Expansion of Elaeagnus umbellata on a gravel bar in the Naka River, Shikoku, Japan

Abstract

The present study seeks to identify the expansion process of the shrub Elaeagnus umbellata Thunb. (Elaeagnaceae) on a gravel bar in the Naka River, Shikoku, Japan, in relation to the hydrogeomorphologic regime of the habitat. The establishment pattern was determined by a series of aerial photographs, and the establishment years were confirmed by examining tree rings taken from five different areas within the population. The topographic change of the river cross‐section was analyzed and it was found that the establishment occurred exactly when and where the riverbed began to stabilize. The three cohorts of 15‐, 10‐ and 4‐year‐olds were recognized, the younger individuals being downstream. The episodic age pattern was significantly synchronized with large floods occurring during the autumn fruit‐ripening season. These spatial and temporal occurrence patterns indicated the probability of hydrochory, which ultimately enhanced the dissemination of endozoochorous E. umbellata seeds into the newly created habitat on the downstream part of the gravel bar. Another cause of quick dominance was its vigorous sprouting ability, which enabled the established E. umbellata to withstand damage and sediment burial by strong floods. The erosion of the deepest part of the riverbed increased the relative elevation of the vegetated stand, which ultimately decreased the frequency and magnitude of disturbance during flood inundation after the 1980s. Consequently the hydrogeomorphic regime of the floods played an important role in habitat creation, seed dispersal, and in the survival of the established individuals, and it determined the population formation and expansion of E. umbellata on the riparian gravel bar.

Effects of complete submergence on seedling growth and survival of five riparian tree species in the warm-temperate regions of Japan

“…Plant distributions in floodplains are determined not only by physiological stress due to floods but also by physical disturbance, soil water availability (Battaglia et al 2000;Hall and Harcombe 2001;Xiong et al 2003), soil nutrient content (Weiher and Keddy 1995), soil composition (Niiyama 1987), seed dispersal limitations (Kohri et al 2002), inter-specific competition (Weiher and Keddy 1995;Xiong et al 2003), and interactions among these factors (Xiong et al 2003). The three Ulmaceae species prefer moderate soil humidity and slightly xeric soil condition but can grow over a wide range of soil conditions (Higa et al 2006;Sakamoto 2009).…”

“…japonica and A. aspera are ornithochory species. Kohri et al (2002) reported that berries of riparian tree species were widely dispersed on a floodplain by floods. Hence, it is unlikely that the seed dispersal of these Ulmaceae species is restricted in the upper areas of floodplains.…”

Spatial–temporal distribution of ornithochorous seeds from an Elaeagnus umbellata community dominating a riparian habitat

“…These results have clarified that an increase in suitable habitat itself within the river system has increased the number of E. umbellata communities, as they tend to distribute on elevated terraces with coarse bed materials in the gravel bars in the Yoshino River (Kohri et al . 2000; Kohri 2008) and Naka River (Kohri et al . 2002).…”

“…Elaeagnus umbellata is highly heliophytic and does not appear as a result of succession, but rather tends to establish linear‐form colonies on bare gravel bars after experiencing late autumnal floods. It does not form a permanent seed bank (Nakagoshi 1985), a gap detection habit is not present, and all seeds germinate only in spring (Kohri et al . 2002).…”

“…This indicates that hydrochory (sensu Johansson et al . 1996) acts as a secondary seed dispersal mechanism (Kohri et al . 2000, 2002; Kohri 2008) to a wider area, and this natural disturbance appears to be effective in creating new safe sites for seed germination (e.g.…”