AUSTRALIAN SAVANNA ECO SYSTEM S
J.J. Mott*, John Williams**, M.H. Andrew*** and A.N. Gillisonf
*
CSIRO Division of Tiopical Crops and Pastures,Cmnirgh;; Laboratory,
St. Lucia, e. 4067,
Australia
**
CSIRO Divisionof Soils,DaviesLaboratory,PrivateMail Bag,P.O. Aitkenvale,
:F:t* CSIRO Division
e. 4g14, Australia
of TropicalCropsand Pastures,
privateBagNo. 44, Winnellie,
Darwin Labor-atories,
NT 5789,Australia.
cSIRo Division of Water and Land Resources,P.O. Box 1666, Canberra
t
City, AcT 2601,
Australia.
SUMMARY
within the Australiansavannas-six.major
groupshave been recognisedon the basis of the graminoid
understorey'I.nall groupsthereis a dominantstimm,ergrowing
season,rrltr,lttt. productivity in winter months.
characterisation
of the mainlimiting.factors
to produciivityrf,o*, thut ttretropicalMonsoonTallgrasssystem
has the most scvereenvironmentwith a winter'droughtani pred<rminani
nutrient limitation to grofih. In the
subhumid svstemsthe higher-soil fertility in the S"ubtropi;Ji;ii;;;
compensatesfor a less favourable
climaticenvironment'but in the inland semi-aridcommunities
bottiincreasingaridity and low nutrientsoils
combineto limit plant production.Throughoutthe subhumid
,uuunnu"ii..,iither naturalor man-inducedis an
almostannualoccurrence.
In termsof functionaladaptations
to the stresses
of the savannaenvironment,
plantsshowmany similarities
to thoseoccurringin speciesin othercontinentalsavaffiasystems.
In ecosystem
functioning
the severenutrient
limitation of the northerngroups closely parallel those
conditiorr .*lrtirg in west and central African
savannas.
vertebratebiomassis low on all savannatypeswith p-roductivity
of both nativeand introducedspeciesbeing
curtailedbv low herbagequality in the driei months.'Invertef;"1";,
;;;;;i"lly termites,reachhigh biomass
levelsandplay a majorpart in tirefunctioningof the ecosystems,
especiallyin termsof nutrientcycling.
SCOPEAND DESCRIPTION
Introduction
Australian savannalandsform a wide continentalarc that extendsacrossthe
northerntropical landsdown to
the sor'rth-eastern
subtropicalzones,and extendfrom the humid coastalareasinland until they
abut the arid
Ti'iodia spp'.hummockgrasslandsand the wetterreachesof the arid,Acacla
shrublands(Figure I ). Theselands
have a graminoidunderstoreyof variablefloristic compositir:n-a ,pi.Oo.inantly
eucalyptupper stratum.
d,
D A RW
O W N S V I L LE
TROPICAL
!
FFFFIM
FFffi
SAVANNAS OF AUSTRALIA
* o n s o o * T A L L G R A S sE l
*,oo"o""
rnorrcAL TALLGRAsS
ffi
r,on"oss oil cLAysorls
S U B - T R O P I C AL
T A L L G NA S S
ru""o"K GRAssLANDs
-
WZ
A R I D / H U t t D Z O N EL t t { E
Figure l. Main vegetationtypesin the Australiansavannazone.
Australian savannas
a, Q. 4067,
[, Australia
4,Winnellie,
ACT 2601,
le graminoid
intermonths.
lgtasssystem
rowth.In the
ls favourable
nutrientsoils
'induced
is an
ry similarities
gverenutrient
ntral African
'l'he:rsualdescriptionfbrsavannalikeformationsinAustraliaisthegenerictermwoodland'Ithasbeenused
(e-'g'Carnahan'1976)'ln this wav it
both for systemsorrt.u.iu.ui".g.laiion "tassifi"^tii"^""a-Limuppin"g
whichmanyauthors(e'g'
giutiioidiltttnt
orirt.
ttrewood] ipp.i-sru,urn ar,tt. e*p'.nse
hasemphasised
element'
functional
dominant
as the
iS83j
BourliereandHadley, 1970;Huntleyand Walker,
(calabv' 1980)' In
"ontiaJt
herbivores
thosein A;;;lt;;;;;ie*.natiu"
In contrastwith African.savannas,
variationin climateover the Quaternary
rr*, d*;;;.ld;able
commonwith other *..ri"ruiurr"r,'tt.r.
etal',1976 VanderHammen'
p e r i o d , b u t i t h a s u . " n r . I u i i i . i y ^ ; ; ; t [ ; ; . t t h ^ . 1 " r ; i o o o o v " " * ( Baithough
o w l e r palvnologicalrecords(Kershaw'
puit.ottfr.*.tti.."-.nt,
19g3).Fire tu. uppo..nil;'iffi il;;
of abJrieinalmansome30'000yearsago'
iJl*itg
importance
in
have'incre'aseC
may
it
1984)indicate
"tival
the subhumidlandshavebeen
majoritv-of
;an, ttre
E;;;;;;
-arrival
over the past 150 v.u.r, ioi'l'JJ"glrt.
in the slashand burn
involved
not
was
"f
man
"*ii"r
aboriginal
of .urop.un'f
burnt in most years.prioi to the
europeanpastoral
recent
More
world'
the
p"artsof
(Mott e/ a/'
agriculturewhich is u tu"t- it ,utanna formatiot;;th*
forage
p*r qualitv of dry season
.onrtrui;;;y';;;
(Walker'
fuel
usec,fsavannu,t u, ,.-uinlJ at a low t"u.t,
of
aciumulation
slo*er
much
of
lggl). Fires are less frequentin the arid:Tli;;;;",1t.
to protectthe valuableforageresource'
might be
lggl) and a deliberat;pJricvorn." prevention
,nlr*res in fire frequencylthe presentecosyste.ms
,"."nt
possible
and
bv
Given increasingaridity
illustrated
is
changes
of th"s"
to be in a statebf rapibecological"h"r;;.tl;p*.ntiui*"gnit,ide
savanna
considered
subhumid
the
part
of
large
a
of
of theunderstorey
to
thechangein thefloristic
from a Themedaaustralis dominatedsystem
""d;ililiEompo.itiJn
understorey
The
south-eastern
of
Queensland.
periodin the late nineteenth
"^ft*gta
fortv-vear
a
loiiortrr rru"ri.*'l"iri;;;
u nrtrroi;in
nredominantly
";.r
f"qutn"y of burning'However'
asthe resultoi tttt"p g'"ting and.incre.attd
Bisset,i955;,
(Shaw
and
ientury
that large sectionsof the
appeari
uititufture.it
'ot'iniJniiut
apart from completeclearingof-some.u..u,
and functionalproperties.
i.iuin tt.i. o.iiinal physiognomic
ffi;;;il
as
tropicaldistribu,i:l :11"*tulian savannas'However'
ttft{rlf i{.f|ir has hishlishtedthe widespread
vegetationwhich are
of
extra-tropical
areas
are
there
Africa,
alsodescribedby Huntre! f r!g3.; for southern
*" in"rudethesetesionsin this review'within
r";;;;;.,u"a
structurallyand functionJi'#ti";;;;i;"i
w:hichare reflectedin a wide
characteristics
r"i;';rd;li;tic
i"
the savannaareastherearemajorvariations
';;rx:t";:;x:;':;:;:"ll,lT*
savannarandscapes
andpedorogicalcharacteristicsof Australian
are
ryg q:gloc1.ul 1963t,;;i.ates that the soilsof manv of theselandscapes
i'9-83'-nr?ur. zi '1.,
(cole, 1983;Beckmann,
status
nutrient
low
the
for
iltir is probablyresponsible
weatheredrelics of earlierpedologicalprocesses^ri'irt"i
planationsurfacesaremorerecent
deeplyweathered
the
with
lnterspersed
surfac-es.
land
of manysoilson these
west from Tertiary weathered
extend
that
puitt'n'
drainage
alluvial depositsin Uroui-f-, oi un"oo.dinuted
intrusions'have formedthe
basaltic
with
iog"trttt
t.dl;;;:;
surfacesin centralQueensland.Argillaceous
nutrientstatusclay soilsin the savannazone'
basisfor the originof tire faigeareasof better
t"
being
species
highbiomass
cycling.
landsdown to
abut the arid
). Theselands
per shatum.
5'1
zV
W N S V I L LE
SOILS OF NORTHERN AUSTRALIA
ffi
ffi
ffi
so''"
"r"ou,o,,ot"
colrnesr
rexruns
c"^c*trcc.e"s
N
[\'s]lJ
s*ererrrsotrs
EEI-III oEEP
li::riiiliil sANDs
-
ABIO/HUMIOZONE LINE
zone'
Figure2. Major soil typesof the Australiansavanna
58
Ecology and Management of the World's Savannas
Soilsof the savannaregionsof Australiafall into five generalgroups(Hubbleet a|.,1983) (Figure2). Tables
2 presentsummariseddata for the moisture regimesof major subsurfacesoil types a1{ th9 nutrient
I a11d
contentofthe upperl0 cm in all soil typesapartfrom shallowstonysoils[seealsoButlerand Hubble(1977)
andWilliams(1983)1.
Thble l. Estimated water holding capacities(WHC) for subsurfacesoils of different textural classesfrom
savannaregionsof Australia.
Soil type
Available
WHC (A)o7o
Clay * silt
meancontent
wt. o/o,
oven dry
hne earth
Non-available
(NA)
Available
(A)
Total WHC
(NA+A)
l0
0.3
1.4
8l
5t
1.0
1.9
64
IJ
2.7
l.l
29
42
Deepsands
Massive
soils
sesquioxidic
Crackingclays
(a) Surfacesealing
(b) Selfmulching
Water-holding capacity
mm/cm depth
3.r
86
2.2
Data from Staceet al. 1968and Stirk 1972.
Table 2. Some chemical properties for surface horizons of several major soil groups of the Australian
Savanna.
Massive
sesquioxidic
soils:
No. of samples
Mean
Standarderror
of mean
Max.
Min.
Texturecontrast
soils:
No. of samples
Mean
Standarderror
of mean
Max.
Min.
Crackingclay soils:
No. of samples
Mean
Standarderror
of mean
Max.
Min.
Deep sands
Mean
o/o
o/o
16
6.10
16
l.0l
Available Total
PfP
ppm
ppm
N
81615
20s
4.5
0.08
0.14
6.90
5.00
0.17
2.30
0.06
0.01
0.l4
0.04
0.5
10.0
1.0
29
6.17
29
1.59
29
0.19
29
25.2
0.t2
8.10
5.10
0.17
4.90
0.29
0.04
0.76
0.02
5.2
130.0
3.0
30
7.46
30
r.57
28
0.14
0.l5
8.80
5.70
0.25
6.50
0.2s
r.20
pH
organic
C
32
570
60
CEC* Exch Ca
meqo/o meqo/o
16
8.70
1.30
16
3.09
0.05
2r.oo
3.13
7.70
0.81
29
14.70
29
4.48
4r
ll50
50
r.66
46.3
t.6
0.63
14.00
0.18
30
58.8
29
486
29
45.00
29
29.9
o.02
0.61
o.o2
r7.2
480.0
2.2
168
4960
50
2.70
74.0
23.0
2.2
sz.O
8.0
0.07
10
29
314
200
Data for Deep Sandsis from Westonet al. (1981). All other data was suppliedby Mr G.D. Hubble, CSIRO,
Division of Soils. Brisbane.The data refersto the 0.10 cm depth interval.
f Available phosphoruswas that extractedwith 0.5M sodiumbicarbonateadjustedto pH 8.5 and shakenfor
l6 hours at2O"C.
