7/25/2019 The Role of Competition in Plant Communities in Arid and Semiarid Regions.pdf
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The Role of Competition in Plant Communities in Arid and Semiarid Regions
Author(s): Norma FowlerReviewed work(s):Source: Annual Review of Ecology and Systematics, Vol. 17 (1986), pp. 89-110Published by: Annual ReviewsStable URL: http://www.jstor.org/stable/2096990.
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Ann. Rev. Ecol. Syst.
1986. 17:89-110
Copyright ?
1986
by
Annual Reviews Inc. All
rights
reserved
THE
ROLE
OF
COMPETITION
N
PLANT COMMUNITIES
N ARID
AND SEMIARID
REGIONS
Norma Fowler
Department
of
Botany, University
of
Texas, Austin,
Texas
78712
INTRODUCTION
The importance,
and even the
existence,
of
competitionamong plants
in
arid
ecosystems
has often been
questioned.
An influential statementof Shreve
(113) assertedthat interspecific competitiondoes not occur
in
deserts, and
Went
(145)
denied that
competition
between
desert
plants occurs at all.
Neitherprovidedevidence for his assertions, althoughShrevepointed out the
diversity
of
habits and
phenologies
found
among
desert
species. He may have
been
responding
to the
strong emphasis placed
on
competition by Clements
and
his
followers
(e.g. 27).
The
importance
of
competition
in
naturalcom-
munities has
recently
been debated
(28, 109, 127).
These reviews
suggested
that terrestrial
plant
communities are
among
the
communities
n which
com-
petition
is
relatively important.However,
the
majority
of
studies upon
which
this
conclusion is based were made
in humid
regions.
Grime
(53) suggested
that
competition
is less
important
in
high
stress habitats
(in
which
he
included
dry habitats),
but he
presented
little evidence from
true
arid or
semiaridenvironments.
This
paper
reviews
the available
evidence
for
competition
in
plant
com-
munities
n
aridand
semiarid
regions;
as is
demonstrated, ompetitioncertain-
ly
occurs in these communities and involves
many
different
species.
In
several instances
t
appears
o be
important
n
the
determination f community
structure.
Competitionmay
be
less
frequent
n
these
communities,though
not
less
important
on that account. This review
also addresses
several
other
questions concerningthe role
of
competition
n
thesecommunities, ncluding:
the role
of
competition
in
determining
the
absence,
or
presence
and abun-
dance, of species in a community, and their spatial arrangement;which
89
0066-4162/86/1
120-0089$02.00
7/25/2019 The Role of Competition in Plant Communities in Arid and Semiarid Regions.pdf
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90
FOWLER
species are
in
competition
with
one
another;
and at which
stages
in the life
cycle they experience competition.In addition,I reviewcurrentknowledge of
the mechanismsof competition
and
the ways
in which
plants partitionniches
in these communities, as well as facilitation
of
one plant by another, and
succession.
Finally, potential
directions
of
future work
are discussed.
DEFINITIONS
I have followed Bailey (6)
in
my use
of
the terms
arid
and semiarid,especially
his
Figures
3.2 and 3.3.
The arid and semiarid
regions
of the
world thus
defined are
collectively nearly
identical to
Walter's
(140)
zonobiomes
III
(subtropicalarid)
and VI
(aridtemperate
with
a cold
winter).
This
definition
s
somewhat broader
han that used
by Noy-Meir (93, 94)
in
previousarticles
n
these volumes. Arid and semiarid
regionsare,
from the
viewpoint
of a
plant
ecologist,
those
in which
an
insufficiency
of
water
frequently
limits or
prevents plant growth
or survival.
Since water
requirements
are
partly
a
function
of
transpiration ates,
which in
turnare
a
function
of
temperature, s
are rates
of
evaporation
rom
the
soil,
the
degree
of
insufficiency
of
water
in
an
ecosystem
is
a function of
temperature
as well as of
rainfall
(6).
Arid
regions are
also
generally
characterized
y very
wide fluctuations
n
precipita-
tion between years (93, 140).
The word
competition
will be
used,
unless
otherwise
noted,
in
the sense of
negative interference,
.e.
any
direct
or
indirect
negative impact
of
one plant
on another
(58). Therefore,
the use
of
the word does not
imply
that mech-
anisms other
than
competition
for
resources
(for example,
allelopathy,
or
the
harboringof predators)
have been eliminated
from
consideration.
There is
some evidence
that
plants may
facilitate each
other's survival andgrowth
in
arid
regions,
i.e.
positive interference,
and this is also
discussed.
EVIDENCE FOR THE OCCURRENCEOF COMPETITION
Studies
of Spatial
Pattern
The
majority
of
tests
for the
occurrence
of
plant competition
in
arid or
semiaridregions
have been studies
of
spatialpattern.
In
manycases evidence
has
been found
that
competition
occurs and
is
important,at least
in determin-
ing spatial pattern.
The
hypothesis underlying
hese studies is that
competition
among neigh-
boring plants
will lead to
density-dependent rowth
and
survival,hence plants
that
are closer
together
will be smaller and more
likely
to die.
