The invisible effects of wildfires in Amazonian forests

Wildfires in humid tropical forests are one of the most critical environment problems of this century, that could define the future of the tropical forest biome and the world’s climate. In Amazonia, the largest tropical forest in the world, forests are being burned at an unprecedented rate. During the 2015/2016 El Nino, an extreme drought event, nearly 10 thousand km2 of forests were burned in the Brazilian Amazon basin (Silva Jr. et al 2019) – approximately half the size of Wales. Although the frequency of wildfires is increasing in Amazonia, we still don’t know much about them such as how the canopy of intact forests responds to the increases of fire and how much greenhouse gases emissions are emitted as a result. In the last 6 years, I have been investigating how Amazonian forests are changing after they burn and what are the consequences of these changes to forest carbon balance.

Understorey low-intensity fire line advancing in the interior of a closed-canopy forest in central-east region of Amazonia. During the dry season forest edges become even drier which allows fire from human activities (eg. deforestation, pasture and agriculture management) to penetrate the forest interior. Fuel in the forest floor, during extreme droughts such as 2010, 2015/2016, becomes very dry creating perfect inflammable conditions for wildfires. Source: Erika Berenguer.

In 2014, I joined the TREES Lab at the Brazilian Space Institute (INPE), in Brazil. There, I led several expeditions into the burned forests across the Brazilian Amazon. Before that, I only had seen old-growth burned forests from satellite images. But the moment I stepped into a forest that was once impacted by wildfire, I noticed the signs of forest degradation that was not captured by satellite imagery. The destruction from the ground was much worse. Fires burn the understory of forests in low intensity and fire-degraded canopies are, in many cases, not evident in the satellite images. While in satellite images burned forests appeared “healthy” few years after the fire event, in the field I saw first-hand how different they were from the pristine forests nearby. The invisible destruction was right in front of me – standing dead trees, multiple piles of broken stems, a bulky stock of dead vegetation decomposing on the forest floor, large gaps in the forest canopy, hotter and drier understory. Over two years, with the great support of local people and the collaboration of researchers, I managed to sample several burned forests sites across five regions in the Brazilian Amazon.

A field assistant stands on top of dead wood burned during the 2015-2016 wildfires at Flona do Tapajós, a forest reserve in central-east Amazonia.  During the burn fine and coarse woody debris releases CO2 to atmosphere, but emissions continues for years as more trees die and decompose. Source: Erika Berenguer.

The joint effort of multiple research institutes resulted in the largest network of permanent forest plots in burned areas. I compiled the measurements taken in burned and undisturbed forests of nearly 10000 trees and started an investigation as part of my PhD at Lancaster University in the UK. The results were published in a special issue about the 2015-2016 El Nino (Silva et al 2018). My co-authors and I found that fire promotes the mortality of large and dense wood trees up to 8 years, which results in the reduction of 25% of carbon stocks for at least 30 years.

These results gave me insights to ask more questions about how long it takes carbon, which is emitted from dead trees in decomposition, to be released in the atmosphere after a fire. I also wanted to know what the relative contribution of these post-fire emissions was compared to immediate emissions, which are derived from the combustion of coarse and fine debris in the forest floor (figure 3). At this point, several studies had only accounted for the carbon emitted in the post-fire phase as a fixed amount, with estimates of post-fire net CO2 emissions across years still unknown. These questions led to further analysis which results were recently published (Silva et al 2020). Here, we estimated the necromass production produced from dying trees across the years and quantified the CO2 emissions by applying a decomposition rate for coarse wood debris (Chambers et a 2000). We then estimated the cumulative post-fire net emissions over a period of 30 years, also accounting for the CO2 taken up by forest regrowth and compared it with the combustion emissions estimated in a previous study (Withey et al 2018).

A field assistant measures a tree in a burned forest 3 years after the 2015/2016 wildfires in the Purus-Madeira interfluve in central Amazonia. The forest understorey is dominated by fallen trees and litter, and regenerating vegetation which is facilitated by gaps opening and sunlight penetration. Measurements were used to estimate the annual carbon balance over the years subsequent the fire event. Source: Aline Pontes-Lopes.

