1. Politecnico di Torino
Ecole Polytechnique
F´ed´erale de Lausanne
Master Thesis
Vegetation growth response to river
hydrodynamic variability: the role of
water table dynamics
Author:
Nicola Testa
Supervisors:
Prof. Paolo Perona
Prof. Francesco Laio
A thesis submitted in fulfillment of the requirements
for the degree of Environmental Engineer
Lausanne - Torino
December 2014

2. Abstract. The biological dynamics of riparian and riverbed vegetation play an impor-
tant role in fluvial morphodynamics. Hydrologic, hydraulic and ecological processes in-
teract and drive the colonization and possible stabilization of alluvial bedforms through
the growth of vegetation roots. Furthermore, vegetation influences the magnitude of
impacting floods on islands and bars and determines specific riverbed morphological
features.
This work completes previous studies which have explored and clarified the water use
(Gorla et al. [2014]), root development (Pasquale et al. [2012], Tron et al. [2014]) and
anchoring (Pasquale et al. [2014]) of salix cutting species, as influenced by different hy-
drologic regimes.
The aim of this experiment, set up in the EPFL campus, is to study the paired effects
of the water table fluctuations and a preferential mechanical stress on the growth of
juvenile riparian vegetation on alluvial material forming river bars and islands. The an-
alyzed vegetation dynamics are biomass distribution of shoots and roots, both radially
and vertically. Periodic monitoring and measurements at the end of the experiment were
done to study the biomass growth.
The focus is on two groups of five cuttings grown in the same environmental conditions
except for the water levels. One group, labeled treatment C, was subjected to water-level
fluctuations (range of oscillation close to the soil surface), while the other group, labeled
treatment B (steady water-table), experienced a constant level. On the other hand, they
were grown in the same conditions regarding cutting biomass, soil composition, weather
variables and mechanical stress induced by the simulated drag force of hydrodynamical
origin.
Treatment B (steady water-table) develops slightly more biomass, during the growing
season, than treatment C in terms of average values. A great variability in terms of
shoot collected data is experienced.
In the below-ground system, roots react to the external forcing by adapting their distri-
bution to permit plant survival. In particular, the main outcome is that oxytropism is
responsible for determining the vertical root-density distribution. These experimental
results confirm some field observations (Pasquale et al. [2012]) and analytical analysis
(Tron et al. [2014]): a water table characterized by a low variability induces root growth
in deeper layers and confined to a narrow range near the minimum water-table elevation
(this is the case for treatment B).
Concerning root-profile radial directionality, in steady water-table conditions the asym-
metry of root profile is less than in simulated river hydrodynamic conditions. In both
cases the direction of this asymmetry is upstream (opposite to the direction of drag
term). In treatment B (steady water table), Z-test and T-test accept the null hypothesis
of not relevant asymmetry, but with low probability.
The effects of the mechanical stress have a minor impact in steady water table conditions,
and this is explained by the fact that the root profile is relatively deeper, given that the
influence of the mechanical stress on the directionality of root growth is expected on the
higher part of the embedded cutting.
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