The expansion of woody shrub vegetation (Elaeagnus umbellata) along a regulated river channel

Abstract

In the past few decades, propagation of woody vegetation in river channels has become conspicuous throughout the lowlands of the temperate zones of the Northern Hemisphere. When attempting to conserve or control populations of riparian woody plant species along regulated river environments, it is important to clarify their colonization patterns and processes and reproductive mechanisms. The expansion processes and mechanisms of many woody species, especially of Salicaceae, have been clarified (e.g. Niiyama 1990; Johnson 1994; Nakamura et al. 1997; Kamada & Okabe 1998; Dixon & Johnson 1999; Karrrenberg et al. 2002; Shin &Nakamura 2005). Natural river channels ar usually disturbance-prone areas where woody species merely survive to maturity and reproduce. However, anthropogenic factors such as dam construction and artificial river embankments have decreased the disturbance frequency and intensity. The stabilized water levels in springtime cause Salix seeds to germinate along water edges, rather than being washed downstream, and controlled flood discharges, in turn, enhance seedling survival and ultimately allow woodland expansion along river channels.