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 st