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A tropical rainforest clearing experiment by biomass burning in the state of Pará, Brazil
Atmospheric Environment 33 (1999) 1991—1998
A tropical rainforest clearing experiment by biomass burning in the state of Para´, Brazil T.M. Arau´jo *, J.A. Carvalho Jr. , N. Higuchi, A.C.P. Brasil Jr., A.L.A. Mesquita INPE, Instituto Nacional de Pesquisas Espaciais, Rodovia Presidente Dutra km 40 12630-000, Cachoeira Paulista, SP, Brazil INPA, Instituto Nacional de Pesquisas da AmazoL nia, Alameda Cosme Ferreira 1756 69083-000, Manaus, AM, Brazil UnB, Universidade de Brası& lia, Departamento de Engenharia MecaL nica 70910-900, Brası& lia, DF, Brasil UFPA, Universidade Federal do Para& , Rua Augusto Correa 01, Guama& , 66075-900, Bele& m, PA, Brazil Received 2 March 1998; accepted 16 November 1998
Abstract Results are described of a forest clearing experiment conducted in Tome´ Ac7 u, located approximately 250 km south of Bele´m, the capital of the Brazilian northern state of Para´. An area of 3 ha of virgin forest was cut in July 1994 and left to dry until October of the same year, when fire was set. Post burning was also performed 30 days after the main fire. The test location biomass content per hectare was measured by indirect methods using formulas with parameters of forest inventories. The carbon content of the several biomass compartments was determined in a CHN analyzer. The combustion completeness was estimated by selecting ten 2;2 m areas and 24 large trunks and examining their consumption rates by fire. The 2;2 m areas were used to determine the completeness of small parts of biomass (those whose characteristic diameters were lower than 10 cm) and the trunks to determine the efficiency of the larger parts (characteristic diameters larger than 10 cm). The overall process combustion completeness was estimated to be 20.1%. Considering that the combustion gases of carbon in open fires contain approximately 90% of CO and 10% of CO in volumetric basis, the emission rates of these gases by the burning process were estimated as 70.2 and 5.0 t ha\, respectively. 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Rainforest clearing; Biomass burning; Carbon balance
1. Introduction Each year forest clearing in the Amazon region starts with the arrival of the dry season, by July, when cutting is performed. In this step, hundreds of tons of biomass per hectare are cut down in several locations of the forest. According to Keller et al. (1991), the emission rates of
*Corresponding author.
CO do not increase in this step because the main gases emitted are nitrogen oxides (NO and NO , together designated as NO ) and nitrous oxide (N O). V The biomass is burned approximately three months after the cut, just before the start of the rain season, when the material lost sufficient moisture to sustain fire. The burning is a fuel-rich combustion process that produces a large variety of combustion products, whose composition depends on the size, moisture content and chemical composition of the biomass on the ground, on the burning temperature and on the aeration rate.
1352-2310/99/$ - see front matter 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 4 2 8 - 2
1992
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
According to Crutzen et al. (1985) and Kirchhoff et al. (1989), it is in this dry season that the increase of the emissions of gases such as CO, O , NO , N O, CH , and V CO is observed. Gases involved in each step of the forest clearing process have been the focus of intense studies because of the influence, sometimes in an indirect way, in the occurrence of the greenhouse effect. There are uncertainties about the concentrations of these gases in the atmosphere. The CO concentrations have been reported as the most uncertain data. That is because the concentrations of CO vary from place to place, being larger, for example, in places with either large industrial activity, or intense biological activity, or where the burning process is employed intensively in land use. The characteristics of a forest clearing fire conducted in typical rainforest vegetation are reported in this paper. The main objective of the work was to estimate the carbon rate released by the process to the atmosphere as carbon dioxide and carbon monoxide. The experiment was performed in a site located in the Brazilian northern state of Para´ and all steps of the burning process were followed directly. The above-ground biomass, its moisture carbon and hydrogen contents and the combustion completeness of the burning process were determined. These data were then used to estimate the carbon, CO and CO emission rates. The results presented in this paper were obtained in a fashion that makes them helpful and useful in estimating CO and CO emissions from deforestation burns in the Amazon region. The term combustion completeness is used here instead of combustion efficiency. Combustion efficiency identifies, for a hydrocarbon fuel, how much carbon reacts to carbon dioxide and how much hydrogen reacts to water. Carbon monoxide represents an energy loss because there is heat involved in the reaction 1 CO#0.5 O P1 CO . Carbon monoxide is, in reality, a source of combustion inefficiency and the term combustion completeness, which identifies how much biomass is converted to gas and flying particulate by the combustion process is, therefore, more appropriate.
2. Experimental procedure The experiment was performed in a small village, named Igarape´ do Vinagre, located about one hour and half by boat from the town of Tome´ Ac7 u, which is 250 km from Bele´m, the capital of the state of Para´. The location of the experiment is given by the following geographic coordinates: 48°0.8 W longitude and 2°30 S latitude. The total available area for the experiment was 3 ha. The area climate is warm and wet. The minimum temperatures in Tome´ Ac7 u vary between 21.0 and 22.5°C and the maximum between 32.8 and 34.8°C. It rains
regularly during 8—9 months per year, being February and March the rainiest months. The vegetation in the experiment area was downed in July 1994 and left to dry until 14 October of the same year, when the main fire was set at 2 : 00 p.m. local time. Before the cut of the vegetation and in the period between the cut and the main fire, several activities were conducted and they are described in the following. Two sample units were selected within the 3 ha area: the first, called unit A (UA), with 1 ha (100;100 m), and the second, called unit B (UB), with 0.2 ha (20;100 m). The location of the units are shown schematically in Fig. 1. Forest inventory was performed in both units. Combustion data were obtained in UA, which was divided in 10 sub-sample units (SS , n"1—10, each measuring 10 L m;100 m) to perform the forest inventory and to facilitate the localization of sample points. Fire in this test area was set with torches by four people, each one positioned along one of the sides of the square. All above-ground biomass in UB was weighed, in a procedure that took six people during three weeks, to find out which of seven different biomass equations, given by Higuchi and Carvalho (1994) and Santos (1996), was the most appropriate to estimate the biomass content of UA. This equation was FW" aD@HA,
(1)
where FW stands for alive above-ground biomass (kg) of individuals with D'10 cm, D is the diameter at breast height (cm), H is the tree total height (m) and a, b and c are regression coefficients, equal to 0.026, 1.529 and 1.747, respectively. The result obtained with Eq. (1) was 0.62% out of that determined by the weighing procedure in UB. Dead above-ground biomass and live aboveground biomass of individuals with D(10 cm were determined by direct measurement in UB and extrapolation to UA. The moisture content of several biomass compartments was determined by drying at 105°C and, then, the dry above-ground biomass was estimated using the results of fresh biomass weight. Care was taken to determine moisture content and fresh weight at the same instance. The mass of carbon distributed in the different categories of biomass was calculated with the determination of the respective carbon contents using a CHN analyzer. Combustion completeness was determined for each biomass compartment. Small branches (DBH(10 cm), leaves, litter, liana had their corresponding completeness estimated by weighing the biomass, before and after the main burning, in 10 2;2 m sampling areas distributed in UA. The sampling areas (A1—A10) were selected prior to the fire.
