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 ofSalixspp. andRobinia pseudoacaciawere 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. pseudoacaciawas easily washed away with the eroded sediment, whereasSalixspp. 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.Salixspp. recruitment was mainly observed in low‐lying areas andR. pseudoacaciain elevated areas. Recruitment from saplings was species‐specific.Salixspp. 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 forR. pseudoacaciawhile declining withSalixspp.