the role of competition in plant communities in arid and semiarid regions.pdf

7/25/2019 The Role of Competition in Plant Communities in Arid and Semiarid Regions.pdf

1/23

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.

Accessed: 22/11/2011 04:40

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at.http://www.jstor.org/page/info/about/policies/terms.jsp

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of

content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

Annual Reviewsis collaborating with JSTOR to digitize, preserve and extend access toAnnual Review of

Ecology and Systematics.

http://www.jstor.org
http://www.jstor.org/action/showPublisher?publisherCode=annrevshttp://www.jstor.org/stable/2096990?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/stable/2096990?origin=JSTOR-pdfhttp://www.jstor.org/action/showPublisher?publisherCode=annrevs


2/23

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


3/23

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


4/23

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).


5/23

92

FOWLER

cn

N

O

~~~~~00

0

-

C~~~~~~~~~~:

m

CZ m

C~~~~~~~~C

CZ

CZ

Z

e

;

E

t X

s

?

s

s,,~~~~~~~~~~~~~:3

:

cE

;

e

r

E

r

z

r

r

E

r

E

E

Y

Y;;

CZ

E

H

E

s,

U

~~~C

C

9

E

tr

E

w

X T

t

~~~~~~~~C

H

v:

f

fo:

o:

o:o:

o:

o:o:

o:

m

X

X

X>


6/23

PLANT

COMPETITION

IN ARID

REGIONS 93

06,06,

00

0

CN 0 Nt

00

C6

C6

CZ

CZ

CZ

Cl.

CZ

CZ

;:I

;:I ;:3

CZ

CZ

CZ

CZ

CZ

CZ

CZ

- -

-

CZ 0

C6

C6

+--

+--

;:I

-t:)

CZ

CZ

CZ

CZ

CZ

CZ

CZ

.- +--

0

= CZ

CZ

>

>

> > -

>

0

CZ

CZ

CZ

CZ CZ

CZ

CZ

0

u

40-

1j

tz

zr to

Q)

to

to to

Q.

Q.


7/23

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


8/23

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-


9/23

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

(


10/23

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-


11/23

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.


12/23

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


13/23

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).


14/23

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.


15/23

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


16/23

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,


17/23

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


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


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.


18/23

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

and

references.

Literature

Cited

1.

Allen, E.

B.

1982.

Water

and

nutrient

competition

between

Salsola

kali

and

two

native

grass

species

(Agropyron

smithiiandBoutelouagracilis). Ecology

63:732-41

2.

Allen, E.

B.,

Allen,

M. F. 1980.

Nat-

ural

re-establishment

of

vesicular-

arbuscular

mycorrhizae

following

strip-

mine

reclamation

n Wyoming.

J.

Appl.

Ecol.

17:139-47

3.

Anderson,

D. J. 1967.

Studies

on

struc-

ture in

plant

communities.

V. Pattern

n

Atriplex

vesicaria

communities

n

south-

eastern

Australia.

Aust. J. Bot.

15:451-

58

4.

Anderson,

D. J.

1971. Pattern

n

desert

perennials.

J. Ecol.

59:555-60

5. Anderson, D. J., Jacobs, S. W. L.,

Malik,

A. R. 1969.

Studies

on structure

in

plant

communities.

VI. The signifi-

cance

of

pattern

valuation

n

some

Aus-

tralian dry-land

vegetation

types.

Aust.

J. Bot.

17:315-22

6. Bailey,

H.

P.

1979.

Semi-arid

climates:

Their

definition

and distribution.

In

Agriculture

in Semi-Arid

Environments,

