Primary Production of a Grassland in Nairobi National Park, Kenya
Author(s): I. Deshmukh
Source: Journal of Applied Ecology, Vol. 23, No. 1 (Apr., 1986), pp. 115-123
Published by: British Ecological Society
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JournalofAppliedEcology(1986) 23,115-123
PRIMARY PRODUCTION OF A GRASSLAND IN NAIROBI
NATIONAL PARK, 'KENYA
BY I. DESHMUKH
ofNairobi,Nairobi,Kenya
ofZoology,University
Department
SUMMARY
(1) Net aerial primaryproductionof a grasslandin Nairobi National Park was
inthethree
estimatedusingtheharvestmethodin conjunctionwithestimatesofmortality
mostabundantgrassspecies.
(2) As a resultof the long rains (March-April)netprimaryproduction(drymatter)
was 651 g m-2and as a resultof theshortrains(October-December),420 g m-2. These
estimatesweremorethantwicethepeak biomassobservedin each season.
Themeda
in theirperformance.
(3) The threemost abundantgrass species differed
triandraForsk. had the highestnet productionrelativeto its biomass and the highest
mortalityrate in all seasons. PennisetummezianumStapf. was lowest in both these
respectsand SetariaphleoidesLeeke intermediate.
(4) Removal by large herbivoreswas too small to detect by exclosuremethods.
Calculationsbased upon populationdensityoftheseanimalssuggestthattheyconsumed
less than4% ofannualnetprimaryproduction.
(5) The grasslandstudiedwas fairlytypicalof otherswithsimilarrainfallin East and
southernAfricawithrespectto peak herbaceousbiomass and averagelarge herbivore
populationdensity.
oftheseresultsformanagement
oftheparkare discussed.
(6) Some implications
INTRODUCTION
production(above ground)in
This studyis thefirstpublishedestimateoftruenetprimary
with
an East Africangrassland.Previousworkhas estimatedpeak biomassof vegetation,
species,and equated this with
littleattentionbeingpaid to the dynamicsof different
of grazingresourcesis important
primaryproduction.However,a properunderstanding
of a conservationarea such as NairobiNationalPark whichis
forpropermanagement
mammals.
becauseofitspopulationsoflargeherbivorous
maintained
primarily
Milner& Hughes (1968) definednet primaryproduction(Ps) in grasslandsforany
periodas,
Pn= AB + L + G,
whereAB is thechangein livingplantmass (biomass),L theloss ofbiomassdue to death
animals.For above
ofplants(or plantparts),and G theloss due to removalbyherbivorous
of
harvests
the
successive
AB
is
estimated
plants.In many
by
usually
groundorgans,
Most studiesof
for
enclosures.
G
been
accounted
Africanstudies has
usinglarge-mammal
&
Joshi
IBP
L
of
have
1979), even
studies,
Singh
ignored (review
tropicalgrasslands
since
the
work
of
has
been
in
its
Wiegert&
regions
apparent
though significance temperate
in
African
savannas
of
herbaceous
the
few
estimates
Evans (1964). Among
production
Guinea
those
in
the
are
account
of
whichtake
savanna,Nigeria(Ohiagu&
plantmortality
&
of
Coast
the
savannas
Wood 1979),
Lamto
Ivory
(Menaut Cesar 1979) and Nylsvley,
&
Toit
SouthAfrica(Grunow,Groenveld Du
reportof thepresent
1980). A preliminary
Presentaddress:USAID, JubaValleyProject,c/o EmbassyoftheUSA, Mogadishu,Somalia.
115
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116
GrassproductioninAfricansavanna
workis theonlypaperto giveestimatesofL forEast Africa(Deshmukh& Baig 1983). In
mostotherAfricanstudiespeak biomass(liveplants)or peak standingcrop (live + dead
plants)have been used as estimatesof netprimaryproduction(compiledby Deshmukh
areas of even such simplemeasuresare difficult
1984). Comparisonsbetweendifferent
because in several cases vegetationwas not clipped at groundlevel, but at some
predetermined
height2-20 cm above the soil surface (e.g. Serengeti:Braun 1973;
& Pigott1978; Nylsvley:Grunow,Groenveld& Du Toit 1980). These
Mweya: Strugnell
estimatesof standingcrop assumedto be availableto
variousclippingheightsrepresent
to understand
theproductionecologyofthe
largegrazingmammalsratherthanattempts
vegetation.