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
1993
Fig. 1. Schematic of the experiment area.
By determining the biomass moisture content before and after the fire, the individual combustion completeness for small size material was calculated in terms of mass of dry material consumed per unit mass of dry material contained in the sample areas. In certain instances, the material inside a 2;2 m area had to be cut and separated from the material outside the area in order to be weighed, which inevitably disturbed their placement and possibly altered their behavior in the fire. Weighing of the remains in the 2;2 m areas was conducted in the day immediately following the burn. The samples used to determine the corresponding moisture contents were collected during the weighing procedure. Trunks and larger branches were consumed only at the surface by the fire. The combustion completeness for this category was estimated based on experimental observation of 24 trunks selected randomly in UA and identified before the fire. The remains from the main fire, which consisted predominantly of small branches (diameter (10 cm), were weighed, piled, and burned in a second fire, which was set three weeks after the main fire. The remains of the second fire were also weighed. The data on combustion completeness and mass of carbon of each dry biomass compartment present in the 1 ha of forest allowed estimation of the quantity of CO
released to the atmosphere by the combustion process. Assuming that about 90% of the combustion products of carbon in open fire is CO and 10% is CO (Crutzen and Andreae, 1990), in volumetric basis, the rates of both gases were estimated in units of t ha\. Hao et al. (1996) reported that the CO percentage varies from 85 to 95%, depending if the burning is at the initial or final phase. In terms of mass of CO , this represents an error of $5.6% in relation to the value calculated using 90% of CO in volumetric basis.
3. Results and discussion The partial and total fresh vegetation above-ground mass (m ) are presented in Table 1. The fresh biomass above-ground density of the test site was 369 t ha\. The values relative to other regions in the Amazon forest vary between 290 and 900 t ha\ (Klinge and Rodrigues, 1973; Higuchi and Carvalho, 1994; Brown et al., 1992, 1995), the average being 327 t ha\ (Fearnside, 1994), which is within 12% of the value found in the present study. Also shown in Table 1, the total dry biomass mass above-ground (m ) of UA was estimated to be 214 t ha\, a value calculated with the average moisture content of each biomass compartment. The moisture
1994
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
Table 1 Total fresh and dry biomass of UA. M is the moisture content (in terms of mass of water per unit mass of fresh biomass), m is the fresh mass, m is the dry mass of biomass, D is diameter at the breast height and is diameter Interval (cm)
Frequency
545 5(D(10 2 511 104D430
53 74 D'30 25
m (t ha\)
M (%)
m (t ha\)
Trunk Leaf Thin branch Thick branch Fallen trunk
13.0 0.6 4.0 — 0.05
41.1 53.0 40.5 40.1 44.0
7.8 0.3 2.4 — 0.03
Trunk Leaf Thin branch
58.2 8.3 9.2
40.1 53.0 40.5
34.9 3.9 5.5
16.3 1.8 2.6
Thick branch Fallen trunk
15.6 9.0
40.1 44.0
9.3 5.0
4.3 2.3
Trunk Leaf Thin branch Thick branch Fallen trunk
100.9 7.7 28.0 47.7 26.7
40.1 53.0 40.5 40.1 44.0
60.4 3.6 16.7 28.4 15.0
28.2 1.7 7.8 13.3 7.0
9.2 22.5 8.6
64.3 40.1 49.6
3.3 13.4 4.3
1.5 6.3 2.0
Category
— D45
"5
— 4519 360
Litter Small trees Liana
Total
4879
—
content, M, is presented in terms of mass of water per unit mass of fresh biomass, so that m "(1!M) m . The average moisture content of the above-ground biomass of the Tome´ Ac7 u site was 42%. The average value found for the above-ground vegetation moisture content near Manaus was 40% (Higuchi and Carvalho, 1994). If this value were used directly in the present work, the dry above-ground biomass would be computed as 221 t ha\, resulting in a 3.5% difference. Table 2 presents the individual and global results of the completeness, where g is the individual complete ness, m is the dry mass, [C] is the biomass carbon content, m is the carbon mass, m is the mass of ! dry biomass burned and m is the mass of carbon ! burned. The average carbon content of the fresh biomass was 44.6% for trunks and branches, 50.4% for leaves, 39.3% for litter, 46.8% for small trees (D(5 cm), and 45.5% for lianas. The total above-ground carbon content was 96 t ha\. The mass of dry biomass and carbon consumed during the main burn were 46.3 and 21.1 t ha\, respectively. Using results of the 2;2 m sample areas, the individual combustion completeness for the small size material was estimated to be 83.4% for leaves and litter, and
