Anthropogenic understory fires affect large areas of tropical forest, particularly during severe droughts. The effects of a single and three annual experimental burns on fuel dynamics, stem mortality, and recruitment were documented in a closed-canopy forest near the southeastern Amazon forest-savanna boundary, within an agribusiness frontier. An 150-ha experimental burn was conducted with a 50-ha control (130), once-burned (B1), and thrice-burned plot (B3) to mimic low-intensity understory wildfires that enter at forest edges. Contrary to expectation, forest flammability declined during a third annual burn—flame heights declined and burned area halved. A slight decline in fine fuels after the second burn, rather than humid microclimate conditions, appears to have limited fire spread and intensity. This reduction in flammability, however, may be short-lived if delayed tree mortality increases surface fuels in future years. Three years after the initial burn treatment, community-wide mortality rates (≥1 cm DBH) in the B1 and B3 plots (5.9% and 7.0% y-1, respectively) significantly exceeded the control (3.2% y-1) by 81% and 116%. Fire-induced mortality increased with a decrease in DBH, but mortality rates varied considerably across species. Overall, the number of burns experienced by an individual stem was the strongest predictor of the probability of mortality. The first burn emitted 22.3 Mg C ha-1 via fuel combustion. During the three years after a single fire, 62 (± 26, SE) Mg ha-1 of live biomass was transferred to the dead biomass pool, compared with 8 Mg ha-1 (± 3) in the control. Recruit stem density recovered within a year after a single burn, but did not recover after multiple burns. By the end of this study, the majority (57%) of new recruits were resprouts, species richness halved, and only a few species dominated. Further, within three years, single and repeat fires prompted grass invasion by species from the adjacent pasture, with a mean incursion distance between 8-10 m into the burned plots. If this agricultural frontier continues to disperse ignition sources, wildfires could substantially degrade this transitional forest, facilitate establishment of savanna or pasture-associated species, and consequently alter the overall carbon storage capacity of southeastern Amazon forests.
|Advisor:||Curran, Lisa, Nepstad, Daniel|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 70/01, Dissertation Abstracts International|
|Keywords:||Amazon, Brazil, Carbon emissions, Fire ecology, Forest fires, Mato Grosso, Tropical forests, Wildfire|
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