The dispersal and deposition of hydrochorous plant seeds in drainage ditches

Abstract

1. Surface water is an important dispersal vector for wetland plant species. However, most previous studies on hydrochory (i.e. water dispersal) have focused on ecosystems with relatively rapid water flow. Therefore, there is a need to study such dispersal in slowflowing or stagnant waterbodies, such as drainage ditches, which might act as dispersal corridors between habitat patches. 2. To gain insight into the mechanisms by which seeds are transported in drainage ditches, the effect of the velocity of wind and water on the rate of transport of floating seeds of three wetland species (Carex pseudocyperus L., Iris pseudacorus L. and Sparganium erectum L.) was investigated. Furthermore, in release and retrace experiments with painted C. pseudocyperus seeds, a number of factors potentially determining the probability of seed deposition were investigated. 3. Net wind speed was found to be the main factor determining the rate at which seeds are transported in drainage ditches. No relation between water flow at middepth in the ditches and seed transport was found. Wind speed and flow at the water surface were positively related. The effect of wind speed on the rate of transport of floating seeds was greater for S. erectum seeds, because a greater ratio of their volume protrudes from the water, than for C. pseudocyperus and I. pseudacorus seeds. 4. The principal factors that determine seed deposition were aquatic plant cover, ditch slope and indentations in the ditch bank. Seeds changed direction if the wind direction changed, or if there was a bend in the ditch. The final pattern of deposition was related to mean net wind speed. Mean transport distance after 2 days varied between 34 and 451 m. 5. Unlike in rivers, seed transport in ditches was determined by wind speed and direction, enabling multidirectional seed dispersal. We conclude that in slow-flowing waters, wind is a more important driver for hydrochorous seed transport than the flow of water. This sheds a new light on hydrochory and has important consequences for the management of otherwise fragmented wetland remnants.

Restoring macrophyte diversity in shallow temperate lakes: biotic versus abiotic constraints

“…Surprisingly, data on the buoyancy of seeds and other propagules from submerged macrophytes are lacking, but recent studies (Xie et al, 2010) reveal that at least vegetative propagules can float for several months. In shallow lakes, wind plays an important role in the dispersal route as the wind-induced currents transport the seeds (Sarneel, 2010;Soomers et al, 2010). Also, for charophytes, wind dispersal may play a role as spores are very light and generally easily dispersed by the wind.…”

Ditch network sustains functional connectivity and influences patterns of gene flow in an intensive agricultural landscape

“…The moisture gradient found along ditch banks provides habitat for various species, especially for wetland taxa, which often suffer greatly from the massive destruction of wetland habitats (Blomqvist et al, 2003;Herzon and Helenius, 2008). Furthermore, the presence of water allows hydrochorous seed dispersal to occur along ditches (Soomers et al, 2010). Hydrochory is an efficient dispersal mode for plant species that may be restricted along small water courses, and enables occasional long-distance gene flow (Kudoh and Whigham, 1997;Pollux et al, 2009;Leys et al, 2014).…”

“…Kudoh and Whigham (1997) and Schleuning et al (2011), for example, documented higher levels of gene flow and lower genetic differentiation among populations connected via flowing waters in a wetland macrophyte and a clonal herb, respectively. Most studies about the role of watercourses on patterns of gene flow focused on rivers and streams with defined current orientation, although hydrochorous seed dispersal can still occur in slow-flowing ditches for various wetland plant species (Gornall et al, 1998;Soomers et al, 2010). Such stagnant networks may form complex associations of watercourses with and without current orientation, leading to partially oriented networks were hydraulic connectivity estimations may be quite different from classical dendritic hydrologic networks.…”

“…Such stagnant networks may form complex associations of watercourses with and without current orientation, leading to partially oriented networks were hydraulic connectivity estimations may be quite different from classical dendritic hydrologic networks. In addition, connectivity may also be impacted by the ditch network structure and the presence of particular elements, like culverts, that may be barriers to seed dispersal (Soomers et al, 2010). In heterogenous habitats like ditches, adjacent land-use and management may further impact micro-habitat quality, which in turn influences species-specific metapopulation dynamics and patterns of genetic structure along linear habitats (Geertsema et al, 2002;Geertsema, 2005).…”

The dispersal capacity of vegetative propagules of riparian fen species

“…In November 2007, two mats were installed in eight ponds, including those sampled before. In fens, dispersal of seeds follows the main wind direction (Soomers et al, 2010;Sarneel, 2010). As the dominant wind direction is SW (Royal Netherlands Meteorological Institute), banks at the NE and SW ends of the ponds are likely to receive different propagule inputs.…”