Competition
will thereforeconvert clumped (aggregated)distributionsof plantsinto ran-
dom
ones,
and randomdistributions nto
regular i.e. overdispersed)
ones.
A
7/25/2019 The Role of Competition in Plant Communities in Arid and Semiarid Regions.pdf
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PLANTCOMPETITION
N ARIDREGIONS
91
variety
of
analyticalmethods have
been used to
determine whether plant
distributionsareclumped,random,or regular 26, 102, 103). Correlations f
the
distance between
neighboringplants
and
their size have
also been calcu-
lated
(101, 102); positive
correlationsare
considered to
be the result of
competition.
Some
of
the
older studies use
the variancevs block
size
methods
of
Grieg-Smith(51).
While some studies of
spatial pattern
have
reportedregular plant dis-
tributions n
at least some
sites, reports
of
random
and
clumped
distributions
are
much more common
(Table
1).
In
contrast,positive correlations
between
plant size andthe distance
between plants
(Table2) have been
found by most
of the workers who report testing for them (but see 55). Such positive
correlationsare
not
limited
to
pairs
of
conspecific
plants,
but also
have been
found between
neighboringplants
of
different
species (157, 158).
Recently,
the
interpretation
f
regularspacing
as
evidence
of
competition
has been
challenged.
Ebert &
McMaster
(34) demonstrated
hat failure to
distinguish
individuals
growing close
together
as
separate
individuals
in-
troduces
a bias towards
regular dispersion
and
towards
positive
correlations
between
plant
size
and
the
distance
between
neighboringplants.
While these
authors
only
addressed
the
conclusions
of
Woodell
et al
(153;
see also
68),
their
warning
is
potentially
applicable
to
all
the studies cited
in
Tables
1
and
2.
Further
nvestigation
of
this
problem
is
clearly
needed,
especially
of
how
common coalesced individuals are and how
the
phenomenon may
be in-
corporated
nto tests
of
statistical
significance.
Nevertheless,
it
seems
rash
and
unnecessary
to
dismiss all
of
the earlier
findings
of
regular
distributions
on
this basis.
A
more fundamental ssue
in
the
interpretation
f
studies
of
spatial pattern
is
that, while
a
regular
distributionof
plantsmay
reasonably
be ascribed
to
competition,
the absence
of
such a
distribution s
not
evidence
for
the absence
of
competition.
As
many
authorshave
pointed
out,
both
spatial
heterogeneity
in the environmentand restricted eed dispersalcan override he tendencyfor
competition
to
produceregular
distributions
f
plants
and
positivecorrelations
between plant size and distance
apart.
Measures of
pattern
are also
scale-
dependent
and hence
dependupon
choice of
quadrat
ize and
other
sampling
decisions.
Therefore,
incorrect
choice
of
sampling
units
may
result in
failure
to detect
existing patterns.
Positive correlations
between the
size
of
competing
plants
and
their
distance
apartmay
also
be absent
simply
because
each
plant
competes
with
many
different
neighboring plants
(40,
117).
In
fact, the
frequency
with which
significant
correlations
are
found
in
desert communi-
ties,
as
compared
with
more
mesic
ones
(e.g. 139),
indicates hat
desert
plants
usually compete
with
relatively
fewer
neighbors
han
do
plants
in
more
mesic
environments
(40).
7/25/2019 The Role of Competition in Plant Communities in Arid and Semiarid Regions.pdf
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92
FOWLER
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7/25/2019 The Role of Competition in Plant Communities in Arid and Semiarid Regions.pdf
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PLANT
COMPETITION
IN ARID
REGIONS 93
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7/25/2019 The Role of Competition in Plant Communities in Arid and Semiarid Regions.pdf
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94 FOWLER
Table 2
Summary
of studies that have found
positive
correlationsbetween
plant
size
and the
distances between pairs of individuals
Species
Habit Location(s) Reference(s)
Ambrosia dumosa shrub Sonoran, Mojave Deserts 101
Atriplex polycarpa
shrub Mojave
Desert 101
Calandrinia arenaria forb
Chile
17
Carnegiea gigantea
cactus
Sonoran Desert 158
Chrysothamnus aniculatus
shrub
Mojave
Desert
101
Croton menthodarus shrub Ecuador
118
Encelia farinosa shrub Sonoran
Desert 35
Eriogonum inflatum forb
Mojave
Desert
155
Fouquieria splendens shrub SonoranDesert 158
Franseria deltoidea shrub
Sonoran Desert 158
Hilaria
rigida grass Sonoran
Desert
91
Larrea
tridentata
shrub
Sonoran, Mojave
Deserts
101, 108,
158
Opuntia acanthocarpa
cactus
Mojave
Desert
157
Opuntia ulgida
cactus Sonoran Desert 158
Opuntia
ramosissima
cactus
Sonoran, Mojave
Deserts
101,
157
Yucca schidigera succulent
Mojave
Desert 157
Studies
of
the Direct
Effects of Competition
A relatively small number of studies of competitionin arid and semiarid
regions have examined the direct effects
of
competitionupon individualplants
and
plant populations.
