The influence of changes in the degree and frequency of disturbance on the annual salt marsh plant (<i>Suaeda maritima, Artemisia fukudo</i>) communities in estuarine tidal flats: A case study of the Kushida River in Mie Prefecture, Japan

In this study, we examined the influence of changes in the degree and frequency of disturbance in estuarine tidal flats on the annual salt marsh plant communities (Suaeda maritima, Artemisia fukudo) in Mie Prefecture, Japan. Suaeda maritima and Artemisia fukudo communities occur in the branch river of the Kushida River. Although the areas occupied by these communities were very small in 2006, the Suaeda maritima community expanded significantly to 3609 m2 in 2008, and the Artemisia fukudo community expanded significantly to 2726 m2 in 2008 and 10,396 m2 in 2010. Before the onset of the investigation period in 2006, the overflow warning water level (3.5 m) and the flood fighting corps standby water level (3.0 m) each occurred on one day in August 2004 and October 2004, respectively;at those times, the water volume exceeded 1000 m3·s-1 and 1500 m3·s-1, respectively. We suggest that because much of the estuarine tidal flat erodes when the water volume exceeds 1000 m3·s&#451, the establishment of the Suaeda maritima and Artemisia fukudo communities is delayed until sufficient substrate is formed by the deposition of new sediment. In contrast, a water level of 2 - 3 m was observed on one day each in 2005, 2007 and 2009, with average water volumes of 488.5, 566.4 and 690.1 m3·s-1, respectively. We suggest that following the repeated disturbances caused by water levels of 1 - 3 m and flow volumes of 500 - 700 m3·s-1 over the bare ground exposed after flooding and erosion, Suaeda maritima is a pioneer species that colonizes on bare ground deposited by sediment transported from upstream and the sea during high tides, and following the same level of disturbance, Artemisia fukudo is secondary colonizer that has germinated and grown on the sediment deposited on the Suaeda maritima community.

Vegetation communities in estuarine tidal flats in the different river and basin environments of the four major rivers of Ise Bay (Suzuka, Tanaka, Kushida), Mie Prefecture, Japan

In this study, we compared and analysed vegetation communities in the estuarine tidal flats of the four major rivers of Ise Bay (Suzuka River, Tanaka River, Kushida River and Miya River) in Mie Prefecture, Japan. Along the Suzuka River, Eragrostis curvula of the exotic plant accounted for 60.0% or more of the entire surface area, and the plant volume was high. Along the Tanaka River, Suaeda maritima community occupied the sand-mud zone in the vicinity of the shoreline on gravel bars, while Phragmites australis community was distributed along a shallow lake upstream. In the Kushida River, a salt marsh plant community (a community type found in areas flooded at high tide) of Suaeda maritima, Phragmites australis and Artemisia fukudo was distributed on the sand-mud surface along the main river. A salt marsh plant community (a community type found in areas that do not flood at high tide) of Phacelurus latifolius accounted for least 50.0% of the entire surface area. Along the Miya River, the area covered by the annual salt marsh plant community type was larger than the area occupied by this community type along the other rivers. The flow volume of the Miya River was high in April, June and August-October of 2006, July and September of 2007 and April-June of 2008. The flow volume was especially high in July 2007, when it reached levels above 1500.0 m3/s;change in flow volume was also large. We suggest that a large-scale disturbance occurred in the estuary, resulting in the formation of a gravelly sandy surface where an annual salt marsh plant community of Suaeda maritime and Artemisia fukudo has been established and grown as the annual precipitation and catchment volume of the basin have increased.

Vegetation Formation in Estuarine Tidal Flats: Influences of Basin Scale and River-Crossing Structures of the Eight Rivers of Ise Bay in Mie and Aichi Prefectures, Japan

In this study, we examined the influences of the differences in basin scale and river-crossing structures of 8 rivers of Ise Bay in Mie and Aichi Prefectures, Japan on the vegetation in the estuarine tidal flats of these rivers. The dominant plant communities of the estuarine tidal flats formed from rivers of large-scale river basins (exceeding 300 km2) were determined. In the Miya River, the dominant plant community was the Suaeda maritima and Artemisia fukudo community. In the Kushida River, the dominant plant community was composed of Phacelurus latifolius, Artemisia fukudo, Phragmites australis, and bamboo. In the Kumozu River, the dominant community was composed of the coastal plants Calystegia soldanella, Lathyrus japonicus, and Carex pumila and the exotic plant of Lolium multiflorum. The plant community of Suzuka River was dominated by the exotic plant of Eragrostis curvula. Among the estuarine tidal flats influenced by a small-scale river basin (50 km2 or less), the plant community of Shinbori River (Fukue tidal flat) was dominated by Suaeda maritima, and the plant communities of the Shio and Harai Rivers were dominated by Phragmites australis. The plant community of Tanaka River was dominated by Phragmites australis and coastal plants. Regarding the relationship between the vegetation and the river environment for each study site, we hypothesised that in a large basin area with few structures crossing the river, the river water catchment in the estuary after heavy rains caused large areas of disturbance and formed bare land, providing suitable habitat for an annual salt marsh plant community. In contrast, in cases with many structures crossing the river, a stable channel, an excavated riverbed and the suppression of runoff and the resulting disturbance of the estuary, flooding did not occur during high tide. Moreover, we hypothesised that in a small basin with many structures crossing the river, disturbance to the estuary was not likely, and the perennial salt marsh plant community of Phragmites australis would be widely distributed, except for a river type such as the Shinbori River, in which tide and river flow were managed by a final closure.