University of Groningen
Biomass and shoot/root allocation of five species from a grassland succession series
at different combinations of light and nutrient supply
Olff, Han; Andel, J. van; Bakker, Jan
Published in:
Functional Ecology
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Publication date:
1990
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Citation for published version (APA):
Olff, H., Andel, J. V., & Bakker, J. P. (1990). Biomass and shoot/root allocation of five species from a
grassland succession series at different combinations of light and nutrient supply. Functional Ecology, 4(2),
193-200.
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Biomass and Shoot/Root Allocation of Five Species from a Grassland Succession Series at
Different Combinations of Light and Nutrient Supply
Author(s): H. Olff, J. Van Andel and J. P. Bakker
Source: Functional Ecology, Vol. 4, No. 2 (1990), pp. 193-200
Published by: British Ecological Society
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Functional
Ecology 1990,
4, 193-200
Biomass and shoot/rootallocation of fivespecies
froma grassland succession series at different
combinations of lightand nutrientsupply*
H. OLFF, J.VAN ANDEL and
J.P. BAKKER
Departmentof Plant Ecology,Biological Center,
Universityof Groningen,PO Box 14 9750 AA
Haren (Gn.), The Netherlands
Abstract. In recent literature the relative importance of interspecific differences in shoot/root allocation patterns and differences in growth rate have
been discussed, especially with regard to the
mechanisms of succession. We studied growth,
leaf area and shoot/root ratio under controlled
conditions at various combinations of light and
nutrient supply of five species from a grassland
successional series. This series was initiated by
stopping the application of fertilizers to agriculbut continuing hay-making.
tural grassland
Species from the later successional stages (lower
nutrient availability, higher level of light at the soil
surface) had a higher shoot/root ratio at most
treatments, contrary to the predictions of the
resource ratio hypothesis. However, a three-way
interaction was also found, and thus the combination of light and nutrient level determined
whether or not a correspondence between successional position and the sequence of species in
shoot/root ratio was found. Some possible explanations are discussed for these results with regard
to the mechanisms of succession.
plant
Allocation,light,mineralnutrition,
Key-words:
succession
resourceinteraction,
strategies,
Introduction
In recent debates on mechanisms which determine
patterns of plant community structure and the
course of succession, emphasis has been laid upon
* Nomenclature
ofspeciesfollowsHeukels,H. & Vander
Meijden, E. (1983). Flora van Nederland. Groningen;
V. & Den
of syntaxafollowsWesthoff,
Nomenclature
193
Held, J.J.(1969). Plantengemeenschappen van Nederland, Zutphen.
the relative importance of differentecological
factors (see Thompson 1987; Tilman 1987ab;
Thompson & Grime, 1988; Berendse & Elberse,
1989). Tilman (1988) hypothesized that the relative abundances of plant species change in
response to changes in the ratio of limiting
resources. Succession oftenproceeds by accumulation ofnutrientsin the soil, and when thisresults
in a higher standing crop (such as in old-field
succession), then a shiftfromnutrientlimitation
towards light limitation might be found. Therefore, during succession on a poor soil the early
successional species would be characterizedby a
higher allocation of available resources to the
roots,which should make thembettercompetitors
for nutrients. The later successional species
would, however, allocate more resources to stems
and leaves, which should make them better
competitorsforlight.This opinion contrastswith
Grime's (1979) view, who considered both a lownutrient level and a low-light level as 'stress
factors'which will select forsimilar lifehistories.
This view was supported by Berendse & Elberse
(1989), who found that species from relatively
poor successional stages had a higher shoot/root
ratio than species fromricherstages,thus contradicting the expectations of the resource ratio
hypothesisof Tilman (1988). The aforementioned
theories have been based on a fixed allocation
patternofresourcesto the rootand shoot,thoughit
has long been known thatthis allocation patternis
very plastic, and especially dependent on light
and nutrient levels (Brouwer, 1962a,b; Wilson,
1988). Tilman (1988) has argued that,on the basis
of simulation results, such interspecific differences in plasticitywould notalterhis conclusions.
