Tree bushes direct/pit planting (Live transplanting)

Category: NBS FOR EROSION CONTROL - Living approach / NBS for erosjonsreduksjon - levende materialer

Description

This technique is used for planting woody vegetation (shrubs, plants, trees) along slopes. The main goals of live planting are reducing the erosion and reinforcing the soil. This is one of the best methods to establish woody vegetation on difficult sites, because the plants do not have to undergo germination. This gives them an advantage compared to seeding of woody vegetation (Thomas and Kropp, 1992). In addition, the survival rate of transplanted woody vegetation is significantly higher than that of seeded woody vegetation. Pits are excavated to accommodate the plants along the slope, then backfilled with soil sometimes mixed with mulch and fertilizer to enhance the vegetation growth. The effectiveness of this system as soil reinforcement depends on the depth at which cuttings can be placed and the depth to which the roots can penetrate. To maximize the protection against erosion, the plants should be placed in horizontal configuration along the slope face (Figure 1).
 

Advantages:

  • Suitable for high steepness slopes and for the worse site conditions;

  • Faster vegetation establishment;

  • It can be used in combination with grass hydroseeding.

Disadvantages:

  • Not suitable for rocky slopes;

  • Local erosion can occur along the trunks and branches during high water flows;

  • Trees can become heavy and cause instabilities along the slope.

 

Figure 1. Pit planting in Nepal (Everest Landscaping and Erosion Control Services: linkedin slideshare)

 



Design methods

Soil reinforcement systems by bushes and trees are described by Gray & Leiser (1982), Copping & Richards (1990). The size of the pit depends on the size of vegetation to be planted (GEO, 2011).

An example of the preferred depth needed to support different types of vegetation is given in Table 2 for live planting in Hong Kong slopes:

Table 2. Soil Depths Needed to Support Different Vegetation (GEO, 2011)

Vegetation type

Minimum Depth of Existing Soil or Placed Soil-mix material over Inert/Rockfill substrate

Preferred soil depth

Grass

150 mm

300 mm

Groundcover/Climbers

300 mm

600 mm

Shrubs

450 mm

900 mm

Trees

800 mm

> 1200 mm

This technique is immediate for erosion control even if it is, in some cases, not highly effective from the beginning since the vegetation planted is still young (young sprouts: Figure 2). However, a proper understanding of the site conditions and surrounding landscape is fundamental to assess the type of plants to be used as well as the density of pits along the slope for a successful vegetation establishment. The species selected should have a root system that penetrates up to the required depth to create favourable conditions for stability. In humid regions, bushes and trees with high transpiration will be more effective in decreasing the water content. In some cases, it is used in combination with grass hydroseeding. The characteristics of the plants to be considered during the selection are divided into three groups, as summarized in Table 3.

 Table 3: Characteristics of plant groups, (after Morgan & Rickson, 1995).

Ecological criteria

Resistance to drought, salt, and temperature extremes

Growth characteristics

Ease of propagation, growth rate requires consideration of cutting material, humidity, temperature, light, soil type and time of propagation

Engineering properties

Root strength, depth and diameter of root systems, water use

 

 
Wherever feasible, native vegetation is preferred and the succession from pioneer to climax bush or tree in the site environment, primarily climate and soil type and moisture, should be considered (Morgan & Rickson, 1995, Gray & Leiser 1982, Schiechtl 1980).

Figure 2 –Volunteers planting for streambank restoration (from riverfriends.org)

 



Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 1 Suitable for controlling surface erosion or slowing runoff flow along a slope/bank.

The main factors are the plant distribution, plant root traits and the depth of the slope slide.
Topple 0
Slide 8
Spread 3
Flow 6

Material type

Descriptor Rating Notes
Earth 8 Mostly for earth and debris sediments in which is possible to excavate for planting. In rock slopes is not possible to excavate pits.

Not possible in rock slopes.
Debris 7
Rock 1

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 9 Vegetation is effective for controlling surface erosion.
Furthermore, the roots provide a reinforce for the soil, improving the slope stability at shallow depths.
Shallow (0.5 to 3 m) 5
Medium (3 to 8 m) 1
Deep (8 to 15 m) 1
Very deep (> 15 m) 0

Rate of movement

Descriptor Rating Notes
Moderate to fast 2 Adequate for movements from extremely slow to slow.
Slow 5
Very slow 7
Extremely slow 9

Ground water conditions

Descriptor Rating Notes
Artesian 7 Applicable irrespective of groundwater conditions. Enough water availability need to be ensured for plants to grow. Indirect effects on groundwater levels due to root-water uptake from plants during evapotranspiration.

Depending on shrub species.
High 8
Low 6
Absent 5

Surface water

Descriptor Rating Notes
Rain 8 Mostly used for contrasting runoff erosion and for entrapping soil sediments.
Despite they are used along stream banks or river banks for rainwater erosion protection, they are not adequate for shoreline erosion.
Snowmelt 6
Localized 4
Stream 1
Torrent 1
River 4

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 8 Direct effect on surface erosion reduction when the vegetation is properly established.
Feasibility and Manageability 10 Easy to install and simple maintenance.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 8 The time of implementation could be relatively fast if the perfect soil conditions and water availability are ensured: fast establishment period.
Environmental suitability 10 The use of indigenous species is always preferred for consistency with surrounding environment
Economic suitability (cost) 10 The costs can be addressed to the supply of plants and depend on the availability of native vegetation. In addition, the plants need to be grown and then transplanted by hand on site.

References

 

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