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Original Research ARTICLE Provisionally accepted The full-text will be published soon. Notify me

Front. Environ. Sci. | doi: 10.3389/fenvs.2019.00124

When does vegetation establish on gravel bars? Observations and modelling in the Alpine Rhine river

  • 1Institute of Hydraulic Engineering, Hydrology and Glaciology, Department of Construction, Environment and Geomatics, ETH Zurich, Switzerland
  • 2Department of Civil, Environmental and Mechanical Engineering, University of Trento, Italy

Predicting when riparian vegetation establishes on river bars is crucial to estimate how morphology and ecology of gravel bed rivers respond to anthropogenic or climatic changes. However, the characteristic timescale required for vegetation establishment on gravel bars remains poorly investigated. The Windows of Opportunity (WoO) concept provides an ideal framework for estimating such timescale by analysing time series of disturbance periods following seed dispersal. Here we present the results of a study conducted in a 41 km long reach of the Alpine Rhine river characterised by the presence of alternate gravel bars, which show differences in morphological activity and vegetated areas. We mapped the time evolution of vegetation cover between 1996 and 2017 by using aerial images and related vegetation occurrence to bar topography. Observations show that vegetation mainly colonised bar surfaces between 2005 and 2008, enduring on bars that showed less morphological activity. Only few patches, which were removed by downstream bar migration, were observed on bars that experienced significant morphological changes. This allowed us to identify conditions that were more favourable to vegetation recruitment and growth. To explain the vegetation pattern observed along the whole reach, we developed a simple modelling framework based on the WoO concept including the effects of flow variability on seed dispersal and seedling recruitment and survival. Model applications demonstrate that vegetation successfully establishes on bare areas if plants can withstand hydrodynamic disturbances for about 85 days after seed dispersal. We also identified timing and location of successful recruitment events and discussed how they are linked to bar morphology, seed dispersal, and riverbed morphodynamics. This study provides a first attempt to quantify the WoO in a gravel bed river with alternate bars, representing a step towards the development of quantitative tools for predicting river morphological trajectories.

Keywords: riparian vegetation, Alternate bar, Gravel bed river, windows of opportunity, Alpine Rhine

Received: 23 May 2019; Accepted: 12 Aug 2019.

Edited by:

Paolo Perona, University of Edinburgh, United Kingdom

Reviewed by:

Teresa Ferreira, University of Lisbon, Portugal
Valentina Bau’, University of Edinburgh, United Kingdom  

Copyright: © 2019 Caponi, Koch, Bertoldi, Vetsch and Siviglia. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Mr. Francesco Caponi, Institute of Hydraulic Engineering, Hydrology and Glaciology, Department of Construction, Environment and Geomatics, ETH Zurich, Zurich, 8093, Zürich, Switzerland, caponi@vaw.baug.ethz.ch