Most
of
these studies involved
manipulatingplant
densities
by
the removal
of
individuals,
and each
found some
evidence of
competition.
Friedmanand his coworkersconducteda series of studies
of
competition
n
the
Negev Desert.
In
one, seedlings
of the shrub
Artemisia herba-alba
were
transplanted round he
codominant
shrub
Zygophyllumdumosum
42).
Both
survival and
growth
of
seedlings
were lower
where
seedlings
were
planted
closer to
adult
shrubs, indicating that competition occurred. Competition
between seedlings of
A. herba-alba
and
adults of the same species was
examined
in
another
study (45) by following
naturallyoccurring seedlings.
Few
seedlings
emerged
under adult
canopies,
and their
death rate was higher
there, again indicating competition.
Densities of
naturallyoccurringannuals
were lower
and
mortality
rates
higher
near
adult A. herba-alba than in the
open,
but
not near
adults
of Z.
dumosum.
However,
numbers
and biomass of
annuals
increased
following
the removal of either shrub
species (46). Two
varieties of the annual
Medicago laciniata had morefruits per plant when all
nearbyplantswere removed
(44),
but
only
when
both varietieswere watered.
In the greenhouse the variety that lost in intervarietalcompetition used
water more
efficiently
and
had a
higher
seed set
(43).
Another
series of studies
was conducted
in
a southern Arizona desert
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PLANT
COMPETITION
N ARIDREGIONS
95
grassland, at the
Santa
Rita Experimental Range.
Removal of Prosopis
juliflora (mesquite) led to an increase in annual and perennialgrasses, es-
pecially
in Trichachne
californica
(cottongrass),
Eragrostis lehmanniana,
and species
of Bouteloua
(20,
66, 96). Removal
of
B. eriopoda,T. californi-
ca,
or
Muhlenbergia
porteri,
another
perennialgrass,
increasedthe
survival
of P.
juliflora seedlings (48).
Addition
of E. lehmanniana educed he density
of the native perennial
grasses,
perhapspartly
due to selective
grazing
of the
natives (66).
Removal
of the subshrubAplopappus
tenuisectus(burroweed),
of
T.
californica,
or
of
annualgrasses
as a
group,
demonstratedhat
each
affected
the
growth
of
the other two, withthe single exception
that the
annual
grasses
did
not
appear
o interfere
with the
growth
of A.
tenuisectus 19, 96).
In similar vegetationin southernAfrica, the removal of all herbaceousplants
increased
rates
of
establishment
and growth
of two Acacia species
but not of
two other woody
species;
the removal
of
the latter two
species
increased
herbaceous
cover
(71).
Sagebrush
(Artemisia
tridentata)
interferes with the
growth
of
perennial
grasses
in
the
American intermontane
desert; sagebrush
removals
led
to an
increase
in
the
dry weight
of individuals
of several
native grasses (105).
Similarly,
removal
of all shrubs
primarily
Larrea
tridentata)
n
a Chihuahuan
desert
site
led to a
significant
increase
in
the cover
of
the
perennial grass
Muhlenbergiaporteri(148). Clippingorremoval of grasses improved herate
of survival
of
seedlings
and
the
growth
and survival
of
2-yr-old
plants
of
the
shrub Gutierreziamicrocephala
(97, 98, 100).
The removal
of
adults
of
this
species
increased
the survival
and
growth
rates
of
conspecific
2-yr-old plants
(97, 100).
Anotherspecies
of Gutierrezia
excludes the forb
Machaeranthera
canescens
from
some sites
(99).
Removal
of
all
grasses
around ndividuals
of
the
grasses Stipa
neomexicana
and
Aristida
glauca
increased recruitment,
survival, growth,
and
reproduction
of both
species
(56).
Inouye (60)
demonstrated ensity-dependent
eduction,
not only
of
survival
and
growth
but
also
of
germination
in
desert
annuals, by experimentally
thinning
stands
of annuals
in
the Sonoran
desert,
and
by
observing natural
stands
of different densities.
In at least one
of
two
sites,
the interactionwas
primarily ntraspecific,
involving
a
single
dominantspecies. In anotherstudy
(61),
the effects
of
thinning
were
reported
to involve
only growth
and
fecundity,
not survival.
Klikoff
(69)
comparednaturally
occurring
stands
of
different densities
of the annual
Plantago
insularis
in
the Sonoran desert.
Stands
of
lower initial
density
had
higher
survival rates when
watered
mod-
erately.
Removal
of all other
species
increased he numberand size
of
plants
of the annual Salsola
inermis
(87).
Some studies have examined the effects of competitionupon the water
status
of the affected
plants
or
upon
soil
water
content,
as well as
upon
measures
of
plant
size
or survival.
Experimental
emovals
of
Larrea tridenta-
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96 FOWLER
ta and/or
Ambrosiadumosa around
argetplants
of each
species improved
he
water potential
of individualsof the other
species (38, 39).
