Sustainability indicators for land uses following forest conversion

“Best bet” Land-use Systems

Country reports

Alternatives To Slash-And-Burn In Indonesia

 

Unique id: IDAEAQCE

Source file: D:\Projects\ASB\ASB Country and Thematic reports\Indonesia PhaseII report\Part II-III.xml

 

Authors: Thomas P. Tomich, Meine van Noordwijk, Suseno Budidarsono, Andy Gillison, Trikurnianti Kusumanto, Daniel Murdiyarso, Fred Stolle, Ahmad M. Fagi, Iswandi Anas, A.F.S. Budiman, Kenneth Chomitz, Rebecca Elmhirst, Chip Fay, Hubert de Foresta, Dennis Garrity, Danan P. Hadi, Suryo Hardiwinoto, Kurniatun Hairiah, Genevieve Michon, Nu Nu San, Cheryl Palm, Soetjipto Partoharjono, Djuber Pasaribu, Eric Penot, Robert Simanungkalit, Martua Sirait, S.M. Sitompul, F.X. Susilo, David Thomas

 

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A set of plot (field) level criteria and indicators was developed to evaluate the sustainability of a range of land use systems which can follow forest conversion (Weise 1998). Sustainability is a complex concept, as there are many reasons why certain land use activities can not be sustained. The original list developed for the ASB project (Van Noordwijk et al., 1998) included criteria at field scale as well as ‘downstream’ and ‘down wind’ environmental effects of certain land use types. Effects of these externalities on broader notions of sustainability are beyond the scope of this phase of research, which is confined to field level sustainability criteria. The main issue then is whether or not farming activities degrade their resource base to a level that impairs future productive use of the land. Three major categories of threats to continued farming are considered:

-          A. not maintaining soil of sufficient structure and biological activity,

-          B. not balancing the budget of nutrient exports and imports,

-          C. letting pest, weed and disease problems reach unmanageable proportions.

Any of these categories can become such a constraint to continued farming that land may have to be (temporarily) abandoned, therefore the most serious category of problems determines the overall sustainability.

            For each of the criteria a number of indicators were developed which can be measured relatively easily, often using data already collected as part of the integrated survey of biodiversity, C stocks and greenhouse gas emissions. These measurements were made for specific land cover types (the FARCI (or ICRAF) series: forest (F), mature agroforest (A), young tree-based systems (R(egrowth)), long-term cassava cropping (C) and temporarily abandoned Imperata grassland (I)), in the Jambi as well as Lampung benchmark area. For the current purpose ‘land use systems’ have to be reconstructed from these measurements, as for example agroforests as a land use have an early as well as a mature phase. All measurements were made in the previously specified benchmark areas, and they thus contain the confounding effects of land use history and current management practices typical for the various actors. For example, continued production of food crops (cassava) is restricted to former transmigration settlements that were cleared from previous forest cover by bulldozer. Current levels of soil compaction may date back to this event regardless of the current land use, but this still forms part of a broader ‘syndrome’ of land use decisions.

            No agricultural land use can consistently harvests produce without putting management efforts into maintenance of the system, so all judgements of sustainability depend on a specified management regime and farmer efforts to overcome obstacles. For each indicator a tentative threshold was developed, which allows a final judgement in three categories:

             0 (RED) = Problems may get beyond the means of farmers to resolve

             0.5 (AMBER) = Additional effort will be needed to address these issues, which may affect the profitability of the land use system, but may otherwise be within the farmer’s management options

             1 (GREEN) = No major problems beyond what normal farm management can deal with.

 

            Before we discuss these indicators a certain ambiguity in the sustainability concept must be mentioned: the final criterion is the possibility to continue farming on a given piece of land, keeping all threats at manageable levels. Continued farming, however, may depend on the ability to change and develop a farm in new directions. Whereas certain land use practices, such as cultivation of very efficient nutrient scavengers such as cassava, may meet the criterion of persistence for a period of say 20 years, this practice is likely to reduce the number of future options, because the soil depletion it induced will require substantial re-investment in soil nutrient stocks before other crops can be grown. The current criteria refer to the field-level land uses per se, as these are measurable while a full land use transition matrix that can only be assessed by other means. We will come back to this in the final section of this chapter.

