Combined effects of invasive plant species and other stressors in streams ecosystems through riverbank-stream interactions (INVASORIAN)

International audience

إنجليزي. Riparian zones are ecotones at the interface between terrestrial and freshwater ecosystems, highly susceptible to the invasion by non-native plant species. Those species have the potential to impact stream and riparian ecosystems negatively by altering fire regime, water use, nutrient cycles and biotic communities. Moreover, those ecosystems are highly vulnerable to other stressors such as climate change, water pollution or the replacement of native forests. Specifically, substitution of native riparian vegetation by non-native plant species can have strong effects on terrestrial and stream communities and on ecosystem processes, such as litter decomposition. The INVASORIAN project aims to analyse the decomposition of invasive species instream and in the riparian zone of streams with natural vs. planted (non-native) riparian forests. We performed a coordinated leaf litter decomposition experiment (litter bag technique) by incubating three invasive species (Arundo donax, Ailanthus altissima and Fallopia japonica) and a common riparian species (control; Alnus glutinosa) in 18 streams running through native and non-native riparian forest at 9 locations in 4 countries (Spain, Portugal, France and Germany). Three different sets of litter bags (n=120) were deployed between riparian and stream ecosystems to analyse terrestrial (inland incubation, 2 months), and aquatic (vertical input: instream, 1 month; and lateral input: inland+instream, 2+1 months) decomposition rates. Additionally, stream discharge, water physicochemistry (temperature, pH, conductivity, and dissolved oxygen), riparian forest soil moisture and organic matter content, and riparian plant communities from each stream reach were characterized. We hypothesised that 1) leaf litter of invasive species will exhibit lower decomposition rates than of the native A. glutinosa; 2) differences in decomposition rates will be higher in ecosystems already subjected to the substitution of the native riparian forest by tree plantations due to: i) decreased inland leaf litter processing and, ii) decreased instream decomposition as a result of less diverse invertebrate communities. This study is pioneer assessing the links between terrestrial and aquatic decomposition of invasive species at a large scale. As biological invasions are becoming more common due to global change, it is essential to understand how these invasions potentially affect the processing of organic matter across natural and anthopogenized landscapes to ensure the conservation of functional ecosystems.