linked aeolian-vegetation systems. i -

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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.

© 2013 Elsevier Inc. All rights reserved.

Author's personal copy

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

phology, Volume 11 http://dx.doi.org/10.1016/B978-0-12-374739-6.00315-8

Linked Aeolian-Vegetation Systems 429


can lead to decreased soil resources for plants and this process

is especially critical for the establishment phase when young

plants are most reliant on soil resources in the top soil layers.

The interactions between aeolian and biotic processes leads to

a rich set of feedbacks in ecosystems that can give rise to sur-

prising dynamics in wind-dominated landscapes.

The purpose of this chapter is to discuss in detail the

interactions between vegetation and aeolian processes and how

these give rise to critical dynamics in wind-erodible landscapes.

The discussion will be in five parts: How vegetation impacts

aeolian transport; How aeolian transport impacts vegetation;

How aeolian transport impacts soil; Feedbacks between vege-

tation and aeolian transport; and, in the final section, the

special case of vegetation in managed ecosystems.

11.22.2 How Vegetation Impacts Sand Transport

Throughout much of the aeolian literature, vegetation is

considered to belong to the class of ‘non-erodible elements’

that influence the flow of air over the surface (e.g., Gillette

and Stockton, 1989; Musick et al., 1996; Shao et al., 1996;

Marticorena et al., 1997; Lancaster, 2004; King et al., 2005). As

such, vegetation protects a portion of the soil surface and also

extracts momentum from the air (Figure 1). In these regards,

vegetation reduces the erodibility of the surface, as do other

non-erodible elements such as large clasts at the soil surface.

Vegetation differs with respect to many other non-erodible

elements insofar as it has the ability to trap sediment carried

by the wind. The porosity of vegetation, compared to a surface

clast of the same size, leads to turbulent flow within the

canopy that both reduces the mean air velocity within the

canopy but also increases the likelihood of collisions between

airborne particles and vegetation elements. Raupach et al.

(2001) have shown that the trapping ability of plants is

maximum when the optical porosity, that is the fraction of

horizontal beams of light that would pass through the canopy,

is approximately 0.2. Below this value, vegetation behaves

increasingly as a bluff body and above this value, air speed

is not decreased sufficiently and collisions are not increased

sufficiently, to trap airborne sediment as efficiently.

2.Extracts momentum from the air

1. Covers the soil surface

Figure 1 Key elements of the impact of vegetation on aeolian processes.role of sparse vegetation in wind erosion. Progress in Physical Geography 1

Conservation of shear stress, resulting from the conser-

vation of momentum, is a key tool for understanding how

vegetation impacts airflow over the surface. Raupach (1992)

and Raupach et al. (1993) created a theoretical basis for

understanding the stress available for particle detachment

and transport (see also Brazel and Nickling, 1987; Wolfe and

Nickling, 1993; Alfaro and Gomes, 1995; Musick et al., 1996;

Wyatt and Nickling, 1997; Gillies et al., 2000, 2006, 2007;

Crawley and Nickling, 2003; Walker and Nickling, 2003; King

et al., 2005, 2006; Leenders et al., 2007). This theory partitions

the total stress to that exerted on the vegetation and that

exerted on the soil surface. Although this theoretical approach

is able to adequately reproduce shear stress ratio (i.e., the

ratio of shear stress experienced by the soil surface to the total

shear stress) measurements from the field and laboratory (see

the compilation of data by King et al. (2005)), the resulting

estimates of threshold shear velocity (i.e., the shear velocity at

which particle movement is initiated) that are produced are

too high: aeolian flux is routinely measured on vegetated

surfaces even when the model predicts no flux due to high

threshold shear velocities (Okin, 2008). The likely cause for

the overestimation of threshold shear velocity of the Raupach

et al. (1993) model is that particle movement is not initiated

everywhere on the surface at the same time. Since the first

version of the model allowed calculation of average shear

stress experienced by the surface, it could not account for the

locations on the surface that might experience highest stress.