 

III.1 Soil structure and biological activity

The following indicators were used:

 

A1. Soil compaction: as evident from soil bulk density (dry weight per unit volume) in the topsoil,

 

A2. Soil carbon saturation: organic carbon (Corg) content relative to that for forest soils of the same texture and pH. This criterion is based on a reference soil C level, Cref, which is estimated from regression analysis of a large soil data set for Sumatra (Van Noordwijk et al., 1997):

 

Cref  = exp(1.333 + 0.00994*Clay% + 0.00699*Silt% - 0.156*pH-KCl)

 

A3. Active Soil Carbon (ASC):

The globally proposed indicator based on microbial biomass relative to soil C could not be used because microbial biomass was not measured in a standardized way. Six other parameters are presented here, however:

-           dry weight of light plus intermediate fraction for the LUDOX size-density fractionationprocedure (Hairiah et al., 1995),

- mineral ammonium and nitrate content of the topsoil during measurements,

- population count of total bacteria (colony forming units), relative to the Corg content (as suggested for the ASC indicator), and relative to the C saturation

- soil respiration (during lab incubation)

All six parameters can be judged against the values obtained for natural forest sites

 

A4. Soil Exposure (SE):

Number of months of low (< 75%) soil cover / length of system cycle in months

 

Available primary data for Lampung and Jambi are summarized in Tables III.1 and III.2. Bulk density data in Tables III.1 and III.2 refer to slightly different sampling depths, but indicate a clear difference between undisturbed forests and land under a cassava/Imperata cycle, with intermediate degrees of compaction under agroforests and other tree-based production system. Serious localized soil compaction was clear in logged-over forest where tracks and logging ramps were compacted beyond easy recovery.  It is easy to compact a soil, but in systems without soil tillage it can take a long time before the soil recovers. Soil compaction can have an impact on water infiltration, root growth and greenhouse gas emissions, but probably stayed below critical levels in all cases observed. For a number of land use systems the overall rating is thus 0.5 (see table III.3).

                        The carbon saturation data  show that no land use systems fully maintain the soil organic matter levels in the top soil of a natural forest (once corrected for soil texture and pH of the site; many values are above 1.0 as the equation for Cref was based on data for the top 10-15 cm of forest soils), but serious declines were only found for the cassava/ Imperata land use type, with the lowest values measured in cassava fields. Reductions of soil organic matter content to this range is evidence of substantial depletion of organic nutrient stocks in the soil and may affect soil physical properties as well as nutrient buffering against leaching. As with soil compaction, problems can be created much faster than they can be solved. For the A2 indicator only the cassava/Imperata cycle gets a warning flag (0.5 score). As mentioned before for soil compaction, the low current value of C saturation may have been partly due to reclamation history as well as current land use (bulldozer land clearing can remove part of the topsoil out of the field boundaries), but frequent fires, low organic inputs through cassava litterfall and frequent soil tillage can account for the low values found.

 

Table III.1  Measured soil fertility indicators for the integrated biodiversity and GHG emission survey in Lampung (L) and Jambi (J) ASB benchmark area (September - November 1996)

 

Land cover type

(number of observations)

Bulk density 2-7 cm, g cm-3

Corg/

Cref

Light + interm. fraction,

g kg-1

Ammonium

Nitra­te

 

Bact. pop/

Corg

Bact. pop. * Cref/

Corg

Soil resp. mg CO2  C kg-1 day-1

Lampung

1.27

0.84

2.25

23

11

17

43

7.0

Jambi

1.09

1.05

3.86

14

12

21

61

15.3

Group 1

L

0 – 5

L

L

L + J

L

L

L

Forest (3)

1.17

1.54

3.22

40

18

12

27

7.9

Agroforest (4)

1.18

1.16

2.48

28

13

16

41

7.2

Regrowing trees (3)

1.32

1.12

2.60

11

8

30

82

8.6

Cassava (3)

1.34

0.71

1.12

16

10

12

27

4.6

Imperata (4)

1.41

1.02

1.88

16

6

17

41

6.7

Group 2

J

5 – 15

J

J

 

J

J

J

Forest (4)

0.91

0.97

7.18

18

 

15

47

17.9

Agroforest (5)

1.01

0.82

3.07

18

 

24

65

16.2

Regrowing trees (2)

1.22

0.74

2.46

8

 

18

43

13.1

Cassava (2)

1.17

0.55

3.11

11

 

30

85

10.6

Imperata (2)

1.28

0.72

3.44

14

 

26

79

14.0

Fprob LUT

<0.001

0.009

0.006

<0.001

0.011

NS

NS

?

