Deforestation, fire susceptibility, and potential tree responses to fire in the eastern Amazon

In the state of Para, Brazil, the authors studied the potential for sustained fire events within 4 dominant vegetation cover types (undisturbed rain forest, selectively logged forest, second-growth forest, and open pasture), by measuring fuel availability, microclimate, and rates of fuel moisture loss; and estimated the potential tree mortality that might result from a wide-scale forest fire by measuring the thermal properties of bark for all trees in a 5-ha stand of mature forest, followed by measurements of heat flux through bark during simulated firest. Partial logging resulted in dramatic increases in downed woody debris. Total fuel mass was significantly greater in the logged forest (180 Mg/ha) compared to the other cover types (30-60 Mg/ha). However, the readily combustible fine-fuels (eg grasses, litter, herbs) were significantly greater in pastures (≥11 Mg/ha) than in all other cover types (≤6 Mg/ha). Anthropogenic disturbance altered microclimate, which in turn affected rates of fuel moisture loss and the dynamic equilibrium of fuel moisture contents. Cattle pastures were the most fire-prone ecosystem. During much of the 6-mo "dry' season (total rainfall: 200-400 mm), sustained combustion was possible in this community within 24 h following and in the second-growth forest after 8-10 d. In contrast, sustained combustion was not possible in the primary forest even after prolonged rainless periods. Only a small percentage of the standing vegetation would likely survive even a low-intensity, surface fire. Even though the autogenic factors in primary forest of the E Amazon create a microcliamte that virtually eliminates the probability of fire, they are currently a common event in disturbed areas of Amazonia. As many as 8 × 10⁶ ha burned in the Amazon Basin. -from Authors