Biomass Burning:
Pressure State Response Indicators

Biomass burning (fire) is used as a tool to aid in a number of land use and related changes, including: the clearing of forests and savannas for agricultural and grazing use; shifting agricultural practices; the control of grass, weeds, litter - and sometimes pests - on agricultural and grazing lands; the elimination of stubble and waste on agricultural lands after the harvest; and domestic use. |  | Pressure State Response

Pressure

The major components of biomass burning are forests (tropical, temperate, and boreal); savannas; agricultural lands after the harvest; and wood for cooking, heating, and the production of charcoal (Box 1). The burning of tropical savannas is estimated to destroy three times as much dry matter per year as the burning of tropical forests. The vast majority of the world's burning is human-initiated, with lightning-induced natural fires accounting for only a small percentage of the total.

Box 1 Types of Biomass Burned: Global estimates of annual amounts of biomass burning and of the resulting release of carbon into the atmosphere
Source of Burning | Biomass burned (Tg dry matter/yr) | Carbon released (Tg dry matter/yr) | Proportion of Total Carbon released (%)
Savannas 3690 1660 42.1 Agricultural wastes 2020 910 23.1 Tropical forests 1260 570 14.5 Fuelwood 1430 640 16.2 Temperate & boreal forests 280 130 3.3 Charcoal 21 30 1.0
World Total 8700 3940 100.
Source: Adapted from Andreae (1991) in Environmental Science and Technology (1995).

Pressures result from two main impacts of burning:

- Increase in greenhouse gas emission - contributing to global warming
- Changes in levels of biodiversity

Increase in greenhouse gas emission: The immediate effect of this burning is the production and release into the atmosphere of gases and particulates that result from the combustion of biomass matter. The instantaneous combustion products of burning vegetation include carbon dioxide, carbon monoxide, methane, non-methane hydrocarbons, nitric oxide, methyl chloride, and various particulates. During the burning of a forest, carbon dioxide that was sequestered for periods ranging from decades to centuries is suddenly released and returned to the atmosphere in a matter of hours. The burning of forests also destroys an important sink for atmospheric carbon dioxide. Hence, burning has both short- and long-term impacts on the global carbon dioxide budget.

If the burned vegetation does not regenerate, the released carbon dioxide remains in the atmosphere. If the burned ecosystem completely regenerates, as the savannas tend to do under the right conditions, the carbon dioxide is eventually removed from the atmosphere via photosynthesis and is incorporated back into new vegetative growth. However, if regeneration is prevented - e.g. by excessive grazing / browsing of growing material, carbon dioxide is not re-incorporated within vegetation and/or the soil. Other gaseous emissions, however, remain in the atmosphere.

The gases produced are environmentally significant. The greenhouse gases Carbon dioxide and Methane influence global climate. Combustion particulates affect the global radiation budget and climate. Carbon monoxide, methane, non-methane hydrocarbons, and nitric oxide are all chemically active gases contributing to global warming or climate change. Methyl chloride is a source of atmospheric chlorine, leading to the chemical destruction of stratospheric ozone. Recently it was discovered that biomass burning is also an important global source of atmospheric bromine in the form of methyl bromine. Bromine leads to the chemical destruction of ozone in the stratosphere and is about 40 times more efficient in that process than is chlorine on a molecule-for-molecule basis.

Measurements have shown that in addition to the instantaneous production of trace gases and particulates resulting from the combustion of biomass matter, burning also enhances the biogenic emissions of nitric oxide and nitrous oxide from soil. It is believed that these emissions are related to increased concentrations of ammonium found in soil following burning. Ammonium, a major nitrogen component of the burn ash, is the substrate in nitrification, which is the microbial process believed responsible for the production of nitric oxide and nitrous oxide. The enhanced biogenic soil emissions of nitric oxide and nitrous oxide may be comparable to or even surpass the instantaneous production of these gases during biomass burning.

Changes in the levels of biodiversity can be illustrated by examples where fire has been used under experimental conditions (see Box 2). In general, continued burning over a number of years results in a long-term reduction in levels of biodiversity. However, some vegetation communities are dependant on fire for their survival. Important considerations are the frequency of fires (e.g. do they occur every year?) and the state of the vegetation - how dry is it? This latter question is related to the temperature at which the fires burn. Fires that burn late in a dry season are hotter and more destructive than fires that occur earlier when the vegetation still has residual moisture.