   LUT*Prov

<0.001

NS

0.021

 

NS

NS

NS

0.026

   LUT*Depth

-

0.021

-

-

-

-

-

-

SED (interaction)

0.08

0.22

1.26

4..1

3.5

10

 

2.8

 

            The various indicators of soil biological activity in Tables III.1 and III.2 may give a partially conflicting signal: the mineral N supply at the time of measurement was higher in the forest and mature agroforests than in other land uses, indicating that N supply from mineralization may have exceeded current N demand from the vegetation around the time of measurement (end of dry season); these same land uses had a relatively high respiration rate, but when  estimates of total microbial population size are scaled by soil organic matter content or by C saturation, the 'active fraction' of the total soil organic matter pool in forests appears to have been lowest. On the basis of this evidence (and other data in the soil biodiversity survey) we conclude that there is no lack of active soil biota in any of the land uses, and Imperata grasslands are not 'depleted' ecosystems from a soil biological perspective, even though their soil organic capital has been reduced.

 

 

Table III.2Additional soil data from intensive biodiversity survey in Jambi (November 1997); data refer to duplicate samples per land cover type

 

Land cover

Bulk density (0 - 5 cm)

Corg/Cref

Ground cover (kg m-2)

Land Use

 

mean

g cm-3

Coeff. variab.

0 - 5 cm depth

Dead

wood

Litter

Green biomass.

 

Natural  forest

0.68

0.224

1.37

12.73

1.33

0.07

Natural forest

 

 

 

 

 

 

 

NTFP extraction

Logged-over Forest

0.77

0.342

1.20

13.40

1.18

0.02

Commercial logging

(Logging ramp)

1.20

0.181

 

 

 

 

 

5 year old Timber Plantation

0.69

0.119

1.23

7.76

0.77

0.03

 

40 year old Rubber AF

1.01

0.131

1.38

7.75

1.41

0.17

Rubber agroforests

10 year old

RubberPlantation

0.73

0.148

0.99

10.0

0.73

0.10

Rubber monoculture

 

 

 

 

 

 

 

Oil palm monoculture

Chromolaena fallow

0.77

0.103

1.16

0

0.56

0.34

Upland rice/ bush fallow rotation

Cassava

Imperata

1.19

1.23

0.069

0.117

0.58

0.81

0

0

0.10

0.05

0.20

0.25

Cassava/Imperata rotation

 

            The indicator of soil cover (A4) requires inferences over the lifespan of the system rather than point measurements. The data in Table III.2 show that the nature of soil cover can shift from dead wood and leaf litter in forests to covers dominated by green biomass. Bare soil is rarely exposed in the landscapes of the peneplains.  In all land use systems with a slash-and-burn land clearing event, soil may be exposed for about 6 months per cycle (or 2% of the time for a rubber system with a 25 year cycle). The only land use system where soil exposure may be an issue is thecassava/Imperata cycle where soil is exposed during the first 3 months of a cassava crop (unless heavily weed-infested or intercropped with crops such as rice, which is not possible at reduced soil fertility), and for about 1 month per year in all cases when the Imperata  fallow is burned. Combined, this may lead to about 10% of the time with incomplete soil cover, when the soil is vulnerable to the direct impact of rain and sun.

 

 

 

 

 

 

 

Table III.3 Sustainability rating of land use systems for Criterion A (maintenance of soil structure and biological activity); 1 = no major problems, 0.5 = problems within farmer management range, 0 = problems beyond what farmers can solve

 

Land use system

A1

Compac­tion

A2

Carbon satu­ration

A3

Acti­ve soil Corg

A4

Soil expo­sure

Over­all

rating

A

Comments on main issue which need attention

Natural forest

1

1

1

1

1

-

Community-based forest management

1

1

1

1

1

-

Commercial logging

0.5

1

1

1

0.5

Soil compaction in ramps and trails

Rubber agroforests

0.5

1

1

1

0.5

Soil compaction?

