INTERNATIONAL SYMPOSIUM TOWARDS SUSTAINABLE LIVELIHOODS AND ECOSYSTEMS IN MOUNTAINOUS REGIONS 7-9 March 2006, Chiang Mai, Thailand

Root effects on slope stability in Sumberjaya, Lampung (Indonesia) Kurniatun Hairiah*, Widianto*, Didik Suprayogo*, Nina Dwi Lestari*, Veronika

Kurniasari*, Arie Santosa*, Bruno Verbist** and Meine Van Noordwijk** *Brawijaya University, Faculty of Agriculture, Jl Veteran, Malang 65145, Indonesia.

Email: K.hairiah@cgiar.org and Safods.unibraw@telkom.net ** World Agroforestry Centre, ICRAF S.E. Asia, Bogor, Indonesia

Abstract Mass movement of soil on steep slopes or ‘landslides’ can be destructive for any vegetation or people in its path and can be a major contributor of sediment load in the river systems. A combination of deep rooted trees for anchoring and shallow rooted grass (for stabilizing topsoil) is generally perceived to stabilize slopes prone to mass movement. In the context of a broad evaluation of the role of agroforestry in maintaining or restoring watershed functions in the humid tropics, we tested two hypotheses: (1) Differences in the distribution of tree roots between species can be used to reduce landslide risks in the context of productive coffee agroforestry systems, (2) Shear strength of soil increases with root length density in the topsoil, regardless of plant species. Surveys were conducted along the riverbank of the Way Ringkih and Way Petai sub-catchments of the Way Besai in Sumber Jaya (West Lampung, Indonesia). About 20 trees species commonly found in multistrata coffee agroforestry systems were selected. The Index of Root Anchoring (IRA) was calculated as Σ Dv

2 /dbh2 where dbh is tree diameter at breast height and Dv is the diameters of vertical roots. Vertical soil shear strength at the top soil layer of 0-5 cm was measured. The results show that land use near the soil scarp along the river bank mostly is a form of coffee-based agroforestry or coffee monoculture; only a few patches of bush fallow were encountered, dominated by grasses and few pioneer tree species such as Ficus padana (semantung), Piper aduncum (sirihan), Trema orientalis (anggrung) and grasses such as Saccharum spontaneum (glagah).

Surprisingly, the highest IRA was shown by unpruned coffee (7.7), indicating that it would probably be able to stabilize river bank, as long as it would be allowed to grow to mature size, without aboveground pruning regime; more frequent pruning stimulates formation of roots in the surface layer, which may in fact reduce erosion. Three other tree species (Artocarpus elasticus, Parkia speciosa and Durio zibethinus) of the coffee agroforestry systems had a high IRA value (>1.0), and tend to grow to larger dbh values, providing more anchoring than coffee on a per tree basis. The common shade trees (legume) in coffee agroforestry system i.e. Gliricidia sepium and Erythina subumbrans and the tree most frequently used for government reforestation programs in the past, Calliandra calothyrsus, have low IRA values, indicating little ‘soil anchoring’; other timber and fruit trees had intermediate IRA values. Trees with a high IRA can probably be used to anchor river banks when grown to mature size. Ideally planting a mix of tree species with different pattern of rooting depth will provide a good protection of the soil surface and also increase river bank stability.

Soil shear strength (kg m-2) depends on soil texture and soil water content; across four plant species (Piper aduncum (‘sirihan’), Saccharum spontaneum (‘glagah’), Coffea canephora (‘kopi’), Bambusa arundinacea (Bamboo), only for one grass species (Saccharum spontaneum) was a weak positive relation found between root length density and shear strength. As the plants tend to occur on soils of different texture, indexing shear strength by texture (clay + 0.5 * silt

was found optimal) clarified the role of fine roots across species. Where root length density (Lrv) in the topsoil is less than 1 cm cm-3 shear strength largely depends on texture, for Lrv > 1 we can expect shear strength to be > 1.5 kg m-2

regardless of texture. Based on this preliminary study, we suggest a mix of tree species with deep roots and grasses with intense fine roots will provide the highest river bank stability in the area. Keyords: Root Reinforcement, Index of Rooting Depth, Root Mapping, Riparian Vegetation, Slope Stability.