* Cation exchangecapacity was determined at pH 7.0 using ammonium chloride and the ammonium
cations.
absorbedin the extractionofthe exchangeable
Australiansavannas
2).Tables
,enutrient
ie (1977)
ssesfrom
dlable
J (A)yo
IWHC
{+A)
8l
64
29
42
{,ustralian
ExchCa
meq%o
l6
3.09
0.05
7.70
0.81
29
4.48
0.63
14.00
0.18
29
29.9
2.2
52.0
8.0
r,CSIRO,
ftakenfor
mmonium
59
are:
Thesesoilsgroups,with the relatedsoil taxonomy(Soil SurveyStaff, 1975)ordersin parentheses
by a shallowstony solum,
l. Shallow Stony Soils(Entisolsand Inceptisols).Thesesoils are characterised
They
generallylessthan 30-50 cm deep.Sandyto loamymaterials- oftenearthyin character- predominate.
but occupya
arevery low in organicmatterandessentialplantnutrientsand supportonly a sparsevegetation,
largeareaof northernAustralia(seeFigure2).
2. Deep Sands(Entisols).Thesesandshaveminimalprofile developmentand are generallymore than I m
of
deep.Water storagecapacityis very low, leachinglosseslrom healryrainsare high and grossdeficiencies
manynutrientelementsaretypical.Therearelargeareasin the westernandcentraldesertsof Australia,where
ofthe surface.Therearealsoextensiveareasofthis
wind erosioncanbe a serioushazardfollowingdisturbance
soiltype in the savannazoneofcentral north Queensland.
resulting
3. CrackingC/ays(Vertisols).A divrrsegroupof fine texturedsoilsthat swellandshrinkseasonally
in cracks,oftento a depthof one metre.They are oftenself mulching,althoughsomeform thin surfaceseals
duringrainfall.
P contentvarieswidely,but in generalthe soilsarepotentiallythe mostfertileof the savannazone.Although
many have high total P and N valuestheseelementsdo not appearto be readily availableundergrassland
und hauebeenfound to be limiting to grassproductionin somesituations(Bishop, 1977). It is
"onditionrthat the soil microbialpopulationmay competefor availableN (R.J.K. Myers,pers.comm.).
postulated
of thesearethe red,yellow
4. Sesquioxidicsoils(Alfisolsand Oxisols,someUltisols).The mostwidespread
and grey earths.They are massive,with sandyor loamy surfacetextureswhich graduallyincreasein clay
contentwith depth.Clay mineralogyis dominatedby kaolinandiron oxide.Most profilesaremoderatelydeep
to deep(l-6 m), but someshallow,loamy forms with largeamountsof gravelcan occur,especiallyin the
Northem Territory.
basiccations.Most are midly
Generallythey are low in solublesalts,organicC, N, P, S and exchangeable
acid at the iurface with pH increasingslightly with depth,but someare calcareousand are alkalinethroughout
to some
materialsarelow but arecompensated
the profile.Waterand nutrientcapacitiesof thesesesquioxidic
by increasedsoil depthandpermeability.
degree
-Geneially
thesesoilsare well drainedbut the yellow and greyearthscan havewatertablesthat rise to the
sufaceduringthe wet season(Williams and Coventry,1919).Althoughthe red, yellow and greyearthshave
permeablesurfaces,with water infiltrationratesof l-2 m day-1,removalof vegetationcan resultin severe
surfacesealingand lower ratesof waterentry (Mott et al., 1979).
5. TbxtureCintrast Soils (Alfisols). Theseare characterizedbya markeddifferencein texturebetweenthe A
andB horizons.Someprofilesfeaturea prominentA2 horizonwhich may rangefrom bleachedand ratherthin,
to pale and thick. The A horizonsvary from 2 to 50 cm in thicknessand rangein texture from sandyloam to
clay loam. The texture changeis normally clear and can be abrupt. The B horizon rangesfrom clay loam to
and red-brownearths
heivy clays. Thesesoils include red and yellow podzolics,solodics,solodized-solonetz,
soils.
brown
and non-calcic
Although the general fertility is low it can vary markedly dependingon the parent material. Marked
in N, P, F and S are common.
dehciencies
The hydrologyofthe texturecontrastprofile in influencedby the depthand surfacestability ofthe A horizon
andthe hydraulic conductivityofthe B horizon.
and many red-brownearths,water ponds abovethe slow-wetting
Among the solodics,solonized-solonetz
runoff and subsurfacemovementalong the top of the B
surface
in
substantial
resulting
B
iorizons,
sodic
horizon. Many of these soils have a small water store (60-80 mm) and the water regimefluctuatesrapidly
betweenwatei loggingandwater defrcit.Surfacesealingfollowingdispersionduringrainfall is often a featureof
thesesoils.
of the potential vegetationresponseto climate can be made from the broad
Climate. An initial assessment
rainfall and temperaturepattems, e.g. the more definedsummerrainfall in the low latitude tropics and the
decreasingrainfall flromcoastalareasto the arid inland (Figure 3 Tothill et al.,this volume)
Water balancemodelswhich incorporatesoil physicalcharacteristicsasthe important intermediarybetween
climate and the plant communityhave been widely used in Australia. (e.g. Fitzpatrick and Nix, 1970;
indices
McAlpine, 1970).We haveusedthe approachof McCown ( l98l a) to evaluatewaterandtemperature
of the six major savannagroupsin Australia. Using long term ( ) 50 years)weekly
for locationsrepresentative
rainfall and evaporationdata the model generatesweekly ratios of actual to potential evapo-transpiration
('water index'). Mean weekly temperaturewas used with the estimatedtemperatureresponseof C4 grasses
(Ivory and Whiteman, 1978) to generatea weekly 'temperatureindex' (Williams and Probert 1984).The
waterandtemperatureindiceswerethen incorporatedin a multiplicativemodelto yield a weeklygrowthindex,
which measuredthe proportionof the weekthat growthwasnot constrainedby eitherwater or temperature(see
Figure4).
Integratedvaluesof the numberof weeksin which growthindex exceeds0.1 (i.e. weekswhen activegrowth
was not constrainedby water or temperature)were as follows: Monsoon Tallgrass20-22; Tropical Tallgrass
14-16; SubtropicalTallgrass10-12; Midgrass6-12; Midgrasson clay soils 8-10; TussockGrassland3-6.
Ecotogy and Management of the World's Savannas
\s
LOCATION MAP FOR GROWTH INDICES
I ?ool
f, rool
Z
E
2 KATHERINE
ol
JS
PENTLAT{D
I DARwTN
f sool
NJMM
8 HUGHENDEN
13
.11
RODDSSAY
9 REDLANDS
15
r,ONT O
l o col-ltxsvlLLE
l6
GAYNDAH
5 wooDsrocK
ll
17 CHARL€VILLE
6 LANsoowN
l 2 E$ERALD
ALPHA
l8
,t oo
BILOELA
4 cRoYDoN
3 coEt{
'12
1lo o
RO X A
1t6'
growth indices
Figure 3. Rainfall patterns of Australian savanna zone. Representative locations for duration of
(
o ).
shown
These values are a measure of the potential annual biological productivity for savanna grasses(Williams and
Probert, 1984), and when consideredtogether with the edapl..icfactors discussedearlier it becomes apparent
that forage production is likely to be spatially and seasonally variable, both within and between the major
savanna groups.
Figure 4 iliustrates the extreme seasonalityof the growing period for the Australian savanna,particularly in
the Monsoon Tallgrass areas, which have very favourable conditions during the summer followed by a very
rapid decline ofgrowth with the onset ofthe winter dry season.A similar pattern exists for other regions except
ttrat the index during the much shorter growth period is usually less favourable; these lower index values are a
reflection of the faci that variability in the length of the growing period is much less in the Monsoon Tallgrass
than in other savanna regions (McCown, 1981b; Williams and Probert, 1984). Although the Monsoon
Tallgrass region has the most favourable growth period of the Australian savannas, it is more variable than
comparable regions in Africa and India (Williams et al., 1984; Nix, 1983).
Within the Tallgrass savanna the relative importance of water balance and low temperature effects on g.rass
production vary greatly (Figure 5). At Katherine in the Monsoon Tallgrass savannas, the major climatic
ionstraint is soil water in contrast to Gayndah in the subtropics,where temperature limits grassproduction for
extended periods during winter. Pentland is intermediate, and winter temperature is often a limitation in the
Tiopical Tallgrass savanna. West of Gayndah towards the Midgrass (Roma) and Tussock Grasslands
(Charleville), winter rainfall and reduced evaporationbecome increasinglyimportant in contributing towards a
more favourable winter water balance. The implications of this climatic change on the nature of the savanna
grasslandssupport the validity ofthe major group boundariesthat have been used in this review.
This climatic analysis helps to explain why biological productivity of the Australian savanna is so strongly
seasonaland highly variablqfrom year to year. However, the variability predicted by our climatic analysis is
much higher than our current measurements suggestfor annual herbage biomass production from the savannas!
and thisiupports our suspicion that nutrient constraint to biological productivity is of prime importance.
The importance of solar radiation to primary production has not been considered. Fitzpatrick and Nix
(19?0) shbwed this to be of minor importance in the subtropics, and McCown (1981a) attached little
Australian savannas
6l
't.0
x
U
0.8
F
0.6
2
B
o
0.4
0'2
x
t{
z
F
5
o
Figure 4. Estimated growth index as a function of week number commencing from January for representative
locations (see Figure 3) in the major vegetation types within the Australian Savanna zone. A : Monsoon
Tallgrass savanna, B : Tropical Tallgrass savanna, C : Subtropical Tallgrass savanna, D : Midgrass
savanna,E : Midgrass on clay soils, F : Tussock Grassland.
rwthindices
A. KATHERI NE
1.0
U
o
/illiamsand
resapparent
n the major
0.8
I
F
B
0.6
tr
rticularlyin
dbyavery
fonsexcept
valuesarea
rn Tallgrass
e Monsoon
riable than
0'4
U
F
02
3
20
WEEKS
30
40
E. CHA R LEVILLE
ctson grass
ior climatic
ductionfor
ationin the
Grasslands
g towardsa
he savanna
U
o
=
.--
WATER INDEX
-
GROWTH INDEX
I
t
tr
::::::::-reupenlruRr
G
u
-
CONSTRAINT
,
sostrongly
r analysis
is
e savannas,
tance.
;k and Nix
achedlittle
WEEKS
Figure 5. The water index/growthindex andtemperatureconstraintas a functionof weeknumbercommencing
:
Januaryeachyear for five locationsin the Australian Savanna.A : MonsoonTallgrasssavanna,B Tropical
:
:
:
Tallgrasssavanna,C SubtropicalTallgrasssavanna,D Midgrasssavanna,E Midgrasson clay soils.
62
of the World'sSavannas
EcologyandManagement
importanceto low radiation during the monsoonseasonsin the Monsoon and Tiopical Tallgrasssavannas.
Wittiams and Probert (1984) have demonstratedthat low radiation during periods of favourablewater and
temperatureenvironmentscan limit pasture production in the Tropical Tallgrassregion west of Charters
hasnot beenexaminedelsewhere.