Rubber agroforests with clonal planting material

0.5

1

1

1

0.5

Soil compaction?

Rubber monoculture

0.5

1

1

1

0.5

Soil compaction?

Oil palm monoculture

0.5

1

1

1

0.5

Soil compaction?

Upland rice/ bush fallow rotation

1

1

1

1

1

-

Cassava/Imperata rotation

0.5

0.5

1

0.5

0.5

Soil compaction, low Corg, lack of soil cover

 

III.2Nutrient balance

Three indicators were developed to judge whether the nutrient balance is (or could potentially be) maintained in a cropping system

B1. Net Nutrient Export (NNE) or nutrients contained in all harvested products minus those in fertilizer inputs for N, P, and K, in kg ha-1 year-1. High net exports indicate the likelihood of depletion, high net surpluses, on the other hand, may indicate excessive fer­ti­lizer use and risks of pollution of ground- and surface water. Nutrient imports include fer­tilizers and N fixation through legumes in the system (none in the land uses considered here). For the net nutrient export, fertilizer inputs are taken at their nutrient value (Table III.4).

 

B2. Nutrient Depletion Time Range (NDTR) If nutrient stocks in soil and vegetation are large relative to net nutrient exports, nutrient offtake can be part of a wise natural resource management strategy; if exports are large relative to stocks, one can expect that yields will decline in the near future, unless nutrient inputs will be increased. Two types of estimates were used for nutrient stocks in the system: total nutrient content of soil plus vegetation and the directly available pool. Neither is directly satisfactory, as measures of the available nutrient pool necessarily use rather arbitrary fractions and there is considerable variation between plants in effectiveness of accessing 'non-available' nutrient sources. As nutrient stocks depend on the soil and vegetation cover, one can not directly assign an NDTR value to a land use system in the peneplains of Sumatra; the soils closer to rivers with a higher clay and silt content will have larger stocks than the sandier soils of the rest of the lowland peneplain. The values (Table III.5) only indicate an order of magnitude.

 

Table III.4 Net Nutrient Export (NNE) based on partial nutrient budgets for different land uses (LU's}, based on yield and input data from farm profitability studies (ChapterIV)

 

 

OUT = harvest,

 kg ha-1

cumulative for 25 yr

 

IN = fertilizer,

kg ha-1

cumulative for 25 year

 

In – Out

kg ha-1 year--11

LU

Pro-ducts

Yield

Mg

ha-1

N

P

K

 

N

P

K

 

N

P

K

NTFP harvesting

Various

 

0.02

0.002

0.03

0

0

0

0

0

0

Logging

Wood

13

63

6

38

0

0

0

-2.5

-0.2

-1.5

Rubber .AF

Rice

0.8

9

28

75

 

 

 

 

 

 

 

Rubber

11.8

78

96

428

 

 

 

 

 

 

 

total

 

87

124

502

0

0

0

-3

-5

-20

Rubber AF, improved

rice

0.8

9

28

75

 

 

 

 

 

 

 

rubber

28.6

189

234

1036

 

 

 

 

 

 

 

total

 

198

261

1111

74

50

0

-5

-8

-44

Rubber.monoculture.

rice

0.8

9

28

75

 

 

 

 

 

 

 

rubber

10.3

68

84

373

 

 

 

 

 

 

 

total

 

77

112

448

149

100

0

3

0

-18

Oil palm

palm oil

268

777

427

1656

2039

980

1794

50

22

6

Sh.Cult.long

rice

6

71

207

559

0

0

0

-3

-8

-22

Sh.Cult.short

rice

4

47

138

373

0

0

0

-2

-6

-15

Cassava

tuber

242

678

244

955

504

160

368

-7

-3

-23

 

1. Nutrient concentrations

kg Mg-1

N

P

K

 

2. Fertilizer use

kg ha-1cycle-1  LUS

Urea

TSP

KCl

Palm oil (bunch)

2.9

0.55

3.9

 

Rubber .agroforest

0

0

0

Rubber (DRC)

6.6

1.2

4.4

 

Rubber agroforests (int.)

