Ecological restoration under pressure from invasive animal species: use of Salicaceae cuttings in a river bank overrun by coypu | Restauration écologique face au développement d'espèces animales invasives : utilisation de boutures de salicacées dans un contexte de berge de rivière dégradée par le ragondin

Numerous studies have established the fundamental ecological role of riparian zones as well as the main threats they face, e.g., by invasive species. Our study concerns a section of a gently sloping river in the western part of France where the coypu, an invasive aquatic rodent, damages streambanks and prevents the regeneration of natural vegetation. The aim was to assess the efficacy of different species used in bioengineering projects for streambank protection and for the restoration of woody habitats. A field experiment was established close to the river bank with cuttings of several woody species belonging to the Salicaceae family. The test consisted in comparing (i) different species (six willows and one poplar), (ii) different cutting sizes, and (iii) the presence or absence of fencing (to protect against coypus). The experiment was conducted over three growing seasons (2009 to 2011). The measured response variables were the survivorship and the growth expressed firstly by the length and the number of shoots, and secondly by the height of plants. The results showed: (i) high growth and survivorship rates for all tested species, notwithstanding some significant differences between species; (ii) positive effects of both the fencing and the planting of longer cuttings. After three growing seasons, Salix viminalis showed the highest growth in exclosures with a mean cumulative shoot length of 7.1 m, whereas outside exclosures Salix purpurea had the highest growth with a mean cumulative shoot length of 3.9 m. Populus nigra and S. purpurea showed the lowest number of stems browsed by the coypu. The different methods were also compared for cost and time-to-establishment: the long cuttings of certain species afford a positive effect as significant as fencing (which is more expensive). Such analyses are useful when it is necessary to strike a balance between cost and efficiency, particularly in the context of invasiveness.