Carbon Sequestration
“Best bet” Land-use Systems
Country reports
Alternatives to Slash-and-Burn in Brazil
Global Environmental Concerns
Unique id: IDADCMZB
Source file: D:\Projects\ASB\ASB Country and Thematic
reports\Brazil country report\ASB Brazil Summary Report.xml
Authors: S. Vosti, C. L.
Carpentier, J. Witcover, . Carvalho dos Santos, E. Muñoz Braz, J.
Ferreira Valentim, S. J. de Magalhães de Oliveira, C. Palm, F. de Souza
Moreira, A. Cattaneo, A. Gillison, A. Mansur Mendes, V. Rodrigues, T. C. de
Araújo Gomes, M. V. Neves d’Oliveira, E. do Amaral, S. Fujisaka, C. Castilla,
T. Tomich, D. Bignell, D. Gonçalves Cordeiro, A. Hermes Vieira, R.S. Correira da
Costa, M. Faminow, M. Locatelli, M. Swift, S. Weise, M. van Noordwijk, N.
Sampaio, I. L. Franke, H. J. Borges de Araujo, L. M. Rossi, E. Barros, B.
Feigl, S.P. Huang, J. Cares, C. Pinho de Sá, . Carneiro, P. Woomer
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Background
The goal of research on this topic was to quantify the
changes in carbon stocks associated with land clearing and the establishment of
different LUS. The net carbon dioxide (CO2) flux from the tropics,
which contributes to climate change, is largely a result of the conversion of
forested land to other LUS (Woomer et al, 2000). When vegetation is removed
through slash-and-burn, carbon is released to the atmosphere as CO2. The net amount released depends on how
quickly forest is converted, the biomass of the cleared vegetation, what
happens to the carbon in the vegetation, the regrowth and biomass of new
vegetation, and the time for which the subsequent LUS remain in place. Much of
the uncertainty over the values of the CO2 flux from the tropics is
a result of inadequate estimates for these parameters (Houghton, 1997). In particular, there is little information on
the carbon sequestration potential of many of the LUS that replace forest in
the humid tropics (Houghton et al, 1993).
ASB scientists in the consortium’s climate change working
group established standardized methods for measuring carbon stocks in forests,
in the traditional LUS established following slash-and-burn clearing and in the
alternative LUS identified as possible options for farmers at the different
sites (Woomer and Palm, 1994; 1998). The
data gathered by these methods were used to calculate both the immediate and
the longer term losses of carbon associated with forest conversion and to identify
the alternatives that sequester the most carbon.
As discussed in detail in the working group’s report (Palm
et al, 2000), comparing the carbon sequestration potential of different LUS
requires knowledge about the average C stored in each system over the period
for which it remains in place, known as its ‘rotation time’. In other words, it
is not necessarily the maximum C stock of the system that is important but
rather the average C stock of the system over time (LUSCta).
A natural forest has a fairly constant C stock, whereas
clearing the forest and establishing a tree plantation, for example, results in
an initial large loss followed by a gradual re-accumulation of C. ASB data indicate that a typical tree plantation
may eventually reach 50 to 80% of the C stock of the forest, but the time it
takes to do so will vary according to the tree species, the management regime,
the soils and the climate. The
time-averaged C stock depends on the carbon accumulation rates, the maximum C
stored in the system, the time it takes to reach maximum C, and the rotation
time of the system (Figures 6a, 6b).[1]
Figure 6. Above-ground C losses and re-accumulation in (a)
traditional swidden agriculture and (b) a tree plantation compared with stocks
in a natural forest
Results
Above-ground carbon
At ASB sites across the humid tropics, the carbon stocks of
selectively logged forests—from which most slash-and-burn clearing occurs—is
only about 50% that of true primary forest. In Brazil,
the average carbon stock (the above-ground vegetation plus litter) of the four
selectively logged forests studied by the group[2] was
approximately 150 tonnes of carbon per hectare (t C ha-1), ranging
from 130 to 175 t C ha-1. The
average value, 150 t C ha-1, was compared to the maximum C stored (Cmax)
and to time-averaged C (LUS Cta) (Table 3, Figure 7) of all the LUS
evaluated. Traditional pastures—the end result of most forest conversion—result
in only 2% of the above-ground C of forest.
