Nordic Society Oikos
Secondary Succession and Summer Herbivory in a Subarctic Grassland: Community Structure
and Diversity
Author(s): Kristjan Zobel, Mari Moora, Valerie K. Brown, Pekka Niemela, Martin Zobel
Source: Ecography, Vol. 20, No. 6 (Dec., 1997), pp. 595-604
Published by: Blackwell Publishing on behalf of Nordic Society Oikos
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ECOGRAPHY 20: 595-604. Copenhagen 1997
Secondary succession and summerherbivoryin a subarctic
grassland:communitystructureand diversity
Kristjan Zobel, Mari Moora, Valerie K. Brown, Pekka Niemela and Martin Zobel
Zobel, K., Moora, M., Brown,V. K., Niemela,P. and Zobel, M. 1997. Secondary
successionand summerherbivoryin a subarcticgrassland:communitystructureand
diversity.- Ecography20: 595-604.
A fieldexperimentwas establishedin a subarcticgrasslandin the FinnishLaplandto
study the role of summer herbivoryin plant community succession. Perennial
vegetationand moss coverwereremovedin an area of 324 m2.The site was divided
into four blocks, of whichtwo were fencedto preventherbivoryby largemammals
(reindeer,hare).
Earlysuccessionalchangesin the vegetationwereassessed.Meanspeciesrichnessper
3 x 3 m plot was consistentlyhigherin the fencedarea,indicatingthat herbivorycan
suppresssmall-scalediversity.Herbivoryaffectedthe heightof severalplant species.
However, there was no correlationbetween frequencyand height of individual
species.Therewas a weak indicationthat tallerspeciesweremore successfulin early
successionwhengrazed.Lightcompetitionis apparentlynot a key processdetermining successionalchange.Thus, in early stage of succession,summerherbivoryhas
little effect on diversityby limitinglight competition,and most speciesare equally
successfulin grazed and ungrazedplots. There was some indirectevidenceabout
competitiveinteractionsin the developingcommunity.However,unlike temperate
grasslands,large mammal herbivoryand competition for light seem not to be
importantdeterminantsof communitychange in this subarcticgrassland(at least
whatconcernesearlysuccessionalstages).This may be explainedby the harshnessof
local climate,and abundanceof light due to the polar day.
K. Zobel([email protected]),
M. MooraandM. Zobel,Deptof BotanyandEcology,Tartu
Univ., 40 Lai St., EE-2400 Tartu,Estonia. - V. K. Brown,InternationalInst. of
Ecology,CABInt., 56 Queen'sGate,London,U.K.SW7 5JR. - P. Niemeli, Fac. of
Forestry,Univ.of Joensuu,Box 111, FIN-80101Jonesuu,Finland.
Several studies have considered the role of herbivory in
plant community succession (see Edwards and Gillman
1987, Gibson et al. 1987, Brown and Gange 1989,
Gibson and Brown 1991, 1992, Davidson 1993, van
Andel et al. 1993a,b). However, the results are variable
and have demonstrated a range of responses from an
acceleration to a retention of succession. In arctic and
subarctic environments, reindeer herbivory retards the
transition from dwarf shrub to lichen community (Oksanen 1980), whereas in grass-rich subarctic communities, herbivory has been shown to inhibit plant
colonization in very early phases of succession (Mc-
Kendrick 1987), or accelerate the replacement of willows by alder in the later stages (Cargill and Chapin
1987). The results of Moen (1990), Quellet et al. (1993)
and Oksanen and Moen (1994) showed that summer
herbivory was apparently unimportant in structuring
plant communities in the subarctic habitats.
We were interested in plant community succession,
with and without herbivory, in a subarctic river floodplain grassland in Finnish Lapland. This grassland is
extraordinarily species-rich due to favourable local microclimate and soil conditions - the number of vascular plant species may be > 150 km-2, while the
Accepted25 March 1997
CopyrightC ECOGRAPHY1997
ISSN 0906-7590
Printedin Ireland- all rightsreserved
ECOGRAPHY 20:6 (1997)
595
mean for the region is 70 km-2 (Heikkinen and
Kalliola 1990). The river valley has for a long time
been used for grazing and hay-making. Now, many
of the grasslands are overgrowing because of the decrease in agricultural activities. However, most communities are still sparsely grazed, mainly by reindeer,
and to a lesser extent by hare.