However, until now this view is not supported by
any experimental evidence. Berendse & Elberse
(1989) determinedthe dependence of the shoot/
rootratioon the nutrientlevel, but did not include
the interactioneffectwith light. This interaction
however seems crucial to the mechanisms proposed by Tilman (1985, 1988), since a resourceratioeffectcan be experimentallytranslatedinto a
resource-interactioneffect (cf. Peace & Grubb,
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194
H. Olffet al.
1982; Van Baalen, 1982). When the statistical
interactioneffectbetween two resources is significant,this implies a significanteffectofany shiftin
theirratio.
The aim of the present study was to determine
the growth and shoot/root ratio allocation of
species from a grassland successional series,
grown individually at differentcombinations of
light *and nutrient supply, and to relate the
growthand allocation pat(treatment-dependent)
terns to the successional position of the species.
Five species froma grassland successional series
were used. This series was initiated by stopping
fertilizerapplication to agriculturalhay-fields,as
partofrestorationmanagementpractices (Bakker,
1987, 1989). This change in managementpractice
has led to a restorationof species-rich communities fromswards which were formerlydominated
by only a few species. The decrease in the availabilityofnutrients(Pegtel,1987) was reflectedby a
substantialdecrease in the above-groundstanding
crop, resultingin an increased lightavailabilityat
the soil surface (Bakker, 1989). This implies that
the directionofsuccession in thiscase is in reverse
order,as described forold fieldsby Tilman (1988),
viz. froma nutrient-rich,
low-lightto a nutrientpoor, high-lightcommunity.However, any theory
about the mechanisms of succession should not
depend on the directionof change.
The vegetationdevelopment of these hay-fields
was recorded duringa 16-yearperiod. These data
will be used to quantifythe successional positions
of these species, which will be related to their
growthand allocation patternsat various combinations of lightand nutrientsupply.
Materialsand methods
Species
Five species were selected forthe present experiment(Table 1); selection was based on differences
in ecological amplitude. Succisa pratensis
Moench is a perennial rosette-forming
species,
while the remaining four species are perennial
grasses. Lolium perenne L. is a characteristic
species of fertilizedmeadows and pastures,while
the otherthreegrass species are found under more
nutrient-poorconditions (Kruijne et al., 1967). S.
pratensis is only found under clearly nutrientpoor conditions (Pegtel, 1986).
Successional position
The experimentalspecies occur in fieldswhich are
situated in the Nature Reserve 'Stroomdallandschap Drentsche A' (53 0N,60 40'E). The management changed from cutting twice a year and
fertilizing(100-200kg Nha-1 year-1) to cutting
once a year (July) without the application of
fertilizers.The vegetation composition thereafter
changed from a Poo-Lolietum via a LolioCynosuretumto a Festuco-Cynosuretum.Standing
crop changed from1000 to 300gm-2 (see Bakker,
1989 forfurtherdetails). The temporal change in
theabundance ofspecies overa 16-yearperiod was
compared to a time-sequence of fields forwhich
timeintervals.
fertilizingwas stopped fordifferent
This comparison made it possible to separate
non-successional fluctuationsin cover percentage
(caused by e.g. extremeweather conditions) from
directional (successional) changes. Five fields
were compared, for which the duration of the
managementpractices (hay-makingwithoutfertilizer application) varied from16 to 42 years. The
cover percentage of all species present was
determinedeach yearin Junefrom1972 until 1988
in several permanent plots in each field. In this
analysis only the moist parts of each field were
used, since the experimental species were frequentlyfound in this habitat (Bakker,1989).