The presentpaper has threemain objectives.First to estimatetotal net primary
productionabove groundof a grasslandin Nairobi National Park, Kenya. Second to
assess theerrorsinvolvedin thevariousmethodscommonlyused to estimatenetprimary
theimportance
Thirdto estimate
production,
butwhichtakeno accountofplantmortality.
mammals.
ofgrassconsumption
bylargeherbivorous
STUDY AREA
10 kmsouthof Nairobi
NairobiNationalPark coversan area of 115 km2approximately
city(latitude1020'S, longitude36050'E, altitude1600 m). Most of theparkis savanna
grasslandwitha dryclimate(sensu Deshmukh1986). Mean annualrainfallin thecityis
850 mm,fallingmainlyin two seasons (Fig. ic), but withlargevariationsfromyearto
in Nairobiis 19.6 OC withmean monthlymaximaand
year. Mean annualtemperature
The park is a dry-season
minimain the ranges 23-28 and 12-14 OC respectively.
area for large herbivorousmammals,most of which disperseinto the
concentration
oftheyear.
Athi-Kapiti
plainsto thesouthand eastfortheremainder
The studysite was situatedclose to the centreof the Park. Perennialtuftedgrasses
Harms
Acacia drepanolobium
butthereare also scattered
comprisemostofthevegetation,
bushes.Of the 233 quadratssampledduringthe study,the frequencyof occurrenceof
grass specieswas: ThemedatriandraForsk. in 85%, Setaria phleoidesStapf.in 71%,
PennisetummezianumLeeke in 21%, Digitaria macroblepharaStapf.in 5%, Panicum
coloratumL. and Ischaeumafrum(Gmel.)Dandy in 3% andAndropogon
gayanusKunth
in 1%. Forbs were foundin 43% of quadrats,the major species beingthe perennials
Indigofera tettenisKlotz. and Commelina africana L. and the annual Ocimum
americanumL. The soils are black clay vertisols(locallyknownas 'black cottonsoils')
whichare oftenwaterlogged
duringtherainsand dryand crackedin thedryseasons.Such
soils are widespreadin the Acacia-Themeda savannas typical of the East African
highlands(Edwards & Bogdan 1951). The vegetationof the studysite was burned
on 26 September1978 (P. P. N. Keiyoro,personalcommunication).
accidentally
METHODS
at 4-6-week
Samplesof herbaceousstandingcrop wereharvestedfromthreetreatments
randompatternfromeach of open
intervals.Ten samples were taken in a stratified
grassland,fromwithinmoveable weldmeshcages (1.2 x 1.2 m) and fromwithina
20 x 20 m chain-link
exclosure.The open grasslandand small
large-mammal
permanent
cage samplesweretakenfromwithinthe same 1 ha square adjacentto the permanent
exclosureand thecages weremovedto newrandomlocationsaftereach sampling.All the
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I. DESHMUKH
117
vegetationin a 0.25 x 0.25 m quadrat was clippedat groundlevel and placed in a
intolivegrasses,forbs
polythene
bag. Theseharvestsamplesweresortedin thelaboratory
and combineddead material.Because of theirsporadicoccurrence,
grassesotherthanT.
triandra,S. phleoidesand P. mezianumand thevariousspeciesof forbsare combinedin
in theremainder
ofthepaper.Each
thecategories'othergrasses'and forbss',
respectively,
was driedat 105 IC to constantweight.
component
has beendescribedelsewhere(Deshmukh
The methodused to estimategrassmortality
clustersof shootsof thethreemostcommonspeciesweremarked
& Baig 1983). Briefly,
withcottontags.Ten clustersofeach specieswereso markedin theopengrasslandand in
exclosureat thetimeof each harvestsample.Dead tissueswerecarefully
the permanent
removed.Whenthenextharvestwas taken,themarkedshootswereremoved,sortedin to
liveand dead portions,
oven-dried
and weighed.
