Carbon Dioxide Emissions from ForestFires in China
(IFFN No. 13 – July 1995, p. 5-8)
With the increasing concern about the rise of atmospheric CO2 content that will incur climate change, the role of forest fires in the global carbon cycle has received increasing interest. However, the frequency and areal extent of forest fires in China, and their implication for CO2 emission to the atmosphere, have not been assessed yet.
The objectives of this paper are to assess the frequency and areal extent of forest fires in China from 1950 to 1992, and to estimate the carbon emission from these forest fires. Two sources of statistical data were compiled: (1) annual forest statistics of China, which identify the frequency and areal extent of forest fires for each province since 1950; and (2) forest growing stock volumes available on a provincial basis as a result of the third countrywide forest survey of 1984-1988.
Here, two forest components have been taken into consideration: the aboveground biomass and litter mass because they are more sensitive to fires than the underground component. For the aboveground tree component, its carbon density was estimated as a product of volume density, wood density, the ratio of total to merchantable biomass, the ratio of aboveground to total biomass and the conversion factor of biomass to carbon. Wood density is in the range of 0.3 to 0.7 t/m3. An average value of 0.5 t/m3 was adopted because the softwood and hardwood areas are approximately equal both on a national and provincial basis. We assumed the ratio of total to merchantable biomass to be 2.7, while the ratio of aboveground to total biomass is assumed to be 0.75. The generally accepted conversion factor of biomass to CO2-C was 0.45. For litter mass, the carbon storage in litter per unit of area (t/ha) was calculated from the litter mass on the forest floor (10 t/ha) and the conversion factor of biomass to carbon, which was taken again to be 0.45 as mentioned above.
The annual carbon emission M from forest fires in a statistical unit is related to the burned area A by the following equation:
M = A x S (Bi x Ei) (1)
where Bi is the average organic carbon per unit area or carbon density (t/ha) of the ith forest component in a statistical unit, and Ei is the burning efficiency of the ith forest biomass. Here we adopted Ei to be 0.4 for the aboveground component. The reason is that the burned area A herein is defined as the forest area in which more than 30% of the trees have been combusted and recurring fire on the same forest was excluded. According to the chronicle of forest fires, the forest area burned was about of half of the forest area influenced by fires. A similar Ei value of 0.4 was used for secondary forest and 0.3 for primary forest by other authors. The Ei for litter mass was assumed to be 0.8, because litter, comprising fallen leaves, small twigs and branches, is easily ignited.
The cumulative occurrence of forest fires in the period of 1950-1992 in China amounted to 632,994, with an annual average of 16,212 (Tab.1). The variations in number of forest fires in all provinces are significant and nearly 60% of provinces have variation coefficients larger than 1. The cumulative burned forest area amounted to 36.31 million ha in the period of 1950-1992, consisting about 29% of the total forest area in China, with an annual average of 946,000 ha (Tab.2). The variation in burned forest area for all provinces is also significant. About 76% of provinces have variation coefficients larger than 1. The cumulative direct carbon emission from forest fires amounted to 343.29 million t in the period of 1950-1992, with an annual average of 20 million t per year, of which 12.4 million t per year (62%) from aboveground biomass combustion (Tab.3). The carbon release from Neimenggu, Helongjiang and Yunnan Provinces accounts for 75.9% of the total release (Tab.3).
The direct carbon release from forest fire is 0.30 million t in 1992 and 20 million t/yr over 1950-1992, which are about 0.04% and 2.94% of the carbon emission from fossil fuel combustion and cement production in 1992 (678 million t/yr), respectively, and about 1.76% and 117% of the carbon emission from forest due to change in land use (17 million t/yr), respectively.
As compared with the temperate and boreal forests in other countries, forest fire in China is significant. According to the annual occurrence of forest fires (number of fires), China ranks second in the world after Russia. Regarding to burned area, China lies the third, following Canada and Russia.
On a global scale, the carbon release from biomass burning was estimated to be 3.0-6.2 billion t/yr, which accounted for 40-60% of the net carbon flux from terrestrial ecosystems. The carbon emission from temperate and boreal forest have been estimated at 130 million t/yr, accounting for 3% of the carbon release from biomass burning. If we take the carbon emission from forest fires in China into account, the global carbon release from temperate and boreal forests increases by 15.4%.
Tab.1. Occurrence of forest fires by the province in China from 1950-1992
Tab.2. Forest area affected by fires for each province of China from 1950 to 1992
Tab.3. Direct carbon emission from forest fires by the province in China
A more detailed version of this article, with bibliographic references, can be obtained from the authors:
Xiaoke Wang, Yahui Zhuang and Zongwei Feng
Research Center for Eco-Environmental Sciences
Chinese Academy of Sciences