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Restoration of Lakeshore Vegetation Using Sediment Seed Banks; Studies and Practices in Kasumigaura,

Jun NISHIHIRO* and Izumi WASHITANI

Graduate School of Agricultural and Life Sciences, The University of , 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan e-mail [email protected] *corresponding author

Abstract Recovery of lost or degraded vegetation and plant diversity is an important but often difficult step in the restoration processes. At , 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 bank on the surface of restoration sites, is recognized as one of the most effective methods (van der Valk & Recently, restoration of a wide variety of wetland Pederson, 1989; van der Valk et al., 1992; Galatowitsch types has been planned and implemented in various & van der Valk, 1994; Brown & Bedford, 1997; Stauffer places of the world. Most cases of restoration include & Brooks, 1997; Middleton, 1999; Bruelheide & Flintop, measures to recover lost or degraded vegetation and 2000; McKinstry & Anderson, 2005). This is because plant diversity. If the target species for restoration are the properly selected seed banks may be expected to sufficiently available in the soil seed bank at the site or contain a variety of species and genetically diverse indi- in propagules dispersed to the site, vegetation restora- viduals (Levin, 1990; van der Valk et al., 1992; Uesugi tion can be achieved by only altering the physical et al., 2007). Furthermore, the recovery of species that environmental conditions which prevent self-recovery have disappeared from the above-ground vegetation of vegetation of the site (Galatowitsch & van der Valk, may also occur, depending on seed species characteris- 1994; Middleton, 1999). However, at sites with low tics and environmental conditions. propagule availability, artificial introduction of plant Among the types of wetland vegetation, that at the materials would be needed. shores of freshwater is one of the most vulnerable Several methods of plant introduction such as trans- (Schmieder, 2004); and has been largely lost from many planting, sowing commercial or collected seeds (Vécrin lakes of the world due various anthropogenic factors, et al., 2002; Rochefort et al., 2003; Bissels et al., 2004) including of lakeshore construction (Elias & Meyer, and introduction of cut hay (Hölzel & Otte, 2003) have 2003), deterioration (Brinson & Malvarez, been proposed and implemented. Among them, usage of 2002; Mäemets & Freiberg, 2005), and the alteration of the soil seed banks, i.e., spreading soil that contains seed a lake’s water level and/or its fluctuation pattern

Global Environmental Research ©2007 AIRIES 11: 171-177 (2007) printed in Japan 172 J. NISHIHIRO and I. WASHITANI

(Crivelli et al., 1995; Shay et al., 1999; Nishihiro et al., lake are 220 km2 and 7 m, respectively. The construction 2004a,b). Recently, lakeshore vegetation restoration pro- of concrete levees over the vegetation along almost all jects have been implemented or planned at many lakes the lakeshore, deterioration of water quality, and an of Europe and the United States (Brouwer et al., 2002; artificially controlled water level to manage floods and Gulati & van Donk, 2002; Nienhuis et al., 2002; secure water for agriculture, households and industry Lauridsen et al., 2003; Chow-Fraser, 2005). Several (Nishihiro et al., 2004a) have heavily damaged the litto- studies have suggested that improvement of water qual- ral vegetation during the last 30 years. Although ity (Lauridsen et al., 2003; Chow-Fraser, 2005; Rip the natural littoral vegetation once consisted of at least et al., 2005) and/or lake water level (Chow-Fraser, 423 ha of emergent and 748 ha of submerged vegetation 2005; Coops & Hosper, 2002) can be effective measures as late as 1972 (Sakurai, 1990; Fig. 2), only small frag- for vegetative restoration at lakes where shore vegeta- ments of emergent vegetation, dominated by Phragmites tion has remained at least in part. However, techniques australis remained in front of the concrete levees by the for vegetative restoration at lakeshores where the 1990s and submerged plants had completely disap- vegetation has completely disappeared have not been peared (Miyawaki et al., 2004; Fig. 2). established. At Lake Kasumigaura, Japan, a restoration The Japanese Ministry of Land, Infrastructure, and project was launched. For vegetation which had com- Transport (MLIT) started a restoration project in 2002. pletely disappeared due to construction of levees and This project aimed to develop techniques for restoration ground erosion. In the project, sediment dredged from of lakeshore vegetation at sites where the natural lake- the lake bottom, which was expected to contain species- shore had been replaced by concrete levees, as well as to rich soil seed banks, was used as a vegetative regenera- restore species rich lakeshore vegetation including tion material. The following is a brief summary of the endangered plants, such as Nymphoides peltata, which methods and achievements of this restoration project. had rapidly declined in the lake (Nishihiro et al., 2001). The soil seed bank in the lake-bottom sediment was 2. Restoration of Lakeshore Vegetation at Lake used as the plant material because previous studies had Kasumigaura suggested that the sediment of the lake contained species rich seed banks (Omura et al., 1999; Nishihiro Lake Kasumigaura is the second largest lake in Japan et al., 2003). and is located approximately 60 km northeast of Tokyo Lake sediment of about 0.5 m depth was dredged (Fig. 1). The open water area and maximum depth of the 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-