Removalby largeherbivores
(ingestionplus otherlosses due to feedingand trampling)
was estimatedby comparisonof standingcrop in the open grasslandwiththatin the
whether
totalabsence
exclosurewas erectedto determine
moveablecages. The permanent
of large grazersaffectedprimaryproductionover the studyperiod.Consumptionby
was not estimated.Qualitativeobservations
smallerherbivores
suggestedthatrodentand
Suchis
to removesignificant
insectpopulationswerelow and unlikely
primary
production.
termites
were
the case in manyotherAfricansavannas (Deshmukh1986). Litter-feeding
affect
estimates
ofthemassofdead grass
absentfromthestudyarea and do not,therefore,
(Deshmukh1985).
Rainfalldata are averagesof dailytotalsforfourstationssituatedaroundtheedge of
NairobiNationalPark (Warden'sCamp to thewest,WilsonAerodrometo thenorth-west,
and CheetahGate to thesouth-east).
KenyattaInternational
Airportto thenorth-east
RESULTS
Figure1 showstheseasonalchangesin herbaceousstandingcropforthethreetreatments
combined,and monthlyrainfalltotals.Combineddata are used because therewere no
differences
betweentreatments
significant
(analysesof variance;P > 0.05) exceptin one
instance(see below).A notablefeatureis thepredominance
of dead materialthroughout
the study(Fig. la). The 'long rains' (March-April)produceda muchhigherand more
prolongedbiomasspeak thanthe'shortrains'(October-December)eventhoughthetotal
rainfallwas only slightlygreater(see also Table 1). The different
species and typesof
plantsshowedsimilar,but not identicaltimingof peak biomass (Fig. lb), althoughthe
proportionalincreases differed.The ratio of peak biomass (June or July) to the
March-Apriltroughwas 4.5 forT. triandra,3 0 forS. phleoides,3 1 forP. mezianum,
3.0 forothergrassesand 1.9 forforbs.Statisticalerrorsfortheseparateplanttypeswere
forthoseoccurring
oftheir
quitelarge,particularly
sporadically.
Expressedas percentages
means on each samplingoccasion, the standard errors of the various vegetation
componentsthroughthe year were: T. triandra 10-17%, S. phleoides 16-23%, P.
mezianum20-47%, othergrasses26-43% and forbs16-49%. Thus resultsforcategories
otherthanthetwodominantgrassesmustbe viewedwithcaution.
of threegrass speciesbetweensamplingdates.
Figure2 shows the rate of mortality
Whilstvariation(particularly
of T. triandra)was largein some seasons,thereis a distinct
in T. triandra,followedby S. phleoidesand thenP. mezianum
patternofhighestmortality
rate
Mortality
(averageratesovertheyearwere33, 20 and 14 mgg-1day-' respectively).
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118
GrassproductioninAfricansavanna
600
(a)
500
400 -
300 -
200 -
100
p
F
0 Ll _
? 250 (b)
l
c
'
'
*
*
*
*
*
,, 200 -
150 -
100
50
-0'
I;. 00-
-~~~~~~~~~~~~~~~~
-~-
0
1
1 50
(C)
E
rr
100
50-
0
J
F
M
A
M
J
J
A
S
0
N
1980
0
J
1981
FIG. 1. Seasonal changes in dry weight of herbaceous vegetation and of rainfall in Nairobi
National Park. (a) Biomass of living plants and standing crop of dead vegetation; (b) biomass of
various components of vegetation; (*), total of grasses; (0), Themeda triandra; (a), Setaria
phleoides; (A), Pennisetum mezianum, (V), other grasses; (0), total of forbs; (c) monthlyrainfall
totals, mean of four stations (see text). Where present,vertical bars are standard errors of means.