Towers,but the importanceof radiationconstraints
Plant communities
By using the dehnition of Johnson and Tothill (this volume), we have extendedthe savannaregionsof
of Cr species.Our definitionof savanna
with a predominance
Gillison (1983) to includeother grasslands
we haverearranged
regionsparallelithe grazinglandtypesderivedby Mott et al. (1981),but for convenience
theseinto subhumidand semi-aridsavannagroups'
Subhumidsavannas.Thesecomprtsefive regions(seeFigure l):
(a) MonsoonTallgrassSavanna.This regionextendsnorth from Cairnsacrossmostof northernAustralia
(Figure 1). As well ai occupyinglargeareasof low fertility earthsand somelimited areaswith clay soilsand
ofvariousshallowstonysoils.
form on expanses
oc"ur in a depauperate
O..p rundr,thesesavannas
Over much of the region,the variableupper stratumof woody perennialsconsistsof low (10-25 m)
eucalyptse.g.EucalyTttuitetrodonta andE. dichromophloia,with paperbarks (Melaleuca spp.)occurringon
more-poorlf drained sites; these trees are interspersedwith leguminouswoody speciesErythrophleum
chloro'staciysand a variety of woody shnfis (Atalaya, Cappais, Petalostigmaetc). The dominantgraminoid
understoreyonheaviersoiiscontainsthe perennialbunchgfassesThemedaaustralis,Sehimanemosumand
Chrysopogonfallax, with tall annualSorghum spp. importanton lighter.soils'
is variablewith swardsdominatedby
$) nipicit TallgrassSavanna.In this regionthe grassunderstorey
contoitus,Themeda australis and Bothiochloa spp. The woody upper stratum is similar in
He'te'ropogon
structureio that in the MonsoonTallgrassareaswith,Eacalyptusdichromophloia,E. crebra andE. alba being
main species.
-Subtropical
TallgrassSavanna. Areas ofdense standsof Heteropogoncontortus occupy a,variety of
(c)
soiis.ttre oveistoreyisirequently dominatedby tall (25-35 m) eucalyptswith -E crebra andE. melanophloia
as the main spccies.
(d) Midgiass Savannas.These occur on the drier edgeof the subhumidsavannaand are supportedby
infertiletexturecontrastsoilsor massiveearths.Thereis lessunderstoreyproductivitythan in the Tallgrass
Savannas,with Aristida, Bothiochloa and Chloris being dominant generain the south, and Aristida and
Chrysopogonin the north.Again,the majorupperstratumspeciesare eucalyptswith -8.loyu!n9|, E. crebra
and'8.milanophlioa theimportant species,andwithEremophila mitchellii a commonshrub.Melaleuca sppzoneof CapeYork Peninsulaand in parts of the outwashplainsof the
predominatein the northernmonsoon
Gulf of Carpentaria.
and
@) Mid[rass on Clay Soi/s. Apart from someareasof the densetall shrublandsof Acacia harpophylla
Most of the
e. cimbag6t,therearefew treeson the extensivecrackingclay soilswhichtypify thesegrasslands.
has been clearedfrom the Acacia areasduring the last forty years creatinga man-made
woody ve"getation
Midgrasssavanna.
Irithe southof this region,the dominantnativegrassspeciesis Dichanthiumsericeumin both naturaland
inducedsavannas,with Paspalidium spp. andChloris spp.commonin the clearedBrigalow (A. harpophylla)
forests.In the north on thiextensive clay soil plainsbelowthe Gulf of Carpentaria,and extendingin small
pocketsto WesternAustralia,Dichanthiumfecundum,andEulalia fulva arethe mainperennialspecieswith
an ephemeralwet seasoncomponentof annualSorghum andIseilema spp.
Semi-aid Savanna.This regioncomprisesthe TussockGrasslands(seeFigure l.) and are restrictedto grey
and brown cracking clays which can have a high nutrient status. The lands extent from south western
but with the exception
into the NorihernTerritory.As with clay soilsin mgrehumidenvironments,
Queensland
containalmostno upperstoreyandconsistof
thesegrasslands
oith. Gidg.. (Acaciacambagei)communities,
an even,ipaisety distributeditand of one or more speciesof Astrebla, with a minor componentof
spp.or Aristida latifolius.The perennialspeciesonly occupyc. 4oloof the basal
Dichanthiu'm
"pp.,Erogrotlrs
rainy periodsthere can be a wide variety of ephemeralgrassesand forbs betweenperennial
area and during
tussocks.
PLANT GROWTH AND COMMUNITY DYNAMICS
Morphologicaladaptationto the environmentalstressessuch as fire and droughthave been highlightedas
important factors aflectingplant growth and community dynamicsin Australian savannas(e.g. Walker and
haveparallelsin othercontinentsandincludequalitiessuch
Gillison, 1983;Gillison, 1983).Theseadaptations
as fire resistantbark and mature leavesadaptedto high radiation and droughtstress.But little attentionhas
been devotedto the correlatedphysiologicallyadaptiveattributesof the plants. These adaptationswill be
highlightedin the following sections.
Regeneration
Seedof the dominantgtassspeciesin the subhumidsavannasgerminatesrapidly at the temperatureof the
see
The
follr
me
t91
con
eta
soi
1
the
sea
the
infr
I
the
fre<
f€$t
feg
spp
l
sus
l9
Grt
sut
an(
(t'
hol
Th
to
se
an(
sp(
sp
ac(
wil
mc
(rw
su
ph
l9
to
wa
wa
an
ph
lea
In
D(
re(
let
eu
mi
fol
M
tht
Australiansavannas
rasssavannas.
ablewater and
)st of Charters
nna regionsof
ion of savanna
rearranged
Lave
thernAustralia
r claysoilsand
soils.
ow (10-25 m)
r.)occurringon
Erythrophleum
nantgraminoid
nervosumand
s dominatedby
m is similarin
dE. albabeing
rpya variety of
, melanophloia
e supportedby
in the Tallgrass
d Aristida and
lnea,E. crebra
Melaleucaspp.
shplainsof the
arpophyllaand
ds. Most of the
tg a man-made
othnaturaland
l. harpophylla)
endingin small
with
ial species
rstricted
to grey
southwestern
h theexception
y andconsistof
componentof
4%ofthebasal
weenperennial
highlightedas
g. Walker and
l qualities
such
r attentionhas
tationswill be
peratureof the
63
Andrew and Mott' 1983)'
seedbedduringsummerif soil moistureis adequate(Tothill, l9'l7b-:Mott, l9?8;
germinate
in the wet season
thus
all.seeds
dormancy,
seed
long-term
of
iir.rl grus siecieshavea low level
and..germination
of
dormancy
variety
a
foifo*iig seedset. In the more arid areas,adaptedspeciesexhibit
(Preece,
impermeability
seed
as
features
such
involve
that
I
Mechanisms
198
(Mott andGroves,
).
-..t
moisture
soil
favourable
until
germination
n'uv ensurelong term survivalof seedin the soil, or delay
iSiil, "rl"r-,
in the soil (Hodgkinson
conditionsoccur(Mott, rlZ+;. ny thesemeanslargenumbersof siedscan accumulate
of the seedpool in the
thusma! not requirecontinualreplenishment
eitabiishment
et al.. lggo\and successful
soil.
awnswhich enablethem to penetrate
The seedof many grassesin the subhumidareaspossesshygroscop^ic
dry
tutf"gg-temperatures
ttigtt
ttt.
bottt
escape
to
them
allows
-duringthe.annual
the soil, and this mechanism
of
much
Similarly'
1978)'
Mott,
(iottrill,^1969;
seasonfires,and dryingoith" r, rfacesoil duringgermination
testa'
impermeable
an
with
seeds
some
of
case
In
the
.r.uptt?i-uge.
also
it. U.fo* giound...0 in
majorgerminaiionevent(Hodgkinsone/ al.,1980).
to a ".eas
ini..qu.nt?r..s may lead".iJ
to the majority of *o6Oy speciesfrom the subhumidsavannas'Most of
impo.ta-nt
Vegetativeregenerationis
as a responseto the
theseregeneratefrom undergroundstorageorgans' u p.o...t which-can be interpreted
this tvpe of
with
sclerophyllv,
as
that'
argued
has
neiore
itseq
frequent dry seasonfi;;-;ith.;gh
shoots
which
parts
from
undergtound
the
regenerationmay be u ffionr" tJ poor soil nutrition. SimiGrly
or
Dioscorea
(e'g'
plants
Ampeloctssus
dicotyledonous
g.optyti"
ephemeral
g.utt..'-a
regrowin perennial
rp-p.),tn"f U. an auoidancemechanismfor both hre and drought'
show ? t*lt.increased
In the less reliable raiJatt regime of the midgrasst"uinnut, woody species
(Walkeret a/''
regeneration
of
method
primary
the
from seedbicomes
to fire, andestablishirent
susceptibility
1983).
Griflin,
and
1981;Hodgkinson
Growth
zone can lead to
The additionofnitrogenousand phosphaticfertilisersin the adequatelywateredsubhumid
(Norman' 19.62;t' Mannetje
types
of
soil
rangl
wide
a
over
grass
understorey
the
of
yield
increases
substantiai
yield responsedeclinedwith time
and Shaw, 1972; Bisho-p,rsif, S."nini, lgsi). In some casesthis
observedfor plantsgrownin glassthat
to
manner
a
similar
in
1981)
Scattini,
fgZZ;
3haw,
(f M;;"tJ" and
throughdroughtstress'
mortality
higher
plants
suffered
feitilised
irorr.. (Henzellet al., lga1Slwhere
fot
Heteropogoncontortus, and
trials
glasshouse
in
was
confirmed
grasses
Ca
N
by
of
use
The eflicient
T. australrs respondedless
that
shorved
also
trials
(
I
These
I
Themedaaustralis,uv WiiJon un? iaydock 97 ).
(1984) observedthat
Mclvor
ciliais'
Cenchrus
Af.ic"n
th.
oi
H.
contirtus
did
P
than
to addedN and
whereasH' contortus
P'
for
requirement
low
a
gayanus
had
seedlingsof Chrysopogornia'llaxandAndropogon
is known that this latter
It
requirement.
high
a
had
ittiarts
Digita"ia
and
interil'ediate
were
andC. citiais
of highernutrition(Torssell,l9?3). Growth ratesof these
,p..1.. is a sienificantinvaderunderconlditions
yieldJfor
C. fallax and the highestfor D' ciliais' and is in
lowest
with,
uptake,
parallel
P
6;;i;r tended"to
of stresstolerantplants'
were.a.feature
growth
rates
slow
ui.orJ *itt Grime's(1919) conclurionthat
grasses
of all savannaregionsappearto
perennial
stress,
moisiure
to
adaptation
to
morpnological
In addition
mechanisms'Under
avoidance
and
tolerance
physiologicaf
withstandseasonal-oirti.. deficits by both
to make osmotic adjustments
solutes
accumulate
grass
E.
iontortus
subclimax
in
the
leaves
stress,
,ooirtur.
-2 MPa and can
(Witron et al., 1980).Thus plants can Jontinue active photosynthesisat water deficits of
iurvivewaterpotentialsas low as -12 MPa.
showed similar responses,with
Measurement of gas exchange in the more exeric Astrebla spp. -3
to -4 MPa (Doley and Trivett,
photosynthesiscontiniringat 40{o of maximal rate at water potentialsof
periods
of moisturestressbut leads
brief
growth
during
plant
continrred
for
I 974). This is an obvious"aluunt"g.
grasslayer rapidly withdraws
to i^.g. leaf water deficits towarithe end of the wet sJason.At this time the
waterfrom its rooting zoneuntil leaf deathis induced'
survive-lnder extreme soil
Among woody specier,G u.uti"t appearbetter equipped-thaneucalyptsto -l2MPa(Slatyer, l96l)'
waterpotentialsof
Acaciaaneurewithstandileaf
Inaridsavannas,
waterdeficits.
at -5.7 MPa (Tunstalland Connor' 1975)' In contrast'
anrdA.harpophylla cai u.tiu.ty photosynthesise
than acacias,allhoughstomatalclosureoccursat high
by
drought
moie
ufeciea
pfrot*Vntfriri'r in eucatyptsls
1976;Connoret al ', 1971)'
leafwaterpotentialsandis effectivein reducingwater'loss(PereiraandKozlowski,
and Galabert (reportedby
Birot
Volta,
of
Upper
sauannas
ii
the
naturalised
f'urolyptu,i-ririro
ir rtrJ'
a relative five-fold
showed
season,
dry
in
the
"
"f
loss
water
O"f.V, i'lAt; fourrAtnat stomatat regulationof
levels.
season
wet
the
with
compared
transpiration
reductionin mid-day
woodlandsand degreeof
Leaf-sheddinghas beenobservedin periodsof severedroughtin the arid Acacia
In
theselatter regionsboth
savannas.