1 Introduction Mass movement of soil on steep slopes or ‘landslides’ can be destructive for any vegetation or people in its path and can be a major contributor of sediment load in the river systems. The mechanics of ‘failure’ are broadly understood in terms of weight of the (wet) soil, slope, critical shear strength depending on texture and decreasing with soil water content, and positive pore water pressure after heavy rainfall (Sidle and Dhakal, 2003). While soil compaction (reducing pore space, enhancing shear strength), reduction of the weight of the vegetation (De Ploey, 1981) and enhancement of overland flow (reducing likelihood of positive pore water pressure after rainfall) will generally reduce the risk of landslides with time after forest conversion, the loss of ‘anchoring’ associated with gradual decay of tree roots may initially (in the first few years after conversion) enhance the risk of landslides.

The most destructive deep landslides may be hardly influenced by vegetation, but a combination of deep rooted trees for anchoring and shallow rooted grass (for stabilizing topsoil) is still generally perceived to stabilize slopes prone to mass movement. Living tree roots can contribute up to 20 kPa to the soil shear strength (O’Loughlin and Watson, 1979). Large trees can increase the shear stress required for sliding by 2.5 kPa (Bishop and Stevens, 1964)) and that their removal can promote landslides. The mechanical effect of root systems in enhancing soil stability is based on three mechanisms (Gray and Leiser, 1982): (a) Fine root systems in the surface layers bind soil particle strongly, increasing in cohesion and through the more stable soil structure reducing the entrainment of soil particles in overland flow of water, (b) The tensile strength of roots in the surface layers enhance shear strength and the risks that small blocks of soil break away on river banks, roadsides, gulleys or natural channels, (c) Deep tree root systems give a good support to tree trunks and act as an anchor to the soil resulting a high root resistance to storm force and reducing the chance that larger soil masses are flushed away once channels are formed. Depending on the relative importance of mechanisms a, b and c, the choice of vegetation and its associated rooting pattern can influence slope stability. In the context of a broad evaluation of the role of agroforestry in maintaining or restoring watershed functions in the humid tropics, we tested two hypotheses:

• Differences in the distribution of tree roots between species can be used to reduce landslide risks in the context of productive coffee agroforestry systems,

• Shear strength of soil under horizontal forces increases with root length density in the topsoil, regardless of plant species.

2 Methods

Site description Surveys were conducted in May-August 2005 along the riverbank of the Way Ringkih and Way Petai sub-catchments in the Way Besai catchments in Sumber Jaya (West Lampung, Indonesia) where a lot of land slides occurred. The Way Ringkih and Way Petai sub-catchments are approximately 785.5 ha and 1421 ha in area (Figure 1). The area (bounded by 104°25'46.50" -

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104°26'51.40" E, 5°01'29.88" - 5°02'34.20" S) has a mean annual rainfall of 2614 mm, an average daily air temperature of 21.2°C, and relative humidity in the range of 81 -89 % (Dariah et al., 2004). On the climatic map of Oldeman et al. (1979), the study site is within zone B1, with 7 months of wet season (>200 mm rainfall) and 1 month dry (rainfall <100 mm). Rainfall data collected in 2001-2002 confirmed the long-term trend, with a rainy season from October until May; the month of February 2002, however, was unusually dry with a total rainfall of only 90 mm (Afandi et al., 2003).

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Figure 1: The distribution of landslides occurred in Way Ringkih and Way Petai sub-catchments

in the Way Besai catchments in Sumber Jaya (West Lampung, Indonesia)

Three components of this study are reported here:

1. Rapid survey to explore landslide events along the river bank in the Way Ringkih and Way Petai. The attention was focus on the role of tree on maintaining slope stability,

2. Study of the role of plant root systems in maintenance of soil shear strength in the top soil,

3. Inventory of the potential of tree root systems to act as an anchor to maintain river bank stability.

Step 1. Characteristics of landslide scarps in the Way Ringkih and Way Petai sub-catchments

By following all the streams of the Way Ringkih and Way Petai sub-catchments, all signs of recent landslides and bank collapses were recorded by position along the slope, type of vegetation on the surrounding slope and characteristics of the soil (texture, bulk density and hydraulic conductivity). The existing landslide scarps were grouped based on 4 aspects:

• Size: (1) Very small (< 20 m3), (2) Small (20-50 m3 ), (3) Medium (50-100 m3 ), (4) Large (> 100 m3). The very small landslides were excluded from subsequent analysis.

• Shape of the valley: relatively wide-bottomed ‘U’ shape or deeply incised ‘V’ shape

• Slope: Landslide scarps on very steep (VS >60o) slope were compared with those on steep slope(S <60o).

• Dominance of vegetation: agroforestry coffee based system, coffee monoculture or fallow

system.

