**5. Potential slope plants**

**Figure 16.** Stomatal conductance in (a) sandy loam and (b) acidic soil of three species studied

**Figure 17.** Tolerance mechanism of plants [30]

534 Environmental Risk Assessment of Soil Contamination

Numerous studies have been conducted to determine the plant species which are suitable as slope plants. Normaniza and Barakbah [1] and Stokes et al. [31] referred that native plants usually increased the success rate of the planting program and reduced the long-term main‐ tenance requirements. Karim and Mallik [32] suggested that the selected plants should be adapted to local climate and be able to prevent landslides or erosion. Mafian et al. [12] showed that the reinforcement of soil by vegetation is highly promising solution and this approach would be more beneficial if the species acutely possessed the mechanical (through reinforce‐ ment of soils by plant root), hydrological (through reduction in runoff and by keeping the slope relatively dry) and environmental (through the increase in carbon sequestration to counter the rising carbon dioxide level in atmosphere) aspects.

Different plant species has different hydro-mechanical characteristics and can perform different roles on slope but certain types of plants are better than others in terms of soil reinforcement and surface protection [33]. Many problems may occur after planting any type of plants which does not fulfill the slope plant characteristics. Therefore, selection of plant species by observing the potential slope plant characteristics is crucial. A set of criteria was formulated to select potential species for plantation on slope [1,9,16,31]. Physiological charac‐ teristics such as the high photosynthetic rate, transpiration rate and growth rate and root profiles, such as high cellulose content in roots, fine roots, root biomass, root volume and root length are considered as major criteria [14,16]. Additionally, the selected plant should exhibit other prominent characteristics such as good plant-water relations and tolerance of wide range of adverse condition with regard to soil acidity and water stress [9].

A list of potential tree and shrubs species was presented in Table 4. Based on the observations, *L. leucocephala* and *P. pterocorpum* showed the higher bioengineering characteristics than *A. mangium and M. malabathricum*. It was discovered that root architecture of *L. leucocephala* and *P. pterocorpum* was VH and R type, respectively. The VH- and R type root architecture was considered to be the most effective root system for slope stabilization and soil reinforcement [34]. The H-types were found to be beneficial for wind resistance. The M-types are regarded to be beneficial for controlling soil erosion. The extensive root growth and tensile strength *L. leucocephala* and *P. pterocorpum* were claimed to be the cause of tremendous enhancement of mechanical impacts on soil. Thus, in terms of root properties, *L. leucocephala* and *P. pterocor‐ pum* were more prominent to play a major mechanical role on soil and their high root tensile strength would ultimately improved soil shear strength as well. These introduced tropical plants will indeed assist eco-engineer to establish bioengineering technique on slope and provide long-term soil reinforcement.

Many legumes, especially woody trees are particularly planted for controlling soil erosion, slope stabilization and restoration in tropical countries [13]. *Leucaena leucoephala* and *Peltopho‐ rum pterocorpum,* have a potential to be slope plants. *L. leucoephala* is one of the most productive fast growing, semi ever green and nitrogen fixing tropical legume trees. In Malaysia, *L.* *leucocephala* is used as a potential slope pioneer and wind protection. It has aggressive taproots reaching six to eight feet deep and half inch in diameter that open pathways deep into the soil (Figure 18). Nodules on the root of plant can fix atmospheric nitrogen and this is perhaps the most notable aspect that sets them apart from other plants. Additionally, *P. pterocarpum* is a woody ornamental plant and has a R type root system [16] (Figure 19). This tree usually is planted along roadsides, parks and slope. It has high atmospheric nitrogen-fixing potentiality.

**Figure 18.** Root profile of a potential slope plant- *Leucaena leucocephala* [18].

*M. malabathricum* produced the M type root system that makes it suitable to grow at slope area (Figure 20). Acidic treated *M. malabathricum* showed a higher root length than non-acidic treated, implying high water absorption to perform a basic metabolic process such as photo‐ synthesis. Plant released the absorbed water to the atmosphere by transpiring through pores on the leaves. As a result, the excessive water were removed and resulted in a drier and more stable slope. Moreover the flowering feature of *M. malabathricum* can help to enhance the florafauna interaction of the slopes by increasing the biodiversity. Different species have different mechanical characteristics and ranges acidic soil rehabilitation capacity. Potential slope plants and their mechanical characteristics were shown in Table 4. Additionally, a list of potential tree and shrubs species for planting in acidic slope was shown in Table 5.

**Figure 19.** Potential slope plant- *Peltophorum pterocarpum* [16].

*leucocephala* is used as a potential slope pioneer and wind protection. It has aggressive taproots reaching six to eight feet deep and half inch in diameter that open pathways deep into the soil (Figure 18). Nodules on the root of plant can fix atmospheric nitrogen and this is perhaps the most notable aspect that sets them apart from other plants. Additionally, *P. pterocarpum* is a woody ornamental plant and has a R type root system [16] (Figure 19). This tree usually is planted along roadsides, parks and slope. It has high atmospheric nitrogen-fixing potentiality.

536 Environmental Risk Assessment of Soil Contamination

**Figure 18.** Root profile of a potential slope plant- *Leucaena leucocephala* [18].

tree and shrubs species for planting in acidic slope was shown in Table 5.

*M. malabathricum* produced the M type root system that makes it suitable to grow at slope area (Figure 20). Acidic treated *M. malabathricum* showed a higher root length than non-acidic treated, implying high water absorption to perform a basic metabolic process such as photo‐ synthesis. Plant released the absorbed water to the atmosphere by transpiring through pores on the leaves. As a result, the excessive water were removed and resulted in a drier and more stable slope. Moreover the flowering feature of *M. malabathricum* can help to enhance the florafauna interaction of the slopes by increasing the biodiversity. Different species have different mechanical characteristics and ranges acidic soil rehabilitation capacity. Potential slope plants and their mechanical characteristics were shown in Table 4. Additionally, a list of potential

**Figure 20.** (a) Plant profile, (b & c) root profile and (d) flowering feature of *M. malabathricum*.

**Table 4.** Potential slope plants and their mechanical characteristics [13,16,35].


**Table 5.** List of species for planting in acidic slope and classified by slope characteristic [13, 18].
