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
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 |
|
|
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 |
|
|
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 |
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 Rubber |
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