165

250

0

Cassava

2.8

0.36

3.9

 

Rubber monoculture

330

500

0

Rice

11.8

2.9

2.7

 

Oil palm

4530

4900

3900

NB Oil palm estimates based on removal of bunches without return of mill effluent; if fruits are sold instead of bunches, NPK exports will be lower

Sh.Cult.long

0

0

0

Sh.Cult.short

0

0

0

Cassava

1120

800

800

 


Table III.5 Nutrient Depletion Time Range.(NDTR) for the net nutrient exports of Table III.4 and an 'available' nutrient stock of 800, 200 and 300 kg ha-1 of N, P and K, respectively, in vegetation, organic and directly accessible mineral forms in soil in a typical lowland rain forest of Sumatra's peneplains, and for a total nutrient stock (including less accessible pools in the soil) of 8000, 1200 and 3000 kg ha-1 respectively. NDTR has the unit time and indicates when nutrient stocks would be zero under a linear extrapolation of current trends. Negative net exports (inputs > exports) lead to negative NDTR values.

 

 

Av.Stock/(Out-In),  (year)

 

Tot.Stock/(Out-In),  (year)

 

N

P

K

 

N

P

K

NTFP harvesting

>10 000

>10 000

>10 000

 

>10 000

>10 000

>10 000

Logging

317

833

197

 

3175

5000

1974

Rubber AF

229

40

15

 

2290

242

149

Rubber AF clones

161

24

7

 

1614

142

68

Rubbermonoculture

-281

424

17

 

-2814

2545

168

Oil palm plantation

-16

-9

-55

 

-159

-54

-545

Sh.Cult.long cycle

283

24

13

 

2825

145

134

Sh.Cult.short cycle

424

36

20

 

4237

218

201

Cassava

115

60

13

 

1152

358

128

Table III.5 shows that the substantial differences between the land use systems in net nutrient exports (Table III.4) are reflected in very different depletion trajectories. The nutrient where the most rapid depletion may occur is potassium (K). If only the directly available pool is considered, depletion within a 25-year time frame may occur for the rubber systems and shifting cultivation as well as cassava production. If total stocks are considered (at least part of non 'available' K can be accessed by plants), the time frame to depletion becomes several decades at least. For N no problems are to be expected for the land uses described here according to this calculation. However, our calculations do not include nutrient losses other than in harvested products and substantial N losses will occur during slash-and-burn clearing of forest lands, as well as by leaching during subsequent periods of low  N demand by the vegetation relative to the N supply from mineralization. A more refined estimate would have to include the full spectrum of processes incorporated in the Century model (Palm et al., 1998) and goes beyond the current sustainability assessment.

            The nutrient balance calculations were based on the technical specifications used for the profitability assessments in part IV. For the cassava/Imperata cycle, a moderate use of fertilizer was assumed, below replacement level, but at least mitigating nutrient depletion. Many farmers in the benchmark area appear to use no fertilizer at all in this system, however. For such no-input versions the nutrient balance is clearly negative. A clear trade-off may exist for this land use type  between sustainability and profitability.

 

B3. The Relative Nutrient Replacement Value (RNRV) relates the export of nutrients in harvested products to the costs of replacing them into the agro-ecosystem in the form of chemical fertilizer. This assessment is based on the harvested products rather than the full production system, but refinements could be made in as far as nutrient recoveries depend on the system context. In the calculations for Table III.6 (long term) nutrient recovery of 25, 20 and 30% has been assumed for N, P and K, respectively, while N fixing trees (petai (Parkia) and jengkol (Pithecelobium), included in the Non timber forest products (NTFP) scenario) are assumed to derive two thirds of their N from the atmosphere.

 

Table III.6 Relative nutrient replacement value for main products of various land use systems (Rupiah prices before July 1997); modified and extended from Van Noordwijk et al. (1997)

 

 

Nutrient removal,

g/kg product

Nutrient replacement value

Rp/kg

Farmgate value of product,

Rp/kg

Relative nutrient replacement value (RNRV)

N

P

K

 

NTFP - rotan

2

0.2

1

10

20000

< 0.001

 

 

NTFP - petai/jengkol

5

0.5

5

24

500

0.05

 

 

NTFP - durian

3

0.3

6

28

1000

0.03

 

 

NTFP - others

 

 

 

 

 

< 0.001

 

 

Timber

2.5

0.25

1.5

13

108

0.12

 

 

Rubber (latex)

6.3

1.2

4.4

42

2000

0.02

 

