Posted by Ted Feldpausch
14 February 2023In our recent paper, “Ancient fires enhance Amazon forest drought resistance” published in Frontiers in Forests and Global Change, we studied if ancient fires can alter the response of Amazonian forests to drought events.
Recent studies have shown that the Amazon forests had fire events centennial or millennial years ago (although in a much lower scale and frequency), mostly associated with drought periods and indigenous people activities (de Oliveira et al., 2020; de Oliveira et al., 2022; Feldpausch et al., 2022). In the last decades, the length and frequency of droughts in the Amazon have increased, leading to higher tree mortality rates, and carbon loss to the atmosphere (Feldpausch et al., 2016; Aragão et al., 2018). Therefore, we aimed to understand whether forests that had higher occurrence or intensity of ancient fires have a different response during current drought events.
Figure 1: Amazon tropical forests were studied to evaluate the interaction between pyrogenic carbon in soil and tree response to drought. Image Credit: Laura Vedovato
When the forest burns, pyrogenic carbon (PyC) is produced by the incomplete combustion of biomass and is deposited onto the soil and can be used as a proxy to understand ancient fires. The PyC can remain in the soil for millennia and have some specific properties, which increase soil fertility and the capacity of the soil to retain water. Forests with higher concentrations of PyC in the soil may then have faster growth rates because of the high soil fertility and higher resistance to drought, due to more water stored in the soil. Furthermore, as a consequence of fire and drought events, forests can change their tree species composition, and it is still unclear for how long this change can persist. Therefore, it is possible that changes caused for ancient events, such as fire, are still persisting and can alter how the forest responds to drought events today.
In our study, we used a database with information on forest dynamics, e.g., taxonomy, growth, recruitment and mortality of trees in 95 different plots distributed throughout the Amazon rainforest. In these same places, we also collected soil samples, in which we carried out laboratory analyses of fertility, texture and concentration of pyrogenic carbon (macro and micro charcoal in the soil). To identify and measure periods of extreme drought between the years 1981 to 2017, we used remote sensing data integrated with precipitation data from local meteorological stations. With this combined database, it was possible to carry out statistical analyses and find out whether forest areas with higher concentrations of pyrogenic carbon in the soil have different dynamic rates in response to current drought events.
Our results show that the forest areas that have higher concentrations of pyrogenic carbon in the soil, that is, with evidence of greater occurrences of ancient fires, maintained their ability to gain carbon in trees at the same rates as years without drought. Although we are still not completely sure of the mechanisms behind this result, we do have some hypotheses.
The first is through the increase in soil fertility caused by the higher concentration of pyrogenic carbon in the soil. Previous studies have already shown that areas of the Amazon forest that have more fertile soils have faster forest dynamics, that is, trees grow faster but also die faster, led by tree species that have low-density wood. Thus, even if tree mortality occurs during periods of drought, there is also recruitment of new trees in a short period, not altering the balance between loss and gain of carbon during periods of drought.
Figure 2: Large canopy emergent trees in Amazon tropical forests are susceptible to severe drought. Image Credit: Laura Vedovato
The second hypothesis is through the ability of pyrogenic carbon to increase water retention in the soil. In this way, forest areas that have higher concentrations of pyrogenic carbon in the soil would have greater availability of water in the soil, allowing these forests to continue growing even in periods of drought.
And the third hypothesis is related to changes in the composition of tree species caused by the constant occurrence of these fires in the past (also associated with drought), and which have persisted to the present day. The high frequency of disturbances such as fire leads to the establishment of pioneer species, which have light wood and fast growth, leading to rapid forest dynamics as explained in the first hypothesis.
It is important to emphasize that one or more hypotheses may be correct and that further studies are needed to better understand the underlying mechanisms. Furthermore, we are entering new climatic regimes accompanied by changes in forest cover. Amazonian forests are increasingly fragmented, deforestation is reducing precipitation, the dry season is lengthening and temperatures are rising. Therefore, there is great uncertainty about how forests are responding to these changes.
The recent increased frequency of fires does not allow the forest enough time to recover and show any benefit from these fires during short-term droughts. Thus, the mechanisms we discuss are therefore unlikely to be valid when considering modern fires but are relevant to help us understand the impacts of ancient fires in the Amazon.
The full open-access paper can be found here: https://www.frontiersin.org/articles/10.3389/ffgc.2023.1024101/full
Text: Laura Vedovato and Ted Feldpausch
Further reading:
Aragão, L.E., Anderson, L.O., Fonseca, M.G., Rosan, T.M., Vedovato, L.B., Wagner, F.H., et al. (2018). 21st Century drought-related fires counteract the decline of Amazon deforestation carbon emissions. Nature communications 9(1), 536.
de Oliveira, E.A., Feldpausch, T.R., Marimon, B.S., Morandi, P.S., Phillips, O.L., Bird, M., et al. (2022). Soil pyrogenic carbon in southern Amazonia: Interaction between soil, climate, and above-ground biomass. Frontiers in Forests and Global Change 5. doi: 10.3389/ffgc.2022.880963.
de Oliveira, E.A., Marimon-Junior, B.H., Marimon, B.S., Iriarte, J., Morandi, P.S., Maezumi, S.Y., et al. (2020). Legacy of Amazonian Dark Earth soils on forest structure and species composition. Global Ecology and Biogeography 29(9), 1458-1473. doi: https://doi.org/10.1111/geb.13116
Feldpausch, T.R., Carvalho, L., Macario, K.D., Ascough, P.L., Flores, C.F., Coronado, E.N.H., et al. (2022). Forest Fire History in Amazonia Inferred From Intensive Soil Charcoal Sampling and Radiocarbon Dating. Frontiers in Forests and Global Change 5. doi: 10.3389/ffgc.2022.815438.
Feldpausch, T.R., Phillips, O.L., Brienen, R.J.W., Gloor, E., Lloyd, J., Lopez-Gonzalez, G., et al. (2016). Amazon forest response to repeated droughts. Global Biogeochemical Cycles 30(7), 964-982. doi: 10.1002/2015GB005133.