An adjustment of the vegetation lateral cover to improve

prediction of the maximum shear stress was suggested in the

original Raupach et al. (1993) paper, but this ‘m parameter’

has proved extremely difficult to pin down. Raupach et al.

(1993) suggested a value B1.0 for m, but a variety of other

values have been reported in the literature, suggesting un-

certainty about the meaning and utility of this parameter

(m¼0.16 Wyatt and Nickling, 1997; m¼0.53–0.58 Crawley

and Nickling, 2003; m¼0.19 and 0.28 depending on vege-

tation King et al., 2006).

To overcome the problems with the Raupach et al. (1993)

model, Okin (2005) used a Monte Carlo approach to predict

flux from a landscape in which surface shear stress varies due

to changes in the vegetation cover, shape, and size. Results

3. Traps soil particles

Reproduced from Wolfe, S.A., Nickling, W.G., 1993. The protective7, 50–68, with permission from Sage.

MAC_ALT_TEXT Figure 1

4000

3500

3000

2500

2000

1500

1000QTo

t (g/

cm/d

ay)

500

00 10 20

CV of cover30 40 50

1%5%

10%15%

20%

30%

40%

50%CV of u*ts

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


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



1.0

0.8

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.



(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

definition, nebkha dunelands display highly heterogeneous

distribution of perennial biomass, with little or no perennial

vegetation between nebkhas. This allows high rates of hori-

zontal aeolian transport in nebkha dunelands that may be

part of a positive feedback in which increased aeolian trans-

port leads to increased sediment transport from interspaces to

nebkhas leading to decreased soil nutrient content in inter-

spaces that suppresses vegetation growth in interspaces,

thereby maintaining or enlarging bare interspaces between

nebkhas, which further increases aeolian transport (Baas and

Nield, 2007; Nield and Baas, 2008; Okin et al., 2009a). This

erosion-vegetation feedback is an important part of land

degradation in arid and semiarid regions (e.g., Okin et al.,

2009b) and even if nebkhas are not always an indication of

land degradation, they may commonly be so.

11.22.3 How Aeolian Transport Impacts Soil andVegetation

Aeolian transport primarily occurs as saltation of large par-

ticles (approximately 50–1000 mm) and suspension of fine

particles (approximately o50 mm). Saltation carries most of

the mass and kinetic energy flux, and thus results in primary

physical effects on soil and vegetation, including abrasion,

burial and pedestaling of vegetation, and coarsening of the

surface texture in areas of deposition (Gibbens et al., 1983;

Hennessy et al., 1986; Okin et al., 2001a; Li et al., 2009b). In

contrast, the suspension of fine particles allows for their long-

range transport and removal from the area of emission. With

their high surface area and preponderance of clay and low-

density organic particles, the emission of suspended particles

(i.e., dust; see also Gillette, 1974; Marticorena and Bergametti,

1995; Gillette et al., 1997; Alfaro et al., 1998), results in the

loss of colloids and organic matter that give rise to a con-

siderable portion of the soil’s function: water holding capacity,

cation exchange capacity, and reactive surface area. Emission

of dust also entails the loss of inorganic nutrients that are

concentrated on the finest fraction, including nitrate, ammo-

nium, phosphate, potassium, calcium, and magnesium

(Finnell, 1951; Fryrear, 1981; Williams et al., 1984; Lyles and

Tatarko, 1986; Leys and McTainsh, 1994; Larney et al., 1998;

Okin et al., 2001b; Li et al., 2007). Although the dust may be

deposited far downwind and contribute to the fertility of

downwind ecosystems (i.e., both terrestrial and marine, e.g.,

Swap et al., 1992; Chadwick et al., 1999; Reynolds et al., 2001,

2006; Baker et al., 2003; Okin et al., 2004, 2011; Mahowald

et al., 2005; Neff et al., 2008; Krishnamurthy et al., 2010),

locally, emission of dust is significant contributor to the loss of

fertility. Some of the key processes and feedbacks resulting

from aeolian transport on wind-erodible ecosystems are

shown in Figure 5.

