Vegetation on Levees: A Botanist's Perspective
By Tom Griggs, Ph.D., Senior Restoration Ecologist
Editor’s Note: In light of the recent attention to the Army Corps of Engineers’ policy on trees and vegetation on levees, we are re-introducing this essay by Tom Griggs, which first appeared in Fall 2009 issue of River Partners Journal.
The recent news from the US Army Corps of Engineers (USACE) that trees growing on California’s river side levees must be removed for safety reasons caught everyone by surprise. To institute this policy not only would be practically cost prohibitive, it also would devastate critically scarce riverside habitat. In short, it would exact a heavy financial AND environmental price. Some of the USACE’s concerns about trees growing on the levees include: (1) trees and shrubs might hinder inspections and be physically in the way during a flood-fight. This is a very real concern that requires careful management designs for trees and shrubs growing in the side of a levee. (2) Trees and shrubs might shelter rodents while they dig through the levee. Burrowing rodents, such as gophers and ground squirrels, prefer open grasslands without trees and shrubs. Burrowing through the soil full of woody roots is not habitat for these rodents. (3) Roots might grow through the levees and allow flood water to flow through and erode it from the inside-out. Evidence for this is very limited to non-existent. As a professional Botanist (TG), the USACE arguments reflect a lack of understanding of how plants grow. In fact, following what botanical science tells us about root development, tree and shrub root growth would improve levee integrity, not compromise it.
Roots are respiring organs. To grow and develop, they require oxygen, moisture, nutrients, and space. Nutrients and soil moisture are required for internal physiological processes and for physical growth. Space within the soil matrix (between soil particles) is essential for the root to grow. Where all four of these conditions—oxygen, moisture, nutrients, space—are met, roots proliferate. When any one of the four is not available, root growth stops, such as the interior of levees.
In perennial woody plants, root growth develops over the years into a characteristic architecture that must physically support the tree and meet its needs for moisture and nutrients. Most (90 percent) of the root biomass is located in the upper two feet of the soil profile. It is in the upper two feet of the soil profile that the four requirements for root growth are met: the soil is mixed and loosened by invertebrates allowing oxygen and water to percolate into it and the roots to easily grow through it. A decades old tree can have horizontal surface roots many feet beyond its dripline.
Trees that grow on the levee will likely grow only into the surface two feet of the levee structure and not into the center where oxygen, moisture and nutrients are much less than at the surface. Thus, the argument that tree roots might grow through a levee and therefore be a source for piping is suspect. Levee soil is very different from the soil on the adjacent floodplain. Levees are constructed to resist the penetration of moisture from rainfall; consequently the interior of the levee is dry and compacted. Contrast this with the soils on the floodplain where loamy soil meets the oxygen, space, nutrient and moisture requirements, trees will develop longer vertical roots to reach the water table.
In fact, since root growth is limited to the surface, a reinforcing structure will develop on levees. Trees roots form a network that functions as a web or net that holds together when under shear stresses, similar to how we use steel rebar to reinforce concrete. Vegetation composed of several species and many individuals growing on a levee will saturate the upper layers of levee with a network of interconnected roots. The importance of this is that the individuals will physically support each other from the shear stresses from high winds or high flows.
Excavations of the roots systems of native trees (Valley oak, elderberry, willow, box-elder) were carried out on a levee on the Sacramento River near the Sacramento International Airport in the early 1990s. The trees had been growing on the levee for many years. Ground squirrels were present. And the levee had experienced many flood events without leaking or breaking. The researchers dug trenches along side the different trees and carefully mapped their root systems and measured the amount of roots (root length) at different depths below the soil surface. Most of the roots were located in the upper 10 inches of soil on the levee. The researchers could find no evidence of roots nor rodent burrows that extended through the levee.
Plant roots have special requirements for their growth: oxygen, moisture, nutrients, and space. While these conditions exist in the rich soils of the floodplain, inside a levee it is a different story. The soil that makes up the levee is compacted so there is little space for the root to grow into and there is no moisture to support its growth. Thus, it is not clear how a root might grow completely through the levee. Furthermore, with most of the root network occupying the upper layer of soil, the levee surface is reinforced so that it can withstand greater shear forces.