The relationships between resource availability, plant succession, and species' life history traits are often considered key to understanding variation among species and communities. Leaf lifespan is one trait important in this regard. We observed that leaf lifespan varies 30-fold among 23 species from natural and disturbed communities within a 1-km radius in the northern Amazon basin, near San Carlos de Rio Negro, Venezuela. Moreover, leaf lifespan was highly correlated with a number of important leaf structural and functional characterisues. Stomatal conductance to water vapor (g) and both mass and area-based net photosynthesis decreased with increasing leaf lifespan (r=0.74, 0.91 and 0.75, respectively). Specific leaf area (SLA) also decreased with increasing leaf lifespan (r=0.78), while leaf toughness increased (r=0.62). Correlations between leaf lifespan and leaf nitrogen and phosphorus concentrations were moderate on a weight basis and not significant on an area basis. On an absolute basis, changes in SLA, net photosynthesis and leaf chemistry were large as leaf lifespan varied from 1.5 to 12 months, but such changes were small as leaf lifespan increased from 1 to 5 years. Mass-based net photosynthesis (A/mass) was highly correlated with SLA (r=0.90) and mass-based leaf nitrogen (N/mass) (r=0.85), but area-based net photosynthesis (A/area) was not well correlated with any index of leaf structure or chemistry including N/area. Overall, these results indicate that species allocate resources towards a high photosynthetic assimilation rate for a brief time, or provide resistant physical structure that results in a lower rate of carbon assimilation over a longer time, but not both.

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In the state of Para, Brazil, in the eastern Amazon, we studied the potential for sustained fire events within four 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. We also estimated the potential tree mortality that might result from a wide-scale Amazon 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 fires.Partial logging resulted in dramatic increases in downed woody debris. Total fuel mass was significantly greater in the logged forest ( 180 Mglha) compared to the other cover types (30-60 Mglha). However, the readily combustible fine-fuels (e.g., grasses, litter, herbs) were significantly greater in pastures(<'!: 11 Mglha) than in all other cover types (:::;6 Mglha).Anthropogenic disturbance altered microclimate, which in turn affected rates of fuel moisture loss and the dynamic equilibrium of fuel moisture contents. In pastures the average midday temperature was almost 10•c greater, and the average midday relative humidity was 30% lower, than in primary forest. There was a sixfold difference in average vapor pressure deficit between the primary-forest and the open-pasture cover types. Given the relatively steep gradient between the vapor pressure of the fuel particles and the surrounding atmosphere in the disturbed communities, fuel moisture loss was more rapid and equilibrium moisture contents were lower than in primary forest.Based on the changes in fuels and microclimate, we determined that 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 rainfall events. Openings in the selectively logged forest would burn after 5-6 rainless days 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 (e.g., > 30 d).Through an examination of bark tissues and simulated fires in primary forest we found that only a small percentage of the standing vegetation would likely survive even a lowintensity, surface fire. Mean (±SE) bark thickness for trees >20 em in diameter was 7.3 ± 0.14 mm (n = 699) with values ranging from 1.5 to 28.9 mm. We found a significant relationship (r = 0. 77) between bark thickness and maximum cambium temperatures during fire simulations, and thereby estimated that in the event of a surface fire in the primary forest, 98% of all stems <' !: 1 em diameter at breast height would be killed.Even though the autogenic factors in primary forest of the eastern Amazon create a microclimate that virtually eliminates the probability of fire, they are currently a common event in disturbed areas of Amazonia. As many as 8 x 10 6 ha burned in the Amazon Basin of Brazil in 1987 alone. In terms of curren...

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