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Studies on the Spatial Distribution Pattern of the Soil Seed Bank and Fluvial Processes along the Middle Reaches of River Arakawa, Japan
Abstract
In order to clarify the distribution pattern of soil seed bank and dispersion of seeds by river water flow, germination test were made by applying the surface soil collected from four sites (main channel side, bank marsh, remnant channel, bank lowland) in floodplain of River Arakawa. The abundance of seed banks in back marsh site is relatively less than any other sites. The seed banks of main channel side site had high seedling density of the species that might be dispersed by water (hydrochory) such as Veronica anagallis-aquatica, Cyperus glomeratus, which did not have special feature adapt to water dispersal. In contrast, the seed banks of the back marsh site apart from main channel had low seedling density of those species. Therefore, the difference of seedling density of the hydrochorous species can be explained by the distance from main river channel. The similarity between the floristic composition of actual vegetation and seed bank was higher in main channel side site than any other sites. It might be reflected that high disturbance by flood had been occurred in main channel side. The spatial heterogeneity of seed bank composition in lowland riparian habitat can be partially attributed to water dispersal.
... They expressed concern that species diversity of the riparian area would consequently decrease. Rivers have a role in the dispersal of plant roots, and water flow may mix alien seeds into natural seed banks (Ito et al. 2003). Moreover, introduced species that have become naturalised along riversides may disturb the existing natural vegetation, particularly if the invading species have extensive seed sources provided from surrounding areas (Muranaka & Washitani 2001Muranaka 2005). ...
We investigated the invasion of Robinia pseudoacacia and subsequently affected riparian vegetation of the Saigawa and Azusagawa Rivers, which are upper tributaries of the Shinano River, the longest river in Japan. Habitats that had been affected by river water disturbance in recent years were located at a height relatively near the river surface and were composed of coarse gravel with numerous annual plants. We found subtrees and shrubs of R. pseudoacacia under willow canopies, but small willow individuals did not occur under the R. pseudoacacia canopies. Furthermore, forest floors in willow communities hosted more wetland and riverside species, including herb species, than those beneath R. pseudoacacia canopies. Mature R. pseudoacacia stands reduced the distribution of wetland and riverside species. Furthermore, the species composition at these sites was not riparian, instead consisting of various forest species. The resulting forest landscape is unlike the natural riparian zone in this area.
We conducted a survey on vegetation and a sampling of topsoils on the riverbed of growing sites of 5 species (Sicyos angulatus L., Ambrosia trifida L., Coreopsis lanceolate L., Rudbeckia laciniata L. and Eragrostis curvula (Schrad.) Nees) in nationwide six rivers. After that, we performed a germination experiment using the sampled topsoils. We also analyzed Veronica anagallis-aquatica L. that germinated in the experiment. As a result, germination was obserbed in the second year of the experiment, with five species except Rudbeckia laciniata L., suggesting the formation of persistent soil seed banks.
We clarified the relationship between species composition and vegetation environment for the rocky Azusa River, in the city of Matsumoto, central Japan, and the sandy Chikuma River. Two endangered species, Ixeris tamagawaensis and Orostachys japonicus, occurred in the rocky riverside habitat of the Azusa River. Several species occurred in the sandy riverside habitat of the Chikuma River, including Phalaris arundinacea and Miscanthus sacchariflorus, which usually grow in wet lowlands, and Cyperus microiria and Portulaca oleracea, which are distributed in crop fields. The dominance of species at all sites was analyzed using the ordination technique of detrended correspondence analysis. The first axis was significantly positively correlated with a short distance from the river, low relative height above the water level, small particle diameter of sand, an increase in therophytes, and a decrease in trees and alien plants. Riverside vegetation was influenced by river flow disturbance, as indicated by the distance from water and relative height above water level. Furthermore, the particle diameters of sand and rocks affected the soil water content at the sites. Consequently, various site factors resulted in differences in the species composition of riverside vegetation.
We evaluated the importance of dispersal for species frequencies and distribution by comparing dispersal properties of vascular plant species with their frequencies along river banks. We assumed that species with long‐floating seeds would be more frequent than species with short‐floating seeds. We compiled data on frequencies of vascular plants and their dispersal properties from ten rivers in northern Sweden and compared these with boreal forests and grasslands in the same region. In all rivers, but in none of the reference areas, there was a positive relationship between floating capacity and frequency of species. A comparison of floating capacity between species with and without certain dispersal devices showed that seeds of vegetatively dispersed species had higher floating capacities than other seeds. For other dispersal categories (animal and wind dispersal), floating time did not differ from contrast groups.
The results indicate that water dispersal has a certain role in structuring the riparian flora, and provide a basis for explaining species distribution patterns from dispersal characteristics. They also suggest that continuous river corridors are important for maintaining regional biodiversity.
1. The hypothesis was tested that intermediate connectivity to a river results in propagule inputs to wetlands, whereas excessive connectivity impedes recruitment, and insufficient connectivity causes less competitive species to be eliminated, with no recruitment of new species. As a consequence, very low or very high nutrient levels should decrease species richness by selecting specialized species, whereas intermediate nutrient levels should favour the co-occurrence of species with contrasting nutrient requirements.
2. Among cut-off channels with high sinuosity and which are infrequently flooded by the river (low flood scouring), one example possesses high species richness because most species are saved from extinction by long-term isolation of the channel and cold groundwater supplies. Other channels are poorly supplied with groundwater and show a lower richness of species, because of low propagule inputs and low recruitment potential.
3. Cut-off channels with low sinuosity and which are flooded at intermediate frequencies were divided into three groups. The first group was species-poor, being closely connected to the river through downstream backflows which maintain nutrient-rich and turbid waters, in keeping with the hypothesis. The second group presents intermediate richness caused by: (i) lower river backflows; and (ii) floods that partly scour substrate and plants, and afford regeneration niches for transported propagules. The third group was species-poor because of excessive groundwater supplies, which probably acted as a limiting factor for species growth and recruitment.
4. The most frequently flooded channel shows the highest species richness, and occurrence of rare and fugitive species, because of floods which compensate competition by scouring sediments and plants, and afford regeneration niches for propagules. In this case, conservation of biodiversity necessitates propagule sources at the level of the river landscape.