 

Oil palm (bunches)

2.9

0.55

3.9

25

60

0.41

 

 

Rice

11.8

2.9

2.7

70

400

0.17

 

 

Cassava

2.8

0.36

3.9

22

50

0.44

 

The Nutrient replacement value is obtained as the sum of nutrient contents and replacement costs per nutrient for N, P and K (neglecting other nutrients):

Replacement price per nutrient exported, Rp/g

2.3

12

2.9

Fertilizer price, Rp/kg

260

480

400

Nutrient  fraction of fertilizer

0.45

0.2

0.46

Nutrient recovery by the crop

0.25

0.2

0.3

 

Most RNRV values are below 10% and this indicates that nutrient replenishment would be within reach of farmers if, when and where actual nutrient responses of the crop make fertilizer use necessary. For rice the value is around 15% and this indicates a range were details of fertilizer use (and the various assumptions on efficiency made here) will be important for farmers' decisions on fertilizer use. For oil palm and cassava the RNRV values are around 45%, indicating that fertilizer costs would be a major part of the farm budget if farmers would have to balance the nutrient budgets (when the 'free lunch' of living off the initial stocks is over). The low RNRV values for both products are caused by their low farmgate price per kg product. For oil palm,marketing of fruits instead of bunches could considerably reduce the nutrient exports and, hence, the RNRV. For cassava only a shift in farmgate prices of the product and/or of fertilizers could make fertilizer use more attractive.

 

The overall judgment for criterion B thus highlights the difficulties in maintaining balanced budgets for cassava at current prices (and based on estimated technical coefficients and recoveries), and indicates a number of concerns for upland rice rotations, oil palm production and the proposed intensified rubber at reduced fertilizer input management. Where the overall evaluation indicates values in the critical range, a more detailed assessment is needed for different soils, management practices etc.

III.3 Crop protection from weeds, pests and diseases

For criterion C two indicators have been proposed, both based on 'expert opinion' rather than direct measurements:

C1.  Potential for Weed Problems:

Weed problems becoming a major constraint in the system, unless addressed by additional labor and/or technical input

C2. Potential for Pest or Disease Problems:

Pest or disease problems becoming a major constraint in the system, unless addressed by additional labor and/or technical input

Weed problems are mostly related to Imperata, which is hard to control without herbicides (too expensive for smallholder food production) or ploughing (Van Noordwijk et al., 1997). Damage by pigs and monkeys to new planting material can be a serious obstacle when clonal (more expensive) planting material is used, whereas the existing system tolerates substantial tree losses by planting at high densities at low costs per seedling. The natural regrowth of  rubber agroforests is probably less problematic as a 'weed' than the grass or fern vegetation which develops under attempts at 'weed control'.

 


Table III.7Indicators of current and potential nutrient balance; NDTR = nutrient depletion time range; RNRV = relative nutrient replacement value; 1 = no major problems, 0.5 = problems within farmer management range, 0 = problems beyond what farmers can solve

 

Land use system

B1

Net export

B2

NDTR

B3

RNRV

Overall

RatingB

Comments on main issue

Natural forest

1

1.0

1

1

 

Community-based forest management

1

1.0

1

1

 

Commercial logging

1

1

1

1

 

Rubber agroforests

1

1

1

1

 

Rubber agroforests with selected planting material

0.5

0.5

1

0.5

Output increased at low input?; K supply needs attention

Rubber monoculture

1

1

1

1

 

Oil palm monoculture

1

1

0.5

0.5

Assumed fertilizer rates may be too high;RNRV rating supposes fruits sold rather than bunches

Upland rice/ bush fallow rotation

1

0.5

0.5

0.5

Fertilizer use required for intensification

Cassava/Imperata rotation

0.5

0.5

0

0

Nutrient balance can not be attained at current prices;K in short supply?