ed.

A.

E.

Hall,

G.

H. Cannell,

H.

W.

Lawton,

pp.

73-97.

Berlin:

Springer-

Verlag

7. Barbour,

M.

G. 1969.

Age

and space

distribution

of

the desert

shrub

Larrea

divaricata. Ecology

50:679-85

8. Barbour,

M. G. 1973.

Desert

dogma

re-

examined:

Root/shoot productivity

and

plant

spacing.

Am.

Midl.

Nat.

89:41-57

9.

Barbour,

M.

G.,

Diaz,

D. V. 1973.

Lar-

rea

plant

communities

on

bajada

and

moisture gradients

in

the

United

States

and Argentina.

Vegetatio

28:335-52

10.

Barbour,

M. G.,

MacMahon,

J. A.,

Bamberg,

S. A.,

Ludwig,

J. A. 1977.

The structure

and distribution

of Larrea

communities.

In

Creosote

Bush:

Biology

and

Chemistry

of

Larrea

in New

World

Deserts,

ed.

T.

J. Mabry,

J. H.

Hunzik-

er,

D.

R. DiFeo,

pp.

227-51.

Strouds-

berg, Penn: Dowden, Hutchinson, &

Ross

11.

Beals,

E. W. 1968.

Spatial

patterns

of

shrubs

on a desert

plain

in Ethiopia.

Ecology

49:744-46

12.

Beatley,

J.

C. 1974.

Phenological

events

and theirenvironmentalriggersin Mo-

jave

Desert ecosystems.

Ecology

55:856-63

13. Bonner,

J. 1950.

The role

of toxic sub-

stances

in the interactions

of higher

plants.

Bot. Rev.

16:51-65

14. Bonner,

J.,

Gaiston,

A.

W.

1944.

Toxic

substances

from the culture

media

of

guayule

which may

inhibit growth.

Bot.

Gaz.

106:185-98

15. Boryslawski,

Z., Bentley,

B.

L.

1985.

The

effect of nitrogen

and clipping

on

interference

between

C3

and

C4 grasses.

J.

Ecol. 73:113-21

16. Brennan,H., Cisse, A. M., Djiteye, M.

A.,

Elberse,

W. Th. 1979/1980.

Pasture

dynamics

and forage

availability

in the

Sahel. lsr.

J.

Bot.

28:227-51

17. Bustamente,

R., Serey,

I., Guerrero,

I.

1978. Competencia

intraespecifica

en

plantas

de las dunas

de

Quintero.

I.

Calandrinia

arenaria Cham.

An. Museo

Hist. Nat. Valparaiso

11:55-60

18. Bustamente,R.,

Serey, I., Leighton,

G.

1981.

Estructura

spacial y

competencia

intraespecifica

en arbustos

de desierto:

Alona carnosa

Lind.

An.

Museo

Hist.

Nat. Valparaiso

14:115-18

19.

Cable,

D. R.

1969.

Competition

n the

semidesert grass-shrub type as in-

fluenced

by

root

systems, growth

habits,

and

soil moisture extraction.

Ecology

50:27-38

20.

Cable,

D.

R.,

Tschirley,

F. H. 1961.

Responses

of native and introduced

grasses

following aerialspraying

of vel-

vet mesquite

in southern

Arizona.

J.

Range

Manage.

14:155-59

21. Caldwell, M. M.,

Eissenstat,

D.

M.,

Richards,

J.

H.,

Allen,

M. F.

1985.

Competition

for

phosphorus:

Differen-

tial

uptake

from dual-isotope-labeled

soil

interspaces

between

shrub

and

grass. Science 229:384-86

22.

Canfield,

R. H. 1957. Reproduction

nd

life

span

of some

perennial

grasses

of


19/23

106

FOWLER

southernArizona. J.

Range Manag.

10:

199-203

23. Cannon, W. 1911.

The Root Habits of

Desert Plants. Publ. Carnegie Inst.

Washington

No.

131, Washington,

DC:

Carnegie

Inst.

24. Chabot, B. F.,

Mooney,

M.

A., ed.

1985.

Physiological

Ecology of North

American

Plant

Communities.

New

York: Chapman

& Hall

25. Chew,

R. M., Chew,

A. E. 1965. The

primary

productivity

of a desert shrub

(Larrea tridentata)

community.

Ecol.

Monogr. 35:355-75

26. Clark, P. J., Evans,

F. C.

1954.

Dis-

tance

to

nearest neighbor

as a measure

of

spatial

relationships

in

populations.

Ecology 35:445-53

27.

Clements, F. E., Weaver,

J.

E.,

Han-

son,

H.

C.

1929.

Plant

Competition.

Carnegie

Inst.

Washington

Publ. 398

Washington,

DC: Carnegie

Inst.

28. Connell,

J. H.