Tallgrass
Monsoon
the
towards
increasing
leaflossvarieswith season,
Tbrminalfa)shedthe
Cochlospermum,
.*ufypt, and many otfreiwoody geneia(e.g.B auhinia, Brachychilon,
regions'these
savanna
in
other
trees
with
deciduous
As
season.
dry
ofthe
treiftrt
ttre
majority of their leavesat
1983)'
(Huntley'
season
dry
the
of
just
onset
prior
to.the
foliar buds and juveniie leaues"areformed
prior to the wet season
that
indicates
eucalypts
juvenile
Australian
of
leaves
in
photosynthesis
Measurementof
photosynthesisand little
they respondin a simita'rwuy to youngleavesin African savannatrees,with reduced
64
Ecology and Management of the World's Savannas
flushof growthafterfall of old leaves,but prior to the
waterlossby transpiration(Doley, I 967).This seasonal
onset of the rains, could be relatedto Fisher's (1978) observationthat in fire defoliatedgrassesof the
SubtropicalTallgrasssavannas,completedefoliation allowedthe crowns to equilibratewater potential with
that of the soil water. Under theseconditionsleaf growth occurreduntil the supply of soil water availableat
higherpotentialswas exhausted.
Burning during the dry seasonin both the subhumidTallgrassSavannasand Arid TirssockGrasslandshas
little effect on perennialplant production(Scanlon, 1980; Mott and Andrew, 1984). In both systems,
restrictionof fire leadsat first to lodgingof senescentmaterial, death of tillers and a subsequentreductionof
annualproduction.Burningduringthe growingseasonis more signifrcantin northernAustralia, becausefires at
of growth,may reduceyield by at least50olo(Smith, 1960).
any time afterthe commencement
In the strongly seasonalenvironmentof Monsoon Tallgrassand arid TussockGrasslands,dry seasonfires
changedthe proportionof vegetativeto floral tillers in the next wet season(Lazarideset al., 1965; Scanlon,
1983),althoughtherewas little effecton plant production.Walker et al. (1984) reportedin the Subtropical
Tallgrassthat there could be differentialmortality of Heteropogoncontortus and Themedaaustralis plants
burnt at differenttimes in the dry season.Protectionof tiller primordia in T. australis tussocksappearsto be
more eflicientthan in 1L contortusearly in the dry seasonbut this is later reversed.This differentialprotection,
togetherwith better recruitmentof H. contortus,rnay explain, in part, the shift in grasscompositionunder a
managementregimewhere both grazingand late dry seasonfires suppressthe dominanceof the existing ?i
australis.
The responseto fire in the more variable rainfall regime of arid regions may be due largely to the
synchronous
timing of hre with the growthactivity phaseof individualspecies(Griffin and Friedel, 1984).
According to these authors grassesare dormant in winter, whereasforbs grow actively. Winter fires thus
damagedforbs and favouredsummergrasseswhereassummerfires had the converseeffect.
but in the infrequentfires
Fire is not the primarycauseofdeath in woodyspeciesofthe TallgrassSavannas,
of Midgrasssavannaa considerableproportion of the matureplant populationscan be killed (Walker et al.,
l98l). Althoughthe protectiveinfluenceof fire resistantbark may play a role in the survivalof sometreesof
the TallgrassSavannas,the main adaptationappearsdue to their better ability to resproutfrom vegetative
materialbelowthe groundsurface(Hodgkinsonand Griffin, 1983).
Reproduction
Heteropogoncontortus and Themedaaustralis are basically short-dayplants (Tothill, 1966; Evans and
Knox, 1969) and thus are late-floweringin the Tiopical and early-floweringin the Subtropical Tallgrass
regions.Thesespeciesprobablyoriginatedin the tropics(Tothill and Hacker,1976;Tothill, 1977a) and have
moved southby evolvinga more variablefloweringbehaviour.Thus the floweringresponseof the subtropical
populationsrangefrom a neutral to a short-daycontrol. Presenceof late-floweringin the Monsoon Tallgrass
and Tiopical Tallgrassregionsmay be advantagousin that good growing conditions are reliable in the late
summer(Tothill, 1966;AndrewandMott, 1983).In the subtropicsthe morevariablerainfallcouldleadto the
selectionof a mixtureof maturitytypesat the sameplace(Tothill, 1966).
Within any of the savannaregions,fire can play an important role in controlling the flowering pattern.
Lazarideset al . (1965) found a 25o/oredttctionboth in the numberof reproductivetillers and later development
of tillers in burnt Themedaaustralis plants comparedwith unburnt individuals;there was little responsein
eitherSehima nervosumor Sorghumplumosum, but in both Chrysopogonfallax and C. latifulius, burning
also reducedthe numberof tillers with little effecton the developmenttime. Fire accelerateddevelopmentin
Heteropogoncontortusandin a low rainfall seasonled to a higherpercentageoftillers completingreproduction
in burnt plots. In the MonsoonTallgrasssavannasunburnt T. australis plantsbehavedin a similar mannerto
that reported for T. triandra by Booysenet al. (1963) in Africa, with considerableregrowthofaboveground,
previouslyinitiated,tillers (Mott and Perry, unpublisheddata).
Dry seasonhres in TussockGrasslandsalso stimulatedthe productionof reproductivetillers in Astrebla
spp.,with in almostl0-fold increasein the numberof seedheads(Scanlon,1980).
As with the understorey,floweringin the upper storeyis periodic in the subhumidzone, and many species
flower during the dry season(e.g.Brachychiton, Cochlospermum,Tbrminalia). Flowering the seedset often
takestwo yearsin eucalypts,with a largeamountof germinableseedbeingreleasedfrom hre-resistantcapsules
(Boland,1980).
The scarcityofvegetativeregenerationin the Midgrasssavannasmeansthat successfulseedset is essential
for the survivalof many species.Studiesin the more arid areassuggeststhat speciessuchasu4.aneura flower
opportunistically
afterrainfall(Preece,l97l). However,thereappearsomeenvironmental
constraintsto the
seasonalpattem of this seed set. ln Eremophila gilesii Burrows (1971) found that the flowers were most
numerousimmediatelyafter rain, with up to threeflushesof flowersoccurringduring one floweringperiod.
Population dynamics
Within the widespreadannualsorghumcommunitiesof the MonsoonTallgrassthe lack of any seedreserve
in the soil meansthat annualseedmust be set eachyear to ensuresurvival of the sward.Andrew and Mott
Australian savannas
ut priorto the
Iassesof the
potentialwith
,r availableat
has
rasslands
rcth systems,
t reductionof
lcausefiresat
y seasonfires
)65;Scanlon,
e Subtropical
rsfralrsplants
to be
appears
ialprotection,
lition undera
re existingZ.
argelyto the
riedel,1984).
rterhres thus
frequentfires
Walkeret al.,
sometreesof
)m vegetative
6; Evansand
ical Tallgrass
r?a)andhave
resubtropical
oonTallgrass
rlein the late
ild leadto the
eringpattern.
'development
in
e response
l/ras,burning
in
rvelopment
,reproduction
lar mannerto
aboveground,
s in Astrebla
manyspecies
leedsetoften
fiantcapsules
et is essential
meuraflower
ltraintsto the
rs weremost
rngperiod.
/ seedreserve
rewand Mott
65
( 1 9 8 3 ) s h o w e d t h a t t l e c o m m u n i t i e s s u r v i v e d b e c a u s e o f t h e h i gfires
h p rhave
e d i clittle
t a b i tnet
ityo
f t h eon
sum
merrainfall.
the
effect
swards,dry season
Althoughwet seasonllres may destroytheseannual
was
also
grass
communities
perennial
rrrein ttre
populationof plantsin tnlroiiJ*itg *"i ,.uroo.-pry season
1985)' But in both the Themedaaustralis
-a-n-nattw,
germination
on
trurott
intluence
little
have
to
found
Oi'nontniumfecundumcommunitiesof the cracking
dominatedpastureson earthtype soilsandthe.perenii
the dry seisonapparentlybeingtoo severe(Mott
popuratlon,
*.." .".-1,1"0 into the aauti
clays,no seedlings
andAndrew,1985).
in SubtropicalTallgrassswards dominatedby
Failure to recruit seedlingshas also been observed
nutrients'
autai *ft.te Cook (1985) showedthat competitionfor
Heteropogoncontortus(i;iTi unpublished
tlt
gtlt:t^
in
introduced
moriality
of-seedlin!
.uu,"
1oI
,I-t
rather than for moistureor light. was the -uin
ln
lncrease
no
was
there
showed
fertilisedconditions
fertility red earthsof the Mo-nsccnTallgrasssavannas
data)'
unpublished
(Mott'
fertilised
seedlingsurvival*t.n ungrur;Jswards-were
TallgrassMvannaslive morethan 8 yearsunder
Individualsof the majorperennialspeciesof tne Monsoon
uigiazed T' australisswardis often low (c' 150
;;;il;
normal low grazingpr.r*..r. Productionof viabl!
Preliminaryresultsin ungrazed'
1Z-Sm-;1unJei opti-it conditions'
seedm-r) andonly u f"* ,..0, g..,,'ln_ut"
by the
is furtherexacerbated
This
low.
plants
is
o'r"o.np.ting
clearedareasindicatethat surviial in the absence
(Mott
et al''
grazing
heavy
by
killed
are
rapid .sealing,of the soii surfacewhich folrows*;;;;rr#lants
andNorman'
(Arndt
slow
was
plants'
but
oiougtrtiilteo
,"u.t.
ot'swardsoccurredut..
1979).Regeneration
"
set is higherand many seedsmay
sribtropics,-seed
1959).In the subclimaxswardsof H. contort t in-tft.
situations'Individualtussocksin
t.g"n"tutionin.disturbed
*piO
*itt,
ilSf;1,
germinate[c. 100 --'('S;;;,
within 5 years(J'C' Tothill' pers'
die
may
of tussocks
this latterregionarerelativelyshortJived,unaup io tb"Z"
ttfrtli;
availableon populationdynamics' Mucha
caseof the woody speciesthere is much less information
p"*i.
60 yearsold' From re-portson early
uUout
*tt"
t.gion
ttt.
in
(1979) estimatedthat tnefldest trees
of the presenceof the extremely
becausb
region
this
clearing,this agemay be closeto the maximum*;;hJi"
infestedheartwoodleading
have
may
trees
the
of
80olo
over
voracioustermiteMastotermesdarwiniensis,and
growth
is slow (McCown and
Katherineiree
at
*giont
ultimatel;,todeathtfo* *Jdf"rii ,lg72\.fn *t.'O.i"i
c' l2O-l4O yearswith the majority
at
tree
oldest
ihe
indicates
-ufyr!