In
a monospecific
stand of Enceliafarinosa,
the removal of all
neighboringplants improved he
water status
of
the remainingplants, as
well as
plant size and seed set (35).
Survival
rates
of
transplanted eedlings
of the
grass
Bromus
setifolius
in-
creased
away
from
shrubs where soil water content
was
greater (122).
The
removal of all vegetation around the grass
Hilaria
rigida
in
monospecific
stands increased
soil
waterpotentials, plant water status, and plant size (104).
EVIDENCE
FOR
THE
FREQUENCY
OF
COMPETITION
The extremely variable climates characteristicof arid and semiaridregions
would lead
one to
expect
that
competition
would be a
relatively infrequent
event there.
Variableclimates
will
producefluctuating
resource
evels,
which
in
turn
could cause the size
of
populationsfrequently
to decrease below
the
level at
which
competition
for
resources would occur.
Wiens (149) has
advanced
this
argument
with
respect
to bird
communities,
and
I found
that,
in
a subhumid
but water-limited
grassland,
a
perennialgrass population
was so
reduced by
a moderate
drought
that
density-dependent
ffects were
greatly
reduced
or
eliminated
(41). Perhapsonly
after
populations
ncrease
during
a
series of good years, and the carrying capacity of the environment then
drops
in
a bad
year,
will
competition
for
resources
become intense.
(It
is
important o bear
in
mind, however, that competitionmay be infrequentand
yet play
an
important
role
in
structuring
communities and
regulating pop-
ulations.)
Despite
its
plausibility,
the
existing
evidence does
not
support
his
hypoth-
esis.
Almost all
of the
experimental
studies
just
described
were short-term
(
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PLANT
COMPETITION
N ARIDREGIONS
97
of competitors
ower
down the
slope
than it did to removals
on
the ridgecrest
(to the extent that estimated populationgrowth rates after removals were
almost equal
in
all
locations),
she concludedthatcompetition
restricted he
distribution
of
S.
mexicana.
The forb Machaeranthera anescens
is absent
or
rare
n
all but
disturbed
ites due to competition
rom Gutierrezia
microcepha-
la (via
herbivory;
99 and
see
below). Competition
may
restrict wo
varieties
of the annual
Medicago
laciniata to
north
and
south slopes
in
the Negev
Desert, respectively.
Relative
fecundities
were
greater
or
each variety
on its
usual
slope,
but
only
in
undisturbed
vegetation
(44).
The effects
of competition
uponthe abundances
of
species present
n a site
are also little known. Few studies have looked directly at the impact of
competition
upon
population
sizes
(19, 20,
66, 96,
148).
Other studieshave
only
measured
the effects
of
competition
on
individuals,
although
one can
infer
thatthese effects
must
often result
in the reduction, f
not theregulation,
of population
size. The nature
and
magnitude
of the effects of both intra-and
interspecific
competitionupon
population
sizes
and population
dynamics
in
arid and
semiarid
regions
remain
to
be
investigated.
STAGES
OF
THE
LIFE CYCLE
AFFECTED
BY
COMPETITION
Studies
of
patternsuggest
that
competition
may
affect a
plant
throughout
ts
adult
life,
althoughperhaps
only
intermittently.
Andersonand coworkers
(5,
74)
and
Grieg-Smith
& Chadwick
(52)
found that smaller
(hence, younger)
plants
had a clumped
distribution,
whereas older
plants
had a
more
random
one.
This
they interpreted
as
evidence
for
competition
among young plants,
with
relatively
more
individuals
eliminated
from
high
density
than from
low
density phases (5).
In these
studies,
the failure
to reach a
regular
distribution
was ascribed
to environmental
heterogeneity,
not to the
absence
of competi-
tion amongolderplants. Phillips& MacMahon 101) divided populationsof
Larrea
tridentata,
Ambrosia dumosa,
and several
other shrubs into size
classes,
and
found that
20 of
22
were
consistentwith the
expected
trend,
from
small
to
large plants,
of
clumped-*random-->regular,
lthough
n
no
case
did
the different
size classes
of
a
single population
demonstrateall three
types
of
distribution.
In
Opuntia
bigelovii (79),
Tidestromia
oblongifolia (54),
and
Eriogonum inflatum
(155),
living
and
dead individuals were
more clumped
than
living
individuals
alone,
which
suggests
adult
density-dependent
mortal-
ity.
Measures
of the
direct effects
of
competition
demonstrated
competition
between
adult
perennials
(35,
38, 39, 56), from adult perennials against
seedling
perennials(42,
45, 97, 98,
100)
and
against
seedling
annuals
(46),
among
seedling
annuals
(60,
61,
144),
and even
among
seeds
(60).
Competi-
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98 FOWLER
tion may affect
survival, growth,
or
reproductionsee above). There are too
few studies to generalize, but the existing results suggest that the effects of
competitionupona species should be looked
for in all stages of the life cycle;
competitive
effects should
not
be assumed a
priori
to
be
absent at
any
stage.
