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Seed bank and seedlings emerging after soil disturbance in a wet semi-natural grassland in
Sweden
Author(s): PER MILBERG
Source:
Annales Botanici Fennici,
Vol. 30, No. 1 (1993), pp. 9-13
Published by: Finnish Zoological and Botanical Publishing Board
Stable URL: http://www.jstor.org/stable/23726336
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Ann. Bot. Fennici 30:9-13, 1993
Seed bank and seedlings emerging after soil disturbance
in a wet semi-natural grassland in Sweden
PER MILBERG
Milberg, P. 1993: Seed bank and seedlings emerging after soil disturbance
in a wet semi-natural grassland in Sweden. — Ann. Bot. Fennici 30:9
13. Helsinki. ISSN 0003-3847
The seed bank and seedlings emerging in (i) undisturbed vegetation, (ii)
after removal of the top soil and (iii) after replacing the existing soil
with seed-free soil were studied in a wet semi-natural grassland in central
Sweden. The treatments were applied in the spring and the seedlings
counted 3 months later. Very few seedlings emerged in undisturbed grass
land vegetation, which indicates the importance of gaps for seed regenera
tion. The seed rain (replacement with seed-free soil) contributed very
little to the seedling ñora. The great majority of seedlings emerging after
top soil removal were recruited from the soil seed bank. However, despite
a species-rich seed bank, few species emerged after disturbance and the
seedlings contributed little to the revegetation.
Key words: Grassland, regeneration, seedling, soil disturbance, soil seed bank, Sweden
P. Milberg, Department of Crop Production Science, Swedish University of Agricultural
Sciences, Box 7043, S-750 07 Uppsala, Sweden
INTRODUCTION
The importance of regeneration from seeds in often increased by disturbance because fugitive spe
undisturbed perennial grassland is unclear, but the cies can exploit the available space (Collins & Barber
seedlings of a number of species have difficulty in 1985, Collins 1989, Reader & Buck 1991).
establishing themselves in dense vegetation (Fenner The soil seed bank originates from the seed rain.
1978, Bakker et al. 1980, Goldberg 1987). Many Nevertheless, the differences between the seed bank
species are, therefore, dependent on gap-creating and the present vegetation can be fairly large, be
disturbance for their germination and establishment. cause species differ in seed production and seed
Under natural conditions, newly dispersed seeds and survival, and the vegetation can change over time,
seeds in the soil are most likely to meet with small- The size and composition of the seed bank or the
scale soil disturbance, such as digging, trampling seed rain have been assessed in relation to regenera
or scratching by animals. The literature on gap re- tion after disturbance in only a few cases (Belsky
generation in grasslands in relation to disturbance 1986, Graham & Hutchings 1988, Viragh & Gerenc
type, time, size and frequency is extensive (e.g. ser 1988). Hence, the relative importance of seed
Loucks et al. 1985, Rapp & Rabinowitz 1985, Gold- dispersal and the seed bank is seldom established,
berg 1987, Collins 1987, 1989, Goldberg & Gross I studied the seed bank and seed rain in relation
1988, Rusch 1988, Martinsen et al. 1990, McLendon to the revegetation after artificial small-scale distur
& Redente 1990, Diemont 1990, Chambers et al. bance. The study was conducted in a fertile semi
1990, Glenn & Collins 1992). Species richness is natural wet grassland in central Sweden.
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10 Per Milberg ANN. BOT. FENNICI30 (1993)
STUDY AREA AND METHODS The number of seedlings in the field was low
The study area is on the southwestern shore of Lake Funbo. compared to the size of the seed bank, 1.0% and
10 km east of Uppsala in central Sweden (59°52'N, 17°51'E). 2.7% after top-soil removal at site 1 and 2, re
This semi-natural grassland was mowed for several centuries spectively (Table 2). Of the 22 taxa identified in
but has been grazed since about 1900. The area is flooded the seed bank, nine were found as seedlings after
each spring and irregular flooding can occur in the autumn. disturbance. Of the seven seed bank species lacking
The dense grass vegetation in the area is dominated by Alo- m thg vegetation before treatment only seedlings of
pecurus pratensis, Carex acuta, Deschampsia cespitosa, Ely- . ° J °
mus repetís, Juricus conglomerate and Phalaris arundinacea Cerastium fontanum were found after disturbance,
(the nomenclature follows that of Tutin et al. 1964-1980). Only a few seedlings of Taraxacum officinale
In April 1990, two areas (site 1 and 2), 25 m apart, domi- emerged in the control plots and after replacing the
natedby'Deschampsia cespitosa andAlopecuruspratensis were existing soil with seed-free soil
fenced in At each of the two sites three different treatments xhere were significant differences between the
were applied to 0.5 m x 0.5 m sub-plots in a randomized block A , r, ^
design with three replicates. A fourth treatment, herbicide [W0 sltes (P<0.001), but there were no interactions
application, was originally included but, since only parts of between the treatment and the site. The data from
the vegetation were killed, the results of this treatment are each site are presented separately since they differ
not reported. However, the pre-treatment vegetation and seed in species content (Table 2).
