Fire in the Venezuelan Amazon 2: environmental conditions necessary for forest fires in the evergreen rainforest of Venezuela

Studied the potential for forest fires near San Carlos de Rio Negro, Venezuela, one of the wetter regions of Amazonia: 3 of the communities were tall, closed-canopy forests; had closed canopies but were short in stature; and 2 had open canopies. Fine fuels reached dynamic equilibrium moisture levels (ie no further net drying given prevailing microclimate) after only 3-4 rainless days in the open-canopy and short-stature communities and after 8-10 days in the tall, closed-canopy forest types. Fine-fuel moisture content at "equilibrium' was generally twice as high in the fall, closed-canopy forest as in the other vegetation types (20% vs 10%). Surface fires were successfully started in the open-canopy and short-stature communities after 4-6 days without rain. An average of 19-27% of the available fuels were consumed by these fires. The tall, closed-canopy forest communities would not burn, 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 c1 day each week, on average, but in tall, closed-canopy forest such levels may occur less than 1 day each year. -from Authors