An estimation of the volume involved in soil loss was done on randomly selected landslide scarps for each stratum, based on a geometric technique as shown in Figure 2.

Figure 2: Schematic approach to soil loss measurement

Observation of the vegetation around the scarp focused on tree species, population density, stem diameter (dbh), distance of trees to the scarp and under storey vegetation.

Qualitative observation on the soil profile at landslide scarps (> 2m depth) was made related to soil horizon, color, texture, structure, rooting depth, consistency, soil porosity and the existence of rocks (as sliding board), whereas quantitative measurement of soil properties were made by taking a composite soil samples of each scarp and analyzing its clay and silt content (soil texture). Undisturbed soil samples were collected to measure soil bulk density and hydraulic conductivity.

Step 2. Study on role of plant root system to maintain soil shear strength at the top soil layer Based on the results of study step 1, further identification of the vegetation type was done in the selected tributary along the river bank in Way Ringkih and Way Petai sub-catchments. Two aspects of the tree root development were made, aimed at understanding slope stability:

1. Root length density measurements of 4 common tree species (Coffea canephora (coffee), Maesopsis eminii (‘pohon afrika’), Arthocarpus heterophyllus (jackfruit), Gliricidia sepium (‘kayu hujan’) and Bambusa arundinacea (bamboo)) were compared to a ‘fallow vegetation’ (dominated by Trema orientalis (‘anggrung’) and Melastoma (‘harendong’)) and the edges of rice fields. The measurement of each species was replicated 3 times.

Soil samples were taken using a double ring sample for measuring root density and soil shear strength. Soil and root samples of trees of different age were taken at top soil layer of 0-5 cm, at different distance to trees i.e. 50, 100, 150, 200 cm. For rice, the sample was taken along the dike at 0-5 cm soil depth. Vertical soil shear strength was measured

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directly (Direct Shear Test of Rose, 2004). Subsequently, roots were separated from the soil by wet sieving. Roots of selected trees were separated from other roots of different crops. Root length density (Lrv) was measured based on the line intersect method (Tennant, 1975), dried in the oven at 80oC for 48 hours for estimating root dry weight (Drv).

2. Root measurements were focus on Coffea canephora (coffee), Bambusa arundinacea (bamboo), compare to less value (economicaly) vegetation which commonly grow along river bank i.e. Artocarpus elasticus (‘bendo’) and Saccharum spontaneum (‘glagah’, a tall grass), Piper aduncum (‘sirihan’). Soil shear strength was measured directly on the vertical wall of the soil profile using a Pilcon Shear Vane Test; the measurement were done in the top soil layer of 0-5 cm, at different distance to trees i.e. 50, 100, 150, 200 cm. Root sampling was done as in root observation 1. Soil texture and soil bulk density were measured in the Soil Science laboratory in Malang, using the pipette technique and gravimetry.

Step 3. Inventory of potential tree root system as an anchor to maintain stability of river bank About 20 trees species were selected among the tree species commonly found in multistrata coffee agroforestry systems. High economic value trees were prioritized for the observations, and compared to bush vegetation. Proximal roots (close to the tree stem) were exposed (Van Noordwijk and Purnomosidhi, 1996) and classified as either ‘horizontal’ or ‘vertical’ (descending at angle of >45o) (Figure 3). Two root indices were calculated i.e Index of Root Anchoring (IRA), and Index of Root Binding of Soil (IRB). IRA was calculated as Σ Dv

2/dbh2

where dbh is tree diameter at breast height (1.3 m height) and Dv is the diameters of all vertical roots (Van Noordwijk, 1999; Akinnifesi et al., 2004); IRB was calculated as ΣDh

2/dbh2, where Dh is the diameters of all horizontal roots.

Figure 3: Schematic diagram of the distribution of proximal roots. Horizontal roots descending at

angle of <45o, vertical roots descending at angle of >45o; D = root diameter

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3 Results

Step 1. Characteristics of landslide scarps in Way Ringkih and Way Petai sub-catchments The Way Ringkih sub-catchments area is smaller than theWay Petai sub-catchment, but a higher number of landslides was encountered. During this study there were 111 landslide scarps were observed, 74 in the Way Ringkih sub-catchment and 37 in Way Petai (Table 2). In Way Ringkih, 3 landslides were observed per km of river, while in Way Petai 2 landslides were seen per km of river. Three possible explanations for this difference are i.e. river length, drainage density and solum depth. River density per unit area in Way Ringkih is three times higher than in Way Petai sub-catchments, and the soil solum along the river bank is also thicker. A deeper soil is more prone to landslides because it likely increases the weight of the soil above the primary plane of weakness (Iversen, 2000).

