Download essentially a monospecific shrub land that is primarily composed of

ECOLOGY
AND
MANAGEMENT
essentially
a monospecific
shrub land that
is primarily composed of single lifeform
patches.
The shrub land has very lo~
obstruction
value and a large percentage
of the rainfall leaves the unit as overland
flow or is transported in rills to ephemeral
stream channels (Fig. 3). Run-off water from
this unit has high sediment loads as a result
of soil detachment by raindrop impact. The
quantity of suspended sediment and textural
composition
of the sediment is dependent
upon variables such as rain killetic energy,
soi I detachabi Iity, characteristics
of soi I
surface, depth of surface water layer, and
the influence
of slope on detachment
(Abrahams et af., 1988; Torri and Poesen,
1992).
Healthy, function ing watersheds support
a diverse
fauna that contribute
to soil
- porosity,
affect soi I bulk density, affect
soil fertility and transport sub-soil to the
surface
where
it is susceptible
to
incorporation
into rLlIl-off as suspended
sediment. Animal species that affect several
soi I properties
have been considered
keystone species in the ecosystems in which
they occur (Whitford,
1997). Several
invertebrate
taxa that are widespread
in
the arid regions of the world have been
described
as playing a keystone role by
affecting the properties and processes of
ecosystem s (Wh itford, 2000). Ants and
term ite affect pedogenesis
by transporting
large quantities of subsoil to the surface.
Because the soil used in the construction
of foraging galleries of some species of
term ites includes term ite feces, the surface
gallery soil is enriched in some nutrients
like nitrogen and phosphorus. Termites and
ants have been reported to transport between
OF ARID
ZONE WATERSHEDS
269
I
22 and 4000 kg ha- i' of subsoil to the
surface
in construction
of nests and/or
foraging galleries (Whitford,
2000). The
quantity of soil turnover by ants and termites
is a function of landscape position, soi I
and vegetation. Ants and termites produce
soi I macropores (continuous tubes or voids
in the soil) that transport flowing water
to the deeper parts of the soil profile faster
than predicted
by infi ltration models of
soils of different particle-size distributions
(Phillips et af., 1989). Other invertebrate
taxa that produce soil macropores
include
ground-nesting
spiders,
cicadas,
earthworms, and isopods.
Subterranean and/or epigeic termites are
widespread and relatively abundant in soils
of arid and semi-arid regions that are not
subjected to periodic inundation. Because
termites process such a large fraction of
the dead plant material in the ecosystems
in which they occur, they have a significant
effect on the soil nutrient patterns of arid
and semi-arid landscapes. In the Chihuahuan
Desert of North America and watersheds
in Tanzania, termites have been shown to
be responsible
for most of the variation
in soil organic matter, soil organic carbon
and associated nutrients (Jones, 1989; Nash
and Whitford, 1995) and to playa critical
role in nutrient
cycling
(Schaefer
and
Wh itford, 1981 ).
Many species of desert mammals affect
soi I properties thereby affecting vegetation
spatial patterns, species composition of plant
communities and the hydrological properties
of ecosystems (Whitford and Kay, 1999).
Burrowing animals eject large quantities of
soil in the construction
of burrows. The
ejected soils have lower bulk density and
270
WllITrORD
different texture and chem istry in comparison
to surface soils. Digging by various species
of mammals increases water infiltration in
the area of the excavations.
Some mammal
species occupy the same burrows or burrow
complexes
for several generations.
Soils
around the burrows and burrow complexes
of central place foragers frequently become
enriched in several soil nutrients over time
(Whitford and Kay, 1999). The ecosystems
of healthy watersheds will support a number
of species of invertebrates and vertebrates
that affect soi I properties
and processes
thereby affecting the spatial patterns and
structural features of the vegetation.
In arid regions, sediment loss over time
frequently results in soil surface with varying
cover of rock fragments. The rock fragments
and rock cover of the soil surface are the
lag materials remaining following the loss
of sediment. Rocks and rock fragments have
different effects on infiltration and run-off,
depending on their position in or on the
soil. Rocks and rock fragments resting on
the soil surface increase infiltration
and
decrease run-off volume.
Rock fragments
that are well embedded in the top-soil:tayer
reduce infiltration and run-off generation is
proportional to the percentages of rock cover
(Poesen el 01., 1990). The rock lag on the
soil surface
of low obstruction
value
vegetation units contribute to the high volume
of water and sediment transferred from these
units into downslope
units.
On portions of watersheds or watersheds
with slopes of one per cent or less, vegetation
may be organized in bands or stripes oriented
across the slope (Mabbutt and Fanning, 1987).
