171

Restoration of Lakeshore Vegetation Using Sediment Seed Banks;Studies and Practices in Lake Kasumigaura, Japan
Jun NISHIHIRO* and Izumi WASHITANI
Graduate School of Agricultural and Life Sciences, The University of Tokyo,
1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japane-mail ajn@mail.ecc.u-tokyo.ac.jp*corresponding author

Abstract
Recovery of lost or degraded vegetation and plant diversity is an important but often difficult step inthe wetland restoration processes. At Lake Kasumigaura, Japan, a project was launched in 2002 to recoverlakeshore vegetation using soil seed banks as the plant material. In this project, lake sediments containing
soil seed banks were spread thinly (~10 cm) on the surfaces of artificial lakeshores with microtopographic
variations, which were constructed in front of the concrete levees. In total 180 species, including six endangered or vulnerable plants and twelve native submerged plants that had disappeared from the above-groundvegetation of the lake, were recorded on five restored lakeshores (totally 65,200 m2) during the first year of
the restoration project. The distribution of each recolonized species suggested the importance of arrangement of ground height for restoration of species-rich lakeshore vegetation. Foreseen changes in the vegetation, such as the disappearance of disturbance-dependent species from the above ground vegetation, replacement of submerged-plant dominated vegetation by Typha-dominated plants, and the invasion of the invasive
exotic, Solidago altissima, were recognized in the early stages of monitoring at the restored sites. Vegetation
management including selective removal of the invasive exotics started as a collaborative activity among
citizens, government and researchers. Here, we summarize the methods and achievements of the vegetation
restoration and management.
Key words: alien plant, aquatic plant, propagule bank, rehabilitation, species diversity,
vegetation management

1. Introduction
Recently, restoration of a wide variety of wetland
types has been planned and implemented in various
places of the world. Most cases of restoration include
measures to recover lost or degraded vegetation andplant diversity. If the target species for restoration are
sufficiently available in the soil seed bank at the site orin propagules dispersed to the site, vegetation restoration can be achieved by only altering the physicalenvironmental conditions which prevent self-recovery
of vegetation of the site (Galatowitsch & van der Valk,
1994; Middleton, 1999). However, at sites with low
propagule availability, artificial introduction of plantmaterials would be needed.
Several methods of plant introduction such as transplanting, sowing commercial or collected seeds (Vécrin
et al., 2002; Rochefort et al., 2003; Bissels et al., 2004)
and introduction of cut hay (Hölzel & Otte, 2003) havebeen proposed and implemented. Among them, usage ofthe soil seed banks, i.e., spreading soil that contains seed
Global Environmental Research
11: 171-177 (2007)
printed in Japanbank on the surface of restoration sites, is recognized as
one of the most effective methods (van der Valk &Pederson, 1989; van der Valk et al., 1992; Galatowitsch& van der Valk, 1994; Brown & Bedford, 1997; Stauffer& Brooks, 1997; Middleton, 1999; Bruelheide & Flintop,
2000; McKinstry & Anderson, 2005). This is because
the properly selected seed banks may be expected tocontain a variety of species and genetically diverse individuals (Levin, 1990; van der Valk et al., 1992; Uesugi
et al., 2007). Furthermore, the recovery of species thathave disappeared from the above-ground vegetation
may also occur, depending on seed species characteristics and environmental conditions.
Among the types of wetland vegetation, that at theshores of freshwater lakes is one of the most vulnerable(Schmieder, 2004); and has been largely lost from manylakes of the world due various anthropogenic factors,
including of lakeshore construction (Elias & Meyer,2003), water quality deterioration (Brinson & Malvarez,
2002; Mäemets & Freiberg, 2005), and the alteration ofa lake’s water level and/or its fluctuation pattern©2007 AIRIES172

J. NISHIHIRO and I. WASHITANI(Crivelli et al., 1995; Shay et al., 1999; Nishihiro et al.,2004a,b). Recently, lakeshore vegetation restoration projects have been implemented or planned at many lakesof Europe and the United States (Brouwer et al., 2002;
Gulati & van Donk, 2002; Nienhuis et al., 2002;
Lauridsen et al., 2003; Chow-Fraser, 2005). Several
studies have suggested that improvement of water quality (Lauridsen et al., 2003; Chow-Fraser, 2005; Ripet al., 2005) and/or lake water level (Chow-Fraser,
2005; Coops & Hosper, 2002) can be effective measuresfor vegetative restoration at lakes where shore vegetation has remained at least in part. However, techniquesfor vegetative restoration at lakeshores where thevegetation has completely disappeared have not been
established. At Lake Kasumigaura, Japan, a restoration
project was launched. For vegetation which had completely disappeared due to construction of levees andground erosion. In the project, sediment dredged fromthe lake bottom, which was expected to contain speciesrich soil seed banks, was used as a vegetative regeneration material. The following is a brief summary of themethods and achievements of this restoration project.