bank estimates from these sub-plots are included in the results. Significantly more seedlings emerged after top
Before the treatment, the species composition in the 12 sub- j, remoyal than ■ other treatmentS (p<0.001
plots at each site was recorded. The treatments were applied . . r
on 6.V.1990. They were: for both pair-Wise comparisons).
1 Unt eat d c t 1 Three months after the treatment, the experi
. n rea e con ro. mental plots were covered with vegetation (averages
2. Replacement of the existing soil with seed-free soil. The ' . , ., , 6
soil to 10 cm depth was replaced with commercial planting a 80% cover after top SOll removal at the
soil (70% peat and 30% sand; samples checked for con- two sites). This vegetation was dominated by re
tamination did not reveal any seeds). With this treatment, growth from below-ground parts and runners while
regeneration from the seed rain was recorded. Seedlings covered only 1% of the ground.
3. Removal of top soil. The upper 1-2 cm of soil was removed
with a knife. In this treatment regeneration occurred from
both the seed rain and the seed bank. Vegetative regrowth
also occurred from remaining below-ground parts. DioUUooiUN
Three months after the treatment, the seedlings were count- Very few seedlings were found in undisturbed
ed and identified and the percentage cover of the vegetation vegetation, which indicates that for most species dis
and of the seedlings was estimated. Before statistical treatment. turbance is necessary to facilitate Seed regeneration
the number of seedlings per sub-plot (x) was transformed ■ . , . - .r. . ,
[ln(l+x)J in this tall grassland vegetation. The only seed rain
species found was Taraxacum officinale which, apart
from being wind-dispersed, flowers and sets seed
Seed bank study early in the summer. Hence, the seed rain during
late spring and summer had little impact on the re
To assess the germinable part of the soil seed bank at the growth after disturbance in the present Study,
time of disturbance, five soil cores (3.3 cm diameter. 6 cm „ .u .c .u a- . u
depth) were taken from each of the 24 sub-plots at the end HoweveT the timing of the disturbance IS very im
of April 1990. portant (Squiers 1989) and soil disturbance in the
The soil from each sub-plot was put on a 1 cm layer of autumn would coincide with the seed dispersal period
heat-sterilized sand in aluminium pots (25 cm diameter) covered of more species. Furthermore, the short duration of
with a plastic film. After 3 months in a germination room the Created gaps also stresses the importance of
(diurnal cycle 16/8 h light/darkness and 20/9°C) all seedlings
were identified and removed.
RESULTS
timing of the disturbance for seed rain regeneration.
Timing is also important for seed bank regeneration,
since the seeds of some species can germinate only
during part of the growing season (Baskin & Baskin
1985).
As in many other grassland studies (e.g. Foerster
Twenty-two taxa were identified in the seed bank, 1956, van Altena & Minderhoud 1972, Schenkeveld
of which seven did not occur in the vegetation. & Verkaar 1984, Ryser & Gigon 1985) several
Twenty-four taxa were recorded in the vegetation species found in the seed bank were not present in
and of these five could not be detected in the seed the vegetation. This difference suggests that new spe
bank. The frequency (presence in sub-plots) of taxa cies could establish themselves after disturbance and
in the vegetation and seed bank before treatment hence, the species richness of the vegetation could
is presented in Table 1. increase. However, not even half of the detected seed
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ANN. BOT. FENNICI30 (1993) Seed bank and seedlings in grassland 11
Table 1, Frequency (max 24) of taxa detected in the seed bank and present in the vegetation in sub-plots before treatment
and of seedlings emerging after disturbance. Data from the two sites studied are lumped.