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I. DESHMUKH
119
TABLE 1. Estimatesof seasonal and annual dry-matter
production(g m-2) in
NairobiNationalPark,for 1980. (a) Estimatesbased upon peak mass (aggregate
peak biomassis thesumofdifferent
plantcategoriesfromFig. lb). (a) Net primary
productionas thesum of biomassincrement
(AB) + mortality
(L) forthreegrass
speciesand biomassincrement
ofothergrassesand forbs.
Longrains
Shortrains
Wholeyear
668
294
332
597
160
160
1265
454
492
455
103
9
250
146
4
705
249
13
42
42
7
13
49
55
1071
(a) Peakmass
peaktotalstanding
crop(live+ dead)
peakbiomass(liveonly)
aggregate
peakbiomass(liveonly)
(b) Netprimary
production
AB + L
Themedatriandra
Setariaphleoides
Pennisetummezianum
AB only Othergrasses
Forbs
Total
60f50
40
40
30cD
20''
10
..
M
1980
A
M
J
J
A
S
O
N
D J
1981
in threespecies of grass fromNairobi National Park
FIG. 2. Averagedaily rates of mortality
P. mezianum.Dry weightdying(mg) per
(mean + SE). (E), T. triandra,(4), S. phleoides;(MM),
ofa period,perday.
g dryweightofbiomassat thebeginning
was not statistically
correlatedwithrainfallin any of the species.However,the highest
ratesdidoccurinthelongdryseasonand thelowestduringthelong-rains
growth
period.
Removalby largeherbivores
was negligible
overthestudyperiodas a whole.The only
occasion upon whichtherewas a significant
difference
betweenopen grasslandand
moveablecage harvestsampleswas in January1981 when130 + 55 (SE) g m-2of dead
matterwas apparentlyremoved(t-test,P < 0.05). Duringthe same periodremovalof
livingplantpartswas notstatistically
significant.
Table 1 givesseveralestimatesof'netprimary
production'
as itis commonly
determined
in otherstudiesof Africansavannas.Note thatoverallpeak biomassand aggregatepeak
biomassfordifferent
planttypesgreatlyunderestimate
truenetprimary
production
which
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120
GrassproductioninAfricansavanna
includeslossesdue to mortality.
This last-namedestimate(AB + L) was calculatedinthe
mannerofthefollowing
example.The biomassof T. triandraincreasedby 119 g m-2 (AB)
between7 Apriland 15 June1980. Duringthe same period,estimatedfrommortality
samples,1 009 g had diedforeverygramoflivegrasspresenton 15 June.Since 153 g m-2
was harvestedon thatdate, 153 x 1.009 = 154 g m-2 (L) had died to givea totalnet
productionof 273 g m-2 duringtheperiod.Whenharvestedbiomassdecreasedby more
in a period,'negativeproduction'
thanthelossesdue to mortality
occurred.Thesenegative
giveninTable 1.
valueswereignoredin summation
ofseasonaland annualnetproduction
Whenmortality
is takenintoaccountit becomesapparentthatproduction
occursover
some parts of the year in whichbiomass appears to be declining(cf. Fig. lb). Thus
productionoccurredin March-April1980, In December1980-January1981 in all three
theyearinP. mezianum.
species,inJune-July
1980 in T. triandraand throughout
Net primaryproductionwas closelyrelatedto the rainfallreceivedbetweensample
dates. Takingthethreegrass speciestogether,
2.5 g m-2 of productionwas realizedfor
each 1 mmof rainfalloverthewholeyear.Duringthegrowingseasonsthisvalue had a
narrowrangeof 1.9-2.6 g m-2 exceptin June-July
when5.0 g m-2was producedper
ofrainfall.
millimetre
DISCUSSION
in NairobiNationalParkwith
Comparisonof truenetprimaryproductionabove-ground
otherEast Africanecosystemsis not possiblebecause otherstudiestake no accountof
In perennialgrasslandswheredead mattermay accumulateoverseveral
plantmortality.
years,peak standingcrop (live + dead) cannotbe used to estimateprimaryproduction.