1983.

On

the

prevalence

and

relative

importance

of interspecific

competition:

Evidence

from field ex-

periments.

Am. Nat. 122:661-96

29. Connell,

J.

H., Slatyer,

R. 0. 1977.

Mechanisms

of succession

in natural

communities

and their role

in

communi-

ty stability

and

organization.

Am. Nat.

111:1119-44

30. Costello, D. F. 1944. Naturalrevegeta-

tion of

abandoned

plowed

land

in

the

mixed prairie

association of northeastern

Colorado. Ecology

25:312-26

31. Crisp,

M.

D.

1978. Demography

and

survival under

grazing

of three Austra-

lian semi-desert

shrubs. Oikos 30:

520-28

32. Danin,

A.

1978.

Plant

species diversity

and

plant

succession

in

a

sandy

area

in

the Northern

Negev.

Flora 167:409-22

33.

Dickinson, C. E., Dodd,

J.

L. 1976.

Phenological pattern

in the short

grass

prairie.Am.

Midl. Nat.

96:367-78

34. Ebert, T. A., McMaster, G. S. 1981.

Regular

pattern

of desert

shrubs:

A sam-

pling

artefact?J. Ecol. 69:559-64

35.

Ehleringer,

J.

R.

1984.

Intraspecific

competitive

effects

on water

relations,

growth,

and

reproduction

in

Encelia

farinosa.

Oecologia

63:153-58

36. Ellison,

L.

1960.

Influence

of

grazing

on

plant

succession

of

rangelands.

Bot.

Rev. 26:1-78

37.

Evans,

R.

A., Young,

J.

A.

1970.

Plant

litter and establishment

of alien annual

weed

sp.cies

in

rangeland

communities.

Weed Sci. 18:697-703

38. Fonteyn, P. J., Mahall, B. E. 1978.

Competition

among

desert

perennials.

Nature 275:544-45

39.

Fonteyn,

P.

J., Mahall,

B.

E.

1981.

An

experimental

analysis

of

structure

n a

desert plant

community.

J. Ecol. 69:

883-96

40. Fowler, N. L. 1984. The role of

germinationdate, spatial arrangement,

and neighbourhoodeffects in competi-

tive interactions n LinumJ. Ecol. 72:

307-18

41. Fowler, N. L. 1986. Density-dependent

population

regulation

in a

Texas

grass-

land

community. Ecology 67:545-54

42.

Friedman,J. 1971. The effect of compe-

tition

by adult Zygophyllum dumosum

Boiss.

on

seedlings

of Artemisia herba-

alba Asso

in

the Negev desert of Israel.

J.

Ecol.

59:775-82

43.

Friedman, J., Elberse,

W.

Th. 1976.

Competition between two desert vari-

eties of

Medicago laciniata (L.) Mill.

under

controlled

conditions. Oecologia

22:321-39

44.

Friedman, J., Orshan,

G.

1974.

Allopatric

distribution f

two

varietiesof

Medicago laciniata (L.) Mill.

in the

Negev desert.

J. Ecol.

62:107-14

45.

Friedman,J., Orshan,G. 1975. The

dis-

tribution, emergence

and

survival of

seedlings

of Artemisia herba-alba

Asso

in

the Negev desertof Israelin relation

to

distance

from the

adultplant.J. Ecol.

63:627-32

46. Friedman, J., Orshan, G., Ziger-Cfir,

Y. 1977.

Suppression

of

annuals by

Artemisia herba-alba in the

Negev

Des-

ert of Israel. J. Ecol.

65:413-26

47.

Glendening,

G. E.

1952. Some

quantita-

tive

data

on

the

increase

of

mesquite

and

cactus

on

a desert

range

in southernAri-

zona.

Ecology

33:319-28

48.

Glendening,

G.

E., Paulsen,

H. A.

1955.

Reproduction

and

establishment

of

velvet

mesquite

as related to invasion

of

semidesert

grasslands.

USDA Tech.

Bull. No. 1127.

Washington, DC:

USGPO

49. Gray, R., Bonner,J. 1948. An inhibitor

of

plant

growth

from

the

leaves of En-

celia

farinosa.

Am. J. Bot.

35:52-57

50.

Gray,

R., Bonner,

J.

1948. Structure

determinationand

synthesis

of a

plant

growth

inhibitor, 3-acetyl-6-methoxy

benzaldehyde,

ound

in the

leaves

of

En-

celia

farinosa. J. Am. Chem. Soc. 70:

1249-53

51.

Grieg-Smith,