Vegetation Seed bank Seedlings after
top-soil removal*
Polygonum amphibium** 12 - -
'
Cardamine pratensis 11 -
Lathyrus pratensis** 4 - -
Ranunculus auricomus 1 - -
Stellaria graminea 1 - -
Ranunculus acris and R. repens 24 21 24
Poa pratensis and P. trivialis 24 21 -
Other Gramineae (Alopecurus pratensis, Elymus repens and Phleum pratense) 24 16 -
Trifolium repens 24 10 16
Deschampsia cespitosa 21 18 -
Juncus spp. 8 7 -
Galium palustre 7 3 4
Potentilla anserina 6 6 -
Leontodon autumnalis 6 4 8
Rumex crispus 5 7 4
Taraxacum officinale group 4 4 20
Veronica serpyllifolia 3 18 12
Carex spp. 3 3 -
Cirsium vulgare 2 1 16
Plantago major 1 15 -
Filaginella uliginosa - 8 -
Sagina procumbens - 5 -
Cerastium fontanum - 4 12
Ranunculus sceleratus - 2 -
Chenopodium polyspermum - 1 -
Fraxinus excelsior - 1 -
Polygonum aviculare - 1 -
Matricaria perforata - - 4
* The frequency has been transformed to correspond to that of the vegetation and seed bank.
** These species were not recorded in the vegetation before the treatment, but emerged later in the growing season. The
frequency values were estimated from the occurrence in untreated sub-plots.
bank taxa were found as seedlings in the field after by the invertebrate fauna. In addition, species differ
disturbance and of the seven species present in the in their germination requirements, and light, for
seed bank but lacking in the vegetation, only one example, can be essential for the germination of
was found after disturbance. Hence, although the buried seeds (Pons 1991). Species with seeds not
seed bank was rich in species, it contributed little present in the uppermost part of the soil and requiring
to the species richness of the vegetation. Studies in light for germination would germinate in the labora
other grasslands have also shown limited response tory, but not in the field,
to minor disturbance (Rapp & Rabinowitz 1985, Car- Despite the relatively large size of the disturbed
son & Pickett 1990). areas in the present study, vegetative regrowth clear
There was no obvious correspondence between ly predominated seedlings,
the seed bank and the seedlings emerging in the field In conclusion, in the year of this study, the seed
in the present study. A similar lack of correlation bank was the main source of the seedlings emerging
has been shown previously in arable land (Jensen after disturbance in the spring, but these seedlings
1969, Roberts & Ricketts 1979). Possible explana- contributed very little to the revegetation. Although
tions are an insufficient or incorrect estimate of the the seed bank was rich in species, the species richness
seed bank, irregular vertical distribution of the seeds of the vegetation was not increased by the distur
in the soil or a selective force, such as herbivory bance.
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12 Per Milberg ANN. BOT. FENNICI30 (1993)
Table 2. Number of seedlings per square metre (mean ± S.E.) emerging after treatment and seed bank density per square
metre estimated from 60 soil cores from each site.
Untreated control Soil substitution Top-soil removal
Site 1
Gramineae - - - 2125
Ranunculus acris and R. repens - - 30.7±17.0 1014
Plantago major - - - 624
Filaginella uliginosa - - — 292
Sagina procumbens - — - 253
Veronica serpyllifolia - - - 195
Trifolium repens - - 12.0±6.9 156
Rumex crispus - - 1.3+1.3 117
Potentilla anserina - - - 97
Juncus spp. - - - 97
Care.x spp. - - - 39
Galium palustre - - - 39
Ranunculus sceleratus - - 39
Taraxacum officinale group - 1.3±1.3 2.7+1.3 19
Polygonum aviculare - - - 19
Cirsium vulgare - - 5.3±1.3 -
Matricaria perforata - - 1.3+1.3 -
Total No. seedlings/m^ 0 1.311.3 53.3123.6 5128
Total No. identified taxa 0 1 6 15
Site 2
Gramineae - - - 3392
Veronica serpyllifolia - - 72.0+50.3 897
Plantago major - - - 780
Juncus spp. - - - 585
Ranunculus acris and R. repens - - 34.7+10.9 507
Trifolium repens - - 20.0114.4 195
Cerastium fontanum - - 18.7+14.7 195
Leontodon autumnalis - - 2.711.3 58
Taraxacum officinale group 4.0+0.0 9.311.3 32.014.0 58
Rumex crispus - - - 39
Potentilla anserina - - - 39
Cirsium vulgare - - 2.7+2.7 19
Galium palustre - - 2.712.7 19
Carex spp. - - - 19
Chenopodium polyspermum - - - 19
Fraxinus excelsior - - - 19
Ranunculus sceleratus - - - 19
Unidentified dicotelydon - - - 19
Total No. seedlings/m2 4.010.0 9.311.3 185.3+34.7 6882
Total No. identified taxa 1 1 8 17
ACKNOWLEDGEMENTS
Grants for this study were given by the Swedish Council for Minnesfond. Âby Gârd kindly allowed me to work on their
Forestry and Agricultural Research and Oscar & Lili Lamms premises and Anna Damstrom revised the English text.
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