Trees and Levees: How and Where Do Tree Roots Grow? - SAFCA

Trees and Levees:
How and Where Do Tree Roots Grow?
Photo: Ed Wallace
Alison M. Berry
Plant Sciences Department
University of California, Davis
The Vegetation Challenge Symposium
August 28, 2007

Tree Roots and Levees: Potential Risks
Levee Seepage
Windthrow and Surface Erosion
1. “Roots 101”: How Tree Root Systems Grow
• Horizontally and vertically in the soil profile.
• Opportunistically-- soil conditions.
• Biomechanically-- whole-tree weight & stress.
• Implications for tree failures.
2. Do Tree Roots Grow Into Levees?
• Mayhew Levee Trenching Project
3. Conclusions & Questions

Tree Root Systems – Classic Concepts
•Tree roots grow horizontally more than vertically –most conditions.
•Root systems can extend >2X the diameter of the canopy.
•80% of the root system is in the upper 1 meter (3-4 ft) of soil.
Root architecture: combination of 3 major woody systems
•Heart roots
•Horizontal roots
•Sinker roots

Roots grow opportunistically
•Moisture & nutrients
•Aeration
•Mechanical impedance
•Other soil properties
•Surface 1-2 ft best conditions for growth
40 in.
Soil surface
Lyford 1980
Excavated root distribution pattern (Red oak)

Common tree structural root architecture (mature ash)
40 inches

Tree species characteristics, soil conditions can
different architectures
photo: Molly Ferrell
Cottonwoods (Cosumnes R.)
heart roots, strongly horizontal roots & sinker roots

Soil texture, depth can influence root architecture
Deep, sandy soil
•poor mechanical stability.
•excessively well-drained.
Roots grow downwards
•do not spread laterally.
•likely follow moisture gradients
downward.
Valley oak (dead) root system excavated
in sandy levee, Sacramento River
D. Gray, D. Shields et al. 1992

Surface root plate – fallen Douglas fir
Shallow soils
(hardpan, claypan)
•high mechanical
impedance.
•poor aeration, probably
saturated.
Roots spread laterally
•Dense but shallow “root plate”.
•poor tree stability.

Soil strength: saturated soil
Whole tree effects: crown asymmetry
Root plate and vertical roots – fallen pine

Tree Roots Grow Biomechanically
Individual roots respond to stresses with compensation
“I-beam”= root strength
crown
asymmetry,
sun exposure,
slope, soil
instability
Buttress root
(Steeply tapered roots)
Whole-tree weight distribution also
produces compensating architecture

Uprooting is a major concern near and on levees
Uprooting
damage
potential:
New Orleans
Different species/conditions/root architectures
may produce different damage levels:
Shallow root plates shallow“divot”
Aggressive root systems severe damage.
Steeply tapered roots fracturing.

Root Turnover and “Channels”: Do dead
roots pose threats?
We need more information on dead woody
root persistence and turnover.
Large-diameter (2”) Douglas fir roots persist
for 50 years or more. But hardwood roots
may decay more rapidly.
In either case, resulting organic material is
likely to be recolonized quickly by other roots
(opportunism).
However, large-scale tree removal does
result in increased soil destabilization.

2. Where Do Tree Roots Grow?
Key question: Do large woody roots grow
into or under levees, and cause increased
seepage?
Excavation is still the best way to find out.

Mayhew Levee Trench Profile Project
trees
300 ft berm
USACE, Sacramento District
Photo: Scotty Mahlum, USACE
•Levee constructed in
1970s (non-USACE)
•Residential homes
•300-ft berm
•Levee to be regraded

Project
Photo: Scotty Mahlum, USACE
•3 mature oak trees.
•14’-30’ from levee toe
(river side).
•Trench-profile method
used.
•Trenches 4’ deep,
•Parallel to levee.
•Length of tree crown
shown =56’.

Levee
Trench 4’ deep
Photo: Scotty Mahlum, USACE

Big Project!
USACE
Alison Berry
Vic Claassen
John Lichter
Molly Ferrell
Photo: Scotty Mahlum, USACE
•EDAW
•Kleinfelder, Inc.
•Hart Restoration
•Research crew (UC
Davis, Sac State)

0
1
2
3
4
Root Trench Profile Excavation –Valley Oak #1
Mayhew Levee (Sacramento, CA)
Horizontal (ft)
48
47
46
45
44
43
42
41
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
40
39
Roots

1.2
1.4
1.6
1.8
Sandy loam
Critical bulk density≈ 1.65
Mayhew levee toe
bulk density at 2’
depth = 1.63
Roots
basically do
not grow into
this
compacted
zone.

One scoop: ≥2” diameter roots were below 4-ft trench floor
Levee side
Large (2”)
root at ~4-5 ft
Trench
floor
Tree side
(To river)
Photo: Scotty Mahlum, USACE

“Virtual Trenches”: Ground Penetrating Radar (GPR)
for further corroboration
John Lichter
making GPR
transect scans
Photo: Scotty Mahlum, USACE

Transect Design Sketch
5 transects,
~3 ft apart
Center
transect:
where
excavated
trench profile
was located.

PRELIMINARY
DATA
Antenna detects roots >1” diameter, up to 8-10’ depth
GPR confirms trench findings: large roots at 4’-6’ depth
-1ft
-3ft
-5ft
The 1’ – 3.5’ zone
is featureless,
indicating
homogeneous fill
soil.
This Text
T. Mucciardi
T. Mucciardi

GPR Data: Overhead View of Transects
Depth range (corrected): 41” – 72”
Toe
Tree
Levee
Slope
T. Mucciardi

TAKE HOME MESSAGES
1. Roots could be growing under the Mayhew levee,
from nearby trees.
2. However, roots avoided well-compacted fill:
keyhole trenches, slurry walls, deep protection
trenches may be effective.
3. There are many unanswered questions. We need
more information, combining trench excavation,
GPR and other methods, to determine the real
picture of roots and levees.
4. Biomechanical & root-shoot considerations:
Pruning to control crown symmetry and size could
reduce risk of root damage considerably.