369.3
—
m (%) 3.6 0.1 1.1 — 0.01
214.2
61% for small trunks and thin branches, as shown in Table 2. Very few trunks were consumed, and only partially, by the fire, and these were not among the 24 selected prior to the fire. Only a superficial layer was consumed by the combustion process around the perimeter of the 24 previously selected trunks. The depth of this layer was variable and in parts of some trunks there was even no burned layer. The maximum depth of the burned layer in the selected trunks was observed to be of the order of 3 mm. A maximum value of the individual combustion completeness of each trunk was calculated by D!(D !2t ) G C , (2) g " G D G where D is the diameter at breast height from the G ith trunk and t is the external thickness at trunk per imeter consumed by the fire (equivalent to 3 mm). The maximum combustion completeness was 6.3% for trunks with D between 10 and 30 cm, and 2.3% for trunks with G D larger than 30 cm. G On the other hand, the minimum value of the combustion completeness for trunks is zero (assuming no burned
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
1995
Table 2 Combustion completeness of the several biomass compartments. g is the individual efficiency, m is the dry mass, [C] is the biomass carbon content, m is the carbon mass, m is the mass of dry biomass burned, m is the mass of carbon burned, D is the ! ! diameter at the breast height and is the diameter Category
Interval (cm)
Small size Trees
5(D(10
545
(10 5(D(10
2
Medium size Trees
Large size Trees
Frequency
10(D(30
511
(10
'10 10(D(30
53
D!30
74
(10
'10 D'30
25
Other D(5
(5
4519 360
Biomass Compartment
g (%)
m (t ha\)
[C] (%)
m ! (t ha\)
Trunk Leaf Thin branch Fallen trunk
61.0 83.4 61.0 61.0
7.8 0.3 2.4 0.03
44.6 50.4 44.6 44.6
3.48 0.15 1.07 0.01
4.76 0.25 1.46 0.02
2.13 0.12 0.64 0.01
2.22 0.12 0.68 0.01
Trunk Leaf Thin branch Thick branch Fallen trunk
3.15 83.4 61.0 3.15 3.15
34.9 3.9 5.5 9.3 5.0
44.6 50.4 44.6 44.6 44.6
15.57 1.97 2.45 4.15 2.23
1.10 3.25 3.36 0.29 0.16
0.49 1.65 1.48 0.13 0.07
0.51 1.52 1.57 0.14 0.07
Trunk Leaf Thin branch Thick branch Fallen trunk
1.15 83.4 61.0 3.15 1.15
60.4 3.6 16.7 28.4 15.0
44.6 50.4 44.6 44.6 44.6
26.94 1.81 7.45 12.67 6.69
0.69 3.00 10.19 0.89 0.17
0.31 1.51 4.53 0.40 0.08
0.32 1.40 4.76 0.42 0.08
Litter Small trees Liana
83.4 61.0 61.0
3.3 13.4 4.3
39.3 46.8 45.2
1.3 6.27 1.94
2.75 8.17 2.62
1.07 3.82 1.19
1.28 3.81 1.22
Total
214.2
96.2
m m ! (t ha\) (t ha\)
43.1
19.6
g (%)
20.1
Note: D: Trunk diameter; : Branch diameter.
layer). The mid point between maximum and minimum represents the best estimation of the mean combustion completeness for trunks, leading to 3.15% and 1.15% for trunks with 10(D (30 cm and D '30 cm, respecG G tively. Adding up the individual contributions of each biomass compartment, the overall mean combustion completeness of the main burn was estimated as 20.1%. If the contribution of trunks were completely neglected, the minimum combustion completeness would be 18.6%. Conversely, assuming that all trunks were consumed in a layer of 3 mm, the maximum combustion completeness would be 21.3%. This represents a fluctuation over the mean of$1.5% in respect to all above-ground biomass and $7.3% in respect to the mean value itself. The combustion completeness is a source of uncertainty in the determination of the CO emission rates. Fearnside (1992) reported that the combustion completeness is 28.4% during the first burn and 69.0% by biological decay. He also stated that with typical three reburnings, 35.0% of the carbon is released by combustion and 61.9% by decay. Fearnside et al. (1993) reported combustion completeness of 27.4% in a forest cleared for pasture near Manaus. Seiler and Crutzen (1980) stated
Table 3 Post burning data. g is the combustion completenss, m is the dry mass, [C] is the medium biomass carbon content, m is
! the carbon mass, m is the mass of dry biomass burned and m is the mass of carbon burned ! g (%)
m (t ha\)
[C]
(%)
m ! (t ha\)
m m ! (t ha\) (t ha\)
97.2
3.5
45.4
1.6
3.4
1.54
that, since trunks and large branches contain nearly 90% of the above-ground biomass, the burning efficiency is of the order of 25%. Kauffman et al. (1995) quantified the above-ground biomass, nutrient pools and the effects of biomass burning in four slashed primary tropical forest in the Brazilian Amazon. The total biomass consumption ranged from 42 to 57%. Carvalho et al. (1995) estimated a combustion completeness of 25.1% for a combustion test conducted at INPA’s forest reserve located 60 km from Manaus. A second test conducted by Carvalho et al. (1998) in the same forest reserve yielded a combustion completeness of 20.5%.
