Different types of savanna formations constitute a very important ecosystem in Venezuela from both ecological and socio-economic perspectives. More than one fourth of the country (approximately 22 million ha) is covered by savannas, found in most geographical regions, but mainly in the Llanos of the central part of the country. The vegetation formations which define these savannas can vary substantially: From the open savannas, with a continuous grass cover, occasionally interrupted by trees and shrubs and no more than four gramineous species to the woody savannas, characterized by the presence of trees and shrubs that can cover up to 15 percent of the area.
Agricultural activities have been traditionally established on a great extension of the savanna area, through a very high dynamic process of land use change, which fluctuates according to the country’s economic situation and specific agricultural policies. Some of the most important crops are grown on savannas such as corn, rice and sorghum, among others, while extensive and intensive cattle raising represents the most significant economic activity of these areas.
Savanna burning during the dry season is a common agricultural practice in the country, mainly to eliminate weeds and pests and encourage growth of new grass for animal grazing through the promotion of nutrient cycling. This periodical burning of a great portion of the savanna areas releases important non-CO2 trace gases: methane, carbon monoxide, nitrous oxide, and nitrogen oxides. Carbon dioxide, which is also emitted in large quantities, is not included in the greenhouse gas inventory because, as knowledged by the IPCC methodology, it is reabsorbed by the vegetation regrowth between the burning cycles. Consequently, in an annual basis, net carbon dioxide emissions from savanna burning is considered to be zero.
The proportion of the savanna areas burned in Venezuela is highly uncertain as there are not reliable national statistics that compile, on a regular basis, the frequency and extent of savanna burning. Consequently, a satellite imagery study (Landsat TM, 1:250,000 scale) was performed on about half of the savanna area of the country in order to determine this fraction and provide a more reliable knowledge of this process.
The dates of the selected satellite images were made to coincide with a time period towards the end of the dry season (March and April), under the assumption that such a time framework would ensure the inclusion of most of the burning which could have occurred within the covered area. In order to choose representative years around 1990, in terms of climatic conditions, the average monthly precipitation of the study area for a ten year series was carefully analyzed so that the dates of the selected images would not coincide with any particularly dry or wet year that could bias the study’s results.
A total of five satellite images at 1:250,000 scale were chosen, covering about 25 percent of the national territory and close to 50 percent of the savanna area. The interpretation was done visually while the area calculation of the different vegetation units and burned portions was performed through the implementation of a Geographical Information System. The results from the study show that approximately 13% of the savanna area was burned on average. However, the fractions burned by region vary widely: from less than 1% in the Nor-oriente Region to almost 50% in the Centro-occidente Region. The corresponding burned fractions, obtained from the study, were extrapolated to the entire savanna area in each region in order to estimate the average burned area in the country. Based on this extrapolation, an average of 3.1 million ha of savannas are annually burned in the country.
Aboveground biomass density data were gathered from different local studies, ranging from 3.3 to 6.1 t/ha dry matter, and have also been assigned to specific regions, based on their proximity and general ecological characteristics. Similar approach was used to extrapolate the values provided by the same studies on dead and live biomass fractions. These local data were used to determine the amount of carbon released by the burnings. These results are very controversial as the proportion savanna burned annually in the country (13%) obtained from the study, appears to be very low, especially when compared to the regional data provided by the IPCC methodology. According to different local studies, conducted mainly in Africa and some parts of tropical America, it has been determined that savannas are burned worldwide every one to four years on average. Discussions with various Venezuelan scientists have also confirmed that savannas are very likely burned in the country in a much higher, but still undetermined, proportion than the fraction reported by the study.
Source: Ministry of Environment and Renewable Resources, Ministry of Energy and Mines, Venezuela. 1995. Preliminary National Greebhouse Gas Inventory: Venezuela. UNEP Project GF/4102-92-40, 52 p. + Literature and Annexes.