1400

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Vegetation are (ha) 400

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0 1972 1982 1997 Fig. 2 Areas of lakeshore vegetation of emergent (red bars), floating leaf (yellow bars), and submerged plants (green bars) in the Nishiura lakebody of Lake Kasumigaura (Fig. 1). Data Fig. 1 Location of Lake Kasumigaura and the restoration were obtained from Sakurai, 1990 (for 1972) sites. Lake Kasumigaura consists of three water and the Kasumigaura Office of the Japanese bodies: Nishiura, Kitaura, and Sotonasakaura. 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 shore vegetation. One life history strategy of many wet- restoration sites and spread thinly (about 10 cm in land plants is to form persistent soil seed banks (Keddy depth) on the surface of the artificial lakeshore in & Reznicek, 1982; Grime et al., 1981). In addition, the March-July 2002. Several types of wave protection such lake-bottom environment, which is characterized by low as the bundled brushwood stuffed between double pile availabilities of light and oxygen with lower but more rows placed parallel to the shoreline (cf. Ostendorp constant temperatures than the above water, is expected et al., 1995) were placed along the lakeward side of to present conditions that promote long persistence of each site to reduce wave action and stabilize the shores. viable propagules without germination. The area of each re-created littoral area ranged from The plant species established at the restoration sites 5,300 to 27,800 m2, and the width of each was 30-69 m. occupied specific ranges within the topographical gradi- ent between the water and the highest point in the area 3. Species Richness of Sediment Seed Banks (Fig. 4; Nishihiro et al., 2006). Submerged and float- ing-leaved plants grew in constantly inundated areas At the restoration sites, emergence of seedlings and (about 0-0.4 m water depth), many wet meadow species, vegetative sprouts began within one month, and most of such as Cyperus spp., became established in areas with the ground surface was covered by wetland vegetation saturated soil (about 0-0.1 m above the lake-water level), within two months after the spreading of the sediment and several agricultural weeds or plants of abandoned (Fig. 3). By the end of the first year of restoration work, fields, such as Digitaria ciliaris, took root in areas with 180 species had been established at the five re-created an elevation > 0.2 cm relative to the lake water (Fig. 4). lakeshores with an area totaling 65,200 m2 (Nishihiro et These results suggests that the microtopographical al., 2006). The temporarily established flora included arrangement and hydrological conditions of restored Chara braunii, Nitella hyalina, Potamogeton pectinatus, lakeshores are among the most important determinants Cyperus exaltatus var. iwasakii, Monochoria korsakowii, of the species composition of the early established and Nymphoides peltata, which are endangered or vegetation. This is because seed germination and seed- vulnerable species listed in the Japanese Red List (Envi- ling establishment of wetland plants are highly sensitive ronment Agency of Japan, 2000), and twelve native sub- to the soil water content and water depth (Baskin et al., merged species of six genera, Chara, Ceratophyllum, 1993; Lenssen et al., 1998; Seabloom et al., 1998; Hydrilla, Nitella, Potamogeton, and Vallisneria, which Baldwin et al., 2001; Nishihiro et al., 2004b). Lakeshore had recently disappeared from the remnant aboveground vegetation with a high diversity of wet-meadow and vegetation of the lake (Nishihiro et al., 2006). aquatic plants can be restored from donor seed banks if This result suggests that lake-bottom sediment can the soil is spread over a ground surface with topographic be a promising source of seeds for restoration of lake- 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 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