1996
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
The main parameters of the post burning step are presented in Table 3. The initial mass of dry biomass was 3.5 t ha\, the mass of carbon was 1.6 t ha\ and the combustion completeness of the process was 97.2%. With the determined mean combustion completeness, the carbon rate emitted to the atmosphere by the main burning process was 19.6 t ha\. It should be pointed out that this value does not include an estimate of charcoal weight or its carbon content. Considering that the carbon product gases contain about 90% CO and 10% CO, the amounts of carbon released as CO and CO were esti mated as 17.7 and 2.0 t ha\, respectively. For each one of the gases, the rates released to atmosphere by biomass combustion were 65.1 t ha\ for CO and 4.6 t ha\ for CO. These values were obtained considering the chemical reaction 1 C#0.95 O P0.1 CO#0.9 CO . The estimated mass of CO released during the post burning process followed the same procedure as that used for the main fire. The carbon rate emitted to the atmosphere by the process was 1.55 t ha\. Consequently, the carbon amounts released by post burn as CO and CO were 1.4 and 0.15 t ha\, respectively. For each one of the gases, the rates released to the atmosphere by biomass combustion were 5.1 t ha\ to CO and 0.4 t ha\ to CO. It should be pointed out that even though the combustion completeness is higher in the second burn, the CO and CO emission rates are signifi
Table 4 Estimate of the emission rates of CO and CO per hectare Stage
CO (t ha\)
CO (t ha\)
Participation (%)
Burning Post burning
4.6 0.4
65.1 5.1
92.7 7.3
Total
5.0
70.2
100.0
cantly lower in this step because of the mass of dry biomass involved in each step (214 t ha\ in the main burn and 3.5 t ha\ in the second). Only the fine remains of the main fire are piled up and lighted in the second burn, which is a practice carried out to provide additional clear space for cultivation. The total released amounts of CO and CO to the atmosphere were estimated considering the results of both steps, being 70.2 and 5.0 t ha\, respectively. The data are summarized in Table 4. The post burning is responsible for 7.3% of mass, in t ha\, of the total released CO and CO to the atmosphere. With the data of the released mass per hectare of CO and knowing the vegetation density of the test site is close to the Amazon average density, the annual mass rate of this gas was estimated applying reported data of deforestation rates of the Amazon region. Deforestation rates, however, vary from year to year. As shown in Table 5, the year that presented the highest rate of deforestation was 1987. Seiler and Crutzen (1980) estimated a mean rate of 2.5;10 ha yr\ for primary forest of the Amazon region from 1966 to 1975. The value of 1.7;10 ha yr\ was found by Houghton et al. (1987), which, according to Fearnside (1994), is very close to the real value of the clearing deforestation rate of 1980. However, Setzer and Pereira (1991) obtained a rate of 3.2;10 ha yr\ for 1987, and Cunha (1989) obtained a rate of 1.8; 10 ha yr\ for 1988. Fearnside (1994) estimated that during the 1978—1988 period the mean deforestation rate was about 2.0;10 ha yr\, which declined to 1.1;10 ha yr\ in the period 1990—1991. Based on the average deforestation rate of those above reported and on the quantities of CO and CO emitted to the atmosphere by the combustion process conducted in the 1 ha area of the experiment, the annual release rates of these gases were calculated. In the calculation procedure the data from the main burning and the post burning were considered both separately and together. Table 5 shows the values obtained.
Table 5 Estimate of the emission rates of CO and CO per year Burning
2.5;10 2.0;10 1.7;10 3.2;10 1.8;10 1.1;10
Post Burning
Total
Deforestation rate (ha yr\)
Period
CO (Pg yr\)
CO (Pg yr\)
CO (Pg yr\)
CO (Pg yr\)
CO (Pg yr\)
CO (Pgyr\)
Seiler and Crutzen (1980) Fearnside (1994) Houghton et al. (1987) Setzer and Pereira (1991) Cunha (1989) Fearnside (1994)
1966—1975 1978—1989 1980 1987 1988 1990—1991
0.012 0.010 0.008 0.016 0.009 0.005
0.17 0.14 0.12 0.22 0.13 0.08
0.0007 0.0009 0.0006 0.0012 0.0007 0.0004
0.010 0.013 0.009 0.016 0.009 0.006
0.011 0.013 0.009 0.016 0.009 0.006
0.15 0.19 0.13 0.24 0.13 0.08
Mean
1966—1991
0.01
0.14
0.0007
0.011
0.011
0.15
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
According to Setzer and Pereira (1991) the deforestation rate of 3.2;10 ha yr\ resulted in a release rate of 1.7 Pg yr\ for 1987. Applying the data from the present study to this same deforestation rate, the maximum value found was 0.22 Pg yr\, a value that does not account for below-ground biomass and biomass gasification by biological decay. It is important to mention that other values in the literature for the emission rates of CO consider the total carbon content of the biomass, including the part which is not consumed by the fire and decomposes later over several years. It is necessary to consider emissions from decomposition and uptakes from regrowth in overall calculations of the impact of deforestation on global change.
4. Conclusion The main results of the experiment conducted at the Tome´ Ac7 u site can be summarized as follows: 1. The total above-ground carbon content on the area was calculated as 96 t ha\. 2. The main fire combustion completeness was estimated to be between 18.6% and 21.3%, with an average value of 20.5%. The average efficiency was used to estimate CO and CO emissions from burning. 3. Considering that the gases found by carbon combustion contain 90% of CO and 10% CO on a volumet ric basis, the released rates of these gases due to the main burning were estimated as 70.2 and 5.0 t ha\, respectively. 4. The released rates of CO and CO due to post burning were estimated in 5.1 and 0.4 t ha\, respectively. These values correspond to 7.3% of mass, in t ha\, of the CO and CO emitted to the atmosphere by post burning process in this experiment. 5. Based on deforestation rate data found in the literature and considering that the mean biomass content of the Amazon region is approximately the biomass content of the studied site, the annual average rate of CO emitted to the atmosphere was estimated as 0.14 Pg yr\, relative only to the main burning.
Acknowledgements The authors are grateful to FAPESP (Fundac7 a o de Amparo a` Pesquisa do Estado de Sa o Paulo) for support of this research through the projects 92/0950-0 and 93/4753-8. The authors are also grateful to FAENQUIL (Faculdade de Engenharia Quı´ mica de Lorena), Lorena, SP, Brazil, for the utilization of the CHN analyzer. The support of Mr. Jose´ Carlos dos Santos is also acknowledged.
1997
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Kauffman, J.B., Cummings, D.L., Ward, D.E., Babbitt, R., 1995. Fire in the Brazilian Amazon: biomass, nutrient pools, and losses in slashed primary forests. Oecologia 104, 397—408. Keller, M., Jacob, D.J., Wofsy, S.C., Harriss, R.C., 1991. Effects of tropical deforestation on global and regional atmospheric chemistry. Climatic Change 19, 139—158. Kirchoff, V.W.J.H., Setzer, A.W., Pereira, M.C., 1989. Biomass burning in Amazonia: effects of atmospheric O and CO. Geophysical Research Letters 16(5), 469—472. Klinge, H., Rodrigues, W.A., 1973. Biomass estimation in a Central Amazonian Rain forest. Acta Cientifica Venezolana 24, 225—237.