The vegetation bands divide a slope into
narrow contours of vegetation that serve
as run-on areas or sinks separated by barren
or sparsely vegetated areas that serve as
rLlI1-off or source areas. Banded vegetation
develops on fine-textured
soils on slopes
0
as small as 0.5 (Dunkerley and Brown.
1995). The fine textured soils have low
infiltration rates and most water falling on
the sparsely vegetated areas runs off. The
upslope edges of the vegetation are called
interception zones (Tongway and Ludwig.
1996). The upslope pOltions of the vegetation
bands are areas of high biological activity
that results in high porosity of the interception
zone soils. Interception of run-off water in
the vegetation bands provides several times
more water per unit area of vegetation than
is available from rainfall. The width of the
vegetation stripes is a function of the average
area needed for infiltration of rainfall plus
the run-off from the bare zone. Despite the
large areas of un vegetated
soil, banded
vegetation represents a healthy, functioning
watershed.
Functional
water sheds with
banded vegetation have been described in
arid and semi-arid
regions of Australia.
Africa, the Middle East and North America
(White, 1971) and are probably present in
other arid and semi-arid regions of the world.
DcseJ'tified Watersheds
Deseltification of watersheds in arid and
semi-arid regions around the world varies
in degree of degradation as a function of
the cultural traditions of the people inhabiting
a region. Land-use patterns frequently change
as a result of changes in the socio-economic
systems of the human inhabitant. Deleterious
human impacts are exacerbated by cl imatic
drought. which is a recurring condition in
arid and semi-arid environments. Since most
of the arid and semi-arid lands are used
primarily for the production of domestic
ECOLOGY AND MANAGEMENT
livestock, over-stocking and inadequate herd
management
strategies
are the most
widespread
agents
of desertification.
Desertification initiated by Iivestock impacts
may include: (1) reduction or loss of desirable
forage plant species, (2) reduction in total
plant cover, (3) increased size of bare soil
patches, (4) soil compaction,
(5) loss of
microbiotic
soil crusts (6) increased bulk
density of soils, (7) reduced plant species
diversity,
(8) changes in abundance
and
species composition of animals, and (9) local
extinction of keystone species (Greenwood
e·t 01., 1997; Whitford,
1995, 1997).
Decreased plant cover and increased size
of bare soil patches result in increased
exposure of soil surface to solar radiation,
increased
so i Item peratu re, increased
raindrop splash erosion, decreased infiltration
rates, increased run-off and sediment loss,
and decreased aggregate stability and soil
cohesion (Greenwood
et 01., 1997). The
reduction in vegetation cover allows run-off
water to traverse large distances with no
obstruction to flow to reduce the erosive
energy of the water (Fig. 4). Soil compaction
by grazing animals reduces soil macropores
and total pore space~ which contributes to
decreased infiltration, increased run-off, and
reduced soil water storage. Vegetation that
survives on areas that are greatly impacted
by livestock may have less resistance to
drought
(higher mortality)
and lowered
resilience
in recolonizing
these areas
following drought (Whitford et 01., 1999).
All of these changes affect the health of
desertified
watersheds
and severely
compromise the capability of the watersheds
to provide needed goods and services to
the human inhabitants.
OF ARID ZONE WATERSHEDS
271
While domestic livestock production is
the primary use of arid and semi-arid lands.
Iivestock are notthe only source of deleterious
impacts on arid and semi-arid watersheds.
In many parts of the world, portions of
watersheds have been modified to support
run-off
agriculture.
Changes
made to
watersheds
in order to develop
run-off
agriculture include but are not limited to:
clearing vegetation
from run-off slopes,
construction of dikes and small dams, clearing
rocks from run-off surfaces, and excavation
of channels connecting productive patches
(Evenari et 01., 1971). Run-off agriculture
areas may be abandoned
if and/or when
crop production falls below what is required.
The changes
imposed
on portions
of
watersheds for rllll-off water crop production
are not reversed by natural means.
Road construction
or the development
of roads from frequent traffic by animals
or motorized vehicles is a major source
of watershed degradation. Roads or vehicle
tracks that fail to follow the contours of
watersheds may become head-cutting gullies
within a few decades. Gullies that develop
on roadways are major sources of sediments
that bury the soils at the base ofthe watershed.
Roadways that cut across slopes interfere
with the natural flow patterns of water from
the upper slopes of a watershed. Interference
with natural patterns of overland flow and/or
rill and channel
flow has numerous
deleterious' effects on the hydrology of the
downslope portions of the watershed. Mining
produces other serious impacts on the health
of watersheds.
Mine tailings may cover
several hectares on portions of watersheds.
