Which strategies enable alien plant species to invade braided river systems?

Braided rivers represent extraordinary wetland ecosystems due to their high disturbance regimes. Their riparian vegetation needs to be adapted to these harsh conditions. At the same time, braided river systems are sensitive to changes in the underlying environmental factors. Additionally, their open gravel and sand banks represent germination sites for invasive alien plant species. There is a risk that invasive species can spread over large areas and thus alter river reaches, even leading to morphological changes in the entire river system.

The aim of this study was to identify similarities and differences in the key functional traits and strategies of invasive alien plants compared to the dominant native plant species in braided rivers around the world. The analysis of what makes invasive species more successful than the native species was carried out in seven globally distributed mountain regions with braided rivers, including parts of the European Alps, the mediterranean Vjosa river system, Iceland, Alaska, Patagonia, Central Asia, and New Zealand. Studied river system cover different climate zones, with varying degrees of human influence.

We used a plot design (10m by 10m) covering all occurring biogeomorphic succession phases (Corenblit et al. 2007) in the investigated river reaches. In each plot, we recorded general information on the habitat conditions and vegetation characteristics. We combined the field data with response and effect trait data from the TRY trait database (Kattge et al. 2020). The traits of the dominant native and invasive alien plant species were compared using a PCA analysis to reveal differences in trait combinations that can explain the success of invasive species.

First findings indicate that successfully invading alien species enter the braided river systems in the pioneer succession phase and comprise traits for dealing with the high hydromorphodynamic disturbances, for example a short lifespan. They have rather smaller specific leaf area and plant height. By decreasing the disturbance impact through dense vegetation cover and trapping of fine sediment they push their growing areas to the following biogeomorphic succession phase. Here, especially invasive ecosystem engineer species improve their own growing conditions and may create dense vegetation covers in the formerly open active channel.

Across our study areas, a wide range of the magnitude of plant invasion was visible: While in the Alaskan rivers with extreme arctic to boreal climate and low human impact no invasive alien plants occured, the braided rivers in the intensively agriculturally used Canterbury region on New Zealand's South Island were highly invaded. Here, many formerly sparsely vegetated active channels were densely covered with alien species and the later succession phases prevailed.

 

References

Corenblit, D., Tabacchi, E., Steiger, J., & Gurnell, A. M. (2007). Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: a review of complementary approaches. Earth-Science Reviews, 84(1-2): 56-86.

Kattge, J., Bönisch, G., Díaz, S., Lavorel, S., Prentice, I. C., Leadley, P., Tautenhahn, S., & Werner, G., et al. (2020). TRY plant trait database - enhanced coverage and open access. Global Change Biology, 26(1): 119-188. doi:10.1111/gcb.14904