In temperate grasslands, the availability of light is
frequently the critical factor determining the presence
of many species, and their successional dynamics. For
example, successional changes due to overgrowing or
fertilization are often connected with the elimination
of some species due to competition for light (Willems
1983, Bakker 1989, Kull and Zobel 1991, ter Heerdt
et al. 1991, Hill et al. 1992, Bobbink and Willems
1993). The success of species in competition for light
is mainly determined by its height (Mitchley 1988,
Keddy 1990). In principle, grazing can moderate
plants' height, and thus make competition more symmetric. Gibson and Brown (1991) found that in early
and mid-successional grassland communities ungrazed
plots had lower species diversity. One can ask, is the
same true also for the succession in subarctic grasslands?
Van Andel et al. (1993) claimed that both abiotic
conditions and grazing may be responsible for the
inhibition of successional changes. Interaction of abiotic factors and grazing could be more evident in the
subarctic communities where abiotic conditions are
stressful for plants.
Little attention has so far been given to the stratification of herbaceous plant canopies during succession. It is evident that the relative vertical position of
species, establishing in a canopy, undergoes successional changes. It may be predicted that herbivory
would modify this. Ver Hoef et al. (1989) have found
that there is little relationship between vertical structure and horizontal pattern in a temperate chalk
grassland. The vertical position species occupy in a
canopy is probably more dependent on the growth
form and phenological stage of the plants, than on
the horizontal distribution or pattern of abundance of
species (Harper 1989). Thus, the vertical distribution
of species is not necessarily a good indicator of successional stage, at least in temperate grasslands.
We address four questions: 1) What are the early
successional dynamics of species richness and composition in a subarctic grassland? 2) What is the role of
light competition in determining early changes in species composition and richness? We focus on the vertical structure of the canopy, as the gradual success of
taller individuals over smaller ones may indicate successful competition for light; 3) Does summer herbivory by large mammals (the study site does not
provide resources for large mammal winter herbivory)
alter the direction and rate of successional changes
and enhance the maintenance of relatively very high
596
species richness in mesic river valley grasslands? 4)
Does the vertical position of certain species in the
grassland canopy depend on summer herbivory and
successional stage?
To answer these questions, we established a field
experiment where the succession of a grassland community on bare ground was studied after sod cutting
in 1988. The latter was used to create uniform initial
conditions. Such a manipulation is also of interest for
nature conservation, because of the removal of the
upper soil layer with mosses is used as a management
tool to reverse overgrowing.
Material and methods
Study site
The study site is located in the River Kevojoki valley,
near the Kevo Subarctic Research Inst. of the Univ.
of Turku (69045'N, 27?00'E), Finnish Lapland. The
experimentally manipulated area is part of an old
semi-natural mesic grassland, inhabiting, according to
Hinneri's (1975) classification of edaphic conditions in
the Utsjoki River System, a eutrophic riverside site.
The grassland is surrounded by birch Betula
pubescens and Scots pine Pinus sylvestris forest.
The climate is quite severe, and the duration of the
growing season is only 110-120 days. The polar day
begins in mid-May and lasts until the end of July.
The temperature sum is 550-600 degree days (>5?C).
The snow depth varies between 40 and 70 cm
(Ohlson 1981). The grassland area has been farmed
more or less continuously since the mid-19th century
(Hustich 1942).
Site preparation
The study site was treated with herbicide (Roundup)
in the autumn of two successive years before sod-cutting (1986, 1987) to destroy perennial vegetation. The
extensive moss layer (3-5 cm thick) was removed
carefully in 1988, to facilitate recruitment from the
soil seed bank. Thus, the succession began on bare
ground in 1988 and colonization was from the local
soil seed bank and immigrant propagules. Fences
were erected in the summer of 1989.