The combined data ofthechanges overthestudy
period and the spatial variation of fields which
were no longer fertilizedfor differenttime intervals, were used to draw smooth curves of the
changes in the first42 years afterthe cessation of
fertilizerapplication. However, an increase or
decrease over the experimental period was only
drawn in Fig. 1 if this change was significantly
differentfrom zero. This significance of change
Table 1. Seed sources ofthe experimentalspecies.
Species
Abbreviation
Population
Location
Anthoxanthumodoratum
Cynosuruscristatus
Holcus lanatus
Lolium perenne
Succisa pratensis
AO
CC
HL
LP
SP
Burgvallen
Burgvallen
Burgvallen
Gasteren
Appelbergen
53 ON60 40 'E
53 ON60 40'E
53ON 60 40'E
53ON 60 40'E
530N 60 37'E
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195
100
Allocation of
80
froma
grassland
6
60
400
plantspecies
succession
series
0
C
X
20
8hoxanthum
succlsa
cynosurus
.
'A
Lo/,um
10
20
30
40
The three nutrientsupply rates were combined
with two irradiances of 130 and 65 Wm-2
respectively, over an 18-h photoperiod, in a
factorialdesign. All plants were harvested after2
months. The total leaf area of each shoot was
determinedusing a Li-Cor photoelectricleaf area
meter. The shoot and root dry weight were
determinedafterdryingat 800C for48 h.
Years after cessation of fertilizer application
Fig. 1. Changes in coverageof five species froma
grasslandsuccessionalseriesafterthecessationoffertilizerapplication.Smoothedcurvesweredrawnforeach
speciesbycomparing
observedtemporal
changesduring
a 16-yearstudyperiodbetweenfieldswhichwerenot
fordifferent
timeintervals
fertilized
(seetext).Theoldest
since1946.
fieldwas notfertilized
over the 16-yearstudy period was not tested in a
simple regressionmodel, since the differencesin
absolute coverage between species could resultin
scaling errors. These scaling errors could be
enlarged by single-year deviations (caused by
fluctuationsin weather) fromsuccessional trends
(cf. Fresco et al., 1989). In order to test the
significance of successional trends the relative
cumulative cover was computed and the significance of change was tested as the second order
regression coefficientin a stepwise regression
analysis with the linear coefficientalready in the
model. All permanentplots in which the summed
coverage of a species ever the study period was
lower than 5% were excluded fromthe analysis.
Climate chamber experiment:growthconditions
The seeds ofeach species were cold stratifiedfor3
monthson wet filterpaper at 5 0Cand subsequently
germinatedat 25/15TC.The seedlings were transferredto pots, one seedling per pot, each containing 800g ofsilversand containing1% (v/v)potting
soil, 1% perlite and 1% peat. Three different
nutrient supply rates were created by adding
differentdilutions of a nutrientsolution twice a
week, as described by Biere (1990). The nutrient
supply ratescorresponded to totaladditions of10,
40 and 160mg N per pot, respectively. Previous
experiments (A. Biere, personal communication)
justifiedthe expectationthatthe biomass accumulation of these species would be a linear function
of N-supply rate, expressed on a doublelogarithmicscale.
Ten replicates per species per treatmentcombination were grown in climate chambers at
23/17TC.Lightwas supplied by fluorescenttubes.
Analyses of variance
The significance of the effectsof light,nutrients
and theirinteractionwas firstcomputed in a full
factorialdesign. The relative contributionof each
effectin this design was assessed as its relative
contribution(%) to the explained variance in the
dependent variable (Winer, 1971). All variables
except the shoot/rootratio were log transformed
prior to analysis, to reduce inhomogeneity of
variances.
It was investigated whether or not the
(treatment-dependent)sequence of responses of
the species in the climate chamber experiment
corresponded to theirposition in the successional
series. This was tested by specifyingpolynominal
contrastsbetween the species. The 4 degrees of
freedomofthefactor-specieswere subdivided into
two terms,where the firsteffect(1 d.f.)represented
the linear relationship between the species
responses and their successional positions. The
second term(3 d.f.) representedthe pooled effect
of all higher-orderpolynomials. The parameter
estimate of the linear polynomial coefficientwas
computed foreach treatmentcombinationby nesting the linear polynomial effectwithin the interaction termLight x Nutrients(Winer, 1971).