MECHANISMS
OF
COMPETITION
Competition or
Water
Most
plant ecologists
working
in
arid
or
semiarid
regions
have
assumed
that
the
principal
form
of competition among plants
is
competition
for
water.
Perhaps because it appears to be obvious, the numberof studies directly
supportinghishypothesis
is
relatively ow, although
a large bodyof work has
demonstrated
hat
plants
of arid and semiarid
regions
are often under
water
stress (24). Watering generally
increases rates
of
growth
and
survival,
con-
firming
that
it is
a
limiting
resource
(e.g. 44, 61,
69, 73).
A
numberof studies
have found that
a reduction
in
the
intensity
of
competition,
in
addition
to
increasing plant size, survival, or fecundity,
also is associated with an im-
provement
n
plant
water status
(35, 38, 39, 104)
or
an
increase
in
soil
water
content
(48, 87, 96, 104, 122). Raising
soil
water contentshifted the outcome
of
competition
between
Salsola kali and
perennialgrasses
in
favor
of
the latter
(1) and led to an increasein the abundancesof warm season grasses and forbs
and a decrease
in
succulents
in
a
dry grassland (73).
It has been suggestedthat competition
for
water
is most intense in deserts
with an intermediate evel
of
rainfall;
his
theory
is
based upon the occurrence
of
clumped, random,
and
regularly
distributed
opulations
of
Larrea
tridenta-
ta
(67, 153;
but see
9).
However,
Anderson
(4) pointed
out
that
since
density
decreases
as rainfall
does,
the
water available
per
plant
does not
necessarily
decrease.
Walter
(140)
presented
evidence that the
water supply per unit
of
transpiring
urface
is
relatively
constant.
Competition or Minerals
Fertilizationwith nitrogen
ncreasedthe size of winterannuals n the Mojave
desert,
but
phosphorus
did not
(152). Nitrogen
addition
also
increased the
biomass
of
most
species
in
a
dry grassland,
where its
principal
effect was
to
magnify
the results
of different
watering
reatments
73). Nitrogenlevels had
no effect
on
the
outcome of
competition
between
the
perennial grasses
Bouteloua
gracilis
and
Agropyron
smithii
(15).
Caldwell
et
al (21) demon-
strated
that
the
shrub Artemisia tridentata took
up
much
more phosphorus
from the
root
space
it shared
with
Agropyron spicatum
than from the root
space it sharedwith Agropyrondesertorum,and thatA. desertorum ook up
more
phosphorus
handid A.
spicatum
when
competing
with A. tridentata.
As
the
authors
were careful to
point out,
these results do not
imply that phospho-
rus is the
only,
or even the most
important,
resource
for
these
plants.
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PLANT
COMPETITION
N ARID
REGIONS
99
Allelopathy
Aqueousextractsof Parthenium ncanum(13, 14), Enceliafarinosa (13, 49,
50,
86),
Ambrosia
dumosa (85,
86),
Thamnosma
montana
(86),
Artemisia
herba-alba
(46),
and
Larrea
tridentata
(70) have
been shown
to have
detri-
mental
effects
on one
or
more plant
species.
Extracts
of
L.
tridentata
were
found
to have
no deleterious
effects
on its
own
germination
or early
growth
(8,
70).
The relevance
of the toxicity
of aqueous
extracts
o plant growth
and
survival
in
the
field
has
been
doubted.
Bonner
(13)
failed
to find any toxicity
in
the
soils of
fields
in
which
P.
incanumhad
been grown;
he concluded
that
the production
of toxic substances
s
relevant
only
in
greenhouses
andperhaps
crowded
nurseries.
Muller
& Muller (86)
found
no
correlation
between
the
degree of toxicity of the aqueousextractfrom a shrubspecies andpatternsof
herb
species'
occurrence
under those
shrub species;
they
concluded
that
toxins
are
ineffectual
as factors
in
competition
between plants
in
deserts.
Herbivory
The harboring
of
predators
or
pathogens
is anotherway
in
which
plants
may
interfere
with
one another;
hence it
is a
form of competition
in
the
broad
sense.
The composite
shrub Gutierrezia
sarothrae excludes
the
composite
forb
Machaeranthera
canescens
from
some sites by
harboring
a
grasshopper
thateatsbothspecies;transplants f the forb survivedonly in exclosures(99).
WHICH
PAIRS
AND
GROUPS
OF SPECIES
COMPETE?
The relative
strengths
of intra-
and
interspecific
competition
are
relevant
to
the
problems
of
species
coexistence
and stability
(28),
for example,
to
identifying
competitively
dominant
species.
Interspecific
competition
was
found to
be
stronger
han
intraspecific
ompetition
n
an
experimental
tudy
of
Larrea
tridentata
and
Ambrosia dumosa,
although
other factors
were
more
important
in determining
these
species'
abundances
and
distributions
(38,
39). The limited evidence from studies of patternon this point, however,
supports
he
opposite
generalization:
nterspecific
competition
s
weaker
than
intraspecific
(157,
158).