Table 2: Site description and landslides scarp occurred in Way Ringkih and Way Petai sub-

catchments

Way Ringkih Way Petai Total area, ha 785 1.421 1. River

• Length, km 22,63 17,01 • Drainage pattern Dendritic • Drainage density, km/ha 0,03 0,01

2. Vegetation Covered by coffee garden (69% of the total area Sumberjaya district)

3. Soil • Depth of solum, cm 134 105

4. Riverbank Landslides Number of landslides 74 37 Size of landslides, % • Big (>100m2) 12 11 • Medium (50- 100 m2) 23 24 • Small (20-50 m2) 65 65 Average, m2 52 58

5. Intensity of landslide scarps • per area of sub-catchments

(landslides as fraction of the total area)

0.09 0.03

• per km of river 3 2 6. Dominant vegetation

• Coffee 63 or 85% 19 or 51% • Fern 7 or 9% 9 or 24% • Calliandra 1 or 1% 2 or 5% • Alang-alang (Imperata) 3 or 4% 7 or 19%

About 65 % of the landslides observed in each sub-catchments were small (soil loss 20-50 m3), 24% were medium (soil loss 50-100 m3) and about 11% were big (soil loss >100 m3); in the data set, scarp size was not dependent on the type of land use system. A rough estimation on the soil loss per sub-catchment was made from average soil loss per landslide, the number of landslide scarps observed and an assumption on the length of time a scarp remains visible (Table 3). If the latter is 12 months, the Way Ringkih landslides contributed about 4414 Mg on 785 ha or 5.6 Mg ha-1 of sediment per year, while the Way Petai yielded about 2544 Mg/1421 ha or 1.8 Mg ha-1

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per year. These estimates increase when the expected ‘observable age’ of landslides decreases, or decrease when landslides remain visible for a longer period of time.

Land use near the soil scarp along the river bank mostly was agroforestry coffee based system (shaded by Gliricidia) or coffee monoculture; only a few patches of bush fallow (‘belukar’) were encountered, dominated by grasses and few pioneer tree species such as Ficus padana (‘semantung’), Piper aduncum (‘sirihan’), Trema orientalis (‘anggrung’) and grasses such as Saccharum spontaneum (‘glagah’) etc.

Table 3: Estimated soil loss from sub-catchments Way Ringkih and Way Petai

Avg. Soil loss per landslide, Mg

Total soil loss per sub-catchments, Mg

Way Ringkih Way Petai

Size Way Ringkih

Way Petai

No of landslides

Total loss, Mg

No of landslides

Total loss, Mg

• Big 194 257 9 1746 4 1028 • Medium 75 82 17 1275 9 738 • Small 29 33 48 1392 24 792

Estimated total soil loss 4413 2558

Step 2. Study on role of plant root system to maintain soil shear strength at the top soil layer

Soil shear strength and root length density Roots add shear strength to soil when the root network penetrates a potential failure surface. The amount of tensile root force contributed to a potential slide mass should increase with increasing area of root intersection. Hence, we explored how the tensile strength estimated by the root length density (Lrv) measurement of various vegetation at different distance to tree trunk in relation to soil shear strength. Tree roots have the ability to resist tension, thereby increasing the shear strength of shallow soils through mechanical reinforcement (Roering et al., 2001). In Sumberjaya, landslide scarps often reveal broken root tendrils, suggesting that the tensile strength of the roots was mobilized during failure.

Results of our measurement on various tree root systems, bush fallow and paddy field shows that a higher root length density (Lrv) in soil surface was not always associated with increased soil shear strength; the relation was found to depend on plant species (Figure 4). Coffee and its shading trees (Gliricidia and Maesopsis) had a low Lrv (<1.0 cm cm-3), bamboo had Lrv ranging from 1.0 to 5.5 cm cm-3, paddy rice fields had 1.0 – 2.0 cm cm-3. Increasing Lrv of bamboo on the soil surface is associated with an increase in soil shear strength (y= 1142.5 x + 3136.1; R2 = 0.6026); it can be estimated that increasing Lrv of bamboo by 0.5 cm cm-3 will increase soil shear strength by about 570 kPa. For trees or paddy field no relation was found. The capability of roots to bind soil particles is determine by roots intensity, root strength (diameter, woody root). Measurement on vetiver grass roots reinforcement reported by Hengchaovanich (2005), shows that vetiver roots are very strong with high mean tensile strength of 75 MPa, dense and massive root networks act in unison, it resemble the behavior of soil nails normally used in civil engineering works. With its innate power to penetrate through hardpans or rocky layers, the action of vetiver roots is analogically likened to ‘living soil nails’ by the author (Hengchaovanich, 1998).