in
The grasslandstudiedwas burnedin September1978 and dead plantmaterialharvested
1980 came fromseveralpreviousgrowingseasons (see Deshmukh1985). Peak biomass
(livingplants)in NairobiNationalParkis similarto othergrasslandsin East and southern
Africa.Comparedwith Table 1, the relationshipof Deshmukh(1984) predictspeak
biomassof 182 g m-2 fromthelongrains,135 g m-2 fromtheshortrainsand438 g m-2for
the whole year,giventhe rainfallreceived.However,peak biomass underestimates
net
primaryproduction
by almost120% (Table 1). This degreeoferroris fairlytypicalwhen
comparedwithotherstudies.In the Lamto savannas of Ivory Coast, the equivalent
underestimation
rangesfrom50 to 190% (Menaut& Cesar 1979). Substantialmortality
was also foundat Nylsvleyin SouthAfricawitha maximumrateof 3.7 g m-2 week-',
of 5.2 g m-2 in thesameweek(Grunow,Goenveld&
comparedwitha biomassincrement
net primary
Du Toit 1980). In temperategrasslands,peak biomass underestimates
productionby between10% (Desmukh 1979) and 350% (Wiegert& Evans 1964) with
most determinations
fallingin the range 50-150% (Williamson1976). Of the methods
where
the one used hereis simpleand appropriate
availableto estimateplantmortality,
thatthemarked
thereis a largeaccumulationof dead grass.The inherentassumptions,
ofthesward,thatno dead grassis lostbetweensamplesand that
shootsare representative
removalof dead partsdoes not affecttheshootsare discussedelsewhere(Deshmukh&
Baig 1983,Deshmukh1985).
in theperformance
of thegrassspeciesstudiedin detail
Thereweredistinctdifferences
withrespectto biomass accumulationand mortality
(Table 2). Besidesbeingthe most
abundant,T. triandrahad the highestrate of productionrelativeto its biomassand the
in both
P. mezianumwas lowestand S. phleoidesintermediate
highestrateof mortality.
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I.
121
DESHMUKH
2. A comparisonof the performance
of threegrass species in Nairobi
NationalPark in two seasons of 1980. Production:biomass
ratio(P/B) compares
net primaryproduction(includingmortality)
withmedianbiomass (mean of pregrowthand seasonal peak biomass) foreach growingseason. Mortality:biomass
ratio (LIB) compareslosses due to shoot death withmedianbiomass foreach
growingseason.Data used incalculationsare g drymatterm2.
TABLE
(a) Longrains
Themedatriandra
Setariaphleoides
Pennisetummezianum
(b) Shortrains
Themedatriandra
Setariaphleoides
Pennisetummezianum
P/B
L/B
4.9
25
1.9
4.1
16
1.0
5.7
4.4
2.0
5.4
4.4
1.0
these respects.The dominanceand highproductionof T. triandrais suggestiveof a
favourableenvironment
a conclusionsupportedby the
(despite the high mortality),
widespreadoccurrenceof thisgrass in similarenvironments
(Edwards& Bogdan 1951).
The species is importantin burnedareas (e.g. Trollope 1982) and probablyowes its
dominanceat thestudysiteto firesincluding
theone ofSeptember1978.Littleis knownof
the autecologyof these grasses and furtherwork on theirgrowthperformance
and
abilitiesunderdifferent
conditions
wouldbe rewarding.
competitive
Net productionwas greaterand more prolongedand mortalitylower duringthe
long-rainsgrowingseason thanduringthe short-rains
growingseason. These differences
are probablyrelatedto greaterwater stressfollowingthe shortrains. Januaryand
Februaryare the hottest,least humidand sunniestmonthson average(mean maximum
27-28 OC,relativehumidity
less than70%, almost10 h day-' ofsunshine).In
temperature
contrast,soon afterthe long rains,July-Augustare the coolest,most humidand least
sunnymonths(meanmaximumtemperature
22-23 0C, relativehumidity
80-90%, around
5 h day-' of sunshine).This climaticinformation
may also explainthe high rate of
productionper millimetreof rainfallduring June-July(see Results). Presumably
in thesemonthswas low enablingthe plantsto draw on the water
evapotranspiration
remaining
fromthelongrainsmoreeffectively.