The
degree
of
reciprocity
of competitive
rela-
tionships
would also
be of
interest,
if relevant
data
existed.
The
presence
and
relative intensity
or
absence,
of
interspecific
competition
among
different
component
species
of a
community
is also
of
interest,
because
these
cast
considerable
ight
upon
the
niche
structure
f the communi-
ty.
Few
studies
of
competition,
however,
have
included
several
species
from
one
community.
Yeaton
and
coworkers
(157,
158)
compared
the
degree
of
correlationof distancesbetweenplants
and
their
sizes,
between
species
pairs.
In the
Mojave
desert
(157)
competition
occurred
among
all
three
pairs
of two
Opuntia
species
and
a
Yucca, apparently
at
equal
intensity.
However,
for
individuals
of a
given
size, pairs
of
plants
of different
species
of
Opuntia
were
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100 FOWLER
closer together than conspecific pairs,
which
Yeaton
et
al interpretedas
suggesting some differences in root systems and hence niche separation.In
the Sonoran desert (158), four of the nine
interspecific pairs tested had
significant positive
correlationsbetween
size
and
distance,
in
addition to all
five
correlations
between
conspecific pairs.
The experiments
conducted
at the Santa Rita
ExperimentalRange also
involved several species
from one
community.
The
shrub Aplopappus
tenuisectus, the perennialgrass Trichachne
californica,
and the annual
grass-
es as
a
group,
were all found to
compete
with one
another, except
that the
annual grasses
did
not
affect
T.
californica
(19).
These
relationshipsamong
A. tenuisectus and
the
grasses
were
judged
consistent with
observed
pheno-
logical patterns. Prosopis juliflora reduced the growth and abundanceof
several grasses (20, 66, 96),
and
in
turnthese
grassesreducedthe survivalof
P. juliflora (48). However,
in
similar African
vegetation, competition be-
tween
herbaceous
and
woody species
was
apparently
not
reciprocal (71).
Here again,
too few studies have been made to
generalize
with
confidence,
but those
to date
generally support
the
competitive relationshipsthat have
been inferred
rom
phenological patterns
and root
location.
They
also
suggest
thatdespitethese
niche
differences,
competitionamong many
or
even most of
the
species pairs
of a
communityprobably
occurs.
NICHE
SEPARATION
The separation
or differentiation
of
niches
among species
is
expected
to
reduce the
intensity
of
competitionamong them;
t
may therebypromote
heir
coexistence.
Niche
separation among
plants primarily
takes
the
form of
separation
of resource use
in
space
and/or
in
time.
The
plant species
of
arid
and semiarid
regions represent
a
very
wide
range
of
adaptations
hat tend to
separate
their
use of water
(113, 114). Many
of
these
adaptations
have
received detailed
study by physiological ecologists (24, 120). Only
a brief
outline of potentialmechanismsof niche separationcan be given here.
Phenology
Plants
may separate
their use
of
water
by being physiologically active at
different times
(113, 119, 120).
Ephemerals, including annuals, algae,
and
lichens, grow only
when water conditions are
favorable.
In
areas
with two
rainy seasons,
such as
the
Sonoran
desert,
there
may
be two
separate
sets
of
ephemerals (119, 144). Furthermore,
he
relative abundances
of
different
annual
species
will
vary
from
year
to
year depending upon
the amount and
timingof rainfall(12, 16, 62, 92, 113, 144). Tidestromiaoblongifoliaseems
to
be an
'ephemeralperennial,' growingrapidly
with
little
waterconservation
and then
dying
when the
soil
water
in
a
temporary
wash channel
is
exhausted
(54).
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PLANTCOMPETITION
N ARIDREGIONS 101
Some perennials,
including
most
perennial grasses
(19, 121), grow
only
during relatively favorable seasons. Leaves die back or are shed during dry
periods, and
this reduces transpiration ates
andwater uptake.These species
maydiffer
in
their use
of small rainfalls
106, 107).
Otherperennialsmaintain
leaves, photosynthesize,
and absorband
transpire
water
even
during
very dry
periods. Both
groupsof perennialsmay further eparate
heir
periods
of active
growth by being
warm-season
or cool-season species, often (but
not always)
following a C4
VS
C3
grouping (12, 15, 33, 56,
64, 65, 80, 81).
With stored reserves
of
water, succulents
fall into none of these groups.
Their primary
period
of
water
uptake
is
limited
to the
time
immediately
following rains,
while
their
period
of
active growth
may extend muchlonger.
Finally, along watercoursessome perennials (phreatophytes)depend upon
water
supplies
that are
more or less
continuously abundant.
They may be
evergreen or deciduous,
with
a variety
of
phenologicalpatterns (89).
Separation
in Horizontal
Space
It
is outside the
limits of
this
article
to review the
many
studies
of
plant
distribution
n
relation
to local environment.