Box 2 Impact of Bushfires on Floristic Diversity of Woodland in Côte d'Ivoire Long-term research has been conducted on the impact of bushfires on floristic diversity of woodland. The effect of bushfire on the natural arboreal vegetation evolution was first studied in 1936 in Kokondekro by A. Aubreville on 7-year-old fallow land that held homogenous vegetation. The experimental field was divided into three plots of 2 hectares each. The first was protected against fire, the second was burnt annually on 15 December (early burning), whilst the third plot was burned on 15 March each year (late burning). Seven surveys completed between 1937 and 1994 indicate the evolution of the natural vegetation. The late burning (hot fires) plot shows a reduction of the number of species and density within the population taller than 1.30 m. In 1994, only a few fire resistant species remain. Their stems are contorted and their crowns are damaged by intensive and repeated fire. The number of species in the early burning (cool fires) plot has more than doubled since 1937. The soil quality is an important factor. On fertile soil the canopy closed up after 30 years and as a result limited the growth of herbaceous plants and retarded their desiccation and reduced fire hazards. The vegetation tended towards a dry woodland. Only the trees grown since the decrease of fire intensity will be suitable for sawn timber. On less fertile soil, the canopy is not yet closed, weeds are abundant and yearly fires continue to destroy the terminal buds. The stems are crooked and are suitable for firewood only. In the first plot (protection from fire), the number of species has nearly tripled. The forest is now a dense pseudoclimax with a light undergrowth. The stems are generally straight and cylindrical and suitable for high value wood processing. It is the only plot where creepers flourish.

State

By combining estimates for the global annual amounts of biomass burning with information on the emission ratios for various compounds produced during burning, estimates of global emissions have been made (Box 3)

State indicators are therefore characterised by:

- Extent of burnt area;
- Characteristics (type and amount) of vegetative growth in burned area;
- Period during season during which burning occurs (e.g. start of dry season or end of dry season) and related to this;
- The intensity (temperature) of fire - influencing the amount of destruction experienced;
- Changes in species composition (increases or decreases in levels of biodiversity).

The worst case scenario - i.e. greatest destruction and highest emissions - would result from burning that is carried out towards the end of a dry season when the combustible material burns at the highest temperatures, causes the greatest levels of biomass destruction and results in the greatest extent of burnt area.

The results of burning are increased levels of atmospheric Carbon dioxide and other emissions related to global warming.

Box 3 The contribution of burning to global emissions. Comparison of global emissions from biomass burning with emissions from all sources, including biomass burning.
Category Carbon dioxided(gross) Carbon dioxide(net) Carbon monoxide Non-methane hydrocarbons Nitric oxide Amonia Sufphur gasses Methyl chloride Hydrogen Trophspheric ozone Total particulate matter Particulate organic carbon Elemental carbon (soot) | Biomass burning(Tg element/yr) 3500 1800 350 38 24 8.5 5.3 2.8 0.51 19 420 104 69 | All sources (Tg element/yr) 8700 7000 1100 380 100 40 44 150 2.3 75 110 1530 180 | Biomass burning as of total (%) 40 26 32 10 24 21 12 2 22 25 38 7 39
| 19 | <22 | 86

Source: Andreae (1991) in Environmental Science and Technology (1995).

Response

The majority of fires in savannas and other grazing areas are anthropogenic in nature. As a result, changes to land use management practices can make significant differences to the important indicators listed above.

Response may therefore be characterised by:

- Reduction in frequency of wild fires through policies geared towards improved understanding of the destructive nature of fire;
- The introduction or enhancement of regulatory and best-practice measures relating to wild fires;
- Research into, and adoption of alternative strategies for dealing with those problems currently being addressed through the use of fire as a tool - e.g. alternative measures to control the incidence of the tick vectors of tick-borne diseases;
- Research into the impact of fire on species and habitat diversity, coupled with feedback of this research into land use management and policy;
- The encouragement of active fire management through proactive use of fire early in the season to control and limit the extent of destructive late-season fires ("early burning", or "prescribed burning");
- Use and regular maintenance of fire-breaks;
- Instigation of a national level fire-monitoring programme that is linked to daily reception of information of fire incidence from satellite imagery and to instruments of management and policy capable of acting on the information provided.

Further Reading:

Andreae, M.O. (1991). In Global Biomass Burning: Atmospheric Climatic and Biospheric Implications; Levine, J.S. E.; The MIT Press, Cambridge, MA.

NASA, 1999. Biomass burning and global change. http://asd-www.larc.nasa.gov/biomass_burn/biomass_burn.html

Levine, J.S. 1994. Biomass burning and the production of greenhosue gases. In: Zepp, R.G. (ed) 1994. Climate Biosphere Interaction: Biogenic Emissions and Environmental Effects of Climate Change. John Wiley and Sons. ISBN 0-471-58943-3. http://asd-www.larc.nasa.gov/biomass_burn/biomass.html

Environmental Science and Technology (1995). Biomass Burning: A Driver for Global Change. http://asd-www.larc.nasa.gov/biomass_burn/globe_impact.html

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