Drought-induced Amazonian wildfires instigate a decadal-scale disruption of forest carbon dynamics

Abstract

Drought-induced wildfires have increased in frequency and extent over the tropics. Yet, the long-term (greater than 10 years) responses of Amazonian lowland forests to fire disturbance are poorly known. To understand post-fire forest biomass dynamics, and to assess the time required for fire-affected forests to recover to pre-disturbance levels, we combined 16 single with 182 multiple forest census into a unique large-scale and long-term dataset across the Brazilian Amazonia. We quantified biomass, mortality and wood productivity of burned plots along a chronosequence of up to 31 years post-fire and compared to surrounding unburned plots measured simultaneously. Stem mortality and growth were assessed among functional groups. At the plot level, we found that fire-affected forests have biomass levels 24.8 ± 6.9% below the biomass value of unburned control plots after 31 years. This lower biomass state results from the elevated levels of biomass loss through mortality, which is not sufficiently compensated for by wood productivity (incremental growth + recruitment). At the stem level, we found major changes in mortality and growth rates up to 11 years post-fire. The post-fire stem mortality rates exceeded unburned control plots by 680% (i.e. greater than 40 cm diameter at breast height (DBH); 5–8 years since last fire) and 315% (i.e. greater than 0.7 g cm −3 wood density; 0.75–4 years since last fire). Our findings indicate that wildfires in humid tropical forests can significantly reduce forest biomass for decades by enhancing mortality rates of all trees, including large and high wood density trees, which store the largest amount of biomass in old-growth forests. This assessment of stem dynamics, therefore, demonstrates that wildfires slow down or stall the post-fire recovery of Amazonian forests. This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.

El Niño impacts on human‐modified tropical forests: Consequences for dung beetle diversity and associated ecological processes

“…However, as expected by the relative severity of the impacts on vegetation, El Niño fire‐affected forests showed greater immediate declines (i.e., in 2016; Figure f–h, m–o) and longer‐term losses in beetle responses (i.e., in 2017; Figure a,b,d, g–i) when compared to those forests that only experienced the drought. These findings provide evidence that both El Niño‐related extreme droughts and fire events can bring drastic consequences not only for plant communities and carbon cycling (Silva et al, ; Withey et al, ) but also for fauna diversity and associated ecological functions.…”

“…With some exceptions (e.g., Barlow et al, ; Cleary & Mooers, ), the current literature on drought‐ and fire‐induced impacts on tropical forests is dominated by plant studies (e.g., Berenguer et al, ; Brando et al, ; Silva et al, ), which show increased tree mortality (Nakagawa et al, ), reduced carbon storage (Brando et al, ), and large physiological changes such as in flower and fruit production (Sakai et al, ). Given the scale of effects observed among the primary producers, it seems likely that invertebrate taxa would also be affected.…”

“…However, forest structure is known to be a key determinant of insect communities (Basset, Charles, Hammond, & Brown, ), and there is presumably a threshold at which point changes in forest structure are so great that invertebrate communities and mediated processes are also affected (e.g., França, Frazão, Korasaki, Louzada, & Barlow, ). The lower rates of dung removal and seed dispersal in post‐El Niño surveys (Figure m–r) suggest this threshold was surpassed by the severe impacts of the 2015–16 El Niño drought and wildfires on vegetation and forest structure in this region (Berenguer et al, ; Silva et al, ; Withey et al, ). These lower rates of ecological functions—which occurred within both drought‐only and fire‐affected forests—could be attributed to the higher vulnerability of large‐bodied dung beetles to forest disturbance (Larsen, Williams, & Kremen, ).…”

“…It is not yet clear why droughts should have such a strong and long‐lasting impact, or why the relative magnitude of effects from fire and drought‐and‐fire for dung beetles should be so different from vegetation. Furthermore, as human activities such as logging and fires can affect forest structure and composition for decades (Osazuwa‐Peters, Chapman, & Zanne, ; Silva et al, ), it is unclear at what point and if the dung beetle‐mediated ecosystem functioning will return to pre‐disturbance levels.…”

“…One key issue is that our pre‐El Niño plots were sampled nearly 5 years before the event. Although assessments of the vegetation suggest minimal differences between 2010 and pre‐El Niño samples in 2016, at least some of the influence of El Niño on invertebrate communities may be obscured by pre‐El‐Niño changes in biodiversity through processes such as succession (e.g., Lennox et al, ), longer‐term disturbance responses (e.g., Silva et al, ), or ecological drift and competition (e.g., Levi et al, ; Ulrich, Puchałka, Koprowski, Strona, & Gotelli, ). As such, while before–after studies hold many advantages when the before assessment is immediately before the disturbance (e.g., Christie et al, ; França, Louzada, et al, ), there is a risk that the ecological signal will become degraded with greater temporal disconnection.…”