Tree root systems architecture in earth dike

Abstract

Trees growing on earth dikes generates safety problems and reduces dike durability. Root systems generate internal and external erosion risks which can be important on dikes wooded for a long time. Erosion risks increase with tree age and stand density. The aim of this study is to analyse root systems characteristics of trees rooted in canal dikes and flood protection dikes. Root systems structure and morphology depend on species and materials. On various species studied, different types of roots can be observed. Short and long roots are characterized by differences in diameter decrease, branching frequency and inclination. A root typology is determined.

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International Symposium “Root Research and Applications”
RootRAP, 2–4 September 2009, Boku – Vienna, Austria
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Tree root systems architecture in earth dike
Caroline Zanetti ac, Michel Vennetier b, Patrice Mériaux a, Paul Royet a, Mireille Provansal c,
Simon Dufour c
a Cemagref, Hydraulics engineering and hydrology Research Unit, 3275 route de
Cézanne, CS 40061, 13182 Aix-en-Provence, cedex 5, FRANCE
b Cemagref, Mediterranean ecosystems and associated risks Research Unit, 3275 route de
Cézanne, CS 40061, 13182 Aix-en-Provence, cedex 5, FRANCE
c CEREGE, Europôle de l’Arbois, BP 80, 13545 Aix-en-Provence, FRANCE
Contact: caroline.zanetti@cemagref.fr, Tel: +33 4 42 66 99 56 – Fax: +33 4 42 66 88 65
ABSTRACT
Trees growing on earth dikes generates safety problems and reduces dike durability. Root systems
generate internal and external erosion risks which can be important on dikes wooded for a long time.
Erosion risks increase with tree age and stand density.
The aim of this study is to analyse root systems characteristics of trees rooted in canal dikes and flood
protection dikes. Root systems structure and morphology depend on species and materials.
On various species studied, different types of roots can be observed. Short and long roots are characterized
by differences in diameter decrease, branching frequency and inclination. A root typology is determined.
Key words: stump structure, root system architecture, root typology, earth dikes safety
INTRODUCTIONS
Trees growing on earth dike generates safety problems and reduces dike durability. Tree roots in earth
dikes generate two types of risks:
- internal erosion witch is related to galleries created by rotten roots in earthfill and also to mechanical
action of live roots which can decompact dike materials or destroy masonry protection works or joints;
- and external erosion witch is often related to tree uprooting (during storms for example).
In order to determine the impact of woody root systems on the structure and the durability of dike
embankments, it is necessary to analyze root systems characteristics for different tree species.
The objectives are to acquire data about tree root systems in dikes (on 2 types of dikes: canal dikes and
flood protection dikes).
The acquisition of knowledge on the structure of woody root systems in dikes - morphology, root network
and architecture - requires digging up trees.
METHODS
In order to determine the impact of woody root systems on dike embankments, 100 trees of
various species including poplar (Populus spp.), willow (Salix alba), black locust (Robinia
pseudoacacia), maple (Acer spp.), ash (Fraxinus excelsior), oak (Querqus spp.), larch (Larix
decidua) and pine (Pinus silvestris) were excavated cautiously with mechanical shovel on 7 sites
(Figure 1 and Table 1).