Experimental design
The experimental site of 324 m2 was divided into 36
3 x 3 m plots. These were divided into 4 blocks (2 x
2). Two blocks (diagonally opposite) were fenced to
prevent herbivory of large mammals (reindeer, hare).
The plots within the two herbivory treatments are
referred to as "series".
ECOGRAPHY 20:6 (1997)
Vegetation sampling
The developingvegetationwas sampledin the middle
of the growing season (end of July/beginningof August) in 1988, 1991 and 1993. It was sampledby point
quadrat pins. Pins were marked at different height
intervals (2 cm intervals from 0-10 cm and 5 cm
intervals thereafter).On every sampling date, 30 (in
1988) or 40 (in 1991, 1993) pins were positionedrandomly in each plot. The number of touches of each
species at each height intervalwas recordedfrom 10
(1988) or 20 (1991, 1993) pins (height profile pins),
whereasthe presence/absenceof plant speciestouching
the pins was recordedon the remaining20 pins.
The number and height of willow and mountain
birchseedlingsweremeasuredin July 1993. Becauseof
the very high number of seedlings, only those taller
than 10.0 cm were included.
Student's t-test was applied to compare the mean
height of species in series with and without herbivory.
The dynamicsof the total numberof plant specieswas
analysedby X2-test.The effect of grazingand successional age on plot richnesswas analysed by a 2-way
ANOVA. Speciescounts were squareroot transformed
to apply ANOVA (Rao 1969). The observed spatial
variabilityof richness,and relativepositionof speciesin
the canopy, were tested against the null model of
independentdistributionof species,by usinga randomization procedure.
Relative position of species in the canopy
The successional change of relative position of species
in the canopy, in relation to herbivory, was studied by
comparing the vertical position of species with the
expectation from the null model of independent distribution of species. The position of a species in the
canopy was characterized by the mean relative number
of touches of non-target species above and below a
touch of the target species (we use the term 'target
species' to notify any species in its turn). If a target
species systematically has more touches of non-target
species above it, than being predicted by the null model,
it will indicate a shade preference. Establishment and
growth of such species are probably enhanced under
the canopy of already established neighbours. If significantly fewer touches of non-target species are registered
above or below the target species, as compared with the
null model, superiority in above-ground competition is
indicated.
Touches of non-target species in the same height
interval as the target species were treated separately and
considered as touches 'beside' the target species. Relative proportions of touches of non-target species below,
beside and above the target species (pbl, pbs, pab,
respectively) were calculated for all the recorded
touches of all species on all pins. The means of pbl, pbs
and pab for all the recorded touches of a target species,
for each treatment, for one sampling year, served as the
empirical measure of vertical position for a species.
Frequencyvs verticaldistributionand time
To answer question 2), we studied whether the frequencyof a speciesis relatedto its verticaldistribution
of biomass. This was done by calculatingthe correlation betweenthe mean heightof a speciesin a plot and
its relativefrequencyin the same quadrat.Mean height
was estimated only for species with three or more
touches on the height-profilepins. Correlationcoefficientswerecalculatedif therewerefive or moreplots in
the serieswherethe mean heightwas calculated.Correlation between height and frequencywas studied for
each of the three years. For the first two sampling
occasions,correlationbetweencurrentheight and frequency the following sampling occasion was also included.
In orderto studythe heightdistributionof speciesin
relationto their successwe assessedvalue of the correlation coefficient between frequency and time r(frequency, year). If the frequencyof a species steadily
increasesduring succession,it can be consideredsuccessful, at least in the short term.
To answer question 3), correlationbetween mean
heightand frequencyof specieswas comparedwith and
without herbivory.
