BACKGROUND

Riparian areas are considered ecotones between rivers and terrestrial ecosystems. This proximity to the rivers provides riparian zones with permanent water availability, that together with the usually high fertility of soils (due to the deposition of sediments transported by rivers), confer to riparian zones a high potential for primary production. The combination of these favorable conditions for plant life with their role as corridors for propagules and a high frequency of natural and/or anthropic disturbances, make riparian zones especially vulnerable to exotic plant invasions.
Invasive plants have been often reported to significantly affect forest ecosystems, as they can alter soil properties, productivity, decomposition rates6, and plant, bacterial, and macroinvertebrate communities. In headwater streams, the high canopy cover reduces the entrance of sunlight limiting primary production; thus, allochthonous leaf litter coming from riparian vegetation stands for the main organic matter input. Consequently, changes affecting the quality and/or quantity of these organic matter subsidies can potentially affect key processes, as leaf litter decomposition or nutrient cycling as well as, microbial and invertebrate decomposers, or vertebrate community structure. Despite that, the study of the effects of invasive plants on fluvial food webs has undergone a strong bias in the selection of invasive species, and most studies have been focused on a few species, such as Eucalyptus globulus or Robinia pseudoacacia. However, other highly
aggressive species as Fallopia japonica and Arundo donax, which have the potential to significantly affect streams functioning, have been set aside. Additionally, plant invasions do not appear as isolated perturbations. Headwater ecosystems are subjected to other stressors as water pollution, changes in land use (e.g., cropland abandonment, forestry practices, or climate change). These anthropogenic disturbances can favor invasive species spreading. Such a context exhibits the need of assessing the effects of potential plant invasions on the functioning of forested streams that are already affected by other stressors, such as the substitution of native forests by plantations of exotic riparian species.
The leaf litter that fuels headwater ecosystems may reach streams directly from riparian plants (i.e., vertical inputs) or by lateral movement after short or long-term periods in the riverbanks (i.e., lateral inputs). These inputs might constitute 81 % of total leaf litter in streams in temperate regions. Moreover, this exposure can modify the degree of integration in food webs, since during terrestrial decomposition the litter quality may be heavily affected. Thus, changes in the riparian forest can alter both the nature and the pre-conditioning of the leaf litter material supply and its subsequent instream decomposition.
On top of that, although most studies agree on the fact that climate change will increase the risk of invasion, there is still some controversy (others have reported that climate change may potentially reduce that risk) especially with the expansion of less-studied species. An evaluation of the effects of potentially invasive species on stream ecosystems subjected to different environmental conditions and anthropogenic impacts will enable us to face the problem from a broad perspective. To achieve that, it would be indispensable the collaboration of other researchers from multiple regions.