300 100

250 80 % annual species 200 60 150 40 100 50 20 Number of species Number 0 0 2002 2003 2004 2005 2006

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.

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. 4 Ranges in ground elevation relative to the normal water level of the lake where seedlings or vegetative sprouts of (Fig. 6a,b). Indeed, although Monochoria korosakowii, each species were established. Horizontal lines represent which is a ‘vulnerable’ species in Japan (Environment 10%-90% of the data, boxes 25%-75%, vertical lines in Agency of Japan, 2000) and supposed to be a distur- the boxes the median of the data, and ‘×’ outliers. The bance-dependent species (Hashimoto et al., 2001), had data were obtained from 286 quadrats at five restoration been observed in relatively high frequency at some sites (A-E in Fig. 1). Species observed in more than 15 quadrats are included and ranked according to mean restoration sites in 2003-2004, it could not be found in value. The original data were included in Nishihiro 2005-2006. The disturbance of vegetation would be rare et al. (2006). 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 logged areas. lakeshore erosion (Ostendorp et al., 1995), moderate The species composition of the seed bank introduced disturbances with appropriate intensity and frequency is another determinant of the early established vegeta- should be introduced through future processes of adap- tion, since the species compositions at the restoration tive management. sites where sediment from adjacent locations had been Several submerged plants, including Potamogeton spread were similar (Nishihiro et al., 2006). In general, malaianus, P. pectinatus, and Vallisneria denseserrulata, the spatial heterogeneity of sediment seed banks in a appeared in the shallow water areas in the depressions lake and factors affecting it are largely unknown, al- of the artificially constructed lakeshores (average and though species diversity in lake-sediment seed banks maximum water depth of 34.5 and 50.0 cm, respective- was reported in several previous studies (Haag, 1983; ly) during 2002-2004, but had disappeared by 2006 de Winton et al., 2000). These issues should be ad- when the shallow water areas became dominated by dressed in future studies to predict the distribution of Typha angustifolia (Fig. 6c,d). In order to clarify the useful seed banks. impact of the Typha invasion on the colonization of the submerged plants, Typha rhizomes were dredged from 4. Temporary Changes in Restored Vegetation one of the shallow water areas at a restoration site (site C in Fig. 1) in April 2007 by the MLIT as an experimen- Within two to three years of completion of the tal form of management. In that area, monitoring of restoration work vegetation monitoring revealed several recolonization of submerged plants and re-growth of early changes in the restored vegetation which had been Typha is planned. foreseen during the planning of the project, i.e., a de- If the transparency of the lake water had been high, crease in the number of disturbance-dependent species, the submerged plants could have colonized a deeper disappearance of submerged plants from shallow water area, where Typha can not grow. However, at present, areas, and appearance of invasive exotic plants at the the transparency of the lake water is very low; the mean restoration sites. transparency at the center of the lake being about 67±

The proportion of annual species to total species has 15 cm (mean ± SD, n=360) during 1996-1998 (National been gradually decreasing during 2002-2006, although Institute for Environmental Science, 2001). Improve- the total species richness gradually increased until 2005 ment of water quality is a substantial issue for sustaining (Fig. 5). This suggests that pioneer plants and/or distur- submerged vegetation in the lake.