Randomization test for studying vertical distribution
of species
A randomization procedure was applied to test the
observed position of a species in the canopy, against the
null hypothesis of independent spatial distribution of
species. First, from the pool of number of touches on
real pins in a particular plot, the number of touches on
hypothetical (or pseudo-) pins was selected randomly
with replacement. The number of hypothetical pins in a
plot was the same as in the real data. In order to reduce
side effects arising from possible spatial dependence in
the data, randomization was always performed within a
plot. For each touch on a hypothetical pin, species and
height interval were identified, by selecting randomly
(with replacement) from the pool of all the observed
touches in this sample plot. After such a randomization
was performed for the 16 sample plots each year, the
means of pbl, pbs and pab for the null hypothesis were
calculated for each species. As in the real data, only
touches of non-target species were registered below,
beside and above the target species. The randomization
was repeated 5000 times. When >97.5% of the expected positions of the studied species were either lower
or higher than the observed one (two-tailed test, p=
0.05), the null hypothesis was rejected.
Statistical methods
ECOGRAPHY 20:6 (1997)
597
Randomization test for studying spatial variability
of richness
To test whether the variability of species richness at a
point differs from that which could be expected from
the null hypothesis of random distribution of species
(variance deficit/excess, Palmer and van der Maarel
1995), we used a randomization procedure, similar to
that described above, but slightly more complicated.
When counting species, touched by a point quadrat pin,
it is important to realize that richness can be severely
overestimated in the null model by randomly selecting
touches from the entire pool. For several species, the
probability that a touch on a pin will be accompanied
by another touch of the same plant can be much greater
than that of a touch by another species (it is quite
common that a pin touches the same individual several
times). Without considering this in the null model, the
expected mean number of species per pin would be
much greater than observed, and variance in species
number, as proportional to the mean, would also be
systematically greater.
We therefore estimated for all target species the
empirical probability of: i) a touch on a pin being
accompanied by a touch of a non-target species, and ii)
a touch on a pin being accompanied by a touch of the
same species (itself). The probabilities were used in the
randomization procedure only if estimated on more
than ten observations (otherwise, the 0.5:0.5 ratio was
used).
The randomization procedure resembled the one described above. The difference was that, after selecting
touches randomly from the pool of all the registered
touches in a sample plot, the selection was accepted
only after an additional random procedure. The selected touch (by a target species) was accepted if a
simultaneously generated random number (evenly distributed between 0 and 1) was smaller than: i) when the
selected touch was by a non-target species - the empirical probability of co-occurring with a non-target species or ii) when the selected touch was by the same
species - the empirical probability of co-occurring with
'itself' (target species). In the case of disapproval of the
selection, the procedure was repeated until approval
(note that the total number of touches on each hypothetical pin was determined beforehand, by selecting it
randomly from the pool of observed numbers of
touches).
Variance in the number of species per pin was calculated as variance around the mean over all pins in the
series, in each year. Significance was estimated in the
same way as in the test of relative position of species.
Results
Species richness
Table 1 presents temporal changes in the total number
of species, recorded in a series of plots with similar
Table 1. The changeof speciescomposition,and the total numbersof species(both recordedby point quadratanalysis).Values
representthe numberof 3 x 3 m plots, in a seriesof 16 plots, with (HV +) or without(HV -) herbivory,in whichthe species
was detected.
1988
HV +
Achillea millefolia
Alchemilla spp.
Betula pubescens
Campanula rotundifolia
Carex brunnescens
Carex juncella
Carex vaginata
Cerastiumfontanum
Deschampsia caespitosa
Equisetum arvense
Equisetum pratense
Festuca ovina
Festuca rubra
Juncusfiliformis
Luzula multiflora
Poa alpigena
Ranunculus acris
Rubus arcticus
Rumex acetosa
Rumex acetosella
Salix hastigiata
Solidago virgaurea
Stellaria graminea
Veronica longifolia
Total number
of speciesrecorded
598
2
13
2
2
2
6
11
3
1
-
9
1991
HV4
-
-14
-12
2
-5
1
3
3
12
15
-
HV +
7
-
1993
HV-
16
-
11
13
16
1
16
7
14
3
2
5
1
16
4
5
15
7
2
7
2
8
9
1
-
1
-
1
1
2
11
-
12
13
17
-
1
1
-2
--
HV +
HV-
9
15
4
16
1
1
2
16
8
8
6
1
16
1
2
1
6
11
1
3
1
14
1
-
19
17
16
1
15
16
16
1
7
9
11
9
1
15
1
ECOGRAPHY 20:6 (1997)
Table 2. Mean plot species richness(and standarderrors), both grazed and non-grazed series (Fig. 2). Surprisrecordedby point quadratanalysis,in serieswith and without ingly, the relationship is significant (p = 0.025) in the
herbivory,for the three samplingyears.
