The Role Of Vegetation to Soil Stability And Enrichment

The Role Of Vegetation to Soil Stability And Enrichment

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Abstract

Vegetation like soil is the product of the same group of independent variables such as climate, relief, organism, time, etc. These variables were used to generate the Jennys (1914, 1958) basic soil formation equation:

Soil = f(climate, parent material, relief, organisms, time)

Vegetation and soil mutually influence each other and neither is the result of the other.
Ecosystem stability and the response of ecosystems to disturbances are of crucial importance for conservation management.

Photo Credit: www.environmentalscience.org

Introduction

Landslide frequency can increase after trees are removed from forested slopes (Croft and Adams, 1950; Kawaguchi and Namba, 1956; Bishop and Stevens, 1964; Swanson and Dyrness, 1975; Wu, 1976). Vegetation can modify soil slope stability by mechanically reinforcing slopes through plant roots, and hydrologically by modifying soil moisture distribution and pore water pressures, adding slope surcharge from the weight of trees, and levering and wedging soil by roots (Gray, 1970).

Soil materials are transported from natural forested slopes to stream channels chiefly by mass erosion. From soil mechanics theory, mass erosion results if the shear stress acting on the material exceeds the available shear strength of that material (Swanston, 1974).
The increased shear stress produced by the weight of a mature forest on an unsaturated cohesionless soil is balanced by an equal increase in soil shear strength by the tree surcharge (Bishop and Stevens, 1964).

Vegetation can be defined as the collection of plants in a given place or environment.
Vegetation helps stabilize slopes in numerous ways. There is really no reason not to plant native species when revegetating or enhancing slopes. Native plants provide wildlife habitat, are adapted to our native soils, local weather and hydrology and are beautiful and diverse as well.

In terms of stability, the processes by which vegetation helps in soil stabilisation can be summarised into two main mechanisms.

1. Mechanical
2. Hydrological.

MECHANICAL MECHANISM (SOIL REINFORCEMENT)

Plant roots can help stabilize slopes by anchoring a weak soil mass to fractures in bedrock, by crossing zones of weakness to more stable soil, and by providing long fibrous binders within a weak soil mass.

In deep soil, anchoring to bedrock becomes negligible and the other two conditions predominate. The reinforcement effect of plant roots intermixed with soil resembles soil cohesion (Endo and Tsuruta, 1969). This means that roots possess the ability to strengthen a soil mass.

The strength of forest soil is difficult to measure directly. The total force required to break a soil mass reinforced by linden (Tilia cordata) roots in a study in the U.S.S.R. was calculated to be about 137 tons. Of this force, 130 tons were required to break the roots and 7 tons to tear the sandy loam soil mass from a bank of the Moscow River.

Individual roots become stronger as they become larger. The logarithm of root shear strength is closely related to that of the root diameter (Ziemer and Swanston, 1977). However, Tree roots are estimated to be one and one-half to three times stronger than grassy plants of the same diameter.

Slope stability problems will likely develop after timber cutting on steep slopes where most of the soil strength is provided by the binding action of roots. As roots decay after clear cutting, the value for their relative reinforcement declines.

The Mechanical Mechanism can be summarised thus:

1. Roots of vegetation, especially trees, reinforce soil increasing soil strength.
2. Tree roots may anchor into firm strata, providing support to the upslope soil mainly through buttressing and arching.
3. The weight of trees surcharges the slope, increasing normal and downhill force components.
4. Roots bind soil particles at ground surface, reducing their susceptibility to erosion.

HYDROLOGICAL MECHANISM (WATER CONTROL)

The occurrence of a major storm and mass erosion is closely correlated. Excess soil water is generally accepted to be the principal factor causing slope failures. Pore water pressure produced by the head of water in a saturated soil can reduce shear strength. Rising pore water pressures can reduce the effective weight of the soil mass by producing an uplift force.

Active pore water pressures can reduce soil shear strength by as much as 60% (Swanston, 1969). Increased soil water may also decrease cohesion of some soils through leaching and eluviation.

Vegetation can remove considerable quantities of soil moisture by evapotranspiration. Resultant negative pore water pressure or capillary tension in unsaturated soil increases intergranular pressure and thereby increases soil strength.

Roots physically resist erosion, and increase infiltration of water into the soil. Roots form physical pathways (little tunnels) that help water infiltrate the soil. Deep, woody roots lock the soil layers together, and lateral roots connect many plants into an interlocking grid. Fine feeder roots form a network through the upper soil layer, preventing surface erosion. Groundcovers and grasses have relatively shallow roots and low biomass, so they prevent surface erosion only, and do not stabilize deep soil. Trees possess deeper roots than shrubs and are essential for slope plantings.

Rainfall saturates the upper soils and then seeps laterally over the glacial till, causing slides. Deep tree roots penetrate into the compacted layer and help tie the layers together, preventing slides. Tree roots occurring at the crest and toe of a slope help to prevent wasting in these susceptible areas where larger slides often start.
Vegetation plays two vital roles in terms of water control:

1. Interception
2. Dewatering.

Interception

Erosion occurs when rainfall dislodges soil particles and carries them off a slope, forming rills and gullies that can trigger landslides. Raindrops hitting the soil surface can also seal the soil particles and make a crust that prevents infiltration and creates runoff. Trees and shrubs intercept precipitation before it hits the soil surface. Most of the intercepted precipitation evaporates back into the atmosphere, and the moisture that drips off the plants causes little soil damage because it has less force. It’s a good idea to include evergreen trees in slope plantings because conifers intercept more moisture than deciduous trees, especially in the rainy season when deciduous plants have lost their leaves. Leaves and branches that fall from the plants shield the soil surface from rain drop impact, slow the movement of water across the soil surface, and encourage rainfall to soak into the soil.

