What are the drivers of riparian vegetation recruitment and establishment in an alpine embanked river?

Julien Godfroy , Laurent Borgniet, Thibault Boissy, Herve Piegay, Gabriel Melun, Philippe Janssen

1 – Rejuvenation operations are often conducted in rivers to manage vegetation encroachment and balance biodiversity benefits and flood risks. Understanding the drivers of vegetation recruitment and establishment is necessary to implement these operations but requires monitoring at spatial and temporal scales that are suited to pioneer riparian environment.
2 – Bi-yearly drone surveys were used to retrieve the trajectory of pioneer vegetation in a six kilometres reach of the Isère River (France) following rejuvenation operations. Visible images acquired at the beginning and the end of six consecutive vegetation periods were classified using a convolutional neural network and changes in vegetation cover through time were characterized using spatial operations across dates.
3 – Vegetation encroachment in the reach was driven by patch expansion and presented a high inter-annual variability. Provided environmental conditions were favourable, vegetation could cover 20% of bar surfaces in a two-year period following rejuvenation actions. Vegetation destruction in the reach was on the other hand mainly driven by vegetation clearing operations.
4 – Vegetation recruitment was found to be mainly driven by the hydrological conditions during the seed dispersal and germination period in May and June. Prolonged bar submersion for flow levels starting at 125 m3.s-1 led to lower recruitment while prolonged dry periods were instead favourable to vegetation encroachment. Under favourable hydrological conditions, recruitment mainly occurred in areas that were covered by fine sediments or water at the beginning of the vegetative period.
5 – Spring and winter floods did not result in significant vegetation destruction when plants had a least a year to establish. The surfaces of vegetation destroyed by the Isère River was always lower than the net gain in vegetation cover for each period, and the only spike in vegetation destruction occurred when most vegetation in the reach was less than one year old.
6 – Synthesis: Summer hydrology played a key role in vegetation encroachment in the reach by providing either favourable (i.e., prolonged dry periods and no high flows late June) or unfavourable conditions for vegetation recruitment and establishment. Timing eco-morphogenic releases to sustain submersion could help prevent encroachment.