Results
Successional position
A good correspondence was found between the
spatial variationin mean coverage ofthe species in
durationofsuccession (up to
plots with a different
42 years) and the changes with time computed
fromdata of monitoringover a period of 16 years.
Smooth curves could, therefore,be drawn of the
changes in the cover percentage of each species
over a 42-yeartime period (Fig. 1). At the moment
of cessation of fertilizerapplication, Holcus lanatus L. was the most abundant species. During the
following 10 years, L. perenne disappeared from
the vegetation,while Anthoxanthumodoratum L.
reached its maximum coverage after these 10
years. H. lanatus decreased duringthe whole time
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196
H. Olffet al.
interval of 41 years. Cynosurus cristatus L.
reached its maximum after30 years. S. pratensis
did not occur in the studied permanentplots, but
was, however,locally abundant in the oldest field.
This species is thereforeassumed to be the last
species in this successional sequence. The relative
successional position of these five species was
determinedfromFig. 1 by combiningthe data on
the maximum and magnitude of change. This
resulted in the followingsequence: L. perenne >
H. lanatus > A. odoratum > C. cristatus > S.
pratensis. This sequence will be used in the
following analysis of the relationship between
successional position and allocation parameters.
shoot/rootratioincreased withthenutrientsupply
rate and decreased with the level of irradiance
(Fig. 2). Like the shoot dry weight, the leaf area
mainly increased with the nutrientsupply rate
(55% explained, Table 2, Fig. 2).
The interactionSpecies x Irradiance was significantforthe root dryweight,shoot/rootratioand
for the leaf area, with the largest effecton the
shoot/rootratio. The Species x Nutrientinteraction was significantforthe shoot/rootratioand for
leaf area. The interaction Species x Nutrient x
Irradiance was only significantforthe shoot/root
ratio (Table 2).
Treatment-dependentspecies - differencesin
relation to the successional sequence
Factorial ANOVA
The aforementionedSpecies x Treatmentinteractions mightbe interpretedas the dependence of
species differenceson the treatmentcombination.
As described in the introduction,we are particularly interestedin a special kind of species differences, namelywhetherthe sequence ofresponse of
the species is related to their sequence in the
successional series. To investigatethe statistical
significanceof this relationship,the effectof the
factor Species was partitioned into two polynomial effects.The firsteffect(Spe[1]) represents
the linear polynomial correlation between the
response in the dependent variable and the
successional order (Table 3). The parameterestimate of this effectwas computed for each treatment combination and is given in Fig. 2.
The Spe(1) effectexplained a significantpart of
the variance in all traits(Table 3). For the shoot dry
weight, this effectrepresented a negative correlation between the sequence of response and the
sequence in succession, in all treatmentcombinations (Fig. 2). However, thiswas clearlycaused by
The resultsofthe factorialanalyses ofvariance for
assessing the effect of irradiance and nutrient
supply rate on the shoot dry weight, root dry
weight, shoot/rootratio and leaf area of the five
species are listed in Table 2. The treatmentmeans
foreach species are presented in Fig. 2. The main
effectswere significantforall characteristics,but
the magnitude of the effectswas different.The
species effectwas verylargeforall traits,except for
the shoot/root ratio. The shoot dry weight
increased with the nutrientsupply rate (Fig. 2)
which explained 40% of the total variance (Table
2). The increase in shoot dryweight,caused by a
higher level of irradiance (Fig. 2) was much less
pronounced (1% explained, Table 2). For the root
dry weight,the irradiance effectand the nutrient
effectexplained an equal portionof total variance
(17 and 13%, respectively). Since the relative
impact of nutrientsand irradiance was different
forthe root and the shoot, both factorshad a large
impact on the shoot/root ratio (Table 2). The
Table 2. Factorial analysis of variance of the effectof the level of irradiance and nutrientson various weight and
allocation parametersof fivegrassland species.
