A new study by Pontes-Lopes et al. 2021 examining the impacts of the record-breaking drought and fires caused by the 2015/2016 El Niño has found that even the wet forests of central Amazonia, forests considered relatively fire-resistant, were affected by fire.
Much of our understanding of how fire affects Amazonian forests comes from studies in regions of Amazonia with longer dry seasons, areas that are currently experiencing the highest rates of selective logging, burning, and deforestation for agriculture.
Forest change and loss are often considered to be driven by people in the area, but changing climate is also a factor. Droughts in Amazonia are becoming more frequent and intense. The dry season is getting longer.
What follows is that forest loss is due directly to individuals on the ground logging, fragmenting forests, and starting fires and indirectly through climate change and drought, which allows fire to burn in forests that are normally too wet to sustain fires.
This study is important because it provides important new information on how wet forests with short dry seasons respond to fire. By studying wet forests in central Amazonia, the study helps us understand how forests that rarely sustain fire are affected when drought and forest fragmentation combine to create conditions that allow fire to spread.
Negative impacts of fire
Wet forests in central Amazonia, with shorter dry seasons than other regions of Amazonia, were not too wet-to-burn during severe droughts such as the 2015/2016 El Niño.
Similar to findings from other studies, fire had a greater impact on smaller diameter (for example, 10 to 20 cm diameter) than larger diameter trees. Smaller stems help the forest regenerate after fire. Losing small trees reduces diversity and delays the recovery of biomass, diversity, and forest structure (e.g., see our research by Prestes et al. 2020)
Some might predict that low-intensity fire could lead to increased tree growth post-fire, for example, there could be less competition for resources and fire can release nutrients that can improve the soil fertility of these nutrient-poor soils. While there was an increase in tree mortality as expected, which might reduce competition, there was no post-fire increase in tree growth or increase in the colonization of new trees.
Positive aspects of research findings
While fire is negatively affecting forests across Amazonia, this study found that the impacts of fire were less severe in central Amazonia where forests are wetter, compared to fire effects in forests in other regions of Amazonia with longer dry seasons.
In these central Amazon forests, fire did not substantially increase the mortality of the largest trees, for example, >50 cm diameter; however, it is important to note that other studies from Amazonia have shown there can be a lag in large tree mortality for up to 8 years following fire. So, considering their study monitored the forest for 3 years, there may still be a large number of large trees that die in the future. Their death would open the canopy to increase the risk of fire and add to the fuel load, which could increase fire severity causing more trees to die with future fires.
Their work is important because it provides multi-year measurements of changes in forests post-fire. This is important to track changes over time. Which trees win or lose? What happens to big trees? Which species of young trees colonize the area following fire-induced die-back?
What happens to small trees during fire?
The Pontes-Lopez et al. 2021 paper highlights that in the wet forests of central Amazonia with a shorter dry season, the mortality of small trees was high, which could have long-term impacts on forest recovery. However, the study did not focus on the trees smaller than 10 cm diameter which can be more severely affected by fire. Research from Dr Ted Feldpausch’s group led by Brazilian MSc student Nayane Prestes specifically focused on evaluating the effect of fire on tree size-classes smaller than 10 cm diameter. The Prestes et al. study found that burning substantially altered forest structure and reduced tree species richness, and the effects were most severe for the smallest tree size classes. The changes in forest structure due to fire increase canopy openness and drying of the understory. Lags in tree mortality increase the presence of dead trees that are fuel for future fires. The result is that areas that already burned are more susceptible to burning again. Those forest conditions may then allow fire to spread, even in non-drought years. The Prestes et al. study concluded that, with wildfire widespread across the southern Amazon region and increasing in frequency, fire may negatively affect tree diversity in remaining selectively logged forests, and affect regional carbon cycling with consequences for the global vegetation carbon sink.
Low precitation and high temperature anomalies are expected to continue in Amazonia, negatively affecting forests across Amazonia. Pontes-Lopez et al. show that the impacts of fire were less severe than for regions of Amazonia with longer dry seasons. However, these burned forests that once would rarely sustain fire, are now at greater risk of burning again, with more open canopies drying the understory and increased fuel load from dead trees.
While their findings have some positive aspects for the future of wet forests in central Amazonia—biomass loss and mortality of large trees was lower than burned forests in drier regions of Amazonia—the conditions that determine whether forests will burn have changed, with changes in climate leading to increasing drought and forests becoming more fragmented. Combined, those conditions increase the risk and severity of fire, causing increased tree mortality and biomass loss.
There is a need for long-term studies to evaluate the risk of delayed large-tree mortality and how this affects diversity, forest structure, and fire fuel load. However, establishing and maintaining long-term measurements is challenging and expensive to do across extensive regions of Amazonia and permanent forest plots in disturbed forests can be lost to future fires. There is a need for more studies of fire impacts in areas that have even shorter dry seasons than the current study, e.g., 0-1 month, areas that would burn only with selective logging, fragmentation, and severe drought.
The authors make very important recommendations to develop new products that can help to support future decision-making to avoid or minimize large-scale impacts of fire. This includes developing maps of Amazon fire risk–for example, based on drivers of fire, which forests are most at risk? And developing maps of potential fire impact–for example, which areas are most sensitive or resistant to fire based on tree characteristics such as wood density, bark thickness, tree size, etc.
This study provides important new insight on how forests that were once considered restively fire resistant are affected by new climate conditions and land use.
Text: Dr Ted R. Feldpausch
Pontes-Lopes, A., C.V.J. Silva, J. Barlow, L.M. Rincón, W.A. Campanharo, C.A. Nunes, C.T. de Almeida, C.H.L. Silva Júnior, H.L.G. Cassol, R. Dalagnol, S.C. Stark, P.M.L.A. Graça, and L.E.O.C. Aragão. 2021. Drought-driven wildfire impacts on structure and dynamics in a wet Central Amazonian forest. Proceedings of the Royal Society B: Biological Sciences 288:20210094. https://royalsocietypublishing.org/doi/10.1098/rspb.2021.0094
Prestes, N.C.C.d.S., K.G. Massi, E.A. Silva, D.S. Nogueira, E.A. de Oliveira, R. Freitag, B.S. Marimon, B.H. Marimon-Junior, M. Keller, and T.R. Feldpausch. 2020. Fire Effects on Understory Forest Regeneration in Southern Amazonia. Frontiers in Forests and Global Change 3. https://www.frontiersin.org/articles/10.3389/ffgc.2020.00010/full
Header image credit: https://media.gettyimages.com/photos/in-this-aerial-image-a-fire-burns-in-a-section-of-the-amazon-rain-on-picture-id1163948503?s=2048×2048