Burial of vegetation is perhaps the most obvious effect that

blowing sand might have on vegetation, but different plants

appear to respond to burial in different ways. The degree to

which buried plants can cope with burial by blowing sand


Burial, abrasionand pedestalingClimate

Soil textureand crusting

Crust sandblasting

Saltation

Coarsening ofsurface texture

Suspension

Loss of soil nutrients

Lower C, N and Pavailability at surface

Long-range dust transportto downwind ecosystems

Decreasedrecruitment

Plant mortality

Biological crustmortality

Land use

Vegetation coverand structure

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.



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).



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

aboveground dry weight, leaf area, plant height, survival,

yield, and fruit quality. Physiological changes to sandblasted

plants include temporary reduction of photosynthesis, in-

creased respiration, increased moisture stress resulting from

cells being ruptured by abrading particles, and increased tissue

N concentrations. Some have suggested that the increase

in tissue N is a result of decreased plant-wide N demand due

to lower aboveground biomass (Armbrust et al., 1974), but

Armbrust (1972) has shown higher foliar N concentrations

even when dry weight was not reduced. Nonetheless, abrasion

stress does increase foliar N concentration. The impact of soil

moisture on survival of plants exposed to wind-borne sedi-

ment is unclear; abraded cotton plants had higher rates

of survival under decreased soil moisture conditions, whereas

abraded tomato survival increased with increased soil

moisture.

In native nondesert vegetation, reports of the effect of wind

or wind-borne particles are sparse. Abrasion by snow has been

suggested to stunted growth forms near tree line (Scott et al.,

1993; Pereg and Payette, 1998; Scott and Hansell, 2002).

Cuticular damage from buffeting and abrasion has been cited

as the cause for increased cuticular conductance in Fagus

sylvatica (European beech), Picea sitchensis (Sitka spruce), and

Pinus sylvestris (Scots pine) (van Gardingen and Grace, 1991,

1992; van Gardingen et al., 1991). Unfortunately, the con-

sequences of these effects at the ecosystem-level have not been

well-studied.

Abrasion is probably at least partially responsible for the

presence of ‘streets’ in some well-developed nebkha dunelands

(Okin and Gillette, 2001). Streets are elongated interdune

areas oriented with the prevailing wind that serve as erodible

high-fetch areas and conduits for sand transport (Figure 8).

They presumably arise when interdune areas become

inhospitable to the establishment of new plants because

shrubby areas without nebkha formation do not display the

same anisotropy observed in adjacent nebkha dunelands. New


00−29°

0.2

0.4

Fre

quen

cy

0.6

0.8 Grasslands n = 560

ShrubIntershrub

30−59° 60−89° 90−119° 120−149° 150−179°

00−29°

0.2

0.4

Fre

quen

cy

0.6

0.8 Mesquite dunes n = 2450

30−59° 60−89° 90−119° 120−149° 150−179°

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.



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

found that there were more subtle effects.





By looking at the flux of N carried in wind borne sediments

together with the nutrients in the soils, the authors were able

to show that at greater than about 50% grass removal, vege-

tation growth was insufficient to replace N lost by aeolian

processes. Plots with less than about 50% vegetation removal

were able to conserve N. This loss of soil N appears to be

related to loss of fine-textured particles, which were signifi-

cantly enriched in both C and N compared to the parent soil.

Furthermore, using geostatistical analyses, the authors were

able to show that enhanced wind erosion led to the de-

struction of small, strong islands of fertility associated with

grass patch and enforcement of larger fertile islands associated

with mesquite bushes. These larger fertile islands, however,

comprised less of the total variance than the grass-centered

small fertile islands, suggesting that in addition to changing

the scale of variability of critical soil nutrients, there was a

net homogenization of them as well. On the downwind plots,

the deposited sediment was significantly depleted of N and

coarser than the parent soil. Although this likely does not

impact the existing vegetation on these plots, this nutrient-

poor surface with depleted water holding capacity is likely to

decrease the probability of successful establishment in the

depositional area. All of the results from the set of papers just

discussed are all the more striking due to the fact that the

major changes in biogeochemical status of the soils happened

in just two or three windy seasons. This rapid change high-

lights the potential of aeolian processes to significantly alter

soil biogeochemistry in the long run as well.