Santos, J., 1996. Ana´lise de Modelos de Regressa o para Estimar a Fitomassa da Floresta Tropical U¨mida de Terrafirme da Amazoˆnia Brasileira. Doctorate Thesis, Universidade Federal de Vic7 osa, Curso de Cieˆncia Florestal, MG, Brasil. Seiler, W., Crutzen, P.J., 1980. Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning. Climatic Change 2, 207—247. Setzer, A.W., Pereira, M.C., 1991. Amazonia biomass burning in 1987 and an estimate of their tropospheric emissions. Ambio 20(1), 19—22.
A tropical rainforest clearing experiment by biomass burning in the state of Para´, Brazil T.M. Arau´jo *, J.A. Carvalho Jr. , N. Higuchi, A.C.P. Brasil Jr., A.L.A. Mesquita INPE, Instituto Nacional de Pesquisas Espaciais, Rodovia Presidente Dutra km 40 12630-000, Cachoeira Paulista, SP, Brazil INPA, Instituto Nacional de Pesquisas da AmazoL nia, Alameda Cosme Ferreira 1756 69083-000, Manaus, AM, Brazil UnB, Universidade de Brası& lia, Departamento de Engenharia MecaL nica 70910-900, Brası& lia, DF, Brasil UFPA, Universidade Federal do Para& , Rua Augusto Correa 01, Guama& , 66075-900, Bele& m, PA, Brazil Received 2 March 1998; accepted 16 November 1998
Abstract Results are described of a forest clearing experiment conducted in Tome´ Ac7 u, located approximately 250 km south of Bele´m, the capital of the Brazilian northern state of Para´. An area of 3 ha of virgin forest was cut in July 1994 and left to dry until October of the same year, when fire was set. Post burning was also performed 30 days after the main fire. The test location biomass content per hectare was measured by indirect methods using formulas with parameters of forest inventories. The carbon content of the several biomass compartments was determined in a CHN analyzer. The combustion completeness was estimated by selecting ten 2;2 m areas and 24 large trunks and examining their consumption rates by fire. The 2;2 m areas were used to determine the completeness of small parts of biomass (those whose characteristic diameters were lower than 10 cm) and the trunks to determine the efficiency of the larger parts (characteristic diameters larger than 10 cm). The overall process combustion completeness was estimated to be 20.1%. Considering that the combustion gases of carbon in open fires contain approximately 90% of CO and 10% of CO in volumetric basis, the emission rates of these gases by the burning process were estimated as 70.2 and 5.0 t ha\, respectively. 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Rainforest clearing; Biomass burning; Carbon balance
1. Introduction Each year forest clearing in the Amazon region starts with the arrival of the dry season, by July, when cutting is performed. In this step, hundreds of tons of biomass per hectare are cut down in several locations of the forest. According to Keller et al. (1991), the emission rates of
*Corresponding author.
CO do not increase in this step because the main gases emitted are nitrogen oxides (NO and NO , together designated as NO ) and nitrous oxide (N O). V The biomass is burned approximately three months after the cut, just before the start of the rain season, when the material lost sufficient moisture to sustain fire. The burning is a fuel-rich combustion process that produces a large variety of combustion products, whose composition depends on the size, moisture content and chemical composition of the biomass on the ground, on the burning temperature and on the aeration rate.
1352-2310/99/$ - see front matter 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 4 2 8 - 2
1992
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
According to Crutzen et al. (1985) and Kirchhoff et al. (1989), it is in this dry season that the increase of the emissions of gases such as CO, O , NO , N O, CH , and V CO is observed. Gases involved in each step of the forest clearing process have been the focus of intense studies because of the influence, sometimes in an indirect way, in the occurrence of the greenhouse effect. There are uncertainties about the concentrations of these gases in the atmosphere. The CO concentrations have been reported as the most uncertain data. That is because the concentrations of CO vary from place to place, being larger, for example, in places with either large industrial activity, or intense biological activity, or where the burning process is employed intensively in land use. The characteristics of a forest clearing fire conducted in typical rainforest vegetation are reported in this paper. The main objective of the work was to estimate the carbon rate released by the process to the atmosphere as carbon dioxide and carbon monoxide. The experiment was performed in a site located in the Brazilian northern state of Para´ and all steps of the burning process were followed directly. The above-ground biomass, its moisture carbon and hydrogen contents and the combustion completeness of the burning process were determined. These data were then used to estimate the carbon, CO and CO emission rates. The results presented in this paper were obtained in a fashion that makes them helpful and useful in estimating CO and CO emissions from deforestation burns in the Amazon region. The term combustion completeness is used here instead of combustion efficiency. Combustion efficiency identifies, for a hydrocarbon fuel, how much carbon reacts to carbon dioxide and how much hydrogen reacts to water. Carbon monoxide represents an energy loss because there is heat involved in the reaction 1 CO#0.5 O P1 CO . Carbon monoxide is, in reality, a source of combustion inefficiency and the term combustion completeness, which identifies how much biomass is converted to gas and flying particulate by the combustion process is, therefore, more appropriate.