.irrg
Mott, unpublisheddata;.?iowttr
data)'
unOeitOb yearsold (Mott' unpublished
and the
of the tree population in the Monsoon Tallgrasssavannas'
With the apparent.;;ltil;;;;r
However'
expected'
be
might
of.seedlings
fr.qu.n.y
productionof largeamountsof viablet..e seeO,aiiih
dominaiedby lignoiubers'often with more than 20
underthe normal annualburning regime,..g.n.*tioiis
f
SSl). Witho;t fire new plantsare recruitedinto the
coppiceshootsfrom .""i"fig""iJU.rii"J "VitgiiiS;tf
at Katherine,whererecruitmentis slowerand trees
canopy.Similarr."ruftr'ui"lfrerved undei io*..'r"iniuff
plants examinedhad arisenfrom lignotubers'
small
almost all
more sparse.In theseungrazed,grasslands
is sparseand may be episodic(Andrew and
g".,ninution
.u.iiypt
i,
,.t,
ieed
of
Althoughconsiderableam6unt
tvpes'In the
patterns
thanthesubhumid
growth
"ThJStJiiTtitt"XXll];..--rnities showmuchlessregular
euJnto"cuts(e'g' Noble and Crisp' 1979)'or else
passbefore^ g;;i;J;;
flora, somey;;;;t
ephemeral
1981)'In
to patches*itf, f"uou.ublemoisturettgi-tt (Mott andGroves'
..rt.iJted
U.
growth-"V
successful
for some
soil
in
the
been
has
which
seed
g.*inution-of
i.o.
the former.ur. r.g.n.r-uiionortn. floia occurs
sites'
reservoir
the
from
of the landscape
time, while in the latter theremay be slow reve^getatioi
may occur infrequentlyand may be arrested
Regenerationeventsln *ooJv vegetationoflhe arid savannas
fire' periodicdroughtcan also
p.rtrrUution,uci ar"intensefires is too frequent'As well as
if man-inducea
problems(Griflin and Friedel'
regeneration
subsequent
with
causelargescaledeathof the woodycomponents
t niro];rr,r.
responses
the potenrralcomplexityof v^egetation
studiesin the arid russock Grassrandsemphasise
spp' and'
As,ebla
of
mortality
tussock
iSAZ).U... Atougittsmay ciuse
6;il;.k.,
(On, 198l; Scanlon
production even in
future
limit
may.also
fiot.,
perennial"gratt
"n!
the
is well as changingthe compositionof
be radically alteredby
in favourable-years-can
favourableseasons.The compositionof the tpft...iuf"nora
of a seriesof higher
influence
the
under
while
tain,
oi
una
minor changesin both the distribution
The ability of
of
annuals'
growth
".ouni
limit
can
tussocks
rainfall years an in"..ur. l' ttt" Uurutarea of p.i.nniuf
of this svstem(Williams
itabilitv
term
long
the
;;sures
in
s""d ;;;;;
As,ebtaseedlingst. g;A;;ut*itv
andMcKaY,1983).
PRIMARY PRODUCTION AND NUTRIENT DYNAMICS
concerningprimary production and nutrient
In the Australian savannas,the majority "riir. irr...ation
estimites are additionallyconfounded
p-a*tivity
dynamicsdealswith *,. fr.it*.ous layer. ir .;;; lut.t
herbivores'
invertebrate
and
tV itt. gturing effectsofvertebrate
'J.'triyr::;::'"i";rrbaceous
(Jnderstorey.Theannualdry seasonfires in the subhumidsavannasresult in the
66
Ecology and Management of the World's Savannas
re
bi
m
^6
rrHi
:
a
- B H€€
c*SE;-qt;
i B e -52 ;- #
^ e '^? ^ ?
g =E
f
,-;E
s
tl
p
:
HE ii q 1E^ESE$ifF
:1
:
;
EEi3;?3:=
E;aiEslE;g€EtE3
z=€ i =€ 6iii Eg oE ofi
c)
o
r
a
-i
.iicd>l
coooo
f;2
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Australiansavannas
6',1
biomass.Thus the conceptof a dynamicequilibriumfor
removalof most of the abovegound herbaceous
where.fire has been excluded'The most
biomassdoes not apply in thJ broader..nr" unJ-L""u.t onty
primary production(Table 3)'
annual
their
observing
is
by
types
,"r.*n"
meaningfulway to compare
--nr-,iight
Catypeswas high,wilh a,naverage
understorey
be expected,maximumshbrtterm yieldsfor all
for the Cr grassesinthe Acacia
recorded
that
than
higher
50olo
was
about
g1nlt
This
d-t.
valueof iore tf,un3
understoreyw-erelower than
herbage
ofthe
shrublands.All the valuesfor annualproductionof root biomast
t979)' -$9w9ver, root
(Sims
Coupland,
and
grasslands
temperate
foi
g
recorded
m-,
itt. iOOO-f000
(Christie,1978)'
grasslands
C:
tire
comparable
for
productionof all in savannasystemswas higherth-anthat
biomassbeing
annual
the
of
907o
least
w_ith_at
gro*ttr
in
ill
ecosystems,
tt
great
seasonailtv
There is
productionin
shoot
Grasslands,
Tussock
In
6).
producedin the summer-months'(December-April)-(Figure
1978)'
(christie
in
summer
g
d-r
m-2
with
3.9
g
compared
m-2
dL-r
0.1
*int.l. *u, about
A. harpophylla scrublandsof the clay solls-of
iiiiuctirity of WooZy Vei*atnn.In the few reimaining
reported(Moore et al' 1967)'
specieihave-been
wood-y
of
t
ha-rbiomass
oilT-40
values
,*Uf,u-la.lgionr,
low
eucalyptwoodlandsabove
in
these
that
indicate
preliminary resultsfrom the MonsoonTallgrasssavanna
muchhig*rervalueswould be expectedin the more
data).
unp-ublirh"d
f
tt"-'1tutott,
l5-20
is
;;;;Jbi&";r
iense woodlandsofthe SubtropicalTallgiss savannas,but no definitivedata are available.
slvqrna;. is reflected
Litter production and iecimposition.TYhe high productivity of the SubtropicalTallgrass
(Table 4). In all the
areas
northern
more
the
for
greater
than
in their annual litter accessionwhich is mucli
material.
leaf
tree
of
is
composed
aicession
litter
of
annual
607o
over
savannas
subhumid
all grasslitter from
The annualdry seasonfrresin both Monsoonand SubtropicalTallg.asssavannasdestroy
fragments
non-combusted
and
as
charred
remain
can
litter
leaf
tree
the
of
to
35olo
year
up
but
preceding
the
ratesin
of
decomposition
examination
However,
Mott).
and
iL"-U ,r at.l igtZ and u'npublisheddata of Filet
in threeyears(Table
reached
turnover
with
957o
rapid,
is
litter
ofgrass
thaibreakdown
reveal
swards
unburnt
(Christie,1979),wheremost of the
5). Decompositionratesare similarto those forwetAstieb/o grasslands
to decay and
grassleaf litter decomposeswithin two wet seasons.The tree Ieaf litter is much more resistant
in the
it
to
decompose
of
most
for
years
necessary
are
I
that
9-1
lxtrapoladon from titter Uagstudiesindicates
data).
unpublished
(Mott,
of
hre
absence
maximumlitter loadof 133g m-'?(Table6) was after
In the episodichre regimeof the TussockGrasslands,
hre, litter loadsin the MonsoonTallgrasslands
(Friedel
l98la).
Without
rainfall
yr".i
uu.i"g.
of
above
.o..
also rlach this value withln two years,and after ten years appearto stabiliseat about 180 g m-'?(Mott,
data).
unpublished
th. high decompositionrate observedin litter bagsduring the w_etseasonreflectsthe high soil respiration
ratesrecordedin a MonsoonTallgrasssavannaof up to 600 mg COz m-'h-t at the heightof the wet season
soil carbonif the litter fall is reduced.A
(Bridgeet al.,1983). This breakdowncan leadto a rapiddepletion<-rf
iedu"iion of 2ZOgcm-, or 40oloofthe carbon contentin the top soil layers occurredover a two year period
when litter produJtionhad ceaseddue to heavy defoliationthat also reducedplant basal area^(Bridgeet al.,
largetermitepopulationsqe foundin all but
1983).As well ascontaininga highpopulationof soildetritivores,
theciackingclay soils.SpainanAniown (1979)reported300-400nestmoundsha-'in the TiopicalTallgrass
savannasuid ri-il"r hgureshavebeenrecordedin the MonsoonTallgrasssavannas(Mott, unpublisheddata;
R. Braithwaite,pers. cimm.). Termitebiomassis unknown,but if the undergroundcolonies.are of the same
density as those reported from comparablearid regions(Watson et al., 1973), the populationdensitiesare
g
equivalentto 100 kg ha-t of termiteiiveweight.Wiih a dry matter consumptionrate of approximatelyI 1.6
-r
40 g .-z yr of
per 3 g termite(t ee andWood, l97l ), this is equivalentto the removalof approximately
n1^-z
materialor over 25o/oof the total litter fall for the region.
of below gound biomassoithe herbagelayer would also be rapid. Christie (1978)
The decomposition
reportedthat ihe grassroots in the Acacia woodlandsmay survive for only 2-3 years and data from the
M'onsoonTallgrasi savannasindicate that roots of these systems have a similar turnover rate (Mott,
data).
unpublished
Nutrient Dynamics
Studies *id.ty scatteredsitesthroughoutthe savannaregionhaveshownthat nutrient accessionis low for
"t a yeaily depositionfor N in the order of I kg l6-t y1-r (Wetselaarand Hutton, 1963; Probert'
all ions with
1976;Hutton, iSSf). 1.h. majorityofdepositionis part ofthe localterrestrialcyclingofnutrientsand cannot
be regardedas true accession.
Another sourceof nitrogen is biological fixation. Although herbaceousnative legumesrarely contribute
morethan 57o of the undeistoreybiomiss, their foliage may contain more than 2o/oN at the end of the wet
theselegumesnodulate,6utno datafor amounlsofN fixed are available.Thereis a varietyofwoody
season;
leguminous,p"Ji., in the upper storey and nodulationhas beenobservedin the widespreadAcacia spp. As
with leguminouscrops,it couia be exiected that the major sourceof nitrogenfrom thesespecies.wouldbe by
decomiositionof leaf material (c.f. Wetselaar,I 9S0).Although thesespeciesmay be preferentiallygrazedby
livestoik, the greaterpart ofthe nitrogenis lost to the atmosphere,especiallyin northernregions(Yallis et al.'
1982).Small imounti of asymbioticnit og"n fixation have also beenreportedfor lichensin soil crustsin arid
Ecology and Management of the World's Savannas
.)
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69
Australian savannas
x00
. o5 _o
.?9.
O trr
-i
ol
c.i
a^l
a.l
Tallgrasssavannasat Katherine
lands of Australia (Rogers and Lange, 1972). Soil crusts exist in the Monsoon
(R. Rogers, pers' comm')' It
fixation
nitrogen
in
important
not
Sll2o,thiix'fiesii',is
species,
but the dominant
the only data available is
but
grass
understorey,
the
dense
of
.pp.".r-,ft"t ri .ogen is fixed in the ririzosphere
potential under field conditions
the
to
relation
little
of
be
would
fixld
amounts
the
and
trials
from laboratory
(Weier,
1978).
'
few weeks oithe summer wet season
As with other tropical grasslands,mineralisation could occur in the first
mineralisation was much increased
that
showed
(19S4)r
lg82).Filetet.al.
(e.g. Penning de Vries -f,b3it.y",
an appreciable amount of the
contained
P
and
fertilised'with
*.r.
ih"t
,*LA.
Tallgrass
in Subtropical
ratio in savannaplant residues
high
C:N
normally
the
Thus
int-au..O legume i[acroptilium atropurpureun.
et al'' 1982)'
may be restriiting the net amount of n.ineralisation in the soil (e.g. de Ridden
in tall grasslands results in low
At the end of the wet season, retranslocation of nutrients to the rootstock
1963; Mclvor, l98l )' Thus although
Norman,
o.2o/o4.47o
range
(e.g.
