Reduced disturbances in regulated rivers can result in stable tree colonisation in downstream reaches. We examined spatial and temporal tree colonisation on a midstream sediment bar, focussing on three dominant taxa: Salix spp. (primarily Salix gilgiana), Robinia pseudoacacia and Albizia julibrissin. We developed allometric relationships for tree height, tree age and tree diameter at breast and knee height, as well as relationships between above and below-ground biomass. The spatial profiles for all individual trees of these taxa were obtained before and after a major flood in September 2007. Pre-flood data indicate that R. pseudoacacia colonize the highest elevations, A. julibrissin lower elevations and Salix spp. the lowest elevations of the three taxa. The oldest trees at each elevation were found to have survived 1.8-2.5 m inundation in past floods. Results of investigations subsequent to the flood event in September 2007 suggest that trees can survive flooding as long as their roots remain embedded in the soil. Individuals of Salix spp. experienced higher levels of inundation, but showed the greatest survival. This is probably due to morphological acclimations, particularly deep root systems and high elasticity of shoots. Considering inundation depths, erosion depths and sediment particle sizes in tree habitats, we found that sediment mobilization was the major cause of tree mortality. Thus, the ability of Salix spp. to colonize coarse sediments greatly contributed to its survival, as coarse sediments result in significantly less tractive force.

Tree mortality and regeneration in riparian areas are greatly influenced by flooding. The elevational distribution of Salix spp. and Robinia pseudoacacia were investigated by observing densities and standing conditions before and after a major flood on a sediment bar in the middle reaches of the Arakawa River in Kumagaya, Japan. The subsequent tree recruitment was also examined. R. pseudoacacia was easily washed away with the eroded sediment, whereas Salix spp. was found to be more tolerant. Both species were able to survive even after collapsing, provided that their roots were fully or partially embedded in the sediment. Re-shooting of collapsed trees, rather than emergence from saplings (either by seeds or vegetative fragments), was observed to be the major method of recruitment after the flood. Therefore, tree density before the flood was unchanged, unless the trees were subject to washing away. Salix spp. recruitment was mainly observed in low-lying areas and R. pseudoacacia in elevated areas. Recruitment from saplings was species-specific. Salix spp. at high elevations originated mainly from shoot fragments as they need high levels of moisture for seed germination and at higher elevations, sediment moisture content is very low. R. pseudoacacia, on other hand, originated mainly from roots and seeds. At a given elevation, past recruitment patterns indicated that the annual recruitment of trees increased with tree density up to a particular threshold of recruitment density. Further increases in tree density beyond that optimum value resulted in a decline in recruitment. Furthermore, threshold density was observed to increase along with elevation for R. pseudoacacia while declining with Salix spp.

-We studied the spatiotemporal trends of Eragrostis curvula (Schrad.) Nees in a flood meadow of a regulated river. The response variables, including the abundance of colonies, colony sizes, flowering spikes, biomass and root structure were checked against a set of environmental variables. These variables included distance from the river, substrate conditions (stony to sandy), soil nutrients (total nitrogen, phosphorous and calcium) and soil moisture. Relationships were evidenced using redundancy analysis. Response variables were found to be distinctive and based on substrate conditions (i.e. stony or sandy). The variables 'distance from the river' and 'soil phosphorous' contributed significantly to the overall variance. Stony habitats were observed to have lower soil nutrient levels and were characterised by monospecific stands of E. curvula, while the high nutrient levels of sandy habitats promoted heterogeneous herbaceous vegetation growth. Primary production, especially belowground, was measured to be significantly higher (t-test, P<0.05) in stony habitats. Spatiotemporal trends suggested that, irrespective of the number of colonies, E. curvula demonstrated significant levels of adaptation to micro-environments through biomass allocation and the morphological plasticity of its roots. The study results suggest that river regulation and the subsequent reduction in the frequency of inundation, in addition to the reduction of nutrients and fine sediment supply downstream, give a competitive edge to E. curvula over other herbaceous species.

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