Higher structural connectivity and resistance against invasions of soil bioengineering over hard-engineering for riverbank stabilisation

Riparian corridors play an important role for the maintenance of regional biodiversity and ecosystem functions. Riparian forests are even the only semi-natural vegetation strips remaining in many agricultural or urbanised landscapes. In such landscapes, the spatial continuity of riparian vegetation is frequently broken by the construction of stabilisation structures engineered for erosion control. Here, we examined the effects of different riverbank stabilisation structures—fascines (soil bioengineering), ripraps (hard engineering), and mixed-technique (lower-bank ripraps with upper-bank plantings)—on the structural connectivity of their respective riverbanks. We first revisited previously studied stabilisation structures to extend their vegetation sampling to their adjacent riverbanks. Then, for each type of stabilisation structure, we compared community composition, richness and abundance of native and invasive alien species (IAS), and cover of vegetation strata (herbaceous, shrub and tree) between stabilised embankments and their upstream and downstream banks. Results indicated that, although the composition of fascine banks differed from that of their adjacent riverbanks, they fitted nicely in the structural continuity of their riparian surroundings. Differences were likely explained by the proportion of fast-growing woody species (e.g. willows) planted in fascines, which also induced strong reductions in IAS richness and abundances; i.e. propagule “sinks”. Conversely, ripraps broke the structural continuity of riverbanks and were heavily dominated by IAS while mixed-technique banks displayed intermediate characteristics. Consequently, we argued that fascines may be the riverbank stabilisation structures displaying highest ecological benefits in terms of habitat quality and connectivity and should be preferred over the other investigated engineering techniques.