Mine tailings typically are leached by rain
water and are a source of toxic chemical
272
WHITFORD
1m
Fig. -I. II desrtiJied grassland WI/lOll (/ desert walershed with arrows depictillg
water flow patterns that are essentially unidirectional.
loads in run-off water on watersheds (Jones
et al., 2000).
Watershed
RestorationlRehabilitation
Because a large fraction of the arid
land areas of the world have been degraded
by human activities,
many watershed
functions
have been compromized
(Whitford, 1995). Degraded, desertified
watersheds present difficult management
challenges because some management
options are lost as watersheds are degraded.
While it may be possible to easily restore
one or a few of the functions of disturbed
watersheds, restoration of one function may
eliminate the possibility ofrestoring another
function. An example of the loss of
management options as a result of restoring
an ecological function of an arid watershed
can be seen in the case study of the Santa
ECOLOGY AND MANAGEMENT
Rita Mountain
Arizona.
watersheds
Large-scale
south of Tucson,
livestock
drought during the last decades
grazing and
of the 19th
century resulted in marked decrease
in grass
cover, increased
cover of shrubs, especially
velvet mesquite
(Prosopis velutina), cholla
and prickly
a large
(Medina,
pear cacti (Opuntia spp.) and
increase
in bare, unvegetated
water run-off increased dramatically
flooding
soil
1996). As a result of these changes,
in the
city
causing
of Tucson.
The most important watershed function
of the Santa Rita watersheds that had to
be restored was the hydrological function.
Restoring the hydrological function required
restoring
vegetation
cover that provided
obstruction to overland flow, reducing the
velocity of overland flow and increasing
small scale, high-infiltration
patches. Grass
tussocks are the most effective structural
features of watersheds in terms of causing
water flow to be tortuous with tLissocks
and hummocks absorbing water in transit
(Fig. I; Tongway
and Ludwig,
1997).
Numerous
native grass species and grass
species acquired from other parts of the
world were tested for large-scale revegetation
efforts. The most effective grass species for
rapid establishment
and survival on the
disturbed
landscapes
was Lehmann's
lovegrass
(Eragrostis lehmallllialla),
a
species from the arid rangelands of South
Africa (Roundy and Biedenbender,
1995).
The Lehmann lovegrass - mesquite savanna
that now dom inates the Santa Rita watersheds
has largely restored the hydrological function
of the piedmont landscapes. However, using
the option of an introduced alien species
in restoring
hydrological
function
has
compromised
the potential for restoring
OF ARID ZONE WATERSHEDS
273
animal
biodiversity
and has replaced
nutritious forage grasses with less nutritious
Lehmann
lovegrass
for Iivestock forage
(Bock et al., 1986; Whitford,
1995).
This example from southern Arizona,
USA, clearly demonstrates that restoration
- rehabilitation
of arid watersheds cannot
be achieved by restoring only one of the
features of a healthy watershed. Restoring
the health
of watersheds
requires
an
understanding the ecosystem processes and
watershed
properties
as the basis for
developing
strategies
for restoring these
processes and properties from the patch scale
to the landscape scale (Whisenant,
1999).
Because desertification
affects not only
vegetation
but also soil properties
and
processes,
it is not possible
to restore
vegetation without restoring soil. Because
arid and semi-arid soils are patchy with
respect
to variables
that affect water
infiltration and storage and also soil nutrients,
initial restoration efforts should focus at the
patch
scale.
Successful
rehabilitation
of soils and
vegetation
at the patch scale does not
necessarily require massive expenditure of
energy and resources.
An inexpensive
approach to restoring patch dynamics of
soils on a desert watershed was described
by Tongway
and Ludwig (1996). After
analyzing
the soils,
vegetation,
and
landscape scale hydrological
processes of
a degraded
Acacia spp. woodland
in
Australia,
Tongway
and Ludwig (1996)
devised a simple but effective method of
restoring
productive
soil patches on an
unproductive watershed. They demonstrated
that establishing brush piles oriented with
the long axis 90° to the slope, obstructed
274
WHITFORD
overland flow, captured sediment, increased
water infiltration, increased soil microbial
populations as measured by soil respiration,
and increased soi I ferti Iity ( nitrogen, organ ic
carbon, avai lable phosphorus,
and cation
exchange
capacity).
The brush pi les
ameliorated the soil environment under the
piles thereby
allowing
higher rates of
decomposition
of trapped organics. Litter
trapped with the sed iments provided food
sources for soil fauna such as termites that
changed the macroporosity
of the soil. The
changes in soil properties provided habitat
patches for establ ishment and growth of
perennial grasses. The fertile, water holding
soil patches allowed high plant survivorship
during
drought
thus providing
stress
resistant patches in the landscape (Ludwig
and Tongway,
1996).