Shoot dryweight Root dryweight
Effect
d.f. F
Nutrients
Irradiance
Species
NutrientX Irradiance
Species x Irradiance
Species x Nutrients
Species x Irradiance x Nutrients
Error
2
1
4
2
4
8
8
1loNS
(0 02)
0
Leaf area
% Var. F
% Var. F
166 3***
152 7***
33653***
24.0***
3.5**
12 6
173
63.9
52
o.9
1*6NS
0.1
302
36 8
4.4
12 4
3*8
6*3
61
% Var. F
684.3*** 39 9
12 9*** 1 0
376 1*** 54*8
20.1*** 3 4
2*4NS
03
3 ONS
06
Shoot/rootratio
1loNS
(0.02)
0
170O0***
138.3***
10.8***
27 0***
5.3***
8 0***
3.3**
(035)
% Var.
1122 5*** 55 0
37.2*** 2 7
302 7*** 36 9
5 9**
08
07
3.5**
14 8*** 0 7
0.7
1l9NS
(0 02)
F-values are given, with theirlevel of significance:* P < 0 05; ** P < 0 01; *** P < 0 001; NS, not significant.The
percentageofthe totalvariance explained was computed foreach effect.The within-cellserrormean square is given for
priorto the analyses.
each effect.All variables except the shoot/rootratio were log-transformed
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Low
197
Allocation of
Nutrients
0)
plant species
10
10
series
1
Intermediate
~~~~~~10
0.1
~~~~~~0.1
b )
()
100
-
-.48***
10
-4
10
*
10:
0) 0.1
0.1
10
1
0.1.
0.1
~~~~~~0.01
0.01
10
0.01
10
10
1
0
1.1
0~~~~~~~~~~~~~~~~~~~~~~~~~~(
0.01
-.59***
10
-.56***
4.6***
.48***
-4
Nutrients
01
1'? (
Q)
High
Nutrients
0
0.1
0.1
0
E 12
O
0
'02
W
I30T
[[
Dt~~~~~~~~~~~~1
<
(a
2
o
0
Fig.
2.
S
ty
i
--
o
0
0
r
(,o
3
4
g.90**
2
1
U)
0
)
-.8
Eioo
10
o3
Q)
.68*.47(
4
3
3
2
2
01
0
-~~~~~1
4
Ft
rq
'-100 (
0
1000
-4*
100
1000
100~F1K'
10
10
1
1
0
o- 1000
1000
100
100
100
10
10
10
'
QI)
-.32
-j~~~~~~~~~~~~~~~~~~~
LP
HL AO CCSP
LP
HL AO CCSP
f
63*
LP
HL AOCC
SP
Species
Fig. 2. Shoot dryweight (a), root dryweight (b), shoot/rootratio (c) and leaf area (d) of fivespecies froma grassland
successional series at various combinationsofirradianceand nutrientsupply rate.For each treatmentcombination,the
linear polynomial coefficient(with level ofsignificance)is presentedas a testforthe correspondencebetween orderof
response and orderin the successional sequence (see text).The species are orderedfromearlyto late accordingto their
occurrenceafterthe cessation offertilizerapplication (see Fig. 1). Levels ofsignificance:* P < 0 05; ** P < 0 01; *** P <
0-001. Species abbreviations:LP, Lolium perenne; HL, Holcus lanatus; AO, Anthoxanthumodoratum; CC, Cynosurus
cristatus;SP, Succissa pratensis.
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198
H. Olffet al.
theverylow shoot dryweightat all treatmentsofS.
pratensis; the other species did not show a negative trend.The same resultwas found forthe root
dryweight (Fig. 2).