2. Experimental procedure The experiment was performed in a small village, named Igarape´ do Vinagre, located about one hour and half by boat from the town of Tome´ Ac7 u, which is 250 km from Bele´m, the capital of the state of Para´. The location of the experiment is given by the following geographic coordinates: 48°0.8 W longitude and 2°30 S latitude. The total available area for the experiment was 3 ha. The area climate is warm and wet. The minimum temperatures in Tome´ Ac7 u vary between 21.0 and 22.5°C and the maximum between 32.8 and 34.8°C. It rains
regularly during 8—9 months per year, being February and March the rainiest months. The vegetation in the experiment area was downed in July 1994 and left to dry until 14 October of the same year, when the main fire was set at 2 : 00 p.m. local time. Before the cut of the vegetation and in the period between the cut and the main fire, several activities were conducted and they are described in the following. Two sample units were selected within the 3 ha area: the first, called unit A (UA), with 1 ha (100;100 m), and the second, called unit B (UB), with 0.2 ha (20;100 m). The location of the units are shown schematically in Fig. 1. Forest inventory was performed in both units. Combustion data were obtained in UA, which was divided in 10 sub-sample units (SS , n"1—10, each measuring 10 L m;100 m) to perform the forest inventory and to facilitate the localization of sample points. Fire in this test area was set with torches by four people, each one positioned along one of the sides of the square. All above-ground biomass in UB was weighed, in a procedure that took six people during three weeks, to find out which of seven different biomass equations, given by Higuchi and Carvalho (1994) and Santos (1996), was the most appropriate to estimate the biomass content of UA. This equation was FW" aD@HA,
(1)
where FW stands for alive above-ground biomass (kg) of individuals with D'10 cm, D is the diameter at breast height (cm), H is the tree total height (m) and a, b and c are regression coefficients, equal to 0.026, 1.529 and 1.747, respectively. The result obtained with Eq. (1) was 0.62% out of that determined by the weighing procedure in UB. Dead above-ground biomass and live aboveground biomass of individuals with D(10 cm were determined by direct measurement in UB and extrapolation to UA. The moisture content of several biomass compartments was determined by drying at 105°C and, then, the dry above-ground biomass was estimated using the results of fresh biomass weight. Care was taken to determine moisture content and fresh weight at the same instance. The mass of carbon distributed in the different categories of biomass was calculated with the determination of the respective carbon contents using a CHN analyzer. Combustion completeness was determined for each biomass compartment. Small branches (DBH(10 cm), leaves, litter, liana had their corresponding completeness estimated by weighing the biomass, before and after the main burning, in 10 2;2 m sampling areas distributed in UA. The sampling areas (A1—A10) were selected prior to the fire.
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
1993
Fig. 1. Schematic of the experiment area.
By determining the biomass moisture content before and after the fire, the individual combustion completeness for small size material was calculated in terms of mass of dry material consumed per unit mass of dry material contained in the sample areas. In certain instances, the material inside a 2;2 m area had to be cut and separated from the material outside the area in order to be weighed, which inevitably disturbed their placement and possibly altered their behavior in the fire. Weighing of the remains in the 2;2 m areas was conducted in the day immediately following the burn. The samples used to determine the corresponding moisture contents were collected during the weighing procedure. Trunks and larger branches were consumed only at the surface by the fire. The combustion completeness for this category was estimated based on experimental observation of 24 trunks selected randomly in UA and identified before the fire. The remains from the main fire, which consisted predominantly of small branches (diameter (10 cm), were weighed, piled, and burned in a second fire, which was set three weeks after the main fire. The remains of the second fire were also weighed. The data on combustion completeness and mass of carbon of each dry biomass compartment present in the 1 ha of forest allowed estimation of the quantity of CO
released to the atmosphere by the combustion process. Assuming that about 90% of the combustion products of carbon in open fire is CO and 10% is CO (Crutzen and Andreae, 1990), in volumetric basis, the rates of both gases were estimated in units of t ha\. Hao et al. (1996) reported that the CO percentage varies from 85 to 95%, depending if the burning is at the initial or final phase. In terms of mass of CO , this represents an error of $5.6% in relation to the value calculated using 90% of CO in volumetric basis.
3. Results and discussion The partial and total fresh vegetation above-ground mass (m ) are presented in Table 1. The fresh biomass above-ground density of the test site was 369 t ha\. The values relative to other regions in the Amazon forest vary between 290 and 900 t ha\ (Klinge and Rodrigues, 1973; Higuchi and Carvalho, 1994; Brown et al., 1992, 1995), the average being 327 t ha\ (Fearnside, 1994), which is within 12% of the value found in the present study. Also shown in Table 1, the total dry biomass mass above-ground (m ) of UA was estimated to be 214 t ha\, a value calculated with the average moisture content of each biomass compartment. The moisture
1994
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
Table 1 Total fresh and dry biomass of UA. M is the moisture content (in terms of mass of water per unit mass of fresh biomass), m is the fresh mass, m is the dry mass of biomass, D is diameter at the breast height and is diameter Interval (cm)
Frequency
545 5(D(10 2 511 104D430
53 74 D'30 25
m (t ha\)
M (%)
m (t ha\)
Trunk Leaf Thin branch Thick branch Fallen trunk
13.0 0.6 4.0 — 0.05
41.1 53.0 40.5 40.1 44.0
7.8 0.3 2.4 — 0.03
Trunk Leaf Thin branch