N
herbage
for
standin!
values
nutrient
limits lossesto about 8-10 kg
fire removes this standing;ateriaiduriig the dry seasonthe low nutrient content
ha-tin eastern regions (Norman and
N
16
kg
to
up
or
Taligrass,
MonsJon
in
grass
litter
standing
ha-,in
N
Monsoon Tallgrass savanna up to
Wetselaar, 1960). Fires also consume part of the tree leaf litter, but in the
-similarly, the N,content of tree leaf
40oloof nutrients remain in the charred lragments (Mott, unpublished data).
was still substantial (2'9 gm-'), and more
litter in an annually burnt site of an Subtr-opical'iallgrasssav_anna
than 507o of that existing in unbumt areas (Lamb et al',1983)'
water after rainfall' Prebble and
Other forms of nutrient redistribution include throughfall and stem flow of
nutrients added by litter fall'
the
equal
could
Sti* iiSAOI found in the Subtropical Tallgrass ,uuunriu that this
improved grass production
of
zones
ring
drip
distinctive
savannas,
Midgrass
the
In the nutrient-poor earths of
and to nutrient release
througmall
fall,
litter
ofdead aid living trees. This is due to
are evident under the
"unopy
1983).
Scanlan,
after tree death (Burrows and
Williams et al. (1980) have found
No values are availabie for lossesdue to leaching and deep drainage,but
in the subhumid savannas, and
zone
rooting
the
component-below
drainage
that there is a considerable
occurs during the monsoonal wet
movement
nitrate
Williams and prt,bert ( 1984) demonitrated ihat substantial
season.
for P and S are scant, but their
This account of nutrient dynamics discussesonly nitrogen movement. Data
growth.
plant
prevent
to
as
reduced
so
be
may
and
lbw
values in soil are extremely
MONSOON TALLGRASS
c.t
I
F
z
u
z
^i
z
U
@
Katherine lat
:
14 O6S
15
.
m
T
o
10
.
..'..-.'
0 .5
E
z
EXTENT OF WET SEASON
I
0
S U B _ T R O P IC A L T A L L G R A SS
*
\
EXTENT OF WET SEASON
2. 5
2000
z
U
z
2. 0
1. f ,
r 500
z
u
o
1. 0
1000
?
2
Narayen lat 25.41S
0' 5
o
I
0
J UNE
JULY
SEP
AUG
-
OCT
NOV
N T T R O G E NC O N T E N T
DEC
JAN
FE B
MAB
APRIL
MAY
'....- D.M.
Katherine,in
Figure6. Seasonaldry matterproductivity andnitrogenconcentrationofthe grassunderstorey-at
thjMonsoon Tallgrais suuattna,and Narayen in the SubtropicalTallgrasssavannafrom (Mott and Tothill'
1984).
@
c.l
v>
G
{!!
ii
500 x
MAY
13
=
-o 6;
(nPr
':i-=a
*(, U
plant production
Environmentalconstraintsto herbaceous
Examinationof the potential growth periods showsthat a climatically favourableregimefor growth may
existfor up to 22 *..kr in thJ tropical MonsoonTallgrasssavannas.However, seasonalgrowth curves
ln tne netd showthat high growthratesof grassesonly occur over the initial eightto ten weeksof the
measured
and0.l5o/o,
wetseason(Figure6). N and P contintsof the greenleaf materialare initially greaterth112.Oo/o
phase.A
growth
initial
this
end
of
phosphorus
the
by
0.08o/o
nitrogen
and
1olo
less
than
rut
io
respectively,
the nutrient contentof greenleaveshas beenrecordedfor the other tallgrasssystems(Shaw and
similarfall-of"na
levelsat this time arehighand soil water
Bissett,1955;Mclvor, l98l). In all ireas radiationandtemperature
'10
of the World'sSavannas
EcologyandManagement
is usually available.The canopyof regrowingplants is open and it only intercepts3G-50o/oof incident light
(Mott, unpublisheddata).Although no data existon the critical minimal foliar contentsof N and P requiredby
the native speciesfor rapid growth, the values recordedare below those regardedby Wilson and Haydock
(1971) and Penningde Vries and Krul (1980) as essentialfor growthof African C4 species.Thus it couldbe
expectedthat nutrient availability rather than climatic variableslimits productionduring the growing season'
especiallyin the Tropical Tallgrasssavannas.The potential magnitudeof this constraintis illustrated by a
comparisonfor severalsitesin the tallgrasssavannas(Figure7) of actualannualproductionof the perennial
grasi swardwith an estimateof potentialproduction;this is basedon the cumulativegrowth period for which
both water and temperatureare 'ron-limiting.The nutrient limitation is apparentlymost important on the low
soilsof the MonsoonandTiopicalTallgrassregions(e.g.KatherineandRedlands).With
fertility sesquioxidic
applicationof P and N at Katherine,actual productionclosely approximatedthe estimatedpotential for that
season.On the betterquality texturecontrastsoils at Lansdown,in the Tiopical Tallgrass,grassproductionat
the study site was not signihcantlylimited by nutrientdeficit. At Calliope,in the SubtropicalTallgrassregionit
is apparentthat in someyears a nutrient(presumablyN) limited yields,while in other yearsof low yield
potentialtherewas suffrcientN to useall availablesoil water.
LANSDOWN
a' c 4
;
!
RODOS BAY
v
BEDLANDS
o
a
49
K A T I - I ER I N E
K A T H E R I N E F E R T I L I Z E DN I P
MEASUREMENT YEAR
c
J
U.
tJ
J
o,
al.
tor
lat
su
wa
Pr
als
wil
wh
st
re(
wa
stc
acl
ha
stc
d(
sa
(T
to
nu
nu
hie
wa
se
pe
nu
as
l9
sol
eal
exl
eit
ar(
andactuat
by waterandtemperature
Figure7. Relationshir;..:ll; rollJi'lt;:.
;;',, ha-r)constrained
(Mclvor,1981);Calliope(Shaw
andLansdown
(Norman,1962);Redlands
grassyield.Sources:
Katherine
andBissett,1955).Potentialgrassyield estimatedas for Figure4 and a maximumshortterm productionrate
as in Table6.
The empirical standardsfor foliar contentsof nitrogenand phosphorusproposedby Penningde Vries and
Krul (1980)for Saheliansavannasystemswereexaminedin the contextof plantgrowthin Australiansavanna
is the primaryelementwhich
regions.The P:N ratio indicatesthat althoughbothelementsarelow, phosphorus
limitesgrowthin the Tallgrasssavannas(Table5). By contrastN appearsto be the main limiting elementin
plantgrowthon the morefertile crackingclay soils.
Interaction of Woodyand HerbaceousSpecies
As in Africa, the studyof competitionfor nutrientsbetweenthe upperand lower stratumof savannasystems
herbagegrowthby removingexistingtrees(e.g.
in Australiahasbeenbasedon the practiceof eitherincreasing
1979).
shrubinvasion(e.g.Harrington
et a|.,1979;Hodgkinson,
Beale,1973;Gillard,1979),or by preventing
Such studies show that the amount of herbagegrowth varies inversely with the amount of upper storey
(e.g.Beale,1973;Walkeret al.1972;Johns,l98l). Tieeremovalcreates
in therateof
an increase
vegetation
herbageproductionwith resultinghighermaximumyields (Walker et al., 19721'Tunstallet al., l98l). ln
ungrazedswards,long term productionhasbeenconfoundedby shrubinvasion(Tirnstallet al ., l98l) or in the
vigourthroughsenescence
andlodging(Mott andAndrew, 1984).
caseof MonsoonTallgrassby decreased
that adult treesand
A model proposedby Walker et al. (1981) for southernAfrican savannassuggests
grasses
utilisecomplementary
soil waterstores,eachutilisingthe upperand lower profilelayersrespectively.
This partitioning doesnot seemclearly definedin Australian savannas,wherethe soil moisturedepth is often
limitedby eitherthe shallowness
of the majorsoil groups,e.g.red earths(Johns,198I ), or by an impermeable
layerof differenttextureof highsolublesalt(McCownet al., 1976;TunstallandConnor,1981).Also Peakeet
alt
dr:
Th
se
se
cul
exi
w
for
ST
soi
nul
fur
Int
bic
for
es
ant
pr(
as
Australian savannas
r ofincident light
md P requiredby
on andHaydock
Thusit could be
I growingseason,
s illustratedby a
r ofthe perennial
periodfor which
ortanton the low
Redlands).
With
potentialfor that
assproductionat
lallgrassregionit
earsof low yield
IZEDN+P
IB
ratureandactual
I Calliope(Shaw
r productionrate
dngde Vries and
lstraliansavanna
ry elementwhich
nitingelementin
savarula
systems
xistingtrees(e.g.
dgkinson,1979).
of upper storey
lasein the rate of
et al.. 1981).In
., l98l) or in the
ndrew,1984).
t adulttrees and
/ersrespectively.
re depthis often
'an impermeable
l). AlsoPeakeet
71
al . (197 5) found in the Subtropical Tallgrasssavannasthat the majority of water use by grassesoccurred in the
top 50 cm ofthe soil profile. In all Tallgrass savannas,the majority ofroots ofthe eucalypt upper storey and
laterals with a high root density in the top metre of soil.
In spite of the above, differential water use by plants is important. This is supported by studies in the
subtropics, which show there is a greater soil water recharge in plots without trees. Once recharged,the soil
water itore in these treelessplots remains higher during wet periods (Tunstall and Walker, I 975; Johns, I 98 I ;
Prebble and Stirk, 1980). The potential for differential water use in an effective soil profile of c. 100 cm was
also shown by Prebble and Stirk (1980) who found that although the surface 40 cm dried quickly in plots both
with and without trees, there was greater water withdrawal in the 60-100 cm zone at the end of a dry period
where trees were still intact.
Trees intercept up to l8olo of rainfall falling on their canopy area (Tunstall and Connor, 1975; Prebble and
Stirk, 1980). This, coupled with increasedrun-off due to sparseherbage cover under the canopy, can lead to
reduced infiltration in drier areas. Once good herbage cover is established there is improved infiltration and
water accession(Johns, 1981). Tunstall and Walker (1975) also reported that in dry periods the soil moisture
store was depleted most rapidly in areas with little woody vegetation. As these gtasses are photosynthetically
active under high water deficits the plants use water rapidly and die back to a dormant crown. On the other
hand trees have relatively little photosynthetic activity after even moderate water stress, and are effrcient at
'strategies'were evident in the survival of trees for long
stomatal regulation of water loss (Doley, 198 I ). These
late
in
the extreme dry seasons of the Monsoon Tallgrass
periods,
leaf
fall
occurred
where
major
drought
savannasor during major droughts ofthe Subtropical Tallgass areas.
As well as water, higher levels of nutrients in areas where trees were killed influence herbage growth
(Tirnstall et al., l98l). They postulated that after death oftrees, soil temperature and water increased,leading
to faster N mineralisation. Johns (l9Sl) also supports the hypothesis that trees and shrubs compete for
nutrients, but points out that both this interaction and the effect ofshrub and tree clearing on the availability of
nutrients and water require clarification. The studies above were all carried out in the Midgrass savannas with a
high probability of moisture stressduring the growing season.In the Monsoon Tallgrass areas, studies on the
witer balance of red earth with or without trees showed that water was available throughout the growing
season,but that there was much greater annual grassproduction under killed trees in the first years (McCown,
pers. comm.). McCown postulated that much of the growth differential can be explained by the release of
nutrients from decomposition of root material from the woody vegetation. Similar suggestions have been made
as to the cause of small areas of increasedgrassproductivity in the Midgrass savannas(Burrows and Scanlon,
1983). This phenomenon would explain the two to three year increase observed in herbage productivity in
some Subtropical Tallgrass lands following the killing of the woody species. Given the low fertility of the
earthy soils ofthe Monsoon Tallgrass, a nutrient-controlled tree/grassinteraction in undisturbed sites could be
expected to influence the distribution pattem ofthe native gtass flora. Although no obvious patterning occurs
either in productivity or in composition of perennial grasseson the widespreadred earth soils, distinct pattems
are observedon yellow earths with restricted drainage (McCown and Jones pers. comm.).