For the shoot/rootratio,the interactionSpecies
x Nutrients x Irradiance (Table 2) could be
explained by the interactionSpe(1) x Nutrientsx
Irradiance (Table 3). This suggeststhatthe magnitude of the correspondence between the order of
response and successional position depended on
the treatmentcombination. The shoot/rootratio
was forall but one of the combinations positively
related to the successional position. Only at the
combination(low irradiance- high nutrients)was
the polynomial coefficientnot significant(Fig. 2).
In the case of positive correlations, all species
showed a trendtowards a highershoot/rootratio
forlatersuccessional species.
The significantSpe(1) effectforthe factorleaf
area (Table 3) could be interpretedas a negative
relationship between the order of response and
successional position (Fig. 2). L. perenne was an
exceptionto thispattern;thisspecies had a smaller
leaf area than H. Janatusat all treatmentcombinations (Fig. 2). The other four species showed a
distincttrend,which was not caused by just one
species. The interactionSpecies x Nutrientsand
Species x Irradiancewere both significantin their
effecton the leaf area (Table 2). These interactions
could be partiallyexplained by the corresponding
interactioneffectsof the linear polynomial effect
(Table 3). This suggests that the Spe(1) effect
depended on both individual factors,but not on
their combination. The magnitude of the correlation between the order of response and successional position increased both with the level of
irradianceand the nutrientsupply rate;the differences between species became more pronounced.
Overviewingtheresultsitcan be concluded thatin
general later successional species had a higher
shoot/rootratio, except at the treatment high
nutrients/low
light.Latersuccessional species had
in general a lower total leaf area.
Discussion
The direction of the relationship between shoot/
root ratio allocation and successional position
shown in most treatmentsofour experimentis the
same as reported by Berendse & Elberse (1989),
who also founda highershoot/rootratioin species
frompoorersuccessional stages. In our fieldstudy
area, Bakker (1989) also found an overall increase
in the ratio above-ground/below-groundproduction of the vegetation aftercessation of fertilizer
application. However, the interactionSpecies x
Nutrients x Irradiance was significant in our
experiment,and thisinteractionwas reflectedby a
deviance fromthe generalpatternin the treatment
highnutrients/lowirradiance.In thistreatmentno
clear relationship was found between shoot/root
ratio allocation and successional position. The
abiotic conditionsin the successional series which
startedafterthe cessation of fertilizerapplication
in the grassland communitiesin the Drentsche A
valley, probably changed from high to lower
availability of nutrients(Bakker, 1987) and from
low- to high-lightconditions on the soil surface
(Bakker,1989). The early stages might,therefore,
be compared with the treatmenthigh nutrients/
low light and the later stages with the treatment
low nutrients/highlight. Since no relationship
Table 3. Analysis ofvariance ofthe effectofthe level ofirradianceand the nutrientsupply rateon various weightand
allocation parameters of five grassland species. The effectof the factorSpecies (d.f. = 4) was partitionedinto two
polynomial terms,in which the firstterm(Species 1) representsthe linear relationshipbetween orderofresponse and
successional position (see text).
Effect
d.f.
Nutrients
2
1
Irradiance
1
Species(1)
3
Species(2)
2
Nutrientsx Irradiance
1
Species(1) x Irradiance
3
Species(2) X Irradiance
2
Species(1) x Nutrients
6
Species(2) X Nutrients
Species(1) x Irradiance x Nutrients 2
Species(2) X Irradiance x Nutrients 6
Error
Shoot dryweight
Root dryweight
Shoot/rootratio
Leaf area
684.3***
12 9***
598.1***
3152 * **
20 1***
166.3***
152 7***
732.9***
216O0***
24O0***
11-8**
1700***
138.3***
36.5***
2 NS
24O0***
5*9***
6.8*
4.8**
1122.5***.