58.2 8.3 9.2
40.1 53.0 40.5
34.9 3.9 5.5
16.3 1.8 2.6
Thick branch Fallen trunk
15.6 9.0
40.1 44.0
9.3 5.0
4.3 2.3
Trunk Leaf Thin branch Thick branch Fallen trunk
100.9 7.7 28.0 47.7 26.7
40.1 53.0 40.5 40.1 44.0
60.4 3.6 16.7 28.4 15.0
28.2 1.7 7.8 13.3 7.0
9.2 22.5 8.6
64.3 40.1 49.6
3.3 13.4 4.3
1.5 6.3 2.0
Category
— D45
"5
— 4519 360
Litter Small trees Liana
Total
4879
—
content, M, is presented in terms of mass of water per unit mass of fresh biomass, so that m "(1!M) m . The average moisture content of the above-ground biomass of the Tome´ Ac7 u site was 42%. The average value found for the above-ground vegetation moisture content near Manaus was 40% (Higuchi and Carvalho, 1994). If this value were used directly in the present work, the dry above-ground biomass would be computed as 221 t ha\, resulting in a 3.5% difference. Table 2 presents the individual and global results of the completeness, where g is the individual complete ness, m is the dry mass, [C] is the biomass carbon content, m is the carbon mass, m is the mass of ! dry biomass burned and m is the mass of carbon ! burned. The average carbon content of the fresh biomass was 44.6% for trunks and branches, 50.4% for leaves, 39.3% for litter, 46.8% for small trees (D(5 cm), and 45.5% for lianas. The total above-ground carbon content was 96 t ha\. The mass of dry biomass and carbon consumed during the main burn were 46.3 and 21.1 t ha\, respectively. Using results of the 2;2 m sample areas, the individual combustion completeness for the small size material was estimated to be 83.4% for leaves and litter, and
369.3
—
m (%) 3.6 0.1 1.1 — 0.01
214.2
61% for small trunks and thin branches, as shown in Table 2. Very few trunks were consumed, and only partially, by the fire, and these were not among the 24 selected prior to the fire. Only a superficial layer was consumed by the combustion process around the perimeter of the 24 previously selected trunks. The depth of this layer was variable and in parts of some trunks there was even no burned layer. The maximum depth of the burned layer in the selected trunks was observed to be of the order of 3 mm. A maximum value of the individual combustion completeness of each trunk was calculated by D!(D !2t ) G C , (2) g " G D G where D is the diameter at breast height from the G ith trunk and t is the external thickness at trunk per imeter consumed by the fire (equivalent to 3 mm). The maximum combustion completeness was 6.3% for trunks with D between 10 and 30 cm, and 2.3% for trunks with G D larger than 30 cm. G On the other hand, the minimum value of the combustion completeness for trunks is zero (assuming no burned
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
1995
Table 2 Combustion completeness of the several biomass compartments. g is the individual efficiency, m is the dry mass, [C] is the biomass carbon content, m is the carbon mass, m is the mass of dry biomass burned, m is the mass of carbon burned, D is the ! ! diameter at the breast height and is the diameter Category
Interval (cm)
Small size Trees
5(D(10
545
(10 5(D(10
2
Medium size Trees
Large size Trees
Frequency
10(D(30
511
(10
'10 10(D(30
53
D!30
74
(10
'10 D'30
25
Other D(5
(5
4519 360
Biomass Compartment
g (%)
m (t ha\)
[C] (%)
m ! (t ha\)
Trunk Leaf Thin branch Fallen trunk
61.0 83.4 61.0 61.0
7.8 0.3 2.4 0.03
44.6 50.4 44.6 44.6
3.48 0.15 1.07 0.01
4.76 0.25 1.46 0.02
2.13 0.12 0.64 0.01
2.22 0.12 0.68 0.01
Trunk Leaf Thin branch Thick branch Fallen trunk
3.15 83.4 61.0 3.15 3.15
34.9 3.9 5.5 9.3 5.0
44.6 50.4 44.6 44.6 44.6
15.57 1.97 2.45 4.15 2.23
1.10 3.25 3.36 0.29 0.16
0.49 1.65 1.48 0.13 0.07
0.51 1.52 1.57 0.14 0.07
Trunk Leaf Thin branch Thick branch Fallen trunk
1.15 83.4 61.0 3.15 1.15
60.4 3.6 16.7 28.4 15.0
44.6 50.4 44.6 44.6 44.6
26.94 1.81 7.45 12.67 6.69
0.69 3.00 10.19 0.89 0.17
0.31 1.51 4.53 0.40 0.08
0.32 1.40 4.76 0.42 0.08
Litter Small trees Liana
83.4 61.0 61.0
3.3 13.4 4.3
39.3 46.8 45.2
1.3 6.27 1.94
2.75 8.17 2.62
1.07 3.82 1.19
1.28 3.81 1.22
Total
214.2
96.2
m m ! (t ha\) (t ha\)
43.1
19.6
g (%)
20.1
Note: D: Trunk diameter; : Branch diameter.
layer). The mid point between maximum and minimum represents the best estimation of the mean combustion completeness for trunks, leading to 3.15% and 1.15% for trunks with 10(D (30 cm and D '30 cm, respecG G tively. Adding up the individual contributions of each biomass compartment, the overall mean combustion completeness of the main burn was estimated as 20.1%. If the contribution of trunks were completely neglected, the minimum combustion completeness would be 18.6%. Conversely, assuming that all trunks were consumed in a layer of 3 mm, the maximum combustion completeness would be 21.3%. This represents a fluctuation over the mean of$1.5% in respect to all above-ground biomass and $7.3% in respect to the mean value itself. The combustion completeness is a source of uncertainty in the determination of the CO emission rates. Fearnside (1992) reported that the combustion completeness is 28.4% during the first burn and 69.0% by biological decay. He also stated that with typical three reburnings, 35.0% of the carbon is released by combustion and 61.9% by decay. Fearnside et al. (1993) reported combustion completeness of 27.4% in a forest cleared for pasture near Manaus. Seiler and Crutzen (1980) stated
Table 3 Post burning data. g is the combustion completenss, m is the dry mass, [C] is the medium biomass carbon content, m is
! the carbon mass, m is the mass of dry biomass burned and m is the mass of carbon burned ! g (%)
m (t ha\)
[C]
(%)
m ! (t ha\)
m m ! (t ha\) (t ha\)
97.2
3.5
45.4
1.6
3.4
1.54
that, since trunks and large branches contain nearly 90% of the above-ground biomass, the burning efficiency is of the order of 25%. Kauffman et al. (1995) quantified the above-ground biomass, nutrient pools and the effects of biomass burning in four slashed primary tropical forest in the Brazilian Amazon. The total biomass consumption ranged from 42 to 57%. Carvalho et al. (1995) estimated a combustion completeness of 25.1% for a combustion test conducted at INPA’s forest reserve located 60 km from Manaus. A second test conducted by Carvalho et al. (1998) in the same forest reserve yielded a combustion completeness of 20.5%.