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Bamboo Coffee GliricidiaMaesopsis Bush Fallow Paddy Field

y = 1142.5x + 3136.1R2 = 0.6026

0100020003000400050006000700080009000

10000

0 1 2 3 4 5 6

Lrv, cm cm-3

Soil

Shea

r Str

engt

h, k

Pa

Linear ("Tree")

Figure 4: Relationship of root length density (Lrv) of trees, bamboo, grass fallow and rice with soil

shear strength along the river bank

Soil shear strength, roots and soil texture

Other factors that influence the soil shear strength are related to soil texture, soil organic matter, bulk density, EC (often positively correlated with aggregation and improved stability) and water content (negatively correlated with stability).

Our measurement shows that soil shear strength (kg m-2) depends on soil texture, and across four plant species only for one species (Saccharum spontaneum; ‘glagah’) a weak relation was found between root length density and shear strength (Figure 5A). As the plants tend to occur on soils of different texture, indexing shear strength by texture (clay + 0.5 * silt was found optimal) clarified the role of fine roots across species (Figure 5B). Where root length density (Lrc) in the topsoil is less than 1 cm cm-3 shear strength largely depends on texture, for Lrc > 1 we can expect shear strength to be > 1.5 kg m-2

regardless of texture.

A. B.

y = 1.0265Ln(x) + 5.7828R2 = 0.2961

0

2

4

6

8

10

12

0 1 2 3

Lrv, cm cm-3

SS/(c

lay

+ 0.

5*si

lt)

4

0-25 cm25-50 cm50-75 cm75-100 cm

Glagah: y = 0.3898x + 0.8931R2 = 0.4479

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 1 2 3 4

Lrv, cm cm-3

SS, k

g cm

-2

Bambu Glagah Kopi Sirihan

Figure 5: (A). Correlation between root length density (Lrv) and soil shear strength (SS) at 0-5 cm

soil depth for four species (Piper aduncum (‘sirihan’), Saccharum spontaneum (‘glagah’), Coffea canephora (‘kopi’), Bambusa arundinacea (Bamboo), (B). Idem for soil shear strength corrected for texture (clay + 0.5 * silt fraction) with data differentiated by distance from the plant.

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Step 3. Inventory of potential tree root system as an anchor to maintain stability of river bank The ecological value of selected tree was shown by a high Index Anchoring Root (IRA) which potentially able to stabilize river bank calculated as ∑ dv2/dbh2. The data was grouped into 3 classes i.e. high when IRA > 1.0, medium 0.1 - 1.0 and low ≤1.0 (Table 4). Results analysis of variance (ANOVA) shows that IRA and IRB of tree species are significantly different. The highest IRA was shown by unpruned coffee (7.7), able to provide an anchor to increase the stability of river bank as long as without aboveground pruning regime. More frequent pruning was found to lead to more formation of roots in surface layer in other tree species as well (Hairiah et al, 2002), which probably reduces erosion. Three other tree species Artocarpus elasticus, Parkia speciosa, Durio zibethinus among the coffee agroforestry systems had a high IRA value. Legume trees commonly used as shade trees in coffee agroforestry system i.e. Gliricidia and Erythrina or in reforestation (Calliandra) provided the least anchoring per unit stem diameter. Most high value tree (timber and fruit trees) had an intermediate IRA value.

Trees with a high IRA can probably be used to anchor river banks substantially beyond what coffee alone can achieve. Ideally planting mix tree species with different pattern on rooting depth will provide a good protection on soil surface and increase river bank stability.

