Consequences of propagule dispersal and river fragmentation for riparian plant community diversity and turnover

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• River margins are depositories of tremendous numbers of propagules (Nilsson and Grelsson 1990, Andersson and Nilsson 2002, Vogt et al. 2006). Some of these are from local sources (<100 m; Levine 2003) and others are transported far (e.g., hundreds of kilometers) by water (Andersson et al. 2000b, Boedeltje et al. 2003). As a consequence, rivers are unique landscape elements in that they biologically (mainly through hydrochory) connect otherwise disconnected meta-populations or meta-communities (Boudell and Stromberg 2008), accelerate colonization, and possibly enhance potential diversity through transporting propagules long distances from throughout a catchment (Nilsson et al. 1991b, 1994, Andersson et al. 2000b). Water dispersal is a secondary dispersal mechanism for many riparian species, but deposition in water can enhance a propagule's chances of reaching a safe site for establishment through dispersing farther from parent plants (decreasing intraspecific competition; i.e., Janzen-Connell effect; Bever 2003). Water dispersal may extend the period over which seeds disperse (“effective dispersal”), and propagules can benefit from directed dispersal if water-transported seeds are deposited nonrandomly in sites more conducive to germination and survival (Merritt and Wohl 2002). Dams alter the delivery of seeds to channel margins and floodplains through governing the timing and magnitude of flows, in some cases decoupling the necessary site conditions for germination and survival with the availability of propagules or other life-history requirements (Mahoney and Rood 1998, Jansson et al. 2000b, Merritt and Wohl 2006). Dams are also barriers to the movement of propagules along river corridors, trapping 75–95% of propagules and reducing the propagule species diversity in rivers along reaches downstream of dams (Andersson et al. 2000b, Jansson et al. 2005). Dams obstruct dispersal of seeds from regional seed sources upstream, which may reduce potential richness of downstream plant communities (Merritt and Wohl 2006, Brown and Chenoweth 2008). Jansson et al. (2000b) concluded that plant communities within impoundments were isolated (and compositionally distinct) from plant communities in impoundments upstream and downstream. Disconnection of riparian zones from their river channels may also affect plant colonization. Leyer (2006) found that disconnected floodplains along the Elbe River in Germany had fewer species-rich plant communities compared to those that remained connected with the river channel. This result is supported by Jansson et al. (2005), who found that recently established plant communities along river margins receiving hydrochores had 36–58% more species than adjacent communities colonized only by wind- and animal-dispersed seeds, and these results did not differ between free-flowing and dam-fragmented rivers

• Leyer (2006)

• Jansson et al. (2000b)