The average rotation time of a pasture is 8 to 10 years before
re-establishment, but the rotation time does not have much effect on C storage
in pastures because of the constant offtake of biomass through grazing. Notably, improving pastures, either through
managing them better or through planting legumes, does not significantly
increase their carbon storage or time-averaged C stocks. Indeed, poorly managed
pastures may sequester more carbon than well managed ones.
Lands planted to simple tree-crop
systems, such as monocultured coffee, attain a maximum C stock of as little as
15 t C ha-1, only 10% of forest C stock. In contrast, multistrata
agroforestry systems, which may have rotation times of up to 20 years, may reach
a maximum of 90 t C ha-1 or 54% of forest C stock. However, the
time-averaged C stocks for these LUS are only 7% and 40% respectively of those
of the forest. For land put into an
annual crop/fallow rotation, the maximum C stock of a natural fallow of 5 years
is approximately 20 t C ha-1, compared with 34 t C ha-1
for an improved tree fallow, which is about 23% that of the forest. However, the time-averaged C stock of the
5-year natural fallow is only 6.86 t C ha-1 or 5% of that of the
forest. The value increases to only 11.5
t C ha-1 for improved tree fallows. The slight increase in C storage
and time-averaged C achieved by the improved fallow is due to its high C
accumulation rate of 6.86 t C ha-1 y-1, compared to 3.91
t C ha-1 y-1 for the natural fallow.[3]The C
accumulation rates of the multistrata agroforestry systems were high and
similar to those of the improved fallow. At the Brazilian benchmark sites as in
the rest of the humid tropics, tree-based
systems offer greater potential for carbon sequestration than do grass-based
systems.
Below-ground carbon
The preceding comparison includes only the above-ground
carbon stocks, because the data on the carbon contents of the samples of roots
and soils examined by the group were extremely variable. The root data, in
particular, were not useful for comparing LUS. Apparently the excavation method
used did not adequately sample large roots, so the values for roots in forests
and other tree-based systems were underestimated. These data are not included
in the report and will not be discussed. The soil data were also variable,
partly because of textural differences in the soils of the chronosequence
sampled at each site, despite attempts to sample similar soils. Differences in
soil C measured in two different LUS could therefore be the result of soil
textural differences, rather than any effect of land use. To account for the
variability caused by differences in soil texture within a site, the soil C
data were normalized using the equation developed by van Noordwijk et al (1997)
for estimating the soil C equilibrium values.
The equation calculates what the equilibrium soil C would be in a
natural, undisturbed system:
Calculated forest soil C = Cref = exp {1.333 +
0.00994*%clay + 0.00699*%silt 0.156*pHKCl}
The calculated reference values for
each site sampled were then compared with the actual C measured (Cact),
to give a relative C value: (Crel) = Cact/ Cref. The Crel values obtained for the
forest sites were not always correctly predicted by the equation, so the Crel
of each LUS was divided by the Crel of the forest within each
site, to serve as an estimate of the percentage change in soil C from a
particular land use transition. Table 4
presents the relative C values of the different LUS, compared with those of the
forest. Interpretation is difficult because soil C losses depend on the length
of time the land has been in a particular use, the soil type and any topsoil
erosion.
Table 4.
Corrected soil carbon values for different LUS compared with forest1
|
LUS
|
C actual
|
C reference
|
C relative
|
Crel land use/
Crel forest
|
|
|
|
|
|
|
|
Forest
|
1.78
|
3.35
|
0.53
|
1.0
|
|
Agroforestry
|
1.52
|
3.51
|
0.43
|
0.81
|
|
Fallows
|
0.96
|
2.80
|
0.35
|
0.65
|
|
Pasture
|
1.12
|
2.84
|
0.41
|
0.77
|
|
Crop
|
1.70
|
3.58
|
0.48
|
0.89
|
1.
Corrected according to the equation of van Noordwijk et al (1997).
Changes
in carbon stocks and land use
Once compiled, data on maximum, minimum and time-averaged C
stocks of the different LUS can be used to determine past, current and future
scenarios of the carbon flux associated with changes in land use over larger
areas. Maps of the vegetative cover, based on remote sensing, are available for
each of the benchmark sites. In most cases, at least two maps are available,
made at different points in time, so it was possible to use changes in cover
over time as the basis for calculating the likely changes in carbon stocks
associated with different LUS. For example, this work showed that, in the state
of Rondônia as a whole, the conversion of 93 000 ha of forest to pasture over a
20-year period, representing a loss of 170 C ha-1, resulted in the
net release of 14 million tonnes of C to the atmosphere (Fujisaka et al, 1998).