grazed, but not in the ungrazed plots. As shown in Fig.
2, the latter is mostly due to two outlying points,
year of experiment with herbivory withoutherbivory
representing Juncus filiformis and Luzula multiflora.
3.75+ 0.079
2.63 + 0.066
1988
Overall, there was relatively little difference in the
7.13 + 0.207
6.56 + 0.275
1991
success of species when grazed or ungrazed. Plotting
1993
7.81 +0.219
8.56+ 0.158
success in grazed vs ungrazed sites, illustrates a similarity in both cases (Fig. 3). One species, Achillea milletreatment, by point quadrat analysis. Total richness
was clearly more successful in ungrazed plots.
folium,
was slightly higher in the fenced series during the first
three years, but in the fifth year there were more species
in the grazed series. However, differences were non-sigVertical position of species in the canopy
nificant (%2-test).The ANOVA showed that grazing
and successional age had significant effects on species Six species were significantly taller in either of the two
richness in 3 x 3 m plots (p <0.001 and p= 0.017, treatments (Fig. 4). Two species, Deschampsia caespirespectively, there was no significant interaction; Table tosa and Carex vaginata, were taller in the grazed plots.
2). Plot richness was consistently greater in the fenced Most probably the growth of these more herbivore
resistant species is less suppressed in the grazed plots
sites.
Table 3 shows the height and numbers of birch and because potential competitors are browsed more acwillow seedlings in plots. Both birch and willow were tively and lose relatively more biomass.
The relative position of several species differed sigtaller in ungrazed plots, but there was no significant
nificantly (p = 0.05) from that expected from the null
difference in the numbers of seedlings.
model of independent vertical distribution of species
(Table 4). The only species which clearly preferred
shade in the first sampling season was Carex brunnesSpecies frequency vs vertical distribution
cens, in fenced plots. This species also had fewer
Correlations between mean height of species and their touches of non-target species than expected beside it, in
frequency (in the same and in the next sampling year) the first season (plots without herbivory), and in the
were calculated for 290 cases. Only in 15 cases were second season (both herbivory treatments). Festuca
significant correlations (p = 0.05) detected, which ovina had more touches of non-target species above it
closely approximates to expectation due to Type I error than expected in the second sampling season. In the
(14.5 = 290 x 0.05). Thus, independent of the treat- third sampling date F. ovina had significantly fewer
ment, there is no evidence that taller species are more touches of non-target species below it, regardless of
frequent. There are no successional trends in the height/ herbivory treatment. Clearly, it grows under the canopy
of other species at an early stage of succession, while
frequency relationship.
later on its tussock growth form does not allow other
species to establish beneath it.
In the third season Veronica longifolia and Poa alpiSpecies success during succession vs vertical
gena
preferred shade in plots with herbivory, and Soldistribution
idago virgaurea in the fenced plots. V. longifolia and P.
For most species the estimated value for success during alpigena also have fewer touches of non-target species
succession - r(frequency, year) - was close to zero (e.g. than expected below them in the third sampling date, in
Fig. 1). This suggests that the majority of species do not plots with herbivory. There were no target species
show consistent changes in frequency during the first which would have significantly more touches of nonfive years of succession. When success is plotted against target species below them than expected. Three species,
mean height a positive relationship can be detected in
Table 3. Effectof herbivoryon the numberand heightof birchand willow seedlingsin the experimentalplots. Only seedlings
tallerthan 10.0 cm were sampled.
