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.
Understorey wildfires harm tropical forests by affecting natural regeneration, but the trajectories of fire-disturbed forests after disturbance are poorly understood. To fill this gap, we conducted experimental burns in a transitional forest between the Amazon forests and the Brazilian Savanna (Cerrado) and investigated their effects on plant community diversity of regeneration. The experiment consisted of three 50-ha plots that between 2004 and 2010 were burned either annually (six times), every three years (thrice) or not at all (Control). To evaluate early post-fire recovery, we recorded grass occurrence and regenerating stems (≤1 cm in diameter at breast height). We noted that high fire-frequency plots had a reduction of species richness (62%) and abundance (84%) and were associated with floristic and structural changes, dominance of few species and increase of grass colonization when compared with low fire-frequency. We observed that resprouts were the main pathway for forest restoration in both burned regimes, particularly in low fire-frequency. However, the forest can recover from fires by means of resprouting, until a threshold in fire frequency is reached, when resprouts and seedlings declined for most of the species, with a few fire-tolerant species becoming dominant.
The trajectory and recovery time of fire-disturbed forests depend on the capacity of seedlings and resprouts to get established over time. Here, we investigated the mechanisms associated with fire effects on post-fire regeneration in the context of a large-scale fire experiment located in southeastern Amazonia. Specifically, we tested the hypothesis that recurrent understory fires reduce forest regeneration capacity by reducing the recruitment of seedlings, the viability of the soil seed bank, the seed rain, and the resprouts. The experiment was comprised of three 50-ha plots: an unburned control, and two plots that were experimentally burned three (triennially) or six (annually) times between 2004 and 2010. Seedlings represented 2.4 and 0.6% of the seed rain and soil seed bank in the Control and burned treatment plots, respectively. Compared to the control, the triennial burns caused more reduction than the annual ones in seed rain (−42 vs. −10%) and seed bank (−78 vs. −50%). These fire-related reduction in seed rain in both treatments explain most of the low post-fire density of seedlings (0.4 and 0.8 ind/m 2 , respectively), compared with the Control (2.9 ind/m 2 ). The 3-year interval between fires contributed to highest mortality of seeds stored in the soil than in the annual burn treatment, where fires were less severe. However, there were 5.5 times more resprouting in the plot burned triennially compared with the one burned annually. In both treatments, though, the number of resprouts declined over time. In conclusion, seeds from both seed rain and seed bank and resprouting were negatively affected by both fire frequencies, reducing the likelihood of native species recovery by altering the regeneration pathways. This situation is expected to be increasingly common in southeastern Amazon forests, particularly under warmer and dryer climate conditions.
Este estudo objetivou avaliar a eficácia de tratamentos pré-germinativos em diferentes substratos para superar a dormência de sementes de Ormosia paraensis Ducke. Foram avaliados quatro tratamentos prégerminativos: 1) escarificação mecânica, 2) escarificação mecânica e embebição em água, 3) choque térmico, e 4) controle. As sementes tratadas foram plantadas em diferentes substratos: areia, serragem e solo florestal. Em cada substrato foram utilizadas três réplicas de 50 sementes para cada tratamento pré-germinativo (total de 1.800 sementes). Para verificar possíveis diferenças e interação entre os tratamentos pré-germinativos e os substratos empregou-se uma ANOVA fatorial. Adicionalmente, aplicou-se o teste de Tukey (p<0,05) para comparação de médias. A germinação das sementes não diferiu entre os diversos substratos testados (p>0,05). Os tratamentos com maiores taxas de germinação foram escarificação mecânica (28,4%) e escarificação mecânica com embebição em água (36,4%), que não diferiram entre si, mas foram diferentes dos tratamentos choque térmico (0,02%) e controle (2%) (p<0,05). Os tratamentos de escarificação aumentaram a germinação em 15 vezes e reduziram o tempo médio de germinação (MGT) em mais de 30 dias em comparação ao controle, indicando que a escarificação mecânica constitui um método viável para superação de dormência de sementes de O. paraensis.
A utilização da diversidade de plântulas de espécies arbóreas, oriundas de áreas cuja vegetação teve a supressão autorizada, no enriquecimento de viveiros, tem sido recomendada como uma técnica alternativa na produção de mudas. Neste trabalho, avaliamos a sobrevivência de plântulas transplantadas da regeneração natural de uma floresta madura para um viveiro de mudas em uma área de transição Amazônia-Cerrado, Mato Grosso, Brasil. Foram alocados três transectos de 10 x 1 m, com 10, 50 e 500 m de distância da borda de onde foram coletados todos os indivíduos lenhosos entre 5-20 cm de altura, no período da manhã (7h às 9h), e sequencialmente plantados em sacos plásticos. Os indivíduos foram identificados, quantificados e classificados quanto ao estágio sucessional. A sobrevivência das plântulas foi avaliada durante quatro meses. No total foram coletados 1.179 indivíduos arbóreos pertencentes a 48 espécies, 31 gêneros e 23 famílias, dos quais 71% sobreviveram. No conjunto dos dados, houve um aumento gradativo na abundância e riqueza de espécies arbóreas da borda para o interior da floresta, sendo maior a 500 m. A abundância e o número de espécies não pioneiras coletadas foram maiores que as pioneiras. Os resultados apontam elevadas taxas de sobrevivência e que a técnica de transplante pode facilitar o enriquecimento de viveiros com espécies regionais de difícil obtenção e de diferentes grupos funcionais.
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