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Chapter 23
Biogeochemistry of Deforestation and Biomass Burning J. Boone Kauffman, Ken M. Till, and Ronald W. Shea Department of Rangeland Resources, Oregon State University, Corvallis, OR 97331 Cutting and burning of the world's forests is occurring at unprecedented levels and is dramatically influencing biogeochemical cycles at local, as well as global scales. Biogeochemical cycles are altered through losses associated with wood export, volatilization, convective transport, and accelerated rates of erosion and leaching losses. These anthropogenic activities are resulting in nutrient losses that far exceed natural rates of reaccumulation. As deforestation alters microclimates and hydrological cycles, internal nutrient cycles can be influenced for decades to centuries. The ultimate results of excessive levels of deforestation and biomass burning include losses in site productivity, desertification and/or species extinctions. Biomass burning is also a significant source ofCO ,CH , ΝΟ , and other products of combustion that influence climate and atmospheric geochemistry. Introduction: The Global Extent of Deforestation and Biomass Burning 2
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Over the past 10,000 years it has been estimated that the area of the earth's surface covered by forests and woodlands has decreased by one-third in order to make way for crops, pastures, and cities (i). However, in the last decades of the second millenium A.D., the rates of forest loss have far exceeded that of any other time period in human history. Associated with these unprecedented rates of deforestation are unprecedented rates of species extinction (2). In addition, deforestation is often accompanied with biomass burning (or slash fires). This not only exacerbates losses in biological diversity, but is a rather dramatic biogeochemical event with local as well as global consequences. The practice of deforestation and biomass burning is common among most forest-based cultures in both developed and third world countries. Deforestation and/or intentionally set fires are used for such purposes as timber extraction and slash disposal, clearing of forests for agricultural uses, controlling weeds, insect or pathogen invasions, enhancing the productivity or value of converted pastures and grasslands, and activities associated with charcoal production and fuelwood gathering for cooking, heating or industrial use. Accurate estimates of deforestation on a global scale are largely unavailable, if not impossible, to ascertain. Since preagricultural times, temperate 0097-6156/92/0483-0426$08.75/0 © 1992 American Chemical Society
23. KAUFFMAN ET AL.
Biogeochemistry of Deforestation & Biomass Burning 4
forests have lost the highest percentage of their area (32-35%), followed by subtropical forests and savannas (24-25%), and tropical forests (15-20%) (3). On regional scales, the rates of deforestation can be astounding. Zhang (4) reported that in the Xishuangbanna area of southwestern China, 10,000 km" of tropical forests were lost in the past 30 years. In India, deforestation occurred at a rate of 1.5 million ha yr" during the early 1980s (5). It is estimated that only 16% of Madagascar's primary forest remains. Similarly, 98% of the Atlantic rainforests of Brazil have been cleared (6). From 1950-1980, 23% of Africa's forests and 40% of the forests of the Himalayan watershed were lost. During this same period, approximately 40% of central America's forests disappeared (7). Regional levels of deforestation may affect hydrological cycles which further disrupts forest-ecosystem dynamics. For example, the majority of forest cover in Panama has been lost as a result of deforestation and a concommitant decline in total precipitation has occurred (8). Deforested regions of Malaysia, India, The Philippines, and The Ivory Coast have also experienced decreased amounts of precipitation during the past 30 years (9). Although differences exist among estimations of the quantities of tropical forest being cleared, even the most conservative are remarkable. Currently, the majority of deforestation is occurring in the tropical forests of the world. Woodwell (10) reported annual rates of deforestation in tropical regions ranged from 3.3 to 20.1 million ha yr" . The average rate of tropical forest loss according to Meyers (9) is 9.5 million ha" yr' . In Amazonia, Meyers (11) estimated that 36,000 km were cleared from 1966-1975. For the year 1987, Kaufman et al. (12) reported 350,000 fires burned >20 million ha in Amazonia alone. Based upon satellite imagery, Setzer et al. (13) suggested that 8 million ha of this burned area was slashed primary forest; the remaining areas were covered by second-growth forest or pasture. Because of changes in government policies and programs, coupled with above average precipitation in the dry seasons, the area of primary forests in Amazonia subjected to deforestation and fire events is estimated to have declined to 4.8 million ha in 1988 and 3 million ha in 1989 (14). Fearnside (75) estimated that by 1990,40 million ha or 8% of the Brazilian Amazon forests have been cleared. Land use changes in the tropics have resulted in a landscape characterized as a mosaic of logged forests, cleared fields, and successional forests. This results in the transformation from extremely fire resistant rainforest ecosystems to anthropogenic landscapes in which fire is a common event (16,17). Fires occur in disturbed tropical forests because deforestation has a dramatic effect on microclimate. Deforestation results in lower relative humidities, increased wind speeds, and increased air temperatures. In addition, deforestation results in increased quantities of biomass that are susceptible to fire. This biomass may be in the form of forest slash, leaf litter, grasses, lianas or herbaceous species (16, 18). Abusive forest practices are neither a recent phenomenon nor limited to developing tropical countries. Deforestation has brought about economic and social declines of civilizations for over 5,000 years (19). Postel and Ryan (1) estimated only 1.5 million ha of primary forests remain out of the 6.2 billion ha 2
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THE SCIENCE OF GLOBAL CHANGE
that existed prior to human dominance on earth. Virtually all of Europe's original forests have been replaced by agriculture, urban environments, and intensively managed tree plantations. In the continental United States (excluding Alaska) 600 kg ha" ) are associated in slash fires in tropical rainforests (Kauffman, J. B. and Cummings, D. L. Oregon State University, unpublished data). 1
1
1
1
1
Erosion and Leaching Losses Following Deforestation and Biomass Burning. Accelerated rates of erosion are another significant pathway of nutrient loss following forest disturbance. Erosion losses may be the most significant pathway of loss for elements with high temperatures of volatilization and particularly following severe, high consumption fires where large quantities of nutrients are present in ash. Following fires in Eucalyptus-dominated forests, Harwood and Jackson (58) reported approximately 50% of aboveground nutrients were in the ash fraction. Similar results were found in dry forests of Brazil where 27-41% of the post fire aboveground Ν pools and 84% of the post fire aboveground Ρ pools were in ash (35). Wind erosion resulted in a loss of 54% of that ash in a little
295
Eucalyptus forest Tazmania Fire
Subalpine forest Australia Fire
30
Chaparral-shrubs USA Fire
74-109
146
10-982
7-173
41 428-530
111 490
1
Ν (Kg ha )
Western hemlock Canada Fire
6 25-32
10 16
1
C (Mg ha" )
234-308 982
12 57-70
29 39
1
Biomass (Mg ha' )
Western hemlock Canada Harvest Fire
Tropical dry forest Brazil Harvest Fire
Second-growth tropical rainforest Costa Rica Harvest Fire
Plant Community/ Disturbance
2-3
10
12
2-77
34-50 16
3 1-21
1
Ρ (Kg ha' )
13-21
51
49
0-76
168-237 37
90 0
1
Κ (Kg h a )
19-36
35
4-211
260467 154
96 0
1
Ca (Kg h a )
57
58
59
60
53
35
37
Reference
Table IV. Biomass and nutrient losses associated with wood harvest (fuel wood or timber export) and fire in selected forest ecosystems.
KAUFFMAN E T A L
Biogeochemistty of Deforestation & Biomass Burning
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