5. Fire in Dipterocarp Forests: Impacts on the Regional and Global Atmosphere
In recent years increasing attention has been given to the impact of tropical fires on regional and global-scale environmental processes, e.g., the role of tropical fires in biogeochemical cycles and especially in the chemistry of the atmosphere (CRUTZEN and GOLDAMMER, 1993). Recent estimates of the magnitude of tropical plant biomass burned in shifting agriculture, permanent deforestation, other forest fires and savanna fires revealed that the prompt [gross] annual release of carbon into the atmosphere from these fires may range between one and four billion tons (e.g., ANDREAE and GOLDAMMER, 1992). Though the amount of carbon remaining in the atmosphere [net release] is not known exactly, it is generally accepted that the annual net release of carbon into the atmosphere from plant biomass burned for permanent conversion of tropical forest into other land uses [“net deforestation”] amounts to ca. 1 billion tons per year.
Fig.9. Most air pollution problems arise from forest conversion. Only a small fraction of the biomass of the primary or secondary forest vegetation is harvested and utilized. The remainder is clearcut and burned.
Fig.8. The mountainous pine forests of South Asia are typical fire climax (subclimax) communities. Short-return interval fires favour pines and regularly suppress the regeneration of Dipterocarps. The photo shows a pure Pinus kesiya stand near Da Lat (Viet Nam) with Dipterocarp understory.
Although the emissions from tropical vegetation fires are dominated by carbon dioxide [CO2], many products of incomplete combustion that play important roles in atmospheric chemistry and climate are emitted as well. Much of the burning is regionally concentrated, occuring mainly during the dry season, and resulting in levels of atmospheric pollution that rival those in the industrialized regions of the developed world. Photochemical reactions, for instance, in the plumes of vegetation fires may be responsible for as much as one third of the global input of ozone into the troposphere. Recent observations of seasonally elevated levels of tropospheric ozone in some tropical regions, particularly over the southern tropical Atlantic Ocean between South America and Africa, have been explained by emissions from tropical wildland fires and subsequent photochemical processes which may play an important role in atmospheric chemistry over that large region of the Earth. The investigation of this phenomenon through an international fire research campaign has veryfied this hypothesis (ANDREAE et al., 1993).
Vegetation fires in South East Asia have additional implications which are not yet entirely understood. The global climate is determined critically by tropical convective air movements, leading to the injection of air masses into high altitudes of the atmosphere and their long-range transport and re-distribution. These global circulation patterns originate at the continental and oceanic surfaces with elevated temperatures. This “warm pool” of the globe is in the maritime continent of the equatorial region of Asia.
In the midst of the warmest region of the world, the Indonesian archipelago, extensive burning of vegetation [shifting cultivation, forest conversion burning, and other agricultural burning] takes place [Fig.9]. Although the impacts of these fires on atmospheric chemistry have not yet been explored, it is assumed that two major patterns of emission distribution from vegetation fires exist:
During the “High Phase” (normal years) of the Walker Circulation low pressure is centered over the hot spots. Air masses with products from biomass burning (aerosols, trace gases) are carried to the high troposphere and exported globally.
During the “Low Phase” the warm waters of the “warm pool” are transported to the eastern Pacific, and high pressure builds up over the Indonesian archipelago. A typical ENSO situation develops during which emissions from forest burning are trapped in the lower troposphere.
The last years with extraordinary fire activities in Indonesia were years characterized by the Low Phase of the Walker Circulation. The fire season of 1982-83 was characterized by escaped land-use fires which caused large-size wildfires on several million hectares. In the following years the situation was different. The smoke emitted from the Indonesian archipelago in 1987, 1991, and 1994 was not primarily caused by wildfires. The main sources were shifting cultivation, a traditional practice, but one that is rapidly expanding, and the systematic application of fire for converting primary and selectively exploited rain forests into plantation forests.
The Indonesian Ministry of Forestry released the following preliminary figures for the extent of burning in 1994. They reveal that a total land area of ca. 5.1 million ha had been affected by fire, thereof
Vegetation / Land-Use Type Affected by Fire Area Burned (ha) Traditional dryland farming 2,800,000 Shifting cultivation 1,500,000 Transmigrant farming 260,000 Plantations 221,000 Transmigrant settlements 39,500 Reforestation areas 20,500 Timber estates 17,000 Natural forests 8,000
As a consequence of the Low Phase (trapping of emissions in the lower troposphere) a regional “smog” situation developed and caused enormous health and safety problems in Indonesia and its neighbour countries of Southeast Asia, particularly in Singapore and Malaysia.
At present a large research program, the South-East Asia Fire Experiment (SEAFIRE) is in the planning stage. It is intended to investigate the fire sources and the ecological and atmospheric chemical effects of burning in Southeast Asia. The active phase of this program will be ca. 1997-1999.