) allows
fluxes, albeit low, at high lateral cover. Thus, although this study
concluded that flux was ‘effectively eliminated’ below 15%
cover, fluxes were measurable above this threshold. In fact,
horizontal fluxes in this study were modeled as an exponential
(rather than thresholded) function of vegetation cover in this
study. Indeed, both modeled and measured horizontal sedi-
ment fluxes make it clear that the presence of vegetation does
not shut down aeolian transport, though it certainly can de-
crease aeolian transport dramatically. So, although vegetated
areas do not experience the levels of aeolian flux exhibited in
wind-erodible vegetation-free areas, these fluxes do, none-
theless, occur in areas with significant biotic activity, and
therefore, they have the potential to impact soil, vegetation, and
dust emission; even very small fluxes, when combined over very
large areas and timeframes, can become significant.
Thus, vegetated landscapes can undergo aeolian transport.
Environments with coppice dunes, or nebkhas, are interesting
cases in which vegetation and aeolian transport are coupled.
Nebkhas are dunes that form from the trapping of wind-
transported sediment within a plant (Tengberg, 1995). They
form when accumulation of sand within plants is outpaced
by vegetation growth or resprouting of vegetation from meri-
stems near the soil surface (i.e., ‘coppicing’). There is little
doubt that nebkhas serve as evidence of aeolian transport
Marshall (1971)Lyles and allison (1975)Musick et al. (1996)Musick et al. (1996) porousMusick and gillette (1990)Wolfe and nickling (1996)Wyatt and nickling (1997)Lancaster and baas (1998)Luttmer (2002)
0.6
0.4
She
ar s
tres
s ra
tio (
SS
R)
0.2
0.00 0.001 0.01
Lateral covers (λ)
0.1
(u*s/u*)x=0 = 0.3
(u*s/u*)x=0 = 0.2
(u*s/u*)x=0 = 0.1
(u*s/u*)x=0 = 0.01
Figure 4 Predicted (lines) and actual (symbols) shear stress ratio. Predictions are using the model of wind erosion on vegetated surfaces.Closed symbols are for bluff bodies and open symbols are for porous bodies. Lateral cover is equal to the number density of roughnesselements (i.e., plants or other nonerodible elements) times the average height and average cross-wind width of the roughness elements. Theparameter (u�s/u
�)x¼0 quantifies the degree of suppression of shear stress in the immediate lee of roughness elements with a value of 0 meaningthat this area experiences no shear stress. Reproduced from Okin, G.S., 2008. A new model for wind erosion in the presence of vegetation.Journal of Geophysical Research – Earth Surface 113, F02S10, with permission from AGU.
Linked Aeolian-Vegetation Systems 431
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(for an alternative view, possibly applying to plant-related
soil mounds smaller than nebkhas see Parsons et al., 1992),
and they are generally considered to be evidence of land
degradation in systems dominated by aeolian transport (e.g.,
Nickling and Wolfe, 1994; Tengberg, 1995; Tengberg and
Chen, 1998; Langford, 2000; Laity, 2003; Wang et al., 2006,
2008), however, Dougill and Thomas (2002) have argued that
the relationship between nebkha formation and land deg-
radation may not be straightforward (that is to say, patterns of
nutrient accumulation in nebkha areas are likely the com-
bined result of aeolian transport to nebkhas and biologically-
mediated N cycling within nebkhas). Nevertheless, almost by
Figure 5 Some key impacts of aeolian transport on terrestrial ecosystems, and potential feedbacks between aeolian processes and bioticprocesses. Reproduced from Okin, G.S., Herrick, J.E., Gillette, D.A., 2006. Multiscale controls on and consequences of aeolian processes inlandscape change in arid and semiarid environments. Journal of Arid Environments 65, 253–275.
Figure 6 Bactrian camels amongst nebkhas (coppice dunes) in Inner Mongolia, China. Photo by the author.
432 Linked Aeolian-Vegetation Systems
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most likely depends on whether they can grow fast enough to
outpace deposition which, due to the capacity of vegetation to
trap wind borne sediment, is concentrated at the plants
themselves. Woody species that have the ability to grow faster
than their burial typically become nebkhas (a.k.a coppice
dunes, Figure 6). In cases where wind spacing between neb-
khas is wide enough, interdune areas can become the loci
of significant deflation with eroded material contributing to
the adjacent dunes (Gillette and Pitchford, 2004). Despite its
obviousness, little work has been done on what controls
plants’ ability to keep pace with burial in deserts, although
there is an extensive body of literature on burial in coastal
dunes (e.g., Wallen, 1980; Eldred and Maun, 1982; Disraeli,
1984; Maun and Lapierre, 1984, 1986; Zhang and Maun,
1990, 1992). A high intrinsic growth rate is certainly necessary,
but other factors, such as the location of growth points could
easily be as crucial. Because the location of the growth points
on grasses tend to be close to the ground whereas the growth
points on shrubs are commonly at the end of elevated stems,
we might infer that shrubs are better suited to cope with burial
by windblown sand than grasses. Nonetheless, considerably
more work needs to be done to understand how plants re-
spond to burial, particularly with regard to grasses and non-
nebkha forming species (Figure 7).
Pedestaling is the opposite of burial. Pedestaling occurs
when deflation leads to exposure of plant roots, particularly
the central vertical root of shrubs (Figure 7). Pedestaling is a
clear indication that wind erosion can occur in vegetated
Figure 7 (Top) Pedestaling. Photo by the author. (Middle) Burial ofgrasses by a tongue of sand. (Bottom) Clear abrasion of mesquite(Prosopis glandulosa) bark due to saltation. Courtesy of P. Kahn(middle and bottom).