The above studies emphasise the fact that trees require water for survival throughout the dry season,
although some can reduce their requirement by stomatal closure, or in extreme casesby leafdrop. During the
dry seasonthere islittle or no competition for water between grassesand trees because grassesare dormant.
These also appearslittle competition for moisture between grassesand trees in the high rainfall of the growing
seasonin the Monsoon Tallgrass savannas.However, with more frequent dry spells occurring during the wet
season in the more arid Midgrass and Subtropical Tallgrass savatrnas, competition for water causes a
cumulative reduction in herbage production over the wet season. As in other savannas the long term coexistenceof both trees and grassesappears to rest on the use of different water stores within the soil profile.
When the woody speciesare removed, the responseofthe herbaceousspeciesmay vary between savannatypes
for different reasons.
SECONDARY PRODUCERS
The low biomass of vertebrates within the Australian savannashas been mentioned earlier. In low fertility
soils,it would appearthat the role of invertebratesassumesgreaterimportance in the rapid biological cycling of
nutrients. Although the vertebrate fauna is often more conspicuous, the invertebrates are critical to the
functional stability of Australian savanna lands.
Invertebrates
Among the soil biota the lowest end of the trophic hierachy is occupied by the microflora. Although no
biomassestimatesare available for these organisms,soil respiration rates of 500-600 mg COz m-2 hr-' found
for a Monsoon Tallgrass savanna indicate high activity during the wet season (Bridge et al., 1983). Initial
estimatesindicate that the soil microflora may exceed 50 g m-'z(Mott, unpublisheddata). In similar grasslands
and savanna systems elsewhere the microflora consume over 80oloof the available energy formed by primary
production (Lamotte and Bourliere, 1983; Lamotte, 1983), and from a comparison of environments one may
assumethe same order for the Australian system.
72
Ecology and Management of the World's Savannas
As well as bacterial and fungal microflora, the arthropod detritivores, earthworms and termites can be
important contributors to high soil respiration rate. Although the limits of earthworm distribution in Australia
are not accurately known, it seems that both high soil temperatures and periodic droughts limit their imporrance
as ecological factors in many savanna areas (Lee, 1983). Apart from rainforest reg-ionsand other areas with
permanent water, their role in pedological processes and organic matter decomposition appears
to have been
replaced by other soil invertebrates such as collembolans and termites.
The population density of arthropod detritivores varies widely. Plowman (1979), working in unburnt lorests
in the wetter region of the Subtropical Tallgrass savanna, reported a population of over 30,000 collembola m-,
and a total invertebrate fauna of rrore than 80,000 m-2. At Katherine, in the Northern Territory, Greenslade
and Mott (1982) recorded 2,500 m-'z in the ungrazed understorey.this latter figure is similar inmagnitude to
that found in other tropicel savannas(e.g. Belfield, 1956; Burnett, 1968). In the Monsoon Tallgrassthe greater
part of the collembolan fauna is composed of two elements, one group of species typical of arid regionJ and a
second group of similar composition to rainforest areas. The dominance of these groups presumably fluctuates
with the season.
At Katherine, absenceof fire resulted in an increase in population density of detritivores (Greenslade and
Mott, 1983). Continued annual burning of small (0.4 ha) plots had no effect on epigaeic invertebrates but did
reduce the soil fauna. These results are contrary to those by Pomeroy and Rwaikiidara (1975) and Athias
(1971) who worked in a less seasonal climate in Africa. It appears that in soils with poor water holding
capacity in Monsoon Tallgrass, annual burning may be too frequent to permit the fauna to recolonise burni
areas during the relatively short wet season.
Although termite mounds are common in the savanna landscape there are few estimates of the biomass of
these vertebrates. As stated earlier a value of c. 100 kg ha-t liveweight has been derived for above ground
colonies in the Katherine area of the Monsoon Tallgrasssavannas.Lee and Wood (1971) give a range o1 tSOI100 ha-t for the number of above ground termitaria in tropical savannas.However neitirJr of thesjestimates
has included below ground colonies.
In savant,asystemsthere are three main types of termites; harvestertermites which gather plant material at a
distance, detritivores feeding on soil detritous and xylophagous termites which feed on dead and living wood,
causing such damage as piping in living trees. O[ 5 species recorded in a survey in the Monsoon Tallgrasi
region 37o/o of the species were classifred as xylophagous, 47o/o as detritivores and 160/oas harvesteri (L.
Miller, pers. comm.).
Harvester termites rarely attack green leaves. Thus they have little direct influence on the understorey ofthe
more humid tallgrass savannas, where annual herbage production is reliable. But in the more arid savanna
lands, droughts and ungulate grazing together may reduce the pasture biomass to levels where subsequent
ungulate grazing may kill the grass tussocks (Watson et al., 1973; Watson and Gay, 1970). this problem is
compounded by impervious termite nest caps which retard grass regeneration. As well as direct damage by
harvester termites in dry savannas, it appears that xylophagous termites attack most of the native woody
speciesin monsoon tallgrass savannas(Fox, 1974), sometimeskilling them.
Termites play an important part in ecosystem function, both through the decomposition of litter and the
redistribution of nutrients. As-well as digesting977o of cellulose in theirfood (Lee, tTl+7,tney can also digest
some lignin. Turnover rates of nutrients in termite mounds are poorly known, but Holt ei at.
li}eOy rugg"Jted
that over a 50-yearperiod one generationofmounds could result in the reworking, and consequentenrichment
in organic matter and plant nutrients, of up to 400 kg ha-r yr-r of soil. As almost no plants grow on Australian
termite mounds, these nutrients are withheld from circulation until the mounds erode and distribute material
over the soil surface.The persistenceof small areasof soil with improved overall nutritional status beneath old
termite mounds may be a cause of the patchy grass distribution observed on some of the lower fertilitv soils of
the Tropical Tallgrass savannas(Holt and Coventry, 1982).
As well as being important to the cycling and redistribution of nutrients, termites have had an important role
.
in pedological processes in northem Australia. Williams (1968) suggestedthat termites may have been
responsible for a l5 cm thick, fine-textured upper layer over stony soiiln Monsoon Tallgrass sivannas, and
postulated m accumulatign rate of 0.0125 mm yr-r. Similar rates of soil redistribution wei also calculated
by
(1971) for other areas in the Monsoon Tallgrass savanna. In red and yellow earths in the
!.e9 and Wood
SubtropicalTallgrass,Holtetal.(1980)foundthattheamountofsoilheldinmoundswouldform alayerl.25
mm thick acrossthe entire soil surface.They conservativelyestimatedrates of accumulation of 0.5-O.025 mm
yr-t, and estimate that the 20 cm thick sandy A horizon ofthe soils has resulted from erosional deeradation of
termite mounds over the last 8000 years. Thus Lee (1983) concluded that much of the northern-Australian
landscape owes its present soil mantle to termite-transported material; indeed many texture contrast soils
resemble African soil profiles thought to have been produced by termite activity.
Other invertebrates can also play an important role as above-ground herbivores in savannas lands (Andrew
et al-,1984). Although there are few quantitative data available fbr Australian savannas,specieswhich swarm
and migrate (including the migratory locust, Locusta migratoia) are common in the region and Friend (pers.
comm.) estimated that the normal biomass of Acididae on the fringing coastal plains in northern Australia was
m
an
thr
an
Ve
ka
tht
l9
sa
reI
sh
pr(
int
the
sa
Ta
So
I
ha19
I
bior
l9E
und
sea
is lc
whe
hav
lonl
co
c
env
lack
con
ferti
resu
grea
arid
two
Australian savannas
rmitescan be
n in Australia
lir importance
rer areaswith
r to havebeen
nbumtforests
rllembola
m-'?
i, Greenslade
magnitude
to
ns thegreater
regionsand a
bly fluctuates
reenslade
and
bratesbut did
i) andAthias
rater holding
oloniseburnt
rebiomassof
aboveground
range
of I 50,eseestimates
I materialat a
llivingwood,
oonTallgrass
(L.
rarvesters
rstoreyof the
arid savanna
e subsequent
is problemis
:t damageby
tativewoody
litter and the
analsodigest
80)suggested
rt enrichment
rn Australian
butematerial
s beneathold
rtility soilsof
nportantrole
y havebeen
avannas,
and
calculated
by
earthsin the
t a layer1.25
5-0.025mm
egradation
of
n Australian
lontrastsoils
nds(Andrew
whichswarm
Friend(pers.
\ustraliawas
IJ
in the order of I kg ha-r. Seed harvestingby both ants and termites is also common (Russell et al., 1967:'Mott
and McKeon, 1977) andmay remove considerableamounts of material from the grassunderstorey.Changesin
the composition ofthe understorey speciesmay lead to concurrent changesin balance between seed harvester
and carnivorous ants (Greenslade and Mott, 1979) and long-term effects on the ecosystem have yet to be
evaluated.
Vertebrotes
Newsome (19S3) emphasisesthe richness of the vertebrate fauna in the savanna regions of Australia with
kangaroosand wallabies being abundant in the Monsoon Tallgrass savannas.But when compared to the rest of
the world's savannas it appears that the biomass of native mammalian herbivores is distinctly low (Calaby'
1980). Apart from the I kg ha-r r.stimate by Frith and Calaby (1969) for the biomass of kangaroos in arid
savannas, no data on biomass ofnative herbivores exist, although it is unlikely that the levels over the savanna
region vary greatly from this estimate. The chief introduced herbivores across the savanna region are cattle,
sheep and water buffaloes. Most water buffaloes are restricted to the mesic subcoastal plains, but cattle are
present on all the savanna systems, with sheep being dominant in the T[ssock Grasslands. Biomass of
introduced cattle is also low, ranging from ca. 2 kg ha-I in northern grasslandsto as much as 6 kg ha-r under
the better nutritiona! level of the Subtropical Tallgrass savanna (Mott e/ al., l98l), values which are of the
same order as those for African savannas(Table 6).
Table 6. Biomass of main unsulate herbivores in Australian and African savanna lands.
Biomass
m2
Ngorongoro
Manyara
Serengeti
Nairobi
Brugen
10.4
7.8
4.2
3.0
1.0
Australia(2\
MonsoonTallgrass
SubtropicalTallgrass
Midgrass
TussockGrasslands
0.8
6.1
2.9
4.4
Sources:(l) Schaller(1972); (2) Mott et al. (1981).
In the more arid Midgrass the biomass of introduced ruminants is again low, and averagesless than 3 kg
ha-r, although the fertile clay soils of the Tussock Grasslands may support ,t-8 kg ha-r of sheep (Mott et al.
1981).
For all native and introduced vertebrate herbivores,the major limiting factor for carrying capacity is not the
biomass of understorey grass produced but rather its extremely low nutritional quality (e.g. Mott and Tothill,
1984). Newsome (1983) reported better breeding success of wallabies grazing on fertilised pastures of
undoubtedly higher nutritional quality, and Andrews (1976) emphasisedthe importance of poor quality dry
seasonforage in determining the low reproductive successin cattle in these regions.The production of biomass
is lower in the drier zones of Midgrass savanna,but dry feed may be of betier quality as in the arid rangelands
where amount rather than quality becomes the limiting factor.
Although a wide variety ofother vertebrates (marsupials, birds and reptiles) occupy the savanna zones we
have little quantitative information about their biomass and it is not possible to speculate about their role in the
long term functioning of the savanna ecosystem.