Restoration of Lakeshore Vegetation Using Sediment Seed Banks 175

a) b)

c) d)

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 40 Generally, soil from artificially altered areas or lake- ) and riversides within urbanized watersheds contains a 2 30 high density of exotic plant species (King & Buckney, 2001; Ajima, 2001; Washitani, 2004). Furthermore, 20 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 10 by exotic plants through seed dispersal (Johnston, 1986), Shoot density (/m even if their seed bank density in the soil used is low. 0 Such invasions were revealed in monitoring the restored 2004 2005 2006 lakeshore at Lake Kasumigaura. Some invasive exotic plants, such as Solidago altissima, Ambrosia trifida, and Fig. 7 Density of above-ground shoots of Solidago altissima in areas where the species was Sicyos angulatus, invaded relatively dry ground which selectively removed (treatment; red lines) and formed at relatively elevated areas on the artificially not removed (control; green line) in 2004. The constructed lakeshores (Nishihiro et al., 2007). To con- plots and bars represent means and standard trol these exotics and enhance the recovery of indige- deviations among ten quadrats (1 × 1 m), nous plants, the invasive alien plants were selectively respectively. The control quadrats were set at an area with similar elevation to that in the removed (site A in Fig. 1) every June or July from treated quadrats. The original data were 2004 to 2007. This activity was conducted as a collabo- included in Nishihiro et al. (2007). 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 6. Conclusions and Ecosystem Restoration.’ Participants in this activity totalled 26, 32, 25, and 21 persons including citizens, The following is a summary of the points which are NPO staff, MLIT officials and researchers in 2004, 2005, thought to be generally applicable to restoration prac- 2006, and 2007, respectively. The selective removal of tices with similar aims. 1) Lake-bottom sediment has S. altissima from an area of ~2,500 m2 was ac- great potential as a material to restore lakeshore vegeta- complished through 2007, and the shoot density of tion including species which have disappeared from the Solidago altissima was reduced to ~20 % (Nishihiro above-ground vegetation. 2) The micro-topography of et al., 2007; Fig. 7). restored lakeshores profoundly affects the species

176 J. NISHIHIRO and I. WASHITANI composition of the restored vegetation; a species rich ling emergence from seed banks of 15 New Zealand lakes with lakeshore vegetation can be restored by spreading soil contrasting vegetation histories. Aquatic Botany, 66: 181-194. containing seed banks on the surface of the reclaimed Elias, J.E. and M.W. Meyer (2003) Comparisons of undeveloped and developed shorelands, northern Wisconsin, and recom- lakeshore with appropriate topographic variations. 3) In mendations for restoration. , 23: 800-816. order to restore lakeshore vegetation dynamics permit- Environmental Agency of Japan (2000) Threatened wildlife of ting the co-occurrence of species of diverse life-forms Japan: Red Data Book, 2nd edn. Japan Wilidlife Research Cen- and life-history strategies including submerged and ter, Tokyo. (in Japanese) disturbance-dependent plants, large-scale measures are Galatowitsch, S.M. and A.G. van der valk (1994) Restoring needed such as the improvement of lake water quality Prairie Wetlands: An Ecological Approach. Iowa State Univer- sity Press, Ames, Iowa. and the resumption of the natural disturbance regimes. Grime, J.P., G. Mason, A.V. Curtis, J. Rodman, S.R. Band, 4) Since vegetation restoration using soil seed banks M.A.G. Mowforth, A.M. Neal and S. Shaw (1981) A compara- entails the risk of invasion of invasive alien plants, tive study of germination characteristics in a local flora. Jour- assessment of soil seed banks is needed before imple- nal of Ecology, 69: 1017-1059. mentation. Furthermore, selective removal can be an Gulati, R.D. and E. van Donk (2002) Lakes in the Netherlands, effective measure for managing alien plants invading the their origin, and restoration: state-of-the-art re- view. Hydrobiologia, 478: 73-106. sites. Haag, R.W. 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(Received 25 September 2007, Accepted 13 December 2007)