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

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Clifford Miles
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1 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

2 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

3 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

4 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)

5 Common tree structural root architecture (mature ash) 40 inches

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

7 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

8 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.

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

10 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

$produce different damage levels: Shallow root plates shallow divot Aggressive root systems severe damage. Steeply tapered roots fracturing.$

11 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.

12 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.

13 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.

14 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).

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

16 Levee Trench 4 deep Photo: Scotty Mahlum, USACE

17 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)

18 Root Trench Profile Excavation Valley Oak #1 Mayhew Levee (Sacramento, CA) Horizontal (ft) Roots <10mm Roots mm Roots 20-39mm 14 ft Tree trunk Horizontal (ft) b.d zone with almost no roots Roots <10mm Roots mm Depth (ft) Depth (ft) 1.38

19 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.

20 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

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

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

23 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 zone is featureless, indicating homogeneous fill soil. This Text T. Mucciardi T. Mucciardi

24 GPR Data: Overhead View of Transects Depth range (corrected): Toe Tree Levee Slope T. Mucciardi

25 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.

Tree Root Architecture: Patterns and Geotechnical Roles in Levees

Tree Root Architecture: Patterns and Geotechnical Roles in Levees Shih Ming Chung and Alison M. Berry University of California, Davis Collaborators Gerald Bawden, U.S.G.S.; Keir Keightley, UC Davis; Sandra