RESUMO
Biotic and abiotic factors are important drivers of the introduction, dispersal and establishment of an invasive species in fluvial corridors. In this study, we propose to better understand the spatial distribution of Asian knotweeds and to model their invasibility at the river basin scale in the Rhône Mediterranean and Corsica regions, France. We implemented a multiscale analysis of biophysical and anthropogenic factors related to the presence of knotweeds. Subbasins were sampled (50-600 km2), a large dataset on knotweed occurrence and biotic/abiotic factors was collected, and logistic regression was applied. A robust logit model (accuracy: 90%; false positive rate: 13%) estimated the probability of the occurrence of knotweeds at the river basin scale. We found clear evidence of: i) spatial scale-dependent water availability for knotweed implantation (e.g., summer vs. winter rainfalls > 250 mm); ii) an important role of hydrogeomorphic forces in dispersal; and iii) interspecific competition in riparian areas. The occurrence of knotweeds is also closely related to human-derived pressures. The management of knotweeds on roads and railways in the vicinity of rivers may be a major source of propagules. Hydraulic infrastructures (dikes and mill weirs) may also have served as locations of knotweed introduction since the end of the nineteenth century and may play a major role in the propagule transfer of knotweed; to date, these infrastructures have provided favourable conditions for knotweed establishment. Despite local water authorities' increasing awareness of invasive plants, local management practices for flood mitigation, low awareness of roads/railway managers, and negative representations of knotweeds have probably largely contributed to their dispersion over decades. The final model intends to integrate these biophysical and human factors by providing an operational tool to help river managers determine the sensitivity of their river basins to knotweed invasion.
RESUMO
In most aquatic ecosystems, hydrodynamic conditions are a key abiotic factor determining species distributions and abundance of aquatic plants. Resisting stress and keeping an upright position often relies on investment in tissue reinforcement, which is costly to produce. Silica could provide a more economical alternative. Two laboratory experiments were conducted to measure the response of two submerged species, Egeria densa Planch. and Limnophila heterophylla (Roxb.) Benth., to dissolved silicic acid availability and exposure to hydrodynamic stress. The results were verified with a third species in a field study (Nuphar lutea (L.) Smith). Biogenic silica (BSi) concentration in both stems and leaves increases with increasing dissolved silica availability but also with the presence of hydrodynamic stress. We suggest that the inclusion of extra silica enables the plant to alternatively invest its energy in the production of lignin and cellulose. Although we found no significant effects of hydrodynamic stress on cellulose or lignin concentrations either in the laboratory or in the field, BSi was negatively correlated with cellulose concentration and positively correlated with lignin concentration in samples collected in the field study. This implies that the plant might perform with equal energy efficiency in both standing and running water environments. This could provide submerged species with a tool to respond to abiotic factors, to adapt to new ecological conditions and hence potentially colonise new environments.
