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Global Environmental Research ©2007 AIRIES 11: 171-177 (2007) printed in Japan

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, Japan

e-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 in

the wetland restoration processes. At Lake Kasumigaura, Japan, a project was launched in 2002 to recover lakeshore 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 endan-gered or vulnerable plants and twelve native submerged plants that had disappeared from the above-ground vegetation 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 arrange-ment of ground height for restoration of species-rich lakeshore vegetation. Foreseen changes in the vegeta-tion, such as the disappearance of disturbance-dependent species from the above ground vegetation, replace-ment 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 and plant diversity. If the target species for restoration are sufficiently available in the soil seed bank at the site or in propagules dispersed to the site, vegetation restora-tion can be achieved by only altering the physical environmental 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 plant materials would be needed.

Several methods of plant introduction such as trans-planting, 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) have been proposed and implemented. Among them, usage of the soil seed banks, i.e., spreading soil that contains seed

bank 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 to contain a variety of species and genetically diverse indi-viduals (Levin, 1990; van der Valk et al., 1992; Uesugi et al., 2007). Furthermore, the recovery of species that have disappeared from the above-ground vegetation may also occur, depending on seed species characteris-tics and environmental conditions.

Among the types of wetland vegetation, that at the shores of freshwater lakes is one of the most vulnerable (Schmieder, 2004); and has been largely lost from many lakes 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 of a lake’s water level and/or its fluctuation pattern

172 J. NISHIHIRO and I. WASHITANI

(Crivelli et al., 1995; Shay et al., 1999; Nishihiro et al., 2004a,b). Recently, lakeshore vegetation restoration pro-jects have been implemented or planned at many lakes of 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 qual-ity (Lauridsen et al., 2003; Chow-Fraser, 2005; Rip et al., 2005) and/or lake water level (Chow-Fraser, 2005; Coops & Hosper, 2002) can be effective measures for vegetative restoration at lakes where shore vegeta-tion has remained at least in part. However, techniques for vegetative restoration at lakeshores where the vegetation has completely disappeared have not been established. At Lake Kasumigaura, Japan, a restoration project was launched. For vegetation which had com-pletely disappeared due to construction of levees and ground erosion. In the project, sediment dredged from the lake bottom, which was expected to contain species- rich soil seed banks, was used as a vegetative regenera-tion material. The following is a brief summary of the methods and achievements of this restoration project.

2. Restoration of Lakeshore Vegetation at Lake

Kasumigaura Lake Kasumigaura is the second largest lake in Japan

and is located approximately 60 km northeast of Tokyo (Fig. 1). The open water area and maximum depth of the

lake are 220 km2 and 7 m, respectively. The construction of concrete levees over the vegetation along almost all the lakeshore, deterioration of water quality, and an artificially controlled water level to manage floods and secure water for agriculture, households and industry (Nishihiro et al., 2004a) have heavily damaged the litto-ral 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 frag-ments of emergent vegetation, dominated by Phragmites australis remained in front of the concrete levees by the 1990s and submerged plants had completely disap-peared (Miyawaki et al., 2004; Fig. 2).

The Japanese Ministry of Land, Infrastructure, and Transport (MLIT) started a restoration project in 2002. This project aimed to develop techniques for restoration of lakeshore vegetation at sites where the natural lake-shore had been replaced by concrete levees, as well as to restore 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 was used 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 the lake in January-March 2002. The sediment was trans-ported to storage sites and stored for 1-2 months to al-low the water to drain off. The artificial littoral areas, with microtopographic variations (averaging 0.05 m and ranging – 0.7 to 0.4 m elevation relative to the lake water level), had been constructed by depositing sand in front of the concrete levees at five locations along the lake-

Fig. 1 Location of Lake Kasumigaura and the restoration

sites. Lake Kasumigaura consists of three water bodies: Nishiura, Kitaura, and Sotonasakaura.

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(red bars), floating leaf (yellow bars), and submerged 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 Trans-port (for other years).

Restoration of Lakeshore Vegetation Using Sediment Seed Banks 173

shore (Fig. 1). The sediment was transported to these restoration sites and spread thinly (about 10 cm in depth) on the surface of the artificial lakeshore in March-July 2002. Several types of wave protection such as the bundled brushwood stuffed between double pile rows placed parallel to the shoreline (cf. Ostendorp et al., 1995) were placed along the lakeward side of each site to reduce wave action and stabilize the shores. The area of each re-created littoral area ranged from 5,300 to 27,800 m2, and the width of each was 30-69 m.

3. Species Richness of Sediment Seed Banks

At the restoration sites, emergence of seedlings and

vegetative 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-created lakeshores with an area totaling 65,200 m2 (Nishihiro et al., 2006). The temporarily established flora included Chara braunii, Nitella hyalina, Potamogeton pectinatus, Cyperus exaltatus var. iwasakii, Monochoria korsakowii, and Nymphoides peltata, which are endangered or vulnerable species listed in the Japanese Red List (Envi-ronment Agency of Japan, 2000), and twelve native sub-merged species of six genera, Chara, Ceratophyllum, Hydrilla, Nitella, Potamogeton, and Vallisneria, which had recently disappeared from the remnant aboveground vegetation of the lake (Nishihiro et al., 2006).