With herbivory
SE
mean
Birches
Height,cm
Numberof seedlings
Willows
Height,cm
Number of seedlings
ECOGRAPHY 20:6 (1997)
16.2
51.6
17.2
99.3
+0.25
+52.7
+0.35
+571.2
Withoutherbivory
SE
mean
22.5
69.1
20.0
135.1
F
p
+0.76
+ 116.5
63.19
2.91
0.0000
0.0936
+0.68
6.20
0.0189
1.12
0.2997
+381.8
599
o
0.12r
Carexbrunnescens
r= 0.094
0. 8
a) 0.1.6
a)
a)
0
*
0..2
0.08
.
0.06
.
0.04
w
?.
n,'
0
>
,l
0 -
0.1
0
0.
J
I
I
^~~~~
^^~~~~~~~~"
/
0~~~
0~~~~~~
Achilleamillefolium ,4'-o 0
0"
0.02
9 -..
"'
o
0n
-0
0
3
2
9 1
-0.02
0.02
0
0.12
0.1
008
0.06
0.04
Success with herbivory
Year of experiment
Fig. 1. The success of a species (Carex brunnescens)during Fig. 3. Success of species during successionr(time, relative
succession,demonstratedby the linearcorrelation(r) between frequency)in grazedand ungrazedplots. Line of unity repretime and relativefrequencyof the speciesin 3 x 3 m perma- sents identicalsuccess
nent plots. Data are from plots with herbivory.
season, the observed variance in richness is significantly
less than expected, in both grazed and ungrazed sites.
Deschampsia caespitosa, Festuca rubra and Rumex acetosella, were the only ones to have fewer touches of
non-target species above them than expected in the
third sampling season. This probably indicates their
ability to compete for light (space), thereby making it Discussion
difficult for other species to establish under their The revegetation of bare ground in a subarctic grasscanopy.
land resembles the 'telescoped succession' of Tansley
The effect of herbivory on the relative position of (1929), 'direct succession' of Whittaker and Levin
species in the canopy lacks consistency. In five cases, (1977), or 'regeneration succession' of van der Maarel
the difference from the null model prediction is signifi- (1988). Most species, characteristic of later successional
cant for both treatments, in a certain year. On the other stages, were present from the very beginning of commuhand, seven species are only affected by one treatment nity development. There were few species which did not
at one particular stage in succession.
grow in the undisturbed grassland surrounding the
experimental area. Such a situation is typical for secondary successions of boreal forests (e.g. Foster 1985,
Zobel 1993) and the possible mechanism corresponds to
Variability of species richness at a point
what was called T (tolerance) model by Connell and
Table 5 presents the results of comparing the observed
the further course of the
variance in the number of species at a point, with Slatyer (1977). However,
succession will depend on the effect of herbivory. If it
expectation from the null model of independent spatial
slows down the development of shrub and tree
distribution of species. In the first year of succession, only
but does not eliminate it, many low-growing
layer
the observed variance matches the expectation in both
of subarctic grasslands will be excluded (see
species
herbivory treatments. In the second sampling season,
Moen 1990) and the changes may be explained rather
the null model predicts a much higher variance than
the F (facilitation) model of Connell and Slatyer
observed in the grazed sites, while in the ungrazed sites by
(1977).
null model prediction holds true. In the third sampling
8
0.12
0
0
0.1
c-
a~~~~~
.o_
I
0.08
?
O
0)
0.06
-1
0.02
~
~ ~
^^^-^rtS^o.
--^^
00 as
*< t
^*"
l
0
so0
0
o
A
2
3
4
5
*A
6
Fig. 2. Success of species during successionr(time, relative
frequency)in relationto meanheightof species(see methods).
Solid symbols and solid regressionline - plots subjectedto
herbivory;open symbols and broken line - plots without
herbivory.