Like other
plant communities,
those
of
arid and semiarid
regions
are
characterized
y
the
separation
f
plants
according to microtopographicand other environmentalvariation. Washes
and
other
drainage
features
(108),
dunes
(32), existing
shrubs
(see
below),
and
the relative amounts
of sandand
clay
in
the soil
(119),
are
of
particular
importance
n
controlling
local
distributionsof
species.
Rooting
Zones
Excavations
of roots
of
a
number
of
species
have demonstrated hat the
species
of arid
and
semiarid
regions
are
characterized
by
several different
patterns
of root distribution
23, 25, 84, 119, 158).
Cacti andother succulents
typicallyhave shallowrootingzones. The rootsof perennialsoccupy rooting
zones that are
both
wider
and
deeper
than
those of
the
annuals, and
different
perennialsmay
root
at
different
depths (e.g.
158). Phreatophytesare often
very deeprooted.Woody plants
tend to root more
deeply
than
grasses,
and
the
resulting separation
of water use can
be
sufficient
to
permit
coexistence
(121,
137, 138).
The extent
to
which the soil
is
fully occupied
with
roots
is usually con-
sidered to
be an indicator
of the
importance
of
competition.Gulmonet al (55)
found that cactus
roots of
adjacent
individuals
met; Bustamente
et al
(18)
made the same observationof a Chileanshrub. Other nvestigators,however,
have
reported inding space
between
adjacent
root
systems (23, 25).
It
is
clear
that
the roots
of almost all individuals extend
much
further
han
their
cano-
pies; the apparent
separation
of
plants
is
very
misleading.
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102
FOWLER
FACILITATION
The apparent
acilitation
of the
establishment
or growth
of other plant
species
by
woody
perennial
shrubs
in arid regions
has
been observedby
number
of
authors
(e.g.
112,
113).
Osborn
et
al
(95)
noted
that
annuals
were
most
abundant
n the
mounds
of sand around
the bases
of
Atriplex
vesicaria
in
Australia.
Went (143)
describeda
complex
set
of associations
between
an-
nuals and
shrubs
n the Mojave
and
Sonoran
deserts,
which
Muller
(85)
later
simplified
to two
groups
of annuals-those
that
are
shrub-independent
nd
those
that
are shrub-dependent.
n contrast, competition,
not
facilitation,
between
shrubs and
annuals
occurs
in the Negev
desert
(46).
The associationof annualswith woody shrubshas been ascribed o higher
soil
organic
content,
shading
(which
could
cause
lower
rates
of
evaporation
and transpiration),
he
trapping
of
windblown
seeds,
birddispersal
of
seeds,
and protection
of seeds
or
seedlings
from predation
86, 95, 112).
Muller
(85,
86)
determined
that
the shrub-dependent
nnuals
grow
abundantly
only
be-
neath
shrubspecies
that
accumulate
a mound
of
organic
matterunderneath
y
trapping
wind-blown
material
to
add to
their own
dead shoots.
The
shrub-
dependent
annuals
were
also found
to
grow
abundantly
n areas with
tempo-
rarily
high
levels
of
organic
matter
but
without
shrubs. Halvorson&
Patten
(57) foundthat the totalbiomass, butnot the density, of annualswas greater
under
shrubs
than
in the
open,
especially
under
shrubs
with a
relatively
high,
open
canopy;
this
implies
that
amelioration
of the
physical
environment,
not
seed dispersal,
was the
cause
of the
relationship.
Plant
litter
has been
shown
to
aid the
establishment
of
several
annual
species
(37).
Associations
between
woody
shrubs
and cacti have
also been noted.
Shreve
(112)
reported
hat Carnegiea
gigantea
(saguaro
cactus)
often
grows
beneath
various
trees
and
shrubs.
This association
has
since been
described
by
a
number
of
authors
78,
88,
123,
124,
125, 126,
129, 130).
Shrubshave
been
described
as
providing
young
C.
gigantea
with
protection
from grazing,
trampling,hightemperatures, reezing, anddrought; ocksalso performall of
these
functions
(88,
123,
124, 125,
129,
130).
The
transplant
xperiments
of
Turner
et al (129,
130)
demonstrated
hat both
shade and protection
from
rodents
are
necessary
for seedlings
of C.
gigantea
to
survive.
Using a
model
that
predicted
issue
temperature,
Nobel
(90)
confirmed
that nurse
plants
can
protect
cactus
plants
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PLANT
COMPETITION
N
ARID
REGIONS 103
size
(131), but
the
long
life of the
cactus and the
importance
of
occasional,
lethalfreezes in determiningpopulationsizes of this species (125, 126) make
such
oscillations
unlikely. Based
upon observations of
the
distribution
and
vigor of
individuals, Yeaton
(156)
postulated that
Opuntia
leptocaulis
preferentially
established under
Larrea
tridentata,thus
reducingthat plant's
vigor and
eventually replacing
it,
until
finally
Opuntia leptocaulis
itself
succumbed to soil erosion
and rodent
burrowing.