11.22.4 Feedbacks between Aeolian Transport andVegetation

Aeolian processes are enhanced when vegetation cover is de-

creased. At the same time, aeolian processes result in detri-

mental physical and chemical effects on plants and soils.

Therefore, strong feedbacks exist between aeolian transport

and vegetation.

In one study of these feedbacks, Okin et al. (2009a)

examined the impact of reduction of soil fertility by wind

erosion on the transition of grasslands to shrublands that has

been observed in drylands throughout the world (a map of

reported instances of shrub encroachment is provided by Ravi

et al. (2009)). In their simple model, grasses and shrubs were

made to compete for soil resources, with grasses having first

access to resources. When the impact of erosion on soil fer-

tility was not taken into account, the model exhibited only a

single stable state and events that caused decreased grass cover,

though associated with greater shrub cover, were reversible.

When grass-carrying capacity was made a function of the

amount of grass, an assumption consistent with the experi-

ments of Li et al. (2007, 2008, 2009a, 2009b) in which de-

creased grass cover was found to lead to decreased soil fertility,

the model exhibited bi-stability. One stable state of the model

was a grass-dominated state whereas the other was a shrub-

dominated state. Events that caused the grass dominated state

to lose significant grass cover, such as grazing, caused an

irreversible shift to the grassland state. The irreversibility of

the grass to shrub transition has been a matter of some debate,

but extensive research at sites worldwide (Peters et al., 2006)

suggests that, barring uneconomical human intervention,

it is very difficult to reestablish the dominance of grasses.

The model also suggested that past climate conditions that

affected the rate of grass reproduction could have been re-

sponsible for transitions between grass- and shrub-dominated

states observed in the paleorecord. The observations and

modeling of Tsoar (2005) confirm the potential hysteresis that

exists in vegetated aeolian landscapes (see also Yizhaq et al.,

2007, 2009; Tsoar et al., 2009). In his model, wind power

competes with vegetation reproduction rate to determine

whether a dunefield is vegetated or unvegetated. In high wind-

power slow-growth scenarios, dunes are always unvegetated.

In low wind-power high-growth scenarios, they are always

vegetated, and vegetation removal is reversible. However, be-

tween these extremes a bistable state exists in which vegetated

dunes, once disturbed through the reduction of vegetation

cover, cannot recover and will remain unvegetated.

11.22.5 Managed Ecosystems

A rich literature exists on aeolian transport in managed eco-

systems, particularly field agriculture and rangeland systems.

Although the aeolian processes at work in managed eco-

systems are the same as those in natural ecosystems, agri-

culture and grazing can lead to significant changes in the

aeolian transport regime. Specifically, changes to vegetation

cover and distribution as well as soil characteristics under

management leads to changes in the rates of aeolian transport

in these systems.

A major effort by the United States Department of Agri-

culture (USDA) to model wind erosion from fields has been

undertaken, resulting in the Wind Erosion Prediction System

(WEPS), which has wind erosion, management practices, and

vegetation growth subsystems (Hagen, 1991; Zobeck, 1991).

WEPS has been extensively tested and generally provides

good estimates of wind erosion (see Larney et al., 1995, 1998;

Hagen, 2004; Feng and Sharratt, 2007; Buschiazzo and

Zobeck, 2008). In field agriculture, native vegetation must first

be removed prior to sowing and crop residue is commonly

removed prior to replanting though no-till methods are used

in some places (Papendick and Parr, 1997). Bare fields are very

large unvegetated gaps that are susceptible to aeolian trans-

port. As a result, simple field geometry and its relationship

with erosive winds are critical in determining the impact of the

field on aeolian transport. Over field-scales, in addition, the

fetch effect, or the tendency of flux to increase with fetch

distance, becomes critical (Gillette et al., 1996). As vegetation

cover increases during the growing season, aeolian transport

from the field is reduced, but following harvest when vege-

tation can be completely removed, aeolian transport can in-

crease again. It is a common practice in areas where wind

erosion from fields is a major management concern for plant

residues to be preserved on fields between plantings to reduce

aeolian transport within and off of fields.