0
-
0
0
1
*
S
a1
Mean height
600
4
3
s 3
co
-1
.02
0 3
._
multiflora
oLuzula
11
.-I
,l
, -1
/
Juncus filiforrnis
_
0
-
~~
11
2 0201
.
,
0.04
3
Un
*,
. 6
a)
5
1
2
3
4
6
5
7
8
Mean height with herbivory
Fig. 4. Comparisonof mean height of speciesin grazedand
ungrazedplots. Specieswhichare significantlytaller(p = 0.05)
in one treatmentare markedwith bigger symbols and numbered: 1) Luzula multiflora; 2) Juncusfiliformis; 3) Deschampsia caespitosa; 4) Carex juncella; 5) Achillea millefolia; 6)
Carexvaginata.Line of unity representsidenticalheight.
ECOGRAPHY 20:6 (1997)
t?n
0
0
D
cr
PO
5?
Table 4. Target plant species with vertical position in the canopy significantly (p = 0.05) different from that predicted by the null h
Target species can have more/fewer touches of non-target species above/beside/below them, as compared with the prediction from
and the three sampling years are distinguished.
1988
A
B
0
V
E
B
E
,S
I
D
E
B
E
L
0
W
More than predicted from H?
1991
Festuca ovina
Veronica longifolia
Poa alpigena
Festuca ovina
Solidago virgaurea
with
herbivory
Carex brunnescens
Fewer tha
1993
1988
without
herbivory
with
herbivory
without
herbivory
with
herbivory
without
herbivory
Festuca rubra
Veronica longifolia
Carex br
Descham
Achillea
Betula pu
Carex brunnescens
Carex br
Descham
Achillea
The dynamics of species richness was different in
grazed and ungrazed sites. The increase in the number
of species was initially more rapid in the fenced sites,
and the mean richness in 3 x 3 m plots was also considerably higher. According to Davidson (1993), herbivory
typically retards succession in earlier phases, but the
role of herbivory changes principally during succession.
In temperate grasslands, grazing is expected to increase
diversity on a local scale, because animals remove
proportionally more leaf canopy of taller species, i.e.
the more powerful competitors for light (Armesto and
Pickett 1985, Mitchley 1988). The different conditions
in a subarctic grassland make the role of light competition in community organization doubtful. This doubt is
confirmed by the analysis of the dynamics of species.
frequency and canopy structure, as there was no evidence of taller species being (or becoming) more frequent. However, in the grazed plots, there was a weak
positive correlation between the 'successional success'
during the five years and the vertical position of
species in the canopy. This correlation most likely
reflects the fact that herbivore resistant (less browsed
and thus taller) species were able to perform better
under summer grazing than the more palatable species.
Because the level of herbivory was not sufficient to
eliminate the regrowth of willows and birches, the
situation may totally change when a closed canopy of
shrubs develops. One may expect that successional
changes will then follow rather F (facilitation) model
than T (tolerance) model, i.e. the mechanism of succession will change within a successional sequence (this
possibility is discussed by Connell et al. 1987 and
Pickett et al. 1987).
Our results conform to the earlier experience (Oksanen and Ericson 1987, Oksanen and Moen 1994)
showing that herbivory is generally unimportant in
relatively productive subarctic habitats. Moreover,
given the design and duration of our experiment, there
can be three possible reasons which could have made
the effect of herbivory smaller than it generally is in
subarctic meadows: i) in subarctic habitats winter resources are usually limiting for large herbivores - in the
Table5. Observedvariancein the numberof speciesper point
quadratpin, as comparedto the expectationfrom the null
model of independentdistributionof species.
year of
experiment
1988
1991
1993
602
with herbivory
withoutherbivory
non-significant
difference
(p = 0.930)
smallerthan
expected
(p < 0.001)
smallerthan
expected
(p < 0.001)
non-significant
difference
(p = 0.246)
non-significant
difference
(p -=0.530)
smallerthan
expected
(p = 0.036)
winter hares browse willows and birches tall enough to
penetrate the snow, reindeers use lichens as the main
food, and our site provides none of these; ii) much of
the dynamics of vascular plants in subarctic meadows
can be due to the impact of microtine rodents (Oksanen
1988, 1990, Oksanen and Moen 1994) which was not
manipulated in our experiment; iii) duration of the
experiment may have been too short for detecting the
impact of summer grazing. However, our results
demonstrate a difference from early successional communities in temperate climate (e.g. Gibson and Brown
1991), where herbivory may have significant effect on
community structure in early stages of succession.