P.

1983. Quantitative

Plant

Ecology. Berkeley: Univ. Calif.

Press. 3rd ed.

52.

Grieg-Smith,P., Chadwick,M.

J.

1965.

Data

on

pattern

within

plant communi-

ties. III. Acacia-Capparis semi-desert

scrub

in the

Sudan. J. Ecol.

53:465-74

53.

Grime,

J.

P.

1977. Evidence for the ex-

istence of three

primary strategies

in


20/23

PLANT

COMPETITION N

ARID

REGIONS

107

plants and its relevanceto ecological

and

evolutionary theory. Am. Nat. 111:

1169-74

54. Gulmon, S. L., Mooney,

H. A. 1977.

Spatial

and

temporal

relationships be-

tween two desert shrubs, Atriplex

hymenelytraand Tidestromiaoblongifo-

lia

in

Death Valley,

California.

J. Ecol.

65:831-38

55. Gulmon,

S.

L., Rundel,

P.

W.,

Ehlerin-

ger,

J.

R., Mooney,

H. A. 1979. Spatial

relationshipsand competition

in a Chil-

ean desert cactus.

Oecologia 44:40-

43

56. Gurevitch,

J.

1986. Competition

and the

local distribution of the grass Stipa

neomexicana. Ecology 67:46-57

57. Halvorson,

W.

L., Patten,

D.

T.

1975.

Productivity

and flowering of winter

ephemerals

n

relation to Sonorandesert

shrubs.

Am.

Midl.

Nat. 93:311-19

58. Harper,

J.

L.

1977. Population

Biology

of

Plants. London: Academic Press

59. Humphrey,

R. R. 1958.

The

desert

grassland.

A

history

of

vegetational

change

and an

analysis

of causes. Bot.

Rev. 24:193-252

60.

Inouye,

R.

S. 1980. Density-dependent

germinationresponse by

seeds

of desert

annuals.

Oecologia

46:235-38

61. Inouye, R. S., Byers, G. S., Brown, J.

H. 1980. Effects of predationand com-

petition

on

survivorship,

fecundity, and

community structure

of desert annuals.

Ecology 61:1344-51

62.

Juhren, M., Went,

F.

W.,

Phillips,

E.

1956. Ecology

of desert plants. IV.

Combined

field and

laboratory

work on

germination

of

annuals

in the Joshua

Tree

National Monument,

California.

Ecology

37:318-30

63.

Kassas, M., Girgis,

W.

A.

1965.

Habi-

tat and

plant

communities of the Egyp-

tian Desert.

VII. The

units

of a

desert

ecosystem. J. Ecol. 53:715-28

64.

Kemp,

P. R.

1983.

Phenological pat-

terns of Chihuahuan esert

plants

n rela-

tion

to the

timing

of water

availability.

J. Ecol. 71:427-36

65.

Kemp,

P.

R., Williams,

G. J.

1980.

A

physiological basis

for niche

separation

between

Agropyron

smithii

(C3)

and

Bouteloua

gracilis (C4).

Ecology 61:

846-58

66.

Kincaid,

D.

R., Holt,

G.

A., Dalton,

P.

D., Tixier,

J.

S.

1959.

The

spread

of

Lehmann

ovegrass

as

affected by

mes-

quite

and native

perennialgrasses.

Ecol-

ogy 40:738-42

67.

King,

T.

J., Woodell,

S.

R. J.

1973.

The causes of

regular pattern

in

desert

perennials.

J. Ecol.

61:761-65

68.

King,

T.

J., Woodell,

S.

R. J.

1984.

Are

regular

patterns n

desertshrubs

artefacts

of sampling?J. Ecol. 72:295-98

69. Klikoff,

L. G. 1966.

Competitive re-

sponse

to

moisture stress of

a winter

annual

of

the Sonoran

Desert. Am.Midl.

Nat.

75:383-91

70.

Knipe, D.,

Herbel,

C. H.

1966.

Germinationand

growthof some

semi-

desert grassland

species treated with

aqueous

extract from creosote

bush.

Ecology 47:775-81

71.

Knoop, W.

T., Walker, B. H.

1985.

Interactions of

woody and

herbaceous

vegetation in a

southernAfrican

savan-

na.

J. Ecol.

73:235-53

72.

Lathrop,E. W.,

Archbold, E. F.

1980.

Plant responses to Los Angeles

Aqueduct

construction

in

the

Mojave

Desert. Environ.

Manage. 4:137-48

73.

Lauenroth,

W.

K., Dodd,

J.

L., Sims,

P. L.

1978. The

effects of water

and

nitrogen-induced tresses on

plant

com-