The effect of field agriculture on soil properties is complex,

as is the relationship between physical and chemical prop-

erties of agricultural soils with wind erosion. In the mid-

twentieth century, a large amount of research was conducted

by the USDA on the effect of various soil properties on wind



erosion, including: surface roughness, non-erodible clod/clast

content, aggregate stability, mechanical structure, and bulk

density (Chepil, 1950a, b, 1951a, b, c). These properties are

not only related to the physical and chemical compositions of

the soil itself, but are modulated, to a considerable extent, by

the timing and type of tillage (e.g., Larney and Bullock, 1994;

Lopez et al., 2000). A review of the impacts of agricultural

practices on wind erosion is beyond the scope of this review,

but the references cited herein provide a suitable entry into the

relevant literature.

In many respects, aeolian transport in rangeland systems is

simpler than in agricultural systems, particularly in light of the

fact that soil disturbances are simpler in the absence of tillage.

However, with regard to vegetation’s ability to modulate ae-

olian transport, rangeland systems are considerably more

complex than agricultural systems. Dryland rangelands that

are susceptible to wind erosion are structurally complex,

commonly with a mix of woody vegetation, perennial grasses,

and annual plants and unvegetated gaps that vary in size and

with time. Modeling systems developed for agricultural ap-

plications are not generally applicable to these structurally

heterogeneous systems.

Grazing has two major impacts on aeolian transport; it

affects vegetation and soil crusts. The effect of grazing on

vegetation goes beyond simply reducing the amount of bio-

mass on the land, with consequent impacts on vegetation

cover, spacing, and height. Grazing is also a major factor in the

encroachment of woody vegetation into grasslands (Van

Auken, 2000), which itself results in significant changes in the

plant spacing and gap-size distribution of affected rangelands

(McGlynn and Okin, 2006; Herrick et al., 2010a) leading to

significant increases in aeolian transport in some systems

(Gillette and Pitchford, 2004). Trampling by cattle, as well as

trampling by people and disturbance by off-road vehicles,

leads to disturbance of both physical and biological soil crusts,

which leads to decreased threshold shear velocities and in-

creased dust emission (e.g., Belnap, 1995; Belnap et al., 2007;

Herrick et al., 2010b; Baddock et al., 2011). Overall, the impact

of grazing in terms of its impacts on both vegetation and soil

properties, appears to be increasing aeolian transport and dust

emission (e.g., Belnap et al., 2009).

11.22.6 Summary

Vegetation provides a major control on the magnitude of ae-

olian processes that occur in a landscape, but aeolian pro-

cesses can also have considerable impacts on vegetation. Here,

the interactions between vegetation and aeolian processes

were briefly reviewed.

Linked vegetation-aeolian systems can be coastal dune-

lands, desert dunelands, stabilized dunes, agricultural systems,

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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dx.doi.org/10.1029/2003GB002145


Biographical Sketch

Gregory S. Okin obtained his B.A. degree from Middlebury College in Middlebury, VT. He received his Ph.D. from

the Department of Geological and Planetary Sciences at the California Institute of Technology, writing a disser-

tation entitled ‘‘Wind-Driven Desertification: Process Modeling, Remote Monitoring, and Forecasting’’. He then as

a postdoctoral researcher in the Department of Geography at the University of California, Santa Barbara before

being appointed as an Assistant Professor in the Department of Environmental Sciences at the University of

Virginia. In 2006, he moved to the Department of Geography at the University of California, Los Angeles, where

he now holds an appointment as Professor. Greg’s work focuses on understanding the dynamics of drylands

worldwide, with special emphasis on plant–soil interactions, aeolian transport, nutrient cycling, and remote


sensing.

linked aeolian-vegetation systems. i -

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