Using brush piles to restore resource-rich
soil patches on a watershed is not the only
approach
that can be used to restore
appropriately
structured
patches
on a
watershed.
Combinations
of mechanical
intervention to produce ridge, mound and
basin l1)icrotopography and the application
of appropriate mulches have been successful
in restoring
soil patches.
This type of
intervention may be necessary on severely
degraded watersheds where water and wind
erosion
have effectively
eliminated
the
natural 111 icrotopography
and where upper
soil horizons have been stripped away. There
are a variety of mechanical
means for
producing micro-catchments
on a watershed.
Land imprinters, root plows, deep rippers,
and roller-choppers
can be employed to
develop a desirable micro- topography on
a degraded watershed (Whisenant, 1999).The
selection of mulch materials to be distributed
in the depressions
is extremely important.
Degraded soils may not support the essential
soil macro- and meso-fauna that produce
macropores,
and regulate
the rates of
decomposition and mineralization of organic
materials. Mulch materials must provide the
necessary carbon source to support a diverse
assemblage of soil bacteria and fungi. In
arid soils, most of the mesofauna feed directly
on the soil bacteria and fungi (Whitford,
1996), therefore establ ishment of an abundant
and diverse m icroflora is an essential first
step in soil restoration. In arid regions where
termites are a significant component of the
soil fauna, the mulch materials must also
provide suitable materials to support termites
because termites play a keystone role in
arid ecosystems by affecting numerous soi I
processes and properties (Whitford, 2000).
Research on restoring resource-rich patches
on open-cast mine spoils in New Mexico
showed that waste materials from lumber
mills (wood chips, bark, small stems, and
sawdust) was the most effective mulch for
restoring soil biota in a short period of
time (Whitford and Elkins, 1986). A variety
of organic materials have been used with
varying degrees of success, e.g., straw, hay,
peat moss, shredded bark, corncobs, sewage
sludge, crop residues, manure and plastic
(Whisenant,
1999). Restoring resource-rich
soil patches on arid watersheds may require
a combination of several types of organic
materials to provide the necessary substrate
for the biota and to ameliorate
the soil
m icrocl imate.
Restoration of resource-rich soi I patches
is only the first step in restoring a watershed
to a functional landscape. Re-establishment
of vegetation with the appropriate structure
for retention of water and sediments may
require more than waiting for seeds of the
ECOLOGY
AND
MANAGEMENT
plants that remain on the watershed to be
dispersed
to the soi I patches, germ inate
and become established
as viable plants.
The germ inants of many plant species may
be susceptible to herbivory by the animals
that thrive on degraded watersheds. In the
Chihuahuan
Deselt, rodents kill a large
percentage
of
mesquite
(Prosopis
glandlilosa) shrub germinants, creosotebush
(Larrea /riden/ala)
germinants,
tarbush
(Flollrensia cernu(1) germinants and grass
germinants
(Whitford,
unpublished
data,
Wh itford e/ al., 200 I). In order to insure
the establishment
of desirable shrubs and
grasses on resource-rich
soil patches, it
may be necessary to grow plants in containers
that can be planted in the desired locations.
Containerized shrubs may provide protection
from herbivory and an amel iorated subcanopy microclimate,
i.e., functioning
as
nurse plants that allow grass plants to
establish
(Franco
and Nobel,
1989;
Livingston
el
al., 1997).
Selection of plant species that lare to
be established on watersheds that require
restoration
interventions
in order to
re-establ ish all of the ecosystem functions
of a healthy watershed must be based on
knowledge of the life histories of the species
and their spatial distributions on watersheds.
While it may be more difficult to establish
native species and require a longer time
period than is needed for establ ishment of
al ien species, the potential deleterious effects
of established al ien (exotic) plants (see above)
could seriously compromise the benefits to
be gained from restoring a watershed to
a healthy condition. Restoration may also
require that certain landuses be suspended
until the vegetation has achieved sufficient
vIgor to withstand use. This is especially
.or
ARID
275
ZONE WATERSHEDS
true of grazing by domestic livestock. We
experienced th~ loss of most of the newly
establ ished vegetation on open-cast mine
spoils because of the grazing, browsing and
trampling by cattle, sheep, and horses that
entered the area through a hole in a fence.
Limiting or eliminating human use of the
resOlirces for a period of time in order to
insure restoration of watershed function is
only one of the costs associated
with
watershed restoration. In arid regions of the
world where increasing human populations
require
the water resources
and other
resources provided by healthy watersheds,
it will be necessary for the users to make
short-term
sacrifices
in order for the
population at large to receive the benefits
of healthy watersheds.
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