 

Table III.8Indicators of problems with crop protection from weeds, pests and diseases;1 = no major problems, 0.5 = problems within farmer management range, 0 = problems beyond what farmers can solve

 

Land use system

C1

Weeds

C2. Pests

&diseases

Comments on main issue

Natural forest

1

1

no problems

Community-based forest management

1

1

 

Commercial logging

1

1

 

Rubber agroforests

1

1

 

Rubber agroforests with selected planting material

1

0.5

pigs & monkeys at replanting; fungal diseases when sensitive clones are used

Rubber monoculture

0.5

0.5

fungal diseases, pigs and monkeys at replanting; ferns as ground cover may be problematic

Oil palm monoculture

1

1

 

Upland rice/ bush fallow rotation

1

0.5

vertebrate and insect pests are a constraint

Cassava/Imperata rotation

0.5

1

Imperata fallows are a weed problem unless farmers have draught power available

 

 

III.4 Synthesis of sustainability indicators

When all indicators are combined (Table III.9) we derive the following assessment:

-           most land use systems considered have one or more aspects which need attention, but most of these stay within the range of solvable problems at farm level,

-the cassava/Imperata  cycle has a number of issues associated with it and one of them (maintaining a nutrient balance) is so serious that it can probably not be resolved at the farm level within the current constraints.

 

III.5 Land use change matrix

Sustainability as defined above indicates the degree of reproducibility of a land use system: does it maintain the conditions required for its own continuation? In the real world, however, it is unlikely that land uses will remain unchanged over more than one (or a few) human generations, and it may thus be interesting to evaluate which options are kept open with a given land use system (Table III.10).

            Natural forest can be used as starting point for all land use types, but in a strict sense can only originate from forests; community-managed forests, some logging techniques and extensive rubber agroforests can lead to a return of a vegetation close to natural forests.  On the other side of the spectrum, the cassava/ Imperata cycle can be started after any land use system, but forms a 'dead end', as it can not maintain its own productivity and it takes substantial efforts and expense (nutrient replenishment and Imperata  control) to return to other (more profitable ands sustainable) land use types.  The various tree-crop systems appear to be freely convertible into each other, but extensive rubber agroforests will change in character once the seedbank of original natural vegetation is depleted and the site is out of reach of seed dispersal. Table III.10 strengthens the conclusion that the cassava/Imperata system is the most problematic of the land use systems considered here.

 


Table III.9Overall assessment of sustainability of various land use systems for the peneplain of Sumatra (compare tables III.3, III.7 and III.8)

 

Land use system

A1

A2

A3

A4

B1

B2

B3

C1

C2

Over-all

Main issues1

Natural forest

1

1

1

1

1

1

1

1

1

1

 

Community-based forest management

1

1

1

1

1

1

1

1

1

1

 

Commercial logging

0.5

1

1

1

1

1

1

1

1

0.5

C

Rubber agroforests

0.5

1

1

1

1

1

1

1

1

0.5

C

Rubber agroforests with selected planting material

0.5

1

1

1

0.5

0.5

1

1

0.5

0.5

C, K, W,P

Rubber monoculture

0.5

1

1

1

1

1

1

0.5

0.5

0.5

C,W,P

Oil palm monoculture

0.5

1

1

1

1

1

0.5

1

1

0.5

C, Fert

Upland rice/ bush fallow rotation

1

1

1

1

1

0.5

0.5

1

0.5

0.5

Fert, P

Cassava/Imperata rotation

0.5

0.5

1

0.5

0.5

0.5

0

0.5

1

0

C, Fert, W

1. C = soil compaction; K = potassium balance; Fert = price of fertilizer; W = weeds; P = pests and diseases

 

Table III.10Table of land use transformations that are feasible in a 20-50 year period;  crosses indicate where transitions from one land use system to another are possible

 

Land use system

1

2

3

4

5

6

7

8

9

 

Comment

1. Natural forest

X

X

X

X

X

X

X

X

X

 

Universal starting point

2. Community-based forest management

?

X

X

X

X

X

X

X

X

 

 

3. Commercial logging

?

X

X

X

X

X

X

X

X

 

 

4. Rubber agroforests

?

X

?

X

X

X

X

X

X

 

 

5. Rubber agroforests with clonal planting material

 

?

?

X

X

X

X

X

X

 

 

6. Rubber monoculture

 

 

 

 

X

X

X

X

X

 

 

7. Oil palm monoculture

 

 

 

 

X

X

X

X

X

 

 

8. Upland rice/ bush fallow rotation

 

X

 

X

X

X

X

X

X

 

 

9. Cassava/Imperata rotation

 

 

 

 

?

?

?

 

?

 

Self incompatible, a 'dead end'