whentheir
That moreremovalby largeherbivoreswas not detectedis not surprising
occupanceof thestudyarea is analysed.Accordingto totalcountscarriedoutbythepark
of largemammalianherbivores
and otherhelpers,total biomass (liveweight)
authorities
rangedfrom1130 to 19 020 kg km-2in thecountingblockswhichcoverthestudysite,
of
to biomassusingtheunitweights
duringthestudyperiod.Numbershavebeenconverted
Coe, Cumming & Phillipson(1976). Using a regressionpredictionrelatingdaily
of theseanimalsto thebodyweightof each species(Deshmukhunpublished
consumption
of herbaceousvegetationforthe year beginingFebruary
data), theirtotal consumption
and
1980 amountsto 40.6 g m-2. This quantityis lessthan4% ofnetprimary
production
less than 10% of the sum of peak grass biomassforthetwo growingseasons. At their
highestdensity(January-February
1981) theseanimalsare likelyto have consumedonly
2% of the total standingcrop of vegetation(live + dead) available to them.In such
it is unlikelythat exclosuretechniqueswill recordsignificant
circumstances
vegetation
is lowerby an orderof magnitude
thanis
removal.This calculatedaverageconsumption
oftensupposedin Africansavannas (e.g. Wiegert& Owen 1971). However,the mean
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122
GrassproductioninAfricansavanna
annualbiomassof largeherbivores
in thepark(8300 kg km-2) is typicalofmanyareas of
East and southern
Africainrelationship
to annualrainfall(see Coe, Cumming& Phillipson
1976). Thereis no doubtthatin some instancesmuchhighervegetationremovaloccurs
locallyand/orforshortperiods(e.g. Strugnell
& Pigott1978; Sinclair,Dublin& Borner
1985). Nevertheless,
underconditionsof averagerainfalland overthewholearea used by
it is unlikelythat average consumption
migratory
herbivores,
rates muchexceed those
forNairobiNationalPark(Deshmukh1986 andunpublished
estimated
data).
No accountof belowgroundproductionwas takenin thisstudy.Root production
has
been largelyignoredin Africansavannas, even thoughavailable data indicatethat
belowgroundproduction
may be higherthanthatabove ground(Menaut& Cesar 1979;
& Pigott1978). Underground
Strugnell
organsarelikelyto be ofgreatsignificance
in areas
firesandheavygrazing.An assessmentofrootproduction
subjectto frequent
is essentialto
of productionecology and of resiliencein Africansavannas and
an understanding
further
warrants
study.
NairobiNationalParkis maintained
as a protectedarea primarily
foritspopulationsof
largemammals.Clearly,an understanding
ofproduction
processesofgrassescan enhance
management
capabilities.For example,thecontinueddominanceof T. triandrais related
to thefrequency
offire(see above). Figure1 and Tables 1 and 2 suggestthatS. pheloides
may be increasingand T. triandradecreasingin abundance,in theabsenceof a firesince
1978. Since T. triandrais bothmoreproductive
and morepalatable(Edwards& Bogdan
1951),occasionalburningshouldimprovegrazingresources.Thisconclusionis reinforced
by thepresenceof a largeaccumulationof dead grassoflow nutritive
valueat thegrazing
intensities
observed.Unless thisgrassis burned,decomposition
and nutrient
cyclingare
veryslow(see Deshmukh1985).
ACKNOWLEDGMENTS
The Dean's Committee,Universityof Nairobi provided partial financialsupport.
to conducttheresearchwas grantedby theOfficeofthePresident,
Permission
theWildlife
Conservation
and ManagementDepartment
and theWarden,NairobiNationalPark,all of
the Kenya Government.
The latterkindlymade game countdata availableand his staff
helpedto constructthe permanentenclosure.Dr M. N. Baig helpedwithsome of the
to the manuscript.
sampling.Two anonymousreviewerssuggestedusefulmodifications
FinallyI wishto thankProfessorG. K. Kinotiforhisexampleand encouragement
whilstI
was a memberoftheZoologyDepartment
at theUniversity
ofNairobi.
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I.
DESHMUKH
123
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