Tree colonization trends on a sediment bar after a major flood

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. Copyright © 2010 John Wiley & Sons, Ltd.