International Symposium “Root Research and Applications”
RootRAP, 2–4 September 2009, Boku – Vienna, Austria
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Then manual measurements were carried out on stumps (length, width, depth) and on root
architecture (diameter, length, branching, direction and angle of the roots).
A root typology was defined according to the architectural characteristics of measured roots.
RESULTS
Types of dike and trees position affect considerably stump structure and root architecture. Stump
structure and volume, root distribution, diameter and length depend on many factors: species,
stump age, dike materials (texture, structure, compactness, organic matter content) and water
availability.
Stump structure and root architecture are different on canal and flood protection dikes. On canal
dikes root systems are composed by few big winding roots compared with flood protection dikes
where root systems are composed by many little straight roots.
Root systems are made up of long horizontal roots, of short slanted roots and sometimes of tap
roots. These three types of roots have different rates of diameter decrease, branching frequency
and angle (Table 2). Robinia and poplar have some big long horizontal roots with a diameter
decrease rate of 2%. This rate is about 8% for short roots and 12% for taproot.
DISCUSSIONS
Stump structure and architectural root type influence risks for dikes. For example, risks are higher
when big roots cross right through the dike body.
Ideally, no trees should grow on dikes and all new tree growth should be stopped.
However, by killing existing trees, the structure of dike materials changes due to rotting roots,
thus creating galleries or heterogeneities leading to internal water erosion.
The recommendation so far is to limit the growth of existing trees but not to kill them. Trees
shouldn’t be devitalized if their stumps aren’t removed, materials recompacted and dikes
repaired.
REFERENCES
- Danjon, F., Reubens, B. 2008. Assessing and analyzing 3D architecture of woody root systems,
a review of methods and applications in tree and soil stability, resource acquisition and allocation.
Plant and Soil. p1-34.
-Danjon, F., Barker, D H., Drexhage, M., Stokes, A. 2008. Using Three-dimensional Plant Root
Architecture in Models of Shallow-slope Stability. Annals of Botany. 101. p1281-1293.
- Meriaux, P., Vennetier, M., Aigouy, S., Hoonakker, M., Zyberblat, M., Diagnosis and
management of plant growth on embankment dams and dikes, Vingt-deuxième Commission
Internationale des Grands Barrages, Barcelone, juin 2006, p.1-21.
- Zanetti, C., Vennetier, M., Mériaux, P., Royet, P., Dufour, S., Provansal, M. 2008.
L’enracinement des arbres dans les digues en remblai : étude des systèmes racinaires et impacts
sur la sécurité des ouvrages, Ingénieries EAT. p 49-57.

International Symposium “Root Research and Applications”
RootRAP, 2–4 September 2009, Boku – Vienna, Austria
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FIGURES
Figure 1: Excavation of a stump on Donzère canal dikes (Green Oak)
Figure 2: Manual measurements on root systems
Figure 3: Different types of roots on Ash: Long Large root (left) and Short Large root (right)

International Symposium “Root Research and Applications”
RootRAP, 2–4 September 2009, Boku – Vienna, Austria
4
TABL ES
Site and
Date Number of
trees (100) Tree species Type of studies
Rhone canal dyke in
Donzère
April 2007 3 Oak
Poplar
Rhone canal dyke in
Montélimar
May 2007 9 Acacia
Oak
Poplar
1- Tree root structure observation according to the
context
2- Architectural characteristics measurements
Rhone canal dyke
(downstream
embankment) in Lyon
July 2007
19 Acacia
Ash, Maple
Poplar, Oak
1- Tree root structure observation according to the
context
2- Architectural characteristics measurements
3- Stumps terrestrial laser scanning
Isere river flood
protection dyke in
Grenoble
April 2008
25 Acacia
Ash, Poplar
Oak, Willow
1- Tree root structure observation according to the
context
2- Architectural characteristics measurements
3- Stumps terrestrial laser scanning
4- Experimentation on root decomposition
Rhone canal dyke in
Lyon (upstream
embankment)
September 2008
14
Alder
Ash
Maple
Dogwood
1- Tree root structure observation according to the
context (trees growing on concrete paving
protection and rip-rap)
Loire river flood
protection dyke in
Cosne
October 2008
22 Acacia, Ash
Poplar, Oak
Maple, Willow
1- Tree root structure observation according to the
context
2- Architectural characteristics measurements
3- Dendrology analyses on roots
4- Electrical measurements on roots
Casterino river dam
dyke in Casterino
June 2009 8 Larch
Sylvestris pine
1- Tree root structure observation according to the
context
2- Architectural characteristics measurements
3- Dendrology analyses on roots
4- Experimentation on root decomposition
Table 1: Sites, number and methods of studied root systems
Type Inclination Branching Morphology Top
diameter
Root
lenght Order
T V ≥ 5 cm > 50 cm 1
ST V / O variable Conical < 5 cm < 1m 1 ; 2
LL H ≥ 5 cm > 2 m 1 ; 2
LM H ]1 ; 5[ > 1m 1 ; 2 ; 3 ; 4
LT H
few branching Linear
]0,3 ; 1] > 50 cm 2 ; 3 ; 4
SL H / O ≥ 5 cm < 2 m 1 ; 2
SM H / O ]1 ; 5[ < 1 m 1 ; 2 ; 3 ; 4
ST H / O
many branching Conical
]0,3 ; 1] < 50 cm 2 ; 3 ; 4
T: Taproot, ST: Secondary Taproot, LL/M/T : Long Large / Medium / Thin, SL/M/T: Short Large /
Medium / Thin
Table 2: Root typology parameters