CONCLUSIONS
Of the savanna types within Australia the tropical Monsoon Tallgrass appears to have the most severe
environment, with a severe winter drought period and plant production in the wet season limited because of the
lack of soil nutrients. These factors, combined with large areas of skeletal soils, constitute a major biological
constraint on productivity over almost all this savanna type. Within the Tallgrass systems, the higher soil
fertility along the eastern coastline more than compensates for a less favourable growth environment which
resultsfrom a lower reliability rainfall and lower winter temperatures.In the drier Midgrass environment, both
greater aridity and low nutrient soils probably combine to limit productivity, while in the Tussock Grasslands
aridity is apparently the main limiting factor on the better quality soils. Seasonal rainfall variability in the latter
two regionscan lead to wide fluctuations in yield. It must be emphasisedthat these conclusions are drawn from
14
of the World'sSavannas
EcologyandManagement
to betterpredict actual
data generatedby simplegrowthmodelsand considerablymore work would be needed
growth curves.
of foliar
The low levelso[ availableN and p within all the savannassoils are reflectedin the rapid depletion
limit primary
apparently
elements
Both
growing
season.
the
during
elements
essential
for
these
levels
limits
productionof the he.bage,Uutttr... is somJevideice to Juggestthat P is often the primary elementthat
grassyield.
grassmaterial
Mineralisationof nutrientsoccursearly in the wet seasonwith apparentrapid breatdown-of
ratios-andthis could
and a much stowerincorporationof foliagefrom woody species.tvtuctrtl-tterhas high C:N
and leadto rapid immobilisationof nutrients.Although fire is
to both ..rtri.t mir,eralisat-ion
U.
""p."t.anutrient loss from these systemsis restrictedbeiause of the low nutrient content of much of the
;;.;.",
play an important part in
biomassduringthe dry season.The nativeherbaceousand woody legumesappearto
use.
land
extensive
the fixation oith. .-ill amountsof nitrogenlost from the systemunder
to
be influencedby the
appears
species
woody
of
biomass
the
regio-ns
In common with other savanna
in both the more
severityof the dry r""ron 1..g.Waltei, 1971).This limitationleadsto reducedtree cover
woody
herbaceous
of
the
control
Long_term
areas.
Midgrass
inland
driei
,.uronut northernregionsunitn.
with removal of the
balanceappearsto be a i.rponr" to a differential rise of the availablewater store, but
in the growthof the
woody upperstorey,the releaseof both nutrientsandmoisturewill createan initial increase
species.
herbaceous
tropics and semiThe increasingseverityof the climate betweenthe subhumidsubtropicsand low latitude
soilsoccupying
shallow
with
nutrient-poor
availabilityof soil nutrients,
ariJ regionsis pialleled Ly decreasing
Monsoonand
the
of
sesquioxidic-soils
in
the
nutrients
oisoil
lack
Australia.lhis
of iar northern
iu.g.
"i.",Tallgrasssavannasis apparently compoundedby a large active microflora, which together with
Tiopical
of nutrients.This low nutrient availability is reflectedin the
G*hing, couid potentiallyrestrict the
in nutrient
"uuit"Uitity
strategiis of the dominant perennial grass species.These strategiesparallel the differences
competitive
the
north
contrasting
the
in
system
australis
f.
stress-toi;rant
nutrieni
availabilitv with the
are
responses
oithe subclimaxH. contortasdominatedsystemsin the subtropics(Table7). These
,t
(1980)'
(1979)
Chapin
and
"t.gv
Grime
by
outlined
growth
strategies
similarto the stress-tolerance
Ihble 7. Growth strategiesand stability under healy grazingpressurefor dominantunderstoreycommunities
(Data from Shaw and Bissett,1955; Tothill'
of the MonsoonTallgrassand SubtropicalTallgrassJauannas.
unpublished')
Tothill,
unpublished;
Mott,
1979;
l9'77b:Mott, 1979;Mott er al.,
Monsoon
Tallgrass
(7. australis
dominated
systems)
Establishment
Growth
Reeeneration
No longterm seed
dormancy
Little effectdry
seasonfires
[,ow germination
(4-8 m-'?)
Longlivedtussocks() 9 Yrs)
Poor
Low nutrientrequirement
Slow growthrate
Short growingperiod
upright growth form
Surface sealing soils
GRAZING INTOLERANT, NUTRIENT STRESSES,CLIMAX COMMUNITY
Good
Shortlivedtussocks(- 5 Yrs)
No longterm seed
Subtropical
dormancy
Tallgrass
Invasive
Medium nutrientrequirement
Promotiveeffect
Rapid growth rate
dry seasonfire
(H. contortus
Long growth rate
High germination
dominated
Prostrate growth form
(300m-')
systems)
undergrazing
GRAZING TOLERANT - SUBCLIMAX, INVASIVE COMMUNITY
Within the savannasthereis a high invertebratebiomasswheretermitesplay a vital part in the formationof
the soil mantleand a continuingrole in nutrientcycling.Althoughhistoricallythe Australian savannasappears
to have maintaineda lower naltivevertebrateherbivoie biomassthan the African systems,this has-changed
since the widespreadintroduction of domestic ungulatesin the late-l9th century. Since then they have
maintaineda biomasswhich is of a similar order to that existingin relatively undisturbedregionsof Africa
(Table6). Although the mobility of the introducedherbivoreshasbeenconstrained,in contrastwith the widei"nging nomadiciative ungulatesof Africa, the pasturesin the subhumidsavannashaveremainedremarkably
of
,tuf,t.-ou.. the last 80 yeais. This stability contiastswith the drier regionswhereconsiderabledegradation
taken
has
the ecosystems
Place.
I
hal
'eu
(M
intt
sev
mo
ma
(Ta
reg
frer
dzu
wo
Au
rai
de
Vri
un
for
se
of
Fi
Ja
At
pa
so
an
wi
off
be
ha
cli
af1
w(
wl
Australiansavannas
erpredictactual
tpletionof foliar
y limit primary
rmentthat limits
rf grassmaterial
rsandthis could
A.lthough
fire is
of muchof the
nportantpart in
fluencedby the
r boththe more
baceouswoody
removalof the
hegrowthofthe
opicsandsemisoilsoccupying
e Monsoonand
h togetherwith
reflectedin the
ces in nutrient
the competitive
e responses
are
75
'dystrophic'savannas
of HuntleyandWalker( 1983),bothproductivityanddensity
In the tallgrassregionor
have been limited by the low herbagequality in the drier months. This contrastswith the drier or more
'eutrophic'savannaswherefeedquality is maintainedat a higherlevel for longerand quantity is restricted
(Mott and Tothill, 1984). Technologyhas been developedto overcomethis natural regulationof grazing
in the more
intensity and the increasedgrazingpressurehas led to an instability of the graminoidunder,storey
mortalityof thegrassplantsis
Sincepreliminarystudiesindicatethat defoliation-induced
severeenvironments.
more severein the early wet seasonin the Monsoon Tallgrasssavannas(Mott, unpublisheddata) this death
may be influencedby the growth strategiesof the perennialgrassesin thesenutrientstressedcommunities
(Table7).
All the subhumiclsavannasare fire-resistantto a large degreeand both herbaceousand woody species
regenerateafter the normal annualfire regime.But in the north there remainsa concemthat the presentmore
major
frequentfire regimesmay causea thinningand breakdownof the tree layer.In the semi-aridsavannas,
resist
grass
to
damage.
appear
species
fires,
although
woody
species
by
episodic
the
to
damagecan be caused
In iheir comparisonof the agriculturalimplications of soils and climate betweensavannaregionsof the
world, Williams et al. (1984) found considerablesimilarity betweenthe low latitudetropical savannasof
Australia, and thoseof Sahelianwest Africa and northern India. They noted however,that althoughsummer
over shorterperiodsit was much less reliablethan elsewhere.The
rainfall in Australia was concentrated
detailedstudy on factorswhich influencethe primary productivity in Sahelianpasturesreportedby Penningde
by Bremenand de Wit (1983) showsthat herbageproductionis
Vries and Djiteye (1982) and summarised
undersimilar constraintsto thosein Australia, with nutrients,particularly P, beingthe primary limiting factor
ior herbageproductionover most areas.With thesesimilar constraintsto production it is not surprisingthat
secondaryproductionas reflectedby ungulateproductivity hasparallelresponsesto Australia, with the quality
of herbagebeingmostimportantin the wetterareaswhile quantityis the prime constraintin the drier regions.
LAMPTO
(7oN)
Monsoon Tallgrass
eycommunities
, 1955;Tothill,
x 05
u
z
lion
0
June
u
June
Jan
;
:alingsoils
Dec
F
ROMA
(250S)
Midgrass
I
F
0.5
TY
Jan
-----
heformationof
vannas
appears
rishaschanged
hen they have
gionsof Africa
;with the wideredremarkably
degradation
of
June
WATER
TNDEX
Jan June
Oec
GROWTH
INDEX
Dec
ffi'"'Ji.H''-f
Figure E. The water index/growthindex andtemperatureconstraintas a functionof weeknumbercommencing
Januaryeachyear for two African sites(Lampto, on the Ivory Coast,and Nylsvley in SouthAfrica), and two
Australiansites(Coen and Roma) of similar latitudes.
The modelusedto estimatepotentialproductivity within Australian savannasprovidesa usefultool for the
that in
It demonstrates
in the'natural'grasslands.
ofenvironmentalconstraints
partitioningandunderstanding
somesoil/climatesystemswater will alwaysbe secondaryto nutrients.In other climatesand soilsthe nutrients
andwaterwill interactso that in someseasonsnutrientswill be the main controllingfactor, whereasin othersit
will be water andtemperature.It is clearthat simulationof the dynamicinteractionbetweenwater andnutrients
offersonemethodthat can assistour understandingofthe functioningofsavannaecosystems.Further data are
beinganalysedand as an initial step to evaluatefunctionalrelationshipsbetweencontinents,growth periods
havJbeenialculated for both Nylsvley and Lampto sitesin Africa (Figure 8). Thesedata showthat in termsof
by the wettest
climateNylsvley falls into the Australian Midgrasssystemwhile Lampto would be characterised
areasof the Monsoon Tallgrassregion. Further analysisof actual primary production data from these sites
wouldenablemuch closerparallelsto be drawn betweenthe functionalcharacteristicsof thesesites.
Uniquely,Australia has uniform sovereigntyover a wide rangeof relativelyundisturbedsavannatypes. As
with other continentsthe major factors controllingplant growth in theseregionsare the seasonalityof growth
76
Ecology and Managementof the World's Savannas
due to both moisture and temperature limitations and the extremely low levels of available nutrients in the soil.
Although some input has been expended in obtaining and analysing information of the climatically controlled
constraints to growth, we have very little information on either the size of the main nutrient pools or the flux
rate of these nutrients through pastoral or agricultural systems. With the great diversity of savanna systems
available for study it is to be hoped that future research priorities will enable Australia to obtain the basic
information neededto understandthe functional processesthat control both production and stability over these
regions and relate the information to efficient land use in like areas throughout the world.
ACKNOWLEDGEMENTS
The senior author would like to acknowledge the many fruitful discussions he has had on aspects of
Australian savannaecology with many workers in the sphere.Especially he recognisesthe long associationand
lively interchangeof both information and ideas with Dr. J.C. Tothill. The generosity of Dr BiHuntley and Dr
J.C. Menaut in supplying African data from Nylsvley and Lampto sites as well as Dr J.G. Milvor for
Australian data for Redlands and Lansdown is also gratefully acknowledged.Finally all authors would like to
acknowledgethe assistanceof Dr R.L. McCown who both provided us with accessto his water balance records
and participated in extensive discussionswith us during the preparation ofthe manuscript.
B
Br
C
Cr
cl
CI
CI
CI
Ct
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