1996
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
The main parameters of the post burning step are presented in Table 3. The initial mass of dry biomass was 3.5 t ha\, the mass of carbon was 1.6 t ha\ and the combustion completeness of the process was 97.2%. With the determined mean combustion completeness, the carbon rate emitted to the atmosphere by the main burning process was 19.6 t ha\. It should be pointed out that this value does not include an estimate of charcoal weight or its carbon content. Considering that the carbon product gases contain about 90% CO and 10% CO, the amounts of carbon released as CO and CO were esti mated as 17.7 and 2.0 t ha\, respectively. For each one of the gases, the rates released to atmosphere by biomass combustion were 65.1 t ha\ for CO and 4.6 t ha\ for CO. These values were obtained considering the chemical reaction 1 C#0.95 O P0.1 CO#0.9 CO . The estimated mass of CO released during the post burning process followed the same procedure as that used for the main fire. The carbon rate emitted to the atmosphere by the process was 1.55 t ha\. Consequently, the carbon amounts released by post burn as CO and CO were 1.4 and 0.15 t ha\, respectively. For each one of the gases, the rates released to the atmosphere by biomass combustion were 5.1 t ha\ to CO and 0.4 t ha\ to CO. It should be pointed out that even though the combustion completeness is higher in the second burn, the CO and CO emission rates are signifi
Table 4 Estimate of the emission rates of CO and CO per hectare Stage
CO (t ha\)
CO (t ha\)
Participation (%)
Burning Post burning
4.6 0.4
65.1 5.1
92.7 7.3
Total
5.0
70.2
100.0
cantly lower in this step because of the mass of dry biomass involved in each step (214 t ha\ in the main burn and 3.5 t ha\ in the second). Only the fine remains of the main fire are piled up and lighted in the second burn, which is a practice carried out to provide additional clear space for cultivation. The total released amounts of CO and CO to the atmosphere were estimated considering the results of both steps, being 70.2 and 5.0 t ha\, respectively. The data are summarized in Table 4. The post burning is responsible for 7.3% of mass, in t ha\, of the total released CO and CO to the atmosphere. With the data of the released mass per hectare of CO and knowing the vegetation density of the test site is close to the Amazon average density, the annual mass rate of this gas was estimated applying reported data of deforestation rates of the Amazon region. Deforestation rates, however, vary from year to year. As shown in Table 5, the year that presented the highest rate of deforestation was 1987. Seiler and Crutzen (1980) estimated a mean rate of 2.5;10 ha yr\ for primary forest of the Amazon region from 1966 to 1975. The value of 1.7;10 ha yr\ was found by Houghton et al. (1987), which, according to Fearnside (1994), is very close to the real value of the clearing deforestation rate of 1980. However, Setzer and Pereira (1991) obtained a rate of 3.2;10 ha yr\ for 1987, and Cunha (1989) obtained a rate of 1.8; 10 ha yr\ for 1988. Fearnside (1994) estimated that during the 1978—1988 period the mean deforestation rate was about 2.0;10 ha yr\, which declined to 1.1;10 ha yr\ in the period 1990—1991. Based on the average deforestation rate of those above reported and on the quantities of CO and CO emitted to the atmosphere by the combustion process conducted in the 1 ha area of the experiment, the annual release rates of these gases were calculated. In the calculation procedure the data from the main burning and the post burning were considered both separately and together. Table 5 shows the values obtained.
Table 5 Estimate of the emission rates of CO and CO per year Burning
2.5;10 2.0;10 1.7;10 3.2;10 1.8;10 1.1;10
Post Burning
Total
Deforestation rate (ha yr\)
Period
CO (Pg yr\)
CO (Pg yr\)
CO (Pg yr\)
CO (Pg yr\)
CO (Pg yr\)
CO (Pgyr\)
Seiler and Crutzen (1980) Fearnside (1994) Houghton et al. (1987) Setzer and Pereira (1991) Cunha (1989) Fearnside (1994)
1966—1975 1978—1989 1980 1987 1988 1990—1991
0.012 0.010 0.008 0.016 0.009 0.005
0.17 0.14 0.12 0.22 0.13 0.08
0.0007 0.0009 0.0006 0.0012 0.0007 0.0004
0.010 0.013 0.009 0.016 0.009 0.006
0.011 0.013 0.009 0.016 0.009 0.006
0.15 0.19 0.13 0.24 0.13 0.08
Mean
1966—1991
0.01
0.14
0.0007
0.011
0.011
0.15
T.M. Arau´ jo et al. / Atmospheric Environment 33 (1999) 1991—1998
According to Setzer and Pereira (1991) the deforestation rate of 3.2;10 ha yr\ resulted in a release rate of 1.7 Pg yr\ for 1987. Applying the data from the present study to this same deforestation rate, the maximum value found was 0.22 Pg yr\, a value that does not account for below-ground biomass and biomass gasification by biological decay. It is important to mention that other values in the literature for the emission rates of CO consider the total carbon content of the biomass, including the part which is not consumed by the fire and decomposes later over several years. It is necessary to consider emissions from decomposition and uptakes from regrowth in overall calculations of the impact of deforestation on global change.
4. Conclusion The main results of the experiment conducted at the Tome´ Ac7 u site can be summarized as follows: 1. The total above-ground carbon content on the area was calculated as 96 t ha\. 2. The main fire combustion completeness was estimated to be between 18.6% and 21.3%, with an average value of 20.5%. The average efficiency was used to estimate CO and CO emissions from burning. 3. Considering that the gases found by carbon combustion contain 90% of CO and 10% CO on a volumet ric basis, the released rates of these gases due to the main burning were estimated as 70.2 and 5.0 t ha\, respectively. 4. The released rates of CO and CO due to post burning were estimated in 5.1 and 0.4 t ha\, respectively. These values correspond to 7.3% of mass, in t ha\, of the CO and CO emitted to the atmosphere by post burning process in this experiment. 5. Based on deforestation rate data found in the literature and considering that the mean biomass content of the Amazon region is approximately the biomass content of the studied site, the annual average rate of CO emitted to the atmosphere was estimated as 0.14 Pg yr\, relative only to the main burning.
Acknowledgements The authors are grateful to FAPESP (Fundac7 a o de Amparo a` Pesquisa do Estado de Sa o Paulo) for support of this research through the projects 92/0950-0 and 93/4753-8. The authors are also grateful to FAENQUIL (Faculdade de Engenharia Quı´ mica de Lorena), Lorena, SP, Brazil, for the utilization of the CHN analyzer. The support of Mr. Jose´ Carlos dos Santos is also acknowledged.
1997
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