Generally, our evidence indicates that competition
for light may not be a key process determining community change during early successional phases. This may
be related to the fact that, in the subarctic, abiotic
environmental conditions exert a more important role
than biotic factors, such as competition (Bliss 1962,
Sonesson and Callaghan 1991, Walker 1995). Billings
(1987) claims that the physical environment can be the
main controller of plant establishment until succession
is well advanced, when biological constraints begin to
have greater effect. Thus, we may expect a clearer effect
of light competition (and hence of herbivory) later in
succession.
There is an additional explanation of the apparently
minor role of above-ground competition in subarctic
grasslands. In polar regions, the difference in light
intensity between day and night is small. The use of
night sunlight for photosynthesis by many subarctic
and arctic species is well known (Kallio 1984, Sonesson
and Callaghan 1991). The abundance of light from all
directions combined with low temperature and short
growing season can make light competition much less
important than it is in temperate Europe. Also, the
reduced importance of light competition explains to
some extent the small effect of herbivory on community
development.
Significantly smaller variance in species richness than
expected from the null hypothesis of species independence has been interpreted as an indication of community organization (strong biotic interactions among
species; Palmer 1987, Zobel and Zobel 1988), of niche
limitation (Wilson et al. 1987), and in some cases of a
nucleation process (sensu Yarranton and Morrison
1974) which can take place in early stages of succession
(Zobel et al. 1993). Though the test for variance deficit/
excess may easily lead to serious misinterpretations
(Palmer and van der Maarel 1995), the most popular
explanation to a deficit in variance of richness is still
through the packing of the environment by species
(niche limitation). If this explanation holds, the results
indicate competition for space among species in both
experimental treatments. Surprisingly, in the grazed
ECOGRAPHY 20:6 (1997)
site, variance deficit was observed already in the second
sampling season, while in the ungrazed area variance
deficit was detectable later. This contrasts with the
situation which one would expect to find in a temperate
grassland, where the presumably stronger competition
for light should cause variance deficit more rapidly in
non-grazed sites.
Thus, the results of the analysis of variability in
richness show that, while there is an indication of the
area being gradually saturated by species (on a small
scale), this is not because of light competition. Of
course, the latter will be true only if above-ground
competition is a priori considered to be more intense in
non-grazed situation.
- , Dawkins, H. C., Brown, V. K. and Jepsen, M. 1987.
Springgrazingby sheep:effectson seasonalchangesduring
early old field succession. - Vegetatio 70: 33-43.
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Marshall, B. and Jarvis, P. G. (eds), Plant canopies: their
growth, form and function. CambridgeUniv. Press, pp.
105-128.
Heikkinen,R. K. and Kalliola,R. J. 1990.The vascularplants
of the Kevo Nature Reserve (Finland); an ecological-environmental approach. - Kevo Notes 9: 1-56.
Hill, M. O., Evans, D. F. and Bell, S. A. 1992. Long-term
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- This work was supportedby Univ. of
Acknowledgements
Turku Foundation,Kevo SubarcticResearchInst., Finnish
Forest ResearchInst., the Finnish Academyof Scienceand
Royal Society in the U.K. The work benefittedmuch from
protocolsdevelopedduringa NERC researchgrantto V. K.
Brown. MarjaanaAlanen and Juha Rantanenhelped in the
field work. We thank Lauri Oksanenfor valuablecomments McKendrick,J. D. 1987. Plant successionon disturbedsites,
on the manuscript.
North Slope, Alaska, USA. - Arct. Alp. Res. 19: 554-
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