munity

structure

in

a semi-arid

grass-

land.

Oecologia

36:211-22

74.

Malik, A.

R.,

Anderson, D. J.,

Myers-

cough,

P. J.

1976.

Studies

on

structure

in

plant communities. VII. Field and ex-

perimental

analyses

of

Atriplex

vesicaria

populations

from

the Riverine

Plain of

New

South Wales. Aust.

J. Bot.

24:265-

80

75.

McAuliffe,

J. R.

1984. Sahuaro-nurse

tree

associations

in

the

Sonoran

Desert:

Competitive

effects of

sahuaros. Oeco-

logia

64:319-21

76.

McAuliffe,

J. R.

1984.

Prey refugia

and

the

distributionsof two

Sonoran

Desert

cacti.

Oecologia 65:82-85

77.

McAuliffe,

J. R.

1986. Herbivore-

limited

establishmentof a

Sonoran

des-

ert

tree,

Cercidium

microphyllum.

Ecol-

ogy

67:276-80

78.

McDonough,

W. T.

1963.

Interspecific

associations

among

desert

plants.

Am.

Midl. Nat. 70:291-99

79.

McDonough,

W.

T.

1965. Pattern

changes

associated with

the

decline of a

species

in

a

desert habitat.

Vegetatio

13:

97-101

80.

Monson,

R.

K., Littlejohn,

R.

O.,

Wil-

liams,

G. J.

1983.

Photosynthetic

adaptation

o

temperature

n

four

species

from the

Colorado

shortgrass

steppe:

A

physiological

model for

coexistence.

Oecologia

58:43-51

81.

Monson,

R.

K.,

Williams,

G.

J. 1982.

A

correlation between

photosynthetic

temperature

adaptation

and

seasonal

phenology patterns in the shortgrass

prairie.

Oecologia 54:58-62

82.

Moore,

P.

D., Bhadresa,

R.

1978.

Pop-

ulation

structure,

biomass

and

pattern

n

a

semi-desert

shrub,

Zygophyllum

eu-


21/23

108 FOWLER

rypterum,

n

the

Turanbiosphere

reserve

of northeastern

Iran. J. Appl.

Ecol.

15:837-46

83. Muller, C. H. 1940. Plantsuccessionin

the Larrea-Flourensia

climax.

Ecology

21:206-12

84. Muller,

C. H. 1946.

Root development

and ecological

relations

of guayule.

USDA

Tech. Bull. No.

923. Washing-

ton,

DC: USGPO

85. Muller,

C. H. 1953.

The association

of

desert

annuals

with shrubs.

Am. J.

Bot.

40:53-60

86.

Muller,

W.

H.,

Muller, C.

H. 1956.

Association patterns

involving

desert

plants

that

contain toxic

products.Am.

J. Bot.

43:354-61

87. Negbi, M., Evenari, M. 1961. The

means of

survival

of

some

desert sum-

mer

annuals.

In

Plant-water

Rela-

tionships

in Arid

and

Semi-arid

Con-

ditions, pp.

249-59.

Paris:

UNESCO

88. Niering,

W.

A.,

Whittaker,

R.

H.,

Lowe,

C. H. 1963.

The saguaro:

A pop-

ulation in

relation to environment.

Sci-

ence

142:15-23

89.

Nilsen,

E.

T.,

Sharifi,

M. R., Rundel,

P. W. 1984.

Comparative

water rela-

tions

of

phreatophytes

n the Sonoran

Desert

of California.

Ecology 65:767-78

90.

Nobel,

P.

S.

1980. Morphology,

nurse

plants, and minimum apical tempera-

tures

for

young

Carnegiea gigantea.

Bot.

Gaz.

141:188-91

91. Nobel,

P. S.

1981. Spacing

and

transpiration

f various

sized

clumps

of

a

desert

grass,

Hilaria

rigida.

J.

Ecol.

69:735-42

92. Noble,

I.

R.,

Crisp, M.

D.

1979/1980.

Germination

and

growth

models

of

short-lived

grass

and

forb

populations

based on

long

term

photo-point

data

at

Koonamore,

South Australia.

Isr.

J.

Bot.

28:195-210

93. Noy-Meir,

I. 1973. Desert

ecosystems:

Environmentand producers.Ann. Rev.

Ecol. Syst.

4:25-51

94.

Noy-Meir,

I. 1974. Desert

ecosystems:

Higher

trophic

levels.

Ann.

Rev. Ecol.

Syst.

5:195-214

95.

Osborn,

T.

G.

B.,

Wood,

J.

G.,

Pal-

tridge,

T.

B. 1932.

On the

growth

and

reaction

to grazing

of

the

perennial

salt-

bush, Atriplex

vesicarium.

An

ecologi-

cal