In one
instance, a
cactus, Opuntia
ulgida,
was shown
to be
a
sheltering
plant,
the bed
of
spine-covered
oints beneath it
providing
protection or
two
smaller
species
of
cacti
by
reducing predation
on
them
(76). Finally,
plants
other
than annuals and cacti
may
be
facilitated. The
seedlings
of
the
semi-
shrubGutierreziamicrocephalaareprotected rom
predationby
neighboring
adults
of
the same
species;
their rate of
survival
decreased
when the
adults
were
removed
(97).
Seedlings
of
the
small tree
Cercidium
microphyllum
suffered
less
herbivory-caused
mortality
under
other
perennials than
in
the
open
(77).
SUCCESSION
There has been considerabledebate as to whethersuccession occurs in arid
and
semiarid
regions.
If
by
succession
one
means an
orderly,
natural eries of
changes
in
vegetation
following
disturbance,
hen
succession
certainly
occurs
(e.g. 2, 30,
63, 72,
83, 111,
132, 133, 134,
135,
141, 142).
In
most
places,
however, areas
of
disturbanceare
colonized
by species
alreadypresent
n
the
community,
often
growing
in
washes or
other
small
disturbances
111, 113,
134,
142). Onlythe relative
abundances
of
the
species are
altered.
Therefore,
if
a strict definition of
succession is
used,
it
may
be
said not to
occur in
arid
and
semiarid
regions (83).
Some authorshave
assumed that
competition
from
later
successional
spe-
cies
reduces
or
eliminates
populations
of
early successional
species. This
has
been
demonstrated
in
the case of
the
early successional
summer
annual
Salsola inermis
(87).
Succession
may
involve the
facilitationof
later
species
by
earlier ones
(sensu
29),
as
it
does,
for
example,
in
the
Mojavelake
beds,
where the
accumulation
of
soil
and the
redistribution f
minerals
by plants
permit a
primaryplant
succession to
occur
(135).
Or the
plants
may them-
selves neither
promote
nor
retard
vegetational
change
(e.g. 63, 83).
Grazing
is
sometimes
considered a
disturbance,
and
the
changes which
follow its
cessation, succession. The
effects
of
grazing and of
the
cessation of
grazing have been documented for many plant communities of arid and
semiarid
regions
(e.g. 22,
31, 32, 36,
47,
59, 115,
116, 146,
147,
150,
151).
Studies conducted
at the
Santa Rita
Experimental
Range,
described
above,
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104
FOWLER
suggest
that
competition
between
palatable,
decreaser pecies (e.g. most
grasses) andunpalatable, increaser pecies (e.g. Prosopis) plays an impor-
tant
role in this
form of succession.
CONCLUSIONS
AND FUTURE
DIRECTIONS
The
importance
of
competitionamong plants
growing
in
arid
and semiarid
regions has been
doubted,
but studies
suggest that
competition
among plants
is both common
andstrong
enough
to
be
readily detected, both in
deserts and
in
dry grasslands.
The
number of
such
studies
is
not
negligible, even if
allowances are made for the reluctanceof authors and journals to report
negative results. It
is
therefore
my opinion that the occurrence
and potential
importance
of
competition
in
these communities can
now be taken
as given.
Studies
should now be directedtowards
determining
he
circumstancesunder
which
competition
occurs
and its effects
upon
plant populations.
The existing studies are too few to
warrant eneralizations
oncerningmost
aspects
of the nature of
competitionamong plants
or
its
effects
on
plant
communities. The available evidence has been
discussed
above;
I
now
summarize
by
outlining
four
questions
that
appear
o be
particularly
nterest-
ing and critical to our understandingof the role of competition in these
communities.
1. How
frequently
does
competition
occur?
2.
Does
competition
determine
community
composition, and,
if
so,
when
and
how?
3.
At which
stage(s)
of
the
life
cycle
are
plant populations
affected by
competition?
4.
Whichgroups
of
species
compete,
and
how
is
competition
avoided among
co-occurringspecies?
None
of
these
questions s,
of
course, unique
to
plant
communities
of
arid and
semiarid
regions,
but the
particular
dearthof
relevant studies in
these
regions
warrants
directing
attention
to
them.
Another
category
of
unanswered,
and at this
point
unanswerable,questions
addresses
the
similarities
and
differences
among
different
regions.
What
generalizations
can
be
made about the nature
and
effects
of
plant
competition
among plant
communities
of different
temperate
arid
or
semiarid
regions
of
similar
climate? Between
plant
communitiesof
temperate
and
tropical
arid
or
semiaridregions?Between arid and humidregions?With regardto the last
question,
the relative commonness
and
importance
of
the
facilitation of one
plant by
another
seems
to
me
to be
particularlynteresting.
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PLANT
COMPETITION
N ARID
REGIONS
105
ACKNOWLEDGMENTS
Supportwas providedby NSF grant8118968. I
thankP. Fonteyn,
J. Gure-
vich,
J. Russell
Holman,
W. Lauenroth,
G. Montenegro,
C. H. Muller,
M.
Price,
0. Sala,
K.
Schwaegerle,
I. Serey,
and N. Waser
for comments
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