2. Restoration of Lakeshore Vegetation at Lake
Kasumigaura
Lake Kasumigaura is the second largest lake in Japanand is located approximately 60 km northeast of Tokyo(Fig. 1). The open water area and maximum depth of thelake are 220 km2 and 7 m, respectively. The constructionof concrete levees over the vegetation along almost all
the lakeshore, deterioration of water quality, and an
artificially controlled water level to manage floods andsecure water for agriculture, households and industry(Nishihiro et al., 2004a) have heavily damaged the littoral vegetation during the last 30 years. Although
the natural littoral vegetation once consisted of at least
423 ha of emergent and 748 ha of submerged vegetation
as late as 1972 (Sakurai, 1990; Fig. 2), only small fragments of emergent vegetation, dominated by Phragmites
australis remained in front of the concrete levees by the
1990s and submerged plants had completely disappeared (Miyawaki et al., 2004; Fig. 2).
The Japanese Ministry of Land, Infrastructure, andTransport (MLIT) started a restoration project in 2002.
This project aimed to develop techniques for restorationof lakeshore vegetation at sites where the natural lakeshore had been replaced by concrete levees, as well as torestore species rich lakeshore vegetation including
endangered plants, such as Nymphoides peltata, which
had rapidly declined in the lake (Nishihiro et al., 2001).
The soil seed bank in the lake-bottom sediment wasused as the plant material because previous studies had
suggested that the sediment of the lake contained
species rich seed banks (Omura et al., 1999; Nishihiro
et al., 2003).
Lake sediment of about 0.5 m depth was dredged
from around fishing ports located on the shore of thelake in January-March 2002. The sediment was transported to storage sites and stored for 1-2 months to allow the water to drain off. The artificial littoral areas,
with microtopographic variations (averaging 0.05 m andranging – 0.7 to 0.4 m elevation relative to the lake waterlevel), had been constructed by depositing sand in frontof the concrete levees at five locations along the lake1400

Vegetation are (ha)

1200
1000
800
600
400
200
0
1972

Fig. 1 Location of Lake Kasumigaura and the restorationsites. Lake Kasumigaura consists of three water
bodies: Nishiura, Kitaura, and Sotonasakaura.

1982

1997

Fig. 2 Areas of lakeshore vegetation of emergent(red bars), floating leaf (yellow bars), andsubmerged plants (green bars) in the Nishiura
lakebody of Lake Kasumigaura (Fig. 1). Data
were obtained from Sakurai, 1990 (for 1972)
and the Kasumigaura Office of the Japanese
Ministry of Land, Infrastructure, and Transport (for other years).

Restoration of Lakeshore Vegetation Using Sediment Seed Banks

shore (Fig. 1). The sediment was transported to these
restoration sites and spread thinly (about 10 cm indepth) on the surface of the artificial lakeshore in
March-July 2002. Several types of wave protection suchas the bundled brushwood stuffed between double pilerows placed parallel to the shoreline (cf. Ostendorp
et al., 1995) were placed along the lakeward side ofeach site to reduce wave action and stabilize the shores.
The area of each re-created littoral area ranged from5,300 to 27,800 m2, and the width of each was 30-69 m.

3. Species Richness of Sediment Seed BanksAt the restoration sites, emergence of seedlings andvegetative sprouts began within one month, and most of
the ground surface was covered by wetland vegetation
within two months after the spreading of the sediment(Fig. 3). By the end of the first year of restoration work,
180 species had been established at the five re-createdlakeshores with an area totaling 65,200 m2 (Nishihiro etal., 2006). The temporarily established flora included
Chara braunii, Nitella hyalina, Potamogeton pectinatus,
Cyperus exaltatus var. iwasakii, Monochoria korsakowii,
and Nymphoides peltata, which are endangered orvulnerable species listed in the Japanese Red List (Environment Agency of Japan, 2000), and twelve native submerged species of six genera, Chara, Ceratophyllum,
Hydrilla, Nitella, Potamogeton, and Vallisneria, which
had recently disappeared from the remnant abovegroundvegetation of the lake (Nishihiro et al., 2006).
This result suggests that lake-bottom sediment canbe a promising source of seeds for restoration of lake-