Dewatering

Soil saturation can trigger erosion and landslides. Plants improve slope stability by removing water from the soil. Plants use water, absorbed through their roots, to perform basic metabolic processes such as photosynthesis. Plants release absorbed water to the atmosphere, by transpiring through pores on the leaves, much as a person sweats. Transpiration cools the plant and helps transport minerals up the stems. The rate of transpiration varies greatly, depending on the plant species, weather, and other factors. A single tree can transpire hundreds of gallons on a hot, dry day.

The processes of the hydrological mechanism can be summarised thus:

1. Vegetation intercepts rainfall by absorption thereby reducing its infiltration for soil erosion.
2. Roots and steps increase roughness of the ground surface and the permeability of the soil, thus increasing infiltration capacity.
3. Roots extract moisture from soil which is transpired to the atmosphere (evapotranspiration) lowering pore-water pressure.

SOIL ENRICHMENT

Vegetation actively decompacts soil through the expansion of the root systems and the addition of organic matter to the site.

Water absorbs more readily into uncompacted soil. Vegetation also encourages soil fauna to thrive. Soil fauna, such as microorganisms, insects and worms, condition the soil as well.
Organic matter from vegetation, decomposers waste products and their remains (humus) not only increase the texture of the soil by binding soil particles, but also serve as nutrients sources in the soil.

For example, nitrobacter found in roots nodules helps replenish the soil with nitrogen enrichment, dead decaying vegetative parts of plants also provides soil enrichment, etc.
The cumulative impacts of these organisms result in healthier soil that is more resilient during storm events.

Summary

Vegetation helps stabilize forested slopes by providing root strength and by modifying the saturated soil water regime. Plant roots can anchor through the soil mass into fractures in bedrock, can cross zones of weakness to more stable soil, and can provide interlocking long fibrous binders within a weak soil mass. In Mediterranean-type climates, having warm, dry summers, forest evapotranspiration can develop a substantial soil moisture deficit which can reduce both piezometric head and slope mass. Pore water pressures change seasonally in response to precipitation and are often the driving mechanism which ultimately leads to slope failure. When trees are cut, the root system begins to decay, and the soil-root fabric progressively weakens.

The loss of root strength or increased soil moisture content or both after-tree removal can lower the slope safety factor sufficiently that a moderate storm and associated rise in pore water pressure can result in slope failure. After trees are removed, the frequency of landslides can increase.

Also without vegetation, soil nutrients that has already been exhausted or washed away by erosion will not be easily replenished by natural means.

Conclusion

Vegetation helps stabilize steep forested slopes chiefly by reinforcing the soil through tree roots and by changing the soil water regime. Most slope failures occur during major storms when the soil is saturated. Pore water pressures within the soil change seasonally in response to precipitation. Soil moisture in areas where the forest has been recently cut is usually greater than in uncut areas. Also following cutting, the tree root system begins to decay, and the soil- root fabric progressively weakens.

However soil moisture depletion and the strength of the soil-root fabric will return to that of uncut forests during forestation as roots from the new forest reoccupy the soil.
Therefore, it's essential to protect the available vegetations as well as key into the concept of increasing vegetation population either in homes or by reforestation.

References

1. www.soundnativeplants.com: Role of vegetation in slope stability.
2. Bishop, D.M. and Stevens, M.E.,, 1964:: Landslides on logged areas in southeast Alaska. U.S Dep. Agric For Serv. Res. Paper NOR-1 Juneau, AK, USA.
3. Bjorkhem, U. , Lundeberg, G. and Scholander, J., 1975: Root distribution and compression strength in forest soils. Research Notes no. 22.
4. Departments of Forest Ecology and Forest Soils. Royal College of Forestry, Stockholm, Sweden.
5. Burroughs, E.R. and Thomas, B.R., 1977: Declining root strength in Douglas-fir after felling as a factor in slope stability. U.S. Dep. Agric. For. Serv. Res. Paper INT-190. Ogden, UT, USA.
6. Croft, A.R. and Adams, J.A., 1950: Landslides and sedimentation in the North Fork of Ogden River, May 1949. U.S. Dept. Agric. For. Serv. and Range For. Res. Paper 21. Ogden, UT, USA.
7. Endo, T. and Tsuruta, T., 1969: The effect of the tree’s roots upon the shear strength of soil. 1968 Annual Report, Hokkaido Branch, Forest Experiment Station, pp. 167-182. English translation by Arata, J.M. and Ziemer, R.R., U.S. Dep. Agric. For. Serv. Arcata, CA, USA.
8. Gray, D.H., 1970: Effects of forest clear-cutting on the stability of natural slopes. Bull. of the Assoc. of Engineering Geologists, vol. VII, nos. 1 and 2, pp. 45-66.
9. Kawaguchi, T. and Namba, S., 1956: Landslides and erosion control. Rin-Go-Shiken- Hokoku, vol. 84, pp. 43-66.
10. O’Loughlin, C.L., 1974: The effect of timber removal on the stability of forest soils. Jour. Hydrol. (N.Z. ) , vol. 13, no. 2, p p . 121-134