0.5NS
3 1*
9 2***
0.8NS
1.gNS
0.6NS
0.5NS
1.1NS
1.2NS
(0 02)
(0 02)
F-values are given, with theirlevel of significance:* P < 0 05;
7.7**
0.6NS
0.8NS
**
P < 0-01;
8.2***
3.2*
3.4**
(0 35)
37.2***
493.8***
246.2***
0 NS
4 5**
25O0***
11.6***
l NS
1lgNS
(0 02)
P < 0 001; NS, not significant.
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199
Allocation of
plant species
froma
grassland
succession
series
between allocation pattern and successional
sequence was foundin theformertreatment,and a
negative relationship was found in the latter
treatment,these results suggest an increasing
importance of differencesin allocation patterns
with successional age. This was also found by
Tilman (1988) who reportedthat during old field
succession on an intermediatelynutrient-rich
soil,
the competitive relationships during the first
successional stages are mainly determined by
differencesin maximum growth rate among the
species already present(transientdynamics). The
faster growing species were better competitors,
because they could overtop slower growing
species. In latersuccessional stagesresource ratioand allocation-dependent competition increased
in importance. Tilman has argued that the duration of the period during which these growth
differencesdetermine the successional sequence
depends on the initial nutrientstatus of the soil.
On veryrich soil the fullsuccessional sequence is
probablydeterminedby these transientdynamics
(Tilman, 1988). Berendse & Elberse (1989) also
stressed the importance of growth differences
between species, but fromanother point of view.
They argued that the growth rate of grassland
species is directly related to their loss rate, a
relation which becomes increasingly important
with decreasing nutrient availability. In the
present study no clear relationship between biomass and successional position was found,but it
should be kept in mind thatrelative growthrates
have notbeen measured, and thatthe lightintensities in our climate chambers were probably lower
than under fieldconditions.
The highershoot/rootratio of species fromthe
poorersuccessional stagesis contraryto the expectationsofTilman (1985, 1988), but the smallerleaf
area of the species fromthe stages with a more
open vegetationstructuredoes correspond to his
theories. Tilman (1988) explains the smaller leaf
area ofthe species frompoor habitatsfroma higher
allocation of carbon and nutrientsto their roots,
which they could affordbecause of littleneed for
competitiveability forlight.However, this is not
the case in our study; the species fromthe poorer
stages showed in general a highershoot/rootratio.
The largerleaf area of the species fromthe richer
stages could possibly be regardedas an adaptation
to the low-lightconditions (Givnish,1988), since it
was found in all treatments.The smaller leaf area
of the species from the high-light habitat is,
according to our experiment, not coupled to a
higherrelativeinvestmentin the roots.Then what
is the advantage of a higher shoot/rootratio of
species in the poorer successional stages? From
the viewpoint of acquisition of nutrientsit is not
advantageous, and the plastic response of the
species in this experimentis also towards a higher
shoot/rootratio at lower lightand a lower shoot/
root ratio at lower nutrientsupply rates (see also
Brouwer1962ab; Wilson, 1988). One possibilityis
thatthehighershoot/rootratiosofthe species from
poor habitats are compensated for by a higher
specific root length,as was found by Berendse &
Elberse (1989). There mightbe an allocation tradeoffwithin the root system, rather than between
root and shoot. Another possibility is that the
carbon allocation patterns do not reflect the
nutrient allocation patterns (Abrahamson &
Caswell, 1982; Reekie & Bazzaz, 1987). Further
experimentsshould testthese hypotheses,as well
as investigatethe importance of these characteristics under competitive conditions, at different
combinations of nutrient and light availability.
Since climate chambers are highly artificialthe
importance of growth and allocation patterns in
competitivedisplacement should also be studied
under fieldconditions.
Acknowledgments
The investigationswere supported by the Foundation for Biological Research (BION), which is
subsidized by the Netherlands Organization for
ScientificResearch (NWO). We are gratefulto J.
Franke and S. Nijdam fortechnical assistance.
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Received 15 February 1989; revised 21 August 1989;
accepted 2 November 1989
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