Vegetation cover at the water surface best explains seed retention in open channels

Abstract

Hydrochorous dispersal through agricultural channels plays a role in structuring plant communities across agricultural landscapes. To date, research on seed retention in vegetated areas has mainly focused on vegetation types with simple architecture (often cylinders), which consequently do not represent real vegetation features. Here, we test the hypothesis that vegetation cover estimated at the water surface best explains floating seed retention in open channels. We therefore proposed an experiment to measure seed retention in a controlled environment across a large range of hydraulic conditions and vegetation architecture types. We used three types of artificial plants with contrasting morphotypes, and real seeds of Rumex crispus. Vegetation metrics were calculated on the basis of 3D plant models. We also tested the additivity of seed retention as a function of the length of vegetated area crossed by the seeds. We developed a semi‐empirical formula for predicting seed retention. The main results of the experiment show that (i) the seed retention rate reacts differently to changes in density according to species, (ii) vegetation cover at the free water surface, potentially in contact with seeds, is a generic predictor of floating seed retention whatever the nature of the vegetated cover, and (iii) 95% of seed retention was reached for a large range of surface vegetation ratios and length of vegetation cover. The proposed formula could be used by stakeholders (farmers and ecologists) to estimate the amount of vegetation needed in a channel to limit or enhance seed dispersal.

Influence of maintenance practices on hydraulic functions of ditches: A trait‐based approach

“…Two effect traits associated with the two studied hydraulic ecosystem functions (water transport regulation and seed retention) were measured at a specific water depth (60 cm), representative of the water depth encountered in ditches during an intense rainfall: (i) the blockage factor as the proportion of the wetted cross section occupied by plant stands (Vinatier et al, 2017) and (ii) the surface vegetation ratio as the area of the plant cover at the free surface of the water (Rudi et al, 2021). These two indicators depend on the height and specific architecture of individual plants (the specific architecture is in this study taken in consideration through the frontal area), as well as on the density of the community.…”

“…These two effect traits were used to assess the studied ecosystem functions (water transport and seed retention). On the basis of the relation between the blockage factor ( ) and the water friction expressed with the manning coefficient ( ) (Vinatier et al, 2017) and surface vegetation ratio ( ) and water permeability to seed expressed at the percentage of retained seeds ( ) (Rudi et al, 2021), we calculated the gain in each indicator by the formulas presented in Equations () and (). Note that the Manning coefficient ( ) is an empirical coefficient, widely used in hydraulics, accounting for the roughness of the surface in contact with the water flow (and therefore largely depending on vegetation characteristics).…”

“…As highlighted in Rudi et al (2020), many studies used the trait framework to study the modification of ecosystem functions following disturbances due to agricultural practices; however, to our knowledge, no trait‐based study focused on ditch plant vegetation and associated ecosystem functions. Consequently, in this study, rather than focusing only on the ability of these maintenance operations to affect the plant species richness and abundance of species in ditches, we investigated the effect of maintenance on the selection of traits influencing water transport, the main driver of ditch functioning, and influencing seed retention, involved in ditch restoration capacity (these two ecosystem functions being described in detail and quantitatively related to parameters and densities of plant communities and ditch hydraulic characteristics in Vinatier et al, 2017 and Rudi et al, 2021). Following the definitions provided by Violle et al (2007), a trait in this study is a morphological feature that can be measured at the individual level.…”

“…Many studies conducted in natural and experimental open channels have highlighted the fact that plant heights, in relation to the density of stems and architecture of their aerial parts, influence hydraulic resistance (Baptist et al, 2007; Busari & Li, 2015; Cheng, 2011; Green, 2005; James et al, 2004; Luhar et al, 2008; Nepf, 2011; Nikora et al, 2008) and permeability to floating and nonfloating seeds (Chambert & James, 2009; Cornacchia et al, 2019; Defina & Peruzzo, 2010; Rudi et al, 2021). By projecting plant aerial parts to the channel section, we access good indicators of hydraulic resistance and difference in permeability of the cover to different particles such as seeds (Nepf, 2012; Rudi et al, 2020, 2021; Vinatier et al, 2017). Therefore, the modification of plant traits and density by maintenance practices in ditches can have consequences for water transport regulation and seed retention.…”

How long do seeds float? The potential role of hydrochory in passive revegetation management

“…Unfortunately, disturbance caused by river restoration activities can also favour the establishment of invasive plant species (Catford & Jansson, 2014; Gilman, Abell, & Williams, 2004; Toth, 2015). For this reason, it is important to understand the different hydrochoric capabilities of seeds from plant species that are either native or exotic (and hence desirable, and undesirable respectively, in a river restoration context) (e.g., Rudi, Bailly, Belaud, & Vinatier, 2018; Rudi, Belaud, Troiano, Bailly, & Vinatier, 2021). It is also important to consider the relative importance of local versus upstream seed sources as a possible regeneration tool and the extent to which the seeds supplied will travel short or long distances.…”