This paper describes the characteristics of fire spread around a forest clearing site located in the Amazonian arc of deforestation. The experiment was carried out in 2001 at the Caiabi Farm, near the town of Alta Floresta, state of Mato Grosso, Brazil, in the Amazon Arc of Deforestation as part of a set of tests that have been performed in the same area since 1997. So far, six test plots were burned. The main goal in the experiments of the first five plots was to determine biomass fire consumption and carbon release rates under different conditions of size of burned area and period of curing. The results regarding these tests were already published (Carvalho et al., 2001).

Special care had to be taken to prevent fire from escaping the clearing site into the adjacent forest in all five experiments. This procedure had not been necessary in previous experiments conducted by the group in Manaus, state of Amazonas (Carvalho et al., 1995, 1998), and in Tomé Açu, state of Pará (Araújo et al., 1999). Therefore, during 2001 a site was prepared and burned to investigate under-story fire generated by the forest clearing process, and results of this work are presented here.

The experiments reported by Carvalho et al. (2001) were conducted in five plots, denominated A, B, C, D, and E. Biomass fire consumption and carbon release rates were determined in the central 1-hectare area of each plot. In 2001, plot F was felled in May and burned on August 20, to investigate the spread of understory fire in the adjacent standing forest. The location and dimensions of plots A to F are shown in the paper. The times of vegetation felling and burning of each plot are also presented in the paper.

Fire spread rates were investigated with a thermocouple grid in one of the adjacent areas of plot F and using stakes, tapes, and a chronometer in the other three adjacent areas. The nomenclature for each of the sides is defined in the text.

The paper also presents the relative humidity and temperature in the test site just before ignition on August 20, 13:47 h, local time. Ignition was performed with torches from the middle of one the sides. The sequence of events following ignition is also presented.

Thermocouple data will be presented in the form of figures at the conference. Data on litter moisture content will also be presented at the conference. Average spread rates in the hours following the burn of plot F are the following:

Average side I: 0.26 m.min-1 (five measurements considered);

Average side II: 0.11 m.min-1 (three measurements considered; fire stopped in other two);

Average side III: 0.17 m.min-1 (six measurements considered; fire stopped in another one);

Average side IV: 0.23 m.min-1 (four measurements considered; fire stopped in another one).

Other observations were: a) under-story fire was always tilted towards the burned side; b) flame length varied from 15 to 30 cm.