Effects of hydropeaking waves' offsets on growth performances of juvenile Salix species

Hydropower production and other human activities change the natural flow regime of rivers (e.g., hydropeaking effects), in turn, impacting the riparian environment. Here we present an experiment conceived to investigate the response of forced offsets in water table dynamics on the temperate floodplain riparian vegetation species of Salix viminalis. Three specific water level regimes, simulating daily hydropeaking effects in terms of daily water table variations and offsets, were applied to three batteries of Salix cuttings growing outdoors in plastic pots. All pots initially were provided the same unlimited water supply. Then the dynamics were changed to include 90-cm daily oscillations of water table. The transitory responses of the cuttings were quantified with measurements of continuous sap flow and water potential. We regularly monitored growth parameters, leaf water potential, and gas exchange measurements, and took pictures of each plant. At the end of the experiment, all cuttings were removed and the biomass, from both above and below ground, analyzed. The 3D root structure was investigated using high-resolution computed tomography. Our results showed a clear link between root distribution and water regime. The strong offset of water table dynamics was followed by root system adjustment and plant adaptation to the new conditions. Macroscopic effects in terms of growth parameters corresponded with water pressure and transpiration measurements, strengthening our interpretation of the results. Daily hydropeaking offset appeared to have only a minor impact on the development and photosynthesis of S. viminalis cuttings, and can probably be neglected in real cases of dam regulation. On the other hand, weekly periodicity (e.g., weekend periods of dryness) could have harsher impacts, and should be evaluated in future studies.