173

shore vegetation. One life history strategy of many wetland plants is to form persistent soil seed banks (Keddy& Reznicek, 1982; Grime et al., 1981). In addition, thelake-bottom environment, which is characterized by low
availabilities of light and oxygen with lower but moreconstant temperatures than the above water, is expected
to present conditions that promote long persistence of
viable propagules without germination.
The plant species established at the restoration sites
occupied specific ranges within the topographical gradient between the water and the highest point in the area
(Fig. 4; Nishihiro et al., 2006). Submerged and floating-leaved plants grew in constantly inundated areas(about 0-0.4 m water depth), many wet meadow species,
such as Cyperus spp., became established in areas withsaturated soil (about 0-0.1 m above the lake-water level),and several agricultural weeds or plants of abandoned
fields, such as Digitaria ciliaris, took root in areas with
an elevation > 0.2 cm relative to the lake water (Fig. 4).
These results suggests that the microtopographicalarrangement and hydrological conditions of restored
lakeshores are among the most important determinantsof the species composition of the early establishedvegetation. This is because seed germination and seedling establishment of wetland plants are highly sensitive
to the soil water content and water depth (Baskin et al.,
1993; Lenssen et al., 1998; Seabloom et al., 1998;
Baldwin et al., 2001; Nishihiro et al., 2004b). Lakeshore
vegetation with a high diversity of wet-meadow andaquatic plants can be restored from donor seed banks if
the soil is spread over a ground surface with topographic
variation including shallowly inundated and water-

a)

b)

c)

d)

Fig. 3 Photographs of the lakeshore restoration sites.
a) Before the restoration, b) immediately after the soil spreading, c) two months after thesoil spreading, d) one year after the soil spreading. a)-c) were taken at site B, but d) was atsite A (Fig. 1). The plant with yellow flowers in d) is Nymphoides peltata which colonizeda shallow water body.

J. NISHIHIRO and I. WASHITANI
300

100

250

80

200

60

150
40

100

20

50
0

2002 2003 2004 2005 2006

% annual species

Number of species

1740Fig. 5 Numbers of indigenous (green bars) and exotic(red bars) plant species observed at three
restoration sites (A-C in Fig. 1). Temporalchanges in the % annual species wererepresented as plots and lines.

Fig. 4 Ranges in ground elevation relative to the normal waterlevel of the lake where seedlings or vegetative sprouts ofeach species were established. Horizontal lines represent
10%-90% of the data, boxes 25%-75%, vertical lines inthe boxes the median of the data, and ‘×’ outliers. The
data were obtained from 286 quadrats at five restorationsites (A-E in Fig. 1). Species observed in more than
15 quadrats are included and ranked according to meanvalue. The original data were included in Nishihiro
et al. (2006).

logged areas.
The species composition of the seed bank introduced
is another determinant of the early established vegetation, since the species compositions at the restorationsites where sediment from adjacent locations had been
spread were similar (Nishihiro et al., 2006). In general,
the spatial heterogeneity of sediment seed banks in alake and factors affecting it are largely unknown, although species diversity in lake-sediment seed banks
was reported in several previous studies (Haag, 1983;
de Winton et al., 2000). These issues should be addressed in future studies to predict the distribution of
useful seed banks.

4. Temporary Changes in Restored Vegetation
Within two to three years of completion of therestoration work vegetation monitoring revealed several
early changes in the restored vegetation which had been
foreseen during the planning of the project, i.e., a decrease in the number of disturbance-dependent species,
disappearance of submerged plants from shallow water
areas, and appearance of invasive exotic plants at therestoration sites.
The proportion of annual species to total species has
been gradually decreasing during 2002-2006, although
the total species richness gradually increased until 2005
(Fig. 5). This suggests that pioneer plants and/or distur-bance-dependent plants which occurred with relatively
high frequency at the early stages of vegetation recovery
had disappeared as dense and tall vegetation developed(Fig. 6a,b). Indeed, although Monochoria korosakowii,
which is a ‘vulnerable’ species in Japan (Environment
Agency of Japan, 2000) and supposed to be a disturbance-dependent species (Hashimoto et al., 2001), hadbeen observed in relatively high frequency at some
restoration sites in 2003-2004, it could not be found in
2005-2006. The disturbance of vegetation would be rare
at restored lakeshores because the water level of the lake
was stabilized artificially (Nishihiro et al., 2004a) andwave protector were placed in front of the planted area.
Although disturbances which are too intense can cause
lakeshore erosion (Ostendorp et al., 1995), moderate
disturbances with appropriate intensity and frequency
should be introduced through future processes of adaptive management.
Several submerged plants, including Potamogeton
malaianus, P. pectinatus, and Vallisneria denseserrulata,
appeared in the shallow water areas in the depressionsof the artificially constructed lakeshores (average andmaximum water depth of 34.5 and 50.0 cm, respectively) during 2002-2004, but had disappeared by 2006
when the shallow water areas became dominated by
Typha angustifolia (Fig. 6c,d). In order to clarify the
impact of the Typha invasion on the colonization of thesubmerged plants, Typha rhizomes were dredged fromone of the shallow water areas at a restoration site (siteC in Fig. 1) in April 2007 by the MLIT as an experimental form of management. In that area, monitoring ofrecolonization of submerged plants and re-growth of
Typha is planned.
If the transparency of the lake water had been high,the submerged plants could have colonized a deeperarea, where Typha can not grow. However, at present,
the transparency of the lake water is very low; the meantransparency at the center of the lake being about 67±
15 cm (mean ± SD, n=360) during 1996-1998 (National
Institute for Environmental Science, 2001). Improvement of water quality is a substantial issue for sustaining
submerged vegetation in the lake.