Table 4: Index anchoring of tree root systems

No

Scientific Name

Local Name

Index Soil Binding (IRB)

Index Anchoring (IRA)

IRA grouping

1 Calliandra calothyrsus Kaliandra 2.82 0.06 2 Gliricidia sepium Kayu Hujan 4.69 0.06 LOW 3 Erythrina subumbrans Dadap 2.37 0.06 ≤0.1 4 Syzygium aqueum Jambu air 3.30 0.08 5 Ficus padana Semantung 3.61 0.13 6 Toona surenii Surian 3.57 0.13 7 Macarangga triloba Mara 2.87 0.14 8 Cinamommum burmanii Kayu Manis 3.31 0.24 9 Croton argyratus Parengpeng 3.76 0.34

10 Nephelium lappaceum Rambutan 3.35 0.34 11 Quercus lineate Pasang 2.29 0.39 12 Aleurites moluccana Kemiri 2.05 0.40 13 Piper aduncum Sirihan 2.00 0.42 MEDIUM 14 Tectona grandis Jati 2.14 0.43 0.1 – 1.0 15 Psidium guajava Jambu biji 2.87 0.44 16 Gmelina arborea Jati Kertas 3.15 0.58 17 Swietenia mahogany Mahoni 1.64 0.59 18 Artocarpus communis Sukun 1.75 0.82 19 Trema orientalis Anggrung 0.59 0.90 20 Artocarpus heterophyllus Nangka 1.03 0.91 21 Maesopsis eminii Kayu Afrika 1.99 1.04 22 Durio zibethinus Durian 1.81 1.19 23 Coffea canephora var robinson Kopi Robinson 4.46 1.31 24 Parkia speciosa Petai 1.07 1.38 HIGH 25 Artocarpus elasticus Bendo 1.10 1.68 > 1.0

26 Coffea canephora var. robusta (with pruning) Kopi Robusta 7.80 2.50

27 Coffea canephora var. robusta (no pruning) Kopi Robusta tunas 9.28 7.71

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Relationship between tree diameter and root diameter

Not all trees with less vertical roots had more horizontal root. Our measurements on roots of various trees species showed only a weak relationship between horizontal roots (∑dh2 ) and vertical roots (∑dv2) (Figure 5A). Tree root distribution closely link to stem diameter (R2 = 0.86), our data shows that increases stem diameter (dbh2) followed by increasing root area (dh2 + dv2) (Figure 6). This data suggested that a simpler estimation on the role of tree root system in binding soil at surface layer can be based on measuring tree stem diameter.

(A)

y = 0.0231x + 42.947R2 = 0.006

020406080

100120140160180200

0 200 400 600 800

dh2, cm2

dv2, c

m2

All trees Trema orientalisArtocarpus heterophyllus Parkia speciosaArtocarpus elasticus Coffea canephora var. robusta Linear (All trees)

(B)

y = 2.81xR2 = 0.86

0

200

400

600

800

1000

1200

0 100 200 300 400

dbh2, cm2

(dv2 +d

h2 ), cm

2

Figure 6: Relationship between horizontal roots and vertical roots (A) and stem diameter with total area of root system (B)

Tree suitability for stabilizing river bank

Trees with high IRA (Index of Root Anchoring) not always had a low IRB (Index of Root Binding of soil particles) or vice versa. Evaluation on suitability of trees as river bank stabilized was made, results shown in Table 4.

Coffee with or without pruning potentially suitable for anchoring and soil surface holding at the river bank, but it has a low root length density (Figure 5). Combination with other trees such as Gliricidia (IRB high) and other grasses probably better to increase slope stability. High economic values tree (fruit and timber trees) as described by Budidarsono and Wijaya (2004) mostly are grouped in medium class. Based on this study, we suggest a mix of tree species with deep roots and grasses with intense and strong fine roots will provide the highest river bank stability in the area.

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Table 4: Classification of trees suitability for stabilizing river bank based on their Rooting Index (IRA = Index of Root Anchoring, IRB = Index of Root on Binding soil particles)

INDEX IRA_Low (<0.1) IRA_ Medium (0.1 – 1.0) IRA_High (>1.0) IRB_Low Durio zibethinus <1.5 Parkia speciosa Artocarpus elasticus IRB_ Medium Macarangga triloba Cinamommum burmanii

1.5 - 3.5 Calliandra calothyrsus Aleurites moluccana

Erythrina subumbrans Quercus lineata Syzygium aqueum Tectona grandis Maesopsis eminii Gmelina arborea Swietenia mahogany Psidium guajava Nephelium lappaceum Artocarpus communis Piper aduncum IRB_High Gliricidia sepium Croton argyratus Coffea canephora var. robinson >3.5 Toona surenii Trema orientalis Coffea canephora var. robusta

Ficus padana Artocarpus heterophyllus Coffea canephora var. robusta (no pruning)

Acknowledgement This activity has been funded by the Australian Center for International Agricultural Research through the World Agroforestry Center (ICRAF)-Southeast Asia.

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