Go with the flow: Fragment retention patterns shape the vegetative dispersal of aquatic plants in lowland streams

Abstract

1. The dispersal of aquatic plant propagules is highly facilitated in streams due to flow. As many aquatic plants predominantly spread through vegetative propagules, the specific retention and thus drift distance of dispersed plant fragments largely contribute to the rapid spread along the course of a stream. 2. We determined fragment retention for four aquatic plant species (Elodea canadensis, Myriophyllum spicatum, Ceratophyllum demersum, Salvinia natans; representing four different common morpho-structural groups) in sections of small to mediumsized German streams with different levels of stream sinuosity. 3. The number of fragments showed a logistic decline over drift distance. In two small streams, 90% of drifting fragments were retained at distances (D 90) of only 5-9 m and 19-70 m, while higher D 90 values of 116-903 m and 153-2,367 m were determined for sections of a medium-sized stream. The likelihood of retention thereby decreased significantly with increasing stream size and was reduced in straightened stream sections. 4. Differences in retention were more strongly related to fragment buoyancy rather than fragment size and morphology. Increasing buoyancy significantly lowered the likelihood of fragment retention over drift distance by a factor of 3-8, whereas contrasting effects were documented for size and morphology of fragments. 5. The relevance of different obstacles was highly stream section-specific and depended on obstacle abundance, distribution, and the degree of submergence/ emergence. 6. Our findings elucidate the dynamic retention patterns of plant fragments and highlight the strong interplay between extrinsic (stream) and intrinsic (fragment) properties. We conclude that straightened lowland streams of intermediate size promote the rapid dispersal of invasive aquatic plants and are particularly prone to invaders producing large amounts of small and highly buoyant plant fragments. Information on the species-specific fragment colonisation dynamics in the field is further required to improve our understanding of the vegetative dispersal capacity of invasive aquatic plants in stream ecosystems.

The hydrochorous dispersal of plant propagules in a giant river reservoir: Implications for restoration of riparian vegetation

“…The channel morphology was also not very relevant to dispersal distance in this study, which is inconsistent with previous studies (Hyslop & Trowsdale, 2012; Merritt & Wohl, 2002; Su et al, 2019). The main reason for this contradiction is that the channel width of the regulated rivers (29.9–163.7 m) is much broader than rivers in previous studies (2.0–4.1 m in Riis & Sand‐Jensen, 2006; 1.5–8.0 m in de Jager et al, 2019; 1.7–4.5 m in Heidbüchel et al, 2020), which makes the chance of propagule mimics interacting with channel boundaries relatively small, and thereby less important.…”

“…The coordinates of the retained propagule mimics were imported into Google Earth software (Google LLC, USA) to measure the channel distance between the propagule mimics' release and the retrieval points. D 90 was used to describe the distances (km) when 90% of retrieved propagule mimics were retained (Heidbüchel et al, 2020). The impoundment water level of the TGR during the experiment was downloaded from the Hydrological Network of the Yangtze River (http://www.cjh.com.cn/).…”

Stability Analysis of Submerged Vegetation Patterns in Rivers

“…The proposed formulation accounts for the positive feedbacks induced by the removal of propagules on the growth rate of the plants. We proved that the onset of vegetation patches from a uniform vegetated riverbed may be explained by the transport and resettlement of uprooted or broken propagules (Eckert et al., 2016; Heidbüchel et al., 2020).…”

“…The growth of vegetation density depends on the deposition and establishment of vegetative propagules, generated by flow induced breakage or uprooting (Gurnell et al., 2007). Propagules are transported downstream within the water column or floating on the water surface before settling (Eckert et al., 2016; Gurnell et al., 2008; Heidbüchel et al., 2020; Nilsson et al., 2010). In the classic equation (Equation ), such processes are intrinsically modeled by the vegetation growth rate, ν g .…”

Retention of viability by fragmented invasive Crassula helmsii, Elodea canadensis and Lagarosiphon major

“…Invasive aquatic macrophytes have been shown to adversely affect freshwater ecosystems and represent a considerable management burden (e.g., Cuthbert et al, 2021; Hussner et al, 2017). Successful dispersal and establishment of invasive macrophytes are largely dependent on fragmentary propagule survival and viability in relation to root and shoot production (Coughlan, Cuthbert, Kelly, & Jansen, 2018; Heidbüchel, Sachs, Hamzehian, & Hussner, 2020). Generally, the abundance and reproductive quality of propagules that are introduced increase the likelihood of viable propagules colonising suitable environments (see Stringham & Lockwood, 2021 for discussion).…”