Restoration of Lakeshore Vegetation Using Sediment Seed Banks

a)

b)

c)

d)

175

Fig. 6 Temporal changes in vegetation in the restoration sites.
a) One year and b) five years after the restoration work at site A. c) Two year
and d) four years after restoration work in a shallow water body at site B.

5. Invasion and Management of Alien Plants
Shoot density (/m2)

Generally, soil from artificially altered areas or lakeand riversides within urbanized watersheds contains ahigh density of exotic plant species (King & Buckney,
2001; Ajima, 2001; Washitani, 2004). Furthermore,since the vegetation under development at restorationsites from soil seed banks spread on bare ground isgenerally sparse, there is considerable risk of invasion
by exotic plants through seed dispersal (Johnston, 1986),
even if their seed bank density in the soil used is low.
Such invasions were revealed in monitoring the restored
lakeshore at Lake Kasumigaura. Some invasive exotic
plants, such as Solidago altissima, Ambrosia trifida, andSicyos angulatus, invaded relatively dry ground which
formed at relatively elevated areas on the artificiallyconstructed lakeshores (Nishihiro et al., 2007). To control these exotics and enhance the recovery of indigenous plants, the invasive alien plants were selectively
removed (site A in Fig. 1) every June or July from2004 to 2007. This activity was conducted as a collaborative program among the NPO ‘Asaza Fund,’ theKasumigaura Office of the MLIT, and the 21st century
COE program of The University of Tokyo ‘Biodiversity
and Ecosystem Restoration.’ Participants in this activity
totalled 26, 32, 25, and 21 persons including citizens,
NPO staff, MLIT officials and researchers in 2004, 2005,
2006, and 2007, respectively. The selective removal of
S. altissima from an area of ~2,500 m2 was accomplished through 2007, and the shoot density ofSolidago altissima was reduced to ~20 % (Nishihiro
et al., 2007; Fig. 7).

40
30
20
10
0
2004

2005

2006

Fig. 7 Density of above-ground shoots of Solidago
altissima in areas where the species was
selectively removed (treatment; red lines) andnot removed (control; green line) in 2004. The
plots and bars represent means and standard
deviations among ten quadrats (1 × 1 m),
respectively. The control quadrats were set at
an area with similar elevation to that in thetreated quadrats. The original data were
included in Nishihiro et al. (2007).

6. Conclusions
The following is a summary of the points which arethought to be generally applicable to restoration practices with similar aims. 1) Lake-bottom sediment has
great potential as a material to restore lakeshore vegetation including species which have disappeared from theabove-ground vegetation. 2) The micro-topography of
restored lakeshores profoundly affects the species

176

J. NISHIHIRO and I. WASHITANI

composition of the restored vegetation; a species richlakeshore vegetation can be restored by spreading soil
containing seed banks on the surface of the reclaimed
lakeshore with appropriate topographic variations. 3) In
order to restore lakeshore vegetation dynamics permitting the co-occurrence of species of diverse life-forms
and life-history strategies including submerged anddisturbance-dependent plants, large-scale measures areneeded such as the improvement of lake water quality
and the resumption of the natural disturbance regimes.
4) Since vegetation restoration using soil seed banks
entails the risk of invasion of invasive alien plants,
assessment of soil seed banks is needed before implementation. Furthermore, selective removal can be aneffective measure for managing alien plants invading thesites.

Acknowledgements
We thank the Kasumigaura Office of the Japanese
Ministry of Land, Infrastructure, and Transport forallowing us to conduct this study and the NPO ‘Asaza
Fund’ for supporting the restoration practices and ourstudies.
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(Received 25 September 2007, Accepted 13 December 2007)