This result suggests that lake-bottom sediment can be a promising source of seeds for restoration of lake-

shore vegetation. One life history strategy of many wet-land plants is to form persistent soil seed banks (Keddy & Reznicek, 1982; Grime et al., 1981). In addition, the lake-bottom environment, which is characterized by low availabilities of light and oxygen with lower but more constant 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 gradi-ent between the water and the highest point in the area (Fig. 4; Nishihiro et al., 2006). Submerged and float-ing-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 with saturated 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 microtopographical arrangement and hydrological conditions of restored lakeshores are among the most important determinants of the species composition of the early established vegetation. This is because seed germination and seed-ling 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 and aquatic 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)

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Fig. 3 Photographs of the lakeshore restoration sites. a) Before the restoration, b) immediately after the soil spreading, c) two months after the

soil spreading, d) one year after the soil spreading. a)-c) were taken at site B, but d) was at site A (Fig. 1). The plant with yellow flowers in d) is Nymphoides peltata which colonized a shallow water body.

174 J. NISHIHIRO and I. WASHITANI

logged areas. The species composition of the seed bank introduced

is another determinant of the early established vegeta-tion, since the species compositions at the restoration sites where sediment from adjacent locations had been spread were similar (Nishihiro et al., 2006). In general, the spatial heterogeneity of sediment seed banks in a lake and factors affecting it are largely unknown, al-though species diversity in lake-sediment seed banks was reported in several previous studies (Haag, 1983; de Winton et al., 2000). These issues should be ad-dressed 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 the

restoration work vegetation monitoring revealed several early changes in the restored vegetation which had been foreseen during the planning of the project, i.e., a de-crease in the number of disturbance-dependent species, disappearance of submerged plants from shallow water areas, and appearance of invasive exotic plants at the restoration 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 distur-bance-dependent species (Hashimoto et al., 2001), had been 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) and wave 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 adap-tive management.

Several submerged plants, including Potamogeton malaianus, P. pectinatus, and Vallisneria denseserrulata, appeared in the shallow water areas in the depressions of the artificially constructed lakeshores (average and maximum water depth of 34.5 and 50.0 cm, respective-ly) 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 the submerged plants, Typha rhizomes were dredged from one of the shallow water areas at a restoration site (site C in Fig. 1) in April 2007 by the MLIT as an experimen-tal form of management. In that area, monitoring of recolonization 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 deeper area, where Typha can not grow. However, at present, the transparency of the lake water is very low; the mean transparency at the center of the lake being about 67± 15 cm (mean ± SD, n=360) during 1996-1998 (National Institute for Environmental Science, 2001). Improve-ment of water quality is a substantial issue for sustaining submerged vegetation in the lake.

Fig. 4 Ranges in ground elevation relative to the normal water

level of the lake where seedlings or vegetative sprouts of each species were established. Horizontal lines represent 10%-90% of the data, boxes 25%-75%, vertical lines in the boxes the median of the data, and ‘×’ outliers. The data were obtained from 286 quadrats at five restoration sites (A-E in Fig. 1). Species observed in more than 15 quadrats are included and ranked according to mean value. The original data were included in Nishihiro et al. (2006).

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

Restoration of Lakeshore Vegetation Using Sediment Seed Banks 175

5. Invasion and Management of Alien Plants Generally, soil from artificially altered areas or lake-

and riversides within urbanized watersheds contains a high density of exotic plant species (King & Buckney, 2001; Ajima, 2001; Washitani, 2004). Furthermore, since the vegetation under development at restoration sites from soil seed banks spread on bare ground is generally 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, and Sicyos angulatus, invaded relatively dry ground which formed at relatively elevated areas on the artificially constructed lakeshores (Nishihiro et al., 2007). To con-trol these exotics and enhance the recovery of indige-nous plants, the invasive alien plants were selectively removed (site A in Fig. 1) every June or July from 2004 to 2007. This activity was conducted as a collabo-rative program among the NPO ‘Asaza Fund,’ the Kasumigaura 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 ac-complished through 2007, and the shoot density of Solidago altissima was reduced to ~20 % (Nishihiro et al., 2007; Fig. 7).

6. Conclusions

The following is a summary of the points which are

thought to be generally applicable to restoration prac-tices with similar aims. 1) Lake-bottom sediment has great potential as a material to restore lakeshore vegeta-tion including species which have disappeared from the above-ground vegetation. 2) The micro-topography of restored lakeshores profoundly affects the species

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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.

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Fig. 7 Density of above-ground shoots of Solidago

altissima in areas where the species was selectively removed (treatment; red lines) and not 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 the treated quadrats. The original data were included in Nishihiro et al. (2007).

176 J. NISHIHIRO and I. WASHITANI

composition of the restored vegetation; a species rich lakeshore 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 permit-ting the co-occurrence of species of diverse life-forms and life-history strategies including submerged and disturbance-dependent plants, large-scale measures are needed 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 imple-mentation. Furthermore, selective removal can be an effective measure for managing alien plants invading the sites.

Acknowledgements We thank the Kasumigaura Office of the Japanese

Ministry of Land, Infrastructure, and Transport for allowing us to conduct this study and the NPO ‘Asaza Fund’ for supporting the restoration practices and our studies.

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(Received 25 September 2007, Accepted 13 December 2007)