Fire in the Venezuelan Amazon 2: Environmental Conditions Necessary for Forest Fires in the Evergreen Rainforest of Venezuela

Abstract

We studied the potential for forest fires in seven vegetation types near San Carlos de Rio Negro, Venezuela, one of the wetter regions of Amazonia. Three of the communities were tall, closed-canopy forests; two communities had closed canopies but were short in stature; and two communities had open canopies. Fine fuels reached dynamic equilibrium moisture levels (i.e., no further net drying given prevailing microclimate) after only three to four rainless days in the two open-canopy and two short-stature communities and after eight to ten days in the three tall, closed-canopy forest types. Fine-fuel moisture content at 'equilibrium' was generally twice as high in the tall, closed-canopy forest as in the other four vegetation types (20% vs 10%). We successfully started surface fires in the two open-canopy and two short-stature communities after four to six days without rain. An average of 19-27% of the available fuels were consumed by these fires. We were unable to burn the three tall, closed-canopy forest communities, even after artificially excluding rain for 41 days. The elevated relative humidities characteristic of tall, closed-canopy forests do not permit fuels to dry to the combustion point. Under current climatic conditions, fire is only possible if the relative humidity drops to 65% or below. In some open-canopy and short-stature communities, relative humidity falls below this threshold about one day each week, on average, but in tall, closed-canopy forest such levels may occur less than one day each year.