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Effects of experimental fuel additions on fire intensity and severity: unexpected carbon resilience of a neotropical forest

Brando, Paulo M., Oliveria‐Santos, Claudinei, Rocha, Wanderley, Cury, Roberta, Coe, Michael T.
Global change biology 2016 v.22 no.7 pp. 2516-2525
carbon, carbon cycle, carbon sinks, combustion, drought, energy use and consumption, fire intensity, fire spread, fires, fuel loading, fuels, fuels (fire ecology), habitat fragmentation, heat, logging, microclimate, mortality, soil respiration, tree and stand measurements, tree growth, tree mortality, trees, tropical forests, understory, wood, Amazonia
Global changes and associated droughts, heat waves, logging activities, and forest fragmentation may intensify fires in Amazonia by altering forest microclimate and fuel dynamics. To isolate the effects of fuel loads on fire behavior and fire‐induced changes in forest carbon cycling, we manipulated fine fuel loads in a fire experiment located in southeast Amazonia. We predicted that a 50% increase in fine fuel loads would disproportionally increase fire intensity and severity (i.e., tree mortality and losses in carbon stocks) due to multiplicative effects of fine fuel loads on the rate of fire spread, fuel consumption, and burned area. The experiment followed a fully replicated randomized block design (N = 6) comprised of unburned control plots and burned plots that were treated with and without fine fuel additions. The fuel addition treatment significantly increased burned area (+22%) and consequently canopy openness (+10%), fine fuel combustion (+5%), and mortality of individuals ≥5 cm in diameter at breast height (dbh; +37%). Surprisingly, we observed nonsignificant effects of the fuel addition treatment on fireline intensity, and no significant differences among the three treatments for (i) mortality of large trees (≥30 cm dbh), (ii) aboveground forest carbon stocks, and (iii) soil respiration. It was also surprising that postfire tree growth and wood increment were higher in the burned plots treated with fuels than in the unburned control. These results suggest that (i) fine fuel load accumulation increases the likelihood of larger understory fires and (ii) single, low‐intensity fires weakly influence carbon cycling of this primary neotropical forest, although delayed postfire mortality of large trees may lower carbon stocks over the long term. Overall, our findings indicate that increased fine fuel loads alone are unlikely to create threshold conditions for high‐intensity, catastrophic fires during nondrought years.