For many plant species, nutrient availability induces important anatomical responses, particularly the production of low-density tissues to the detriment of supporting tissues. Due to the contrasting biomechanical properties of plant tissues, these anatomical responses may induce important modifications in the biomechanical properties of plant organs. The aim of this study was to determine the effects of nutrient enrichment on the anatomical traits of two freshwater plant species and its consequences on plant biomechanical performance. Two plant species were grown under controlled conditions in low versus high nutrient levels. The anatomical and biomechanical traits of the plant stems were measured. Both species produced tissues with lower densities under nutrient-rich conditions, accompanied by modifications in the structure of the aerenchyma for one species. As expected, nutrient enrichment also led to important modifications in the biomechanical properties of the stem for both species. In particular, mechanical resistance (breaking force and strength) and stiffness of stems were significantly reduced under nutrient rich conditions. The production of weaker stem tissues as a result of nutrient enrichment may increase the risk of plants to mechanical failure, thus challenging plant maintenance in mechanically stressful or disturbed habitats.

Secondary dispersal of terrestrial plant species through watercourses increases the dispersal range of floating seeds but their exposure to water may also challenge their viability and colonisation potential. Many terrestrial invasive species benefit from watercourses to extend their invasive range because water exposure can favour seed germination and early survival of seedlings for species sensitive to dry conditions and water flow enhances their dispersal distance downstream. Floatability, germination rate and survival after water exposure are seed traits involved in water dispersal and their variability may lead to different dispersal strategies. To address the question of the variability of achene traits involved in dispersal, we experimentally measured these three traits for Fallopia × bohemica achenes collected from 10 different stands. The floatability of achenes varied between stands from 2.5 to 5 days and their germination in water extended this period to 20 days through the emergence of floating seedlings. Germination proportions also differed between stands from 0.3 to 0.95 and were negatively related to the median germination time. Seedling survival after planting in soil was affected by their exposure time. This effect differed between stands, but a short germination time did not explain a better survival of some stands. These results show that achenes may differ in their dispersal potential in terms of floatability, as well as tolerance of seedlings to water exposure during dispersal. This study also highlights the fact that achenes of some stands present a combination of traits particularly adapted to long distance dispersal through watercourses, which can contribute to the extension of the invaded area.

Aims: Landscape fragmentation exhibits strong negative consequences on biodiversity. In networks of linear elements, connectivity loss results in a decreased length of connected elements and increased potential barriers, directly impacting the ability of plants to disperse. However, species vary in their tolerance to connectivity loss, likely due to differences in dispersal strategies. We investigated whether species tolerance to decreased ditch network connectivity is determined by seed traits. We selected as a case study, water-dispersed plant species in a ditch network.Location: Ditch network established in an intensive agricultural area in northern France. Methods:We selected 27 sites of 500 x 500 m, where we calculated connectivity indices based on the length of connected ditches, intersection and culvert number. For each parameter, we calculated plant tolerance levels by analysing species changes in occurrence in response to change in connectivity values. Concurrently, we measured in laboratory conditions five seed traits involved in plant movement and establishment in standing aquatic systems and analysed their explanatory power in plant tolerance to fragmentation.Results: All traits were significantly related to at least one component of ditch network connectivity. We interpreted the following two strategies in plant tolerance to connectivity loss from the results: (1) in networks where the connected network length was short plants displayed short-distance dispersal with less efficient sexual reproduction, probably in favour of local vegetative multiplication; and (2) in networks with a high density of culverts or intersections, plants displayed seeds with reduced local retention, where seeds had the capacity to overcome long and frequent trapping events. In highly branched networks, plants exhibited also higher germination rates, promoting seed establishment when trapped along the banks. Seed capacity to be dispersed by wind at the water surface was only a marginal factor in plant tolerance to fragmentation. Conclusions:Connectivity loss acted as a filter on species seed traits. The results of our study offer an enhanced understanding of plant dispersal in fragmented standing aquatic networks and emphasise the importance of developing functional approaches in landscape studies.