To understand the effects of disturbance on vegetation, bending and cutting experiments were conducted on two rhizomatous plant species, Phragmites australis and Miscanthus sacchariflorus, in a floodplain area of the Arakawa River, Japan. The plants were damaged in the late development growth stage on 3 August 2004 (August disturbance) and in the middle development growth stage on 29 June 2005 (June disturbance). The severity of the damage was evaluated based on shoot morphology and belowground biomass. The recoveries of the two plants from the flood-like artificial disturbance were compared with undisturbed stands. The morphological response of the shoot was higher after the June disturbance than after the August disturbance in both plants. In contrast, the recovery of belowground biomass in P. australis at the end of the growth season was higher after the August disturbance (87 and 72% for bending and cutting, respectively) compared with the June disturbance (84 and 60% for the same). The recoveries in M. sacchariflorus for the two disturbances showed the opposite trend (73 and 59% for bending and cutting, respectively, after the August disturbance, and 90 and 73% after the same disturbance in June). The study demonstrated that an event like flooding, whether it is breaking or bending, will cause greater damage if it occurs at the late development growth stage in M. sacchariflorus compared with the middle development growth stage. In contrast, P. australis tolerated disturbances up to a certain magnitude; after that, the effect was more severe in the middle development growth stage compared with late development growth stage.
The coastline of Bangladesh is mostly exposed to extreme meteorological and hydrological conditions where cyclones and storm surges cause devastating effects including loss of human lives and destruction of properties. Coastal vegetation has been considered as a low-cost and natural protection to reduce the energy of current and surge. Present study explored the effectiveness of coastal vegetation against cyclonic storm surge based on species composition, forest width and near-shore run-up slope revealed by field investigations and numerical simulations. A calibrated hydrodynamic numerical model based on modified one-dimensional depth-averaged non-linear long wave differential equations was used to simulate the storm surge mitigation effected by the coastal vegetation. Considering two different types of coastal species, mangrove species, Rhizophora apiculata and beach species, Casuarina equisetifolia, numerical simulations were conducted to assess the effect of coastal forest on the storm surge mitigation. This analysis showed that double layers of wide vegetation belt (300 m) in the vertical direction with R. apiculata and C. equisetifolia on mild slope (1:500) exhibited a strong potential to decrease surge wave height and velocity. However, water depth reduction was low compared with flow velocity reduction. The maximum water depth and current velocity reduced to 1.4m (22% reduction) and 1.2m/s (49% reduction), respectively, behind the vegetation in comparison with the case without vegetation. Wide coastal vegetation belt with mild slope might be suitable for storm surge energy reduction; however, a doubling or tripling of forest width (from 100 m to 200 m or 300 m) did not produce two-fold or three-fold increase of wave reduction with negligible additional velocity reduction. For the same vegetation density the wave energy reduction by R. apiculata was not increased significantly compared to the C. equisetifolia. But young densely C. equisetifolia found more effective to reduce storm surge energy. The information would be of value to policy and decision makers for coastal landscape planning, rehabilitation and coastal resource management.
A dynamic model that includes regrowth after harvesting aerial shoots of an emergent macrophyte, Typha angustifolia L., was applied to evaluate the nitrogen (N) budget and the N uptake by the plant from sediment in Shibakawa Pond, Japan. Under natural conditions (control/ uncut stands), the analysis showed that the annual uptake of N from sediment was 26.6 gN/m 2 and harvesting Typha shoots at their growth peak removed 29.0 gN/m 2 from the system (142 days in summer). Harvesting in winter after weathering of leaves removed only 13.9 gN/m 2 . To evaluate the N budget considering regrowth shoot characteristics, three sets of harvesting experiments were done on 16 May, 8 July, and 5 August 2003. Our study revealed that May, July, and August harvesting removed 9.4, 21.9, and 16.3 gN/m 2 , respectively. Further, combining the first harvesting from spring to summer and the second harvesting in autumn (before the start of senescence of regrowth shoots), the annual total N removals in stands cut in May and autumn and July and autumn were 34.7 and 36.0 gN/m 2 , respectively-higher than that in stands cut in August and autumn (22.2 gN/m 2 ) or that in uncut stands (13.9 gN/m 2 ). At the same time, the amounts stored in rhizomes by stands cut in May and autumn, July and autumn, and August and autumn were 9.1, 8.4, and 4.4 gN/m 2 , respectively, lower than that in uncut stands (18.8 gN/m 2 ). Our results suggest that summer harvesting, especially in July to August, improves N removal efficiency and decreases the translocation of N from primary shoots to rhizomes, which is important for the sustainable management of Typha-dominated wetlands. Combined summer and autumn harvests further increase the removal efficiency but drastically reduce the storage of N. This might be useful when we need to control the plants properly.
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