We found that over a 30 years period, gross emissions by the decomposition of dead trees make up 73% of all emissions, and post-fire regrowth offsets only 35% of that emission. These results showed us that burned forests are a large source of carbon for decades. This is because trees mortality after a fire increase and take several years to return to pre-disturbance levels. Our temporal estimates show a peak in the annual net emissions (balance between CO2 emissions from decomposition and uptake from regrowth) 4 years after combustion emissions end. This means that the CO2 emissions from forest fires occurring now will increase to its maximum by 2024.

(a) CO2 fluxes (Mg ha-1 y-1) from wildfires. Gross emissions (red line) are the total emissions derived from necromass decomposition each year after the burn, CO2 uptake (blue line) is the CO2 taken up through above-ground biomass growth, and net CO2 (black line) is the balance between gross CO2 emissions and uptake. (b) Cumulative CO2 (Mg ha-1) emissions and uptake over 30 yr. Emissions from combustion (dark red) represent a single emission during the burn while gross decomposition emissions (light red) are the cumulative decomposition from all necromass stocks produced in 30 yr, accounting for 73% of total gross emissions. Uptake (blue) offsets part (35%) of total emissions resulting in above baseline values (81 Mg CO2 ha-1) of net emissions (grey). Adapted from Silva et al 2020.

Another major concern that our study highlighted is that CO2 emissions from burned Amazonian forests are not yet incorporated into national and international carbon accounting systems, significantly underestimating how much CO2 is released to the atmosphere. At the moment carbon accounting is only focused on deforestation fires, because forest wildfires are either assumed as non-anthropogenic fires, or that they are carbon neutral in the long term with regrowth offsetting respiration of woody debris and litter. This, however, is not the case in Amazonian humid forests. First, humid Amazonian forests do not burn naturally. Forest wildfires in this region is a consequence of the combination between the use of fire by humans (illegal and legal) and the intensification of dry season because of climate change. Second, we show evidence of decadal carbon deficit in burned forests, clearly demonstrating that regrowth is slow and do not compensate for the emissions produced for decades.

While conservation policies should focus on avoiding forest wildfires in Amazonia, it is also important that emissions from this source get incorporated into national emission. Ensuring accurate carbon accounting means we develop better climate mitigation policies. The good news is recent advances in techniques for mapping burned forests will enable us to upscale our estimates for the Brazilian Amazon. Soon we will be able to reveal the dimensions of the invisible destruction of wildfires in Amazonia and hopefully provide more accurate information so leaders and societies can direct efforts to the right sectors to tackle the problem.

About the Author: Ms. Camila Silva is a Brazilian PhD candidate at Lancaster Environment Centre in UK. In 2019 she was awarded the Dean’s prize for PhD excellence. Her submitted thesis focused on understanding the long-term effects of wildfires in Amazonian forests by assessing a large-scale dataset of permanent plots. She graduated in Forest Engineering at University of Brasilia and received her Msc in Remote Sensing at National Institute of Space Research (INPE) in Brazil. Her research aims combining ground observations from permanent forest plots with satellite-derived data to understand dimensions of the effects from anthropogenic disturbances in tropical forests carbon stock. Twitter: @camilaflorestal1

References

Silva Junior C H L, Anderson L O, Silva A L, Almeida C T, Dalagnol R, Pletsch MA J S, Penha T V, Paloschi R A and Aragao L E O C 2019 Fire responses to the 2010 and 2015/2016 Amazonian droughts Front. Earth Sci. 7 97

Silva C V J et al 2018 Drought-induced Amazonian wildfires instigate a decadal-scale disruption of forest carbon dynamics Phil. Trans. R. Soc. B 373 20180043

Silva C V J et al 2020 Estimating the multi-decadal carbon deficit of burned Amazonian forests Env. Res. Letters 15 11

Chambers J Q, Higuchi N, Schimel J P, Ferreira L V and Melack J M 2000 Decomposition and carbon cycling ofdead trees in tropical forests of the central Amazon Oecologia 122 380–8

Withey K et al 2018 Quantifying immediate carbon emissions from El Niño-mediated wildfires in humid tropical forests Phil. Trans. R. Soc. B 373 20170312

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