An increasing amount of fire usage, coupled with large areas of forest vulnerable to fire creates a new threat to the integrity and sustainability of the tropical forests in Amazonia and elsewhere in the tropical world. Forest wildfires that start form from escaped agriculture and rangeland fires are becoming more common throughout the tropical world. Fire has been observed to spread under the canopy for long distances and smolder for long periods on rotten trunks where it can remain active for long periods.
This paper presents the results of a study conducted during 1998, 1999, 2001, and 2003. The study is conducted as part of a series of small experimental burns to study biomass combustion and carbon release rates from deforested Amazon forest. This particular study monitored the change in vulnerability of the interface between primary forest and recent deforested patches near Alta Floresta in the state of Mato Grosso. In the course of the experiment, we monitored fuel moisture drying rates, microclimate, regional weather, and changes in canopy closure. Those conditions were monitored along a transect from the deforested patch into the forest during the entire dry season from the time of in May until the experimental burns occurred in late August/early September of the same year. This logging-drying-burning protocol is a common practice used by farmers in Amazonia. During the slash fire, we monitored fire behavior and depth of fire penetration in the undisturbed forest on the edge of land clearing. The objective is to identify the conditions necessary to self-sustain fire spread in closed-canopy forests in the Amazon forest. The purpose of establishing the flammability experiment on the edge of the biomass combustion experimental burns is also to measure the effect of heating from burning in adjacent clearings on the flammability of primary forest fuels.
These types of recently created interfaces suffer drastic microclimate changes soon after the logging. Difference between the edge and inside the forest becomes sharper by the end of the dry season. Air temperature at 50-meters inside the forest can be 10 degrees C lower than the edge and relative humidity can be 30% higher inside the forest. Leaf area index also decreases seasonally but is more evident near the edges. These conditions are reflected in litter moisture as well. Fires can penetrate from few meters to over 100 meters inside the forest in these recently created interfaces. We have also observed fires in the region that escape from grassland fires and propagate in the understory for several kilometers by the end of the dry season.