study

of

the

biotic factor.

Proc. Linn.

Soc. New

South

Wales 57:377-402

96.

Parker,

K.

W., Martin,

S. C.

1952. The

mesquite problem

on

southern

Arizona

ranges.

USDA

Circular No.

908.

Washington,

DC: USGPO

97. Parker,M. A. 1982. Association with

mature

plants protects

seedlings

from

predation

in an arid

grassland

shrub,

Gutierrezia microcephala. Oecologia

53:276-80

98. Parker,

M. A.

1985. Size-dependent

herbivoreattack and the demographyof

an arid grassland

shrub.

Ecology

66:850-60

99. Parker,

M. A., Root, R. B. 1981. Insect

herbivores

imit habitatdistributionof

a

native composite

Machaeranthera

canescens.

Ecology

62:1390-92

100. Parker, M. A., Salzman,

A. G. 1985.

Herbivore exclosure

and

competitor

re-

moval: Effects

on

juvenile

survivorship

and growth

in

the shrub Gutierrezia

mi-

crocephala.

J.

Ecol.

73:903-13

101. Phillips,

D.

L., MacMahon,

J.

A. 1981.

Competition

and

spacing patterns

of des-

ert shrubs. J. Ecol. 69:97-115

102. Pielou, E.

C. 1960. A single mechanism

to account

for

regular,

random, and

aggregated

populations.

J. Ecol.

48:575-84

103. Pielou, E.

C. 1962. The use of plant-to-

neighbour

distances for the detection

of

competition.

J. Ecol. 50:357-67

104. Robberecht,R., Mahall, B. E.,

Nobel,

P. S. 1983.

Experimental removal

of

intraspecific

competitors-effects

on

waterrelations

and

productivity

of a des-

ert

bunchgrass,

Hilaria

rigida.

Oecolo-

gia

60:21-24

105. Robertson, J. H. 1947. Responses of

range grasses

to

different intensities

of

competition

with

sagebrush (Artemisia

tridentata Nutt.). Ecology

28:1-16

106. Sala,

0. E., Lauenroth, W. K. 1982.

Small

rainfallevents:

An

ecological

role

in

semiarid regions. Oecologia

53:301-

4

107.

Sala,

0.

E., Lauenroth,

W.

K., Reid,

C.

P.

P.

1982.

Water

relations:

A new di-

mension

for niche

separation

between

Bouteloua gracilis and

Agropyron

smithii in North American semi-arid

grasslands.

J.

Appl. Ecol. 19:647-57

108. Schlesinger,W. H., Jones, C. S. 1984.

The

comparative

mportanceof overland

runoff and mean

annual

rainfall to

shrub

communitiesof

the

Mojave

Desert.

Bot.

Gaz.

145:116-24

109. Schoener, T. W. 1983.

Field ex-

periments

on

interspecific competition.

Am.

Nat. 122:240-85

110. Serey, I., Bustamente, R.,

Guerrero,

I.

1980. Competencia intraspecifica

en

plantas

de las dunas de Quintero.

II.

Baccharis concava Pers. An. Museo

Hist.

Nat. Valparaiso 13:129-32

111.

Shantz, H.

L.

1917. Plant succession

on

abandoned oads

in

eastern

Colorado.

J.

Ecol. 5:19-42

112.

Shreve,

F.

1931. Physical

conditions

in

sun and shade.

Ecology

12:96-104


22/23

PLANT COMPETITION N

ARID

REGIONS 109

113. Shreve, F. 1942. The desert vegetation

of North America. Bot. Rev. 8:195-246

114. Shreve, F. 1951. Vegetation of the

Sonoran Desert. Carnegie Inst. Wash-

ington

Publ.

591, pp. 1-192. Washing-

ton, DC: Carnegie Inst.

115. Shreve, F., Hinckley, A. L. 1937. Thir-

ty years of change in desert vegetation.

Ecology 18:463-78

116. Silander, J. A. 1983. Demographicvari-

ation in the Australiandesert cassia un-

der grazing pressure. Oecologia

60:227-

33

117. Silander, J. A., Pacala, S. W. 1985.

Neighborhood predictors

of

plant per-

formance.

Oecologia

66:256-63

118. Smith, A. P. 1979. Spacing patternsand

crown size

variability

in

an

Ecuadorian

desert shrub

species. Oecologia

40:203-

5

119. Solbrig, 0. T., Barbour,

M.

the role of competition in plant communities in arid and semiarid regions.pdf

Documents