Linked Aeolian-Vegetation Systems 433
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areas, and indeed, from directly underneath plants themselves.
Though pedestaling is a relatively common site in shrubby
wind eroded areas, it is rare to see a plant that has clearly been
killed by this phenomenon (Pyke et al., 2002; Okin et al.,
2006; Okoba and Sterk, 2006). Nonetheless, there has been
very limited research on the impact of pedestaling on desert
vegetation, and considerably more research is required to
ascertain its importance.
Saltation, because it is responsible for the vast majority of
mass flux in aeolian transport not only is responsible for
burial and pedestaling, but it can also dramatically affect
vegetation through sandblasting (Figure 7). Work by Okin
et al. (2001a) and Schauer et al. (2001) has shown that salt-
ating particles can strip off leaves and cambium of plants in
areas of high saltation flux. Despite this, there are to date no
measurements of the impact of saltation on the leaf- or plant-
level physiology of native dryland species. However, there have
been several studies in the past half-century on the influence
of wind and wind-borne sediment on agricultural plants.
These studies were integrated and reviewed by Armbrust and
Retta (2000). The following results are cited there unless
otherwise noted. Wind, in the absence of saltating particles,
reduces plant growth by several mechanisms. At low wind
speeds, the effect seems to be an increase in transpiration,
which results in water stress. This stress causes the plant to
adapt by decreasing leaf area and internode length, whereas
increasing root growth and stem diameter. As the wind speed
increased, cell and cuticular damage occurs, followed by plant
tissue death, and a gnarled appearance becomes apparent.
Abrasion of plants by wind-borne particles decreases
Figure 8 Angular distribution of shrubs and inter-shrub areas in agrassland (with B10% shrub cover) and a well-developed coppiceduneland. Both shrubs and inter-shrub areas display a strong biastoward a direction of 0–601 (clockwise from north), which isconsistent with the direction of the prevailing wind. Reproduced fromMcGlynn, I.O., Okin, G.S., 2006. Characterization of shrub distributionusing high spatial resolution remote sensing: ecosystem implicationfor a former chihuahuan desert grassland. Remote Sensing ofEnvironment 101, 554–566.
10
Jul 7 Aug 2 Sept 16
30
40
50
60 Dusted nonirrigated
Undusted nonirrigatedDusted irrigated
10
(a)
(b)
(c)
Mai
n sh
oot l
engt
h (c
m)
Tot
al s
hoot
leng
th (
cm)
Num
ber
of la
tera
l sho
ots
12
14
15
Figure 9 Plant-level impact of dusting on the desert shrub Larreatridentata. Reproduced from Sharifi, M.R., Gibson, A.C., Rundel,P.W., 1999. Phenological and physiological responses of heavilydusted creosote bush (larrea tridentata) to summer irrigation in themojave desert. Flora 194, 369–378.
434 Linked Aeolian-Vegetation Systems
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recruits in the interdune area, with their entire biomass in the
saltation zone, would be exceedingly vulnerable to abrasion.
However, other factors could contribute to the development
of streets: absence of microsites in interdune areas and fast
transport of seeds through interdune areas, removal of sandy
topsoil and exposure of a hard argillic horizon, and depletion
of nutrients in the interdune area. Considerably more research
is required to fully understand the presence of streets in
nebkha dunelands and to quantify their presence in nebkha
dunelands worldwide.
A study by Sharifi et al. (1997) has shown that dust, de-
posited on leaves of desert shrubs, has the potential to affect
the physiological performance of some species (Figure 9).
This study found that maximum rates of net photosynthesis of
dusted leaves were reduced to 21–58% of those of control
plants, depending on the species. Maximum leaf conductance,
transpiration, and water use efficiency were also shown to
be reduced, whereas dusted leaves and photosynthetic stem
temperatures were 2–3 1C higher than those of control plants
due to greater absorption of infrared radiation. Heavily dusted
shrubs in this study had smaller leaf areas and greater leaf-
specific masses suggesting that the short-term effect of reduced
photosynthesis and decreased water use efficiency may cause
lowered primary production in dusted desert shrubs. This
conclusion is largely supported by the later study of Sharifi
et al. (1999), which investigated the plant-level effects of
dusting. Although these authors found significant impacts on
shoot length in affected individuals of Larrea tridentata, their
result suggested that the impact may be obviated in the pres-
ence of ample water.
Besides the physical impacts of blowing sand and de-
posited dust on plant physiology, wind erosion has significant
impacts on the biogeochemical status of soils and the distri-
bution of soil resources. In an experiment aimed at under-
standing interactions between soil, vegetation, and aeolian
processes, Li et al. (2007, 2008, 2009a, 2009b) showed sig-
nificant, and sometimes unexpected, impacts of wind erosion
on vegetated systems. The experiment reported in these papers
consisted of upwind plots, on which grass was removed in
different proportions and adjacent downwind plots on which
the impact of increased flux from the upwind plots could be
examined. Not surprisingly, the horizontal aeolian flux was
greatest on the plots with grass removal. However, the authors
or simply rangelands with wind-erodible soils. In each of these
cases, both vegetation and aeolian processes determine the
degree to which the surface is mobilized. Vegetation cover
almost always suffers as a result of aeolian erosion, transport,
and deposition, though in the case of nebkhas, vegetative
resilience may mask the negative effects. By the same token,
aeolian transport is greatest under conditions of minimal
vegetative cover. This set of feedbacks often gives rise to hys-
teresis that explains the degree of stability of these systems.
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