Extreme dry weather coupled with high-intensity wildfires is already beginning to damage large areas of the Amazon rainforest, a situation that is being made worse by continued deforestation and logging.
Uncontrolled wildfires are normally associated with scrubland vegetation in hot, dry regions such as Australia and south-western United States. However, they may soon become a more familiar feature in other parts of the world, including the Amazon rainforest, as drought conditions become more common with climate change.
During normal, non-drought years, vegetation in the rainforest rarely catches fire, even during the dry season. The humidity is still so high that leaf litter and small twigs, which act as ‘fuel’ for most forest fires, cannot burn. If a forest fire does somehow ignite, it seldom burns for more than a few hours and usually extinguishes in the cooler night-time temperatures.
However, in the last decade there have been two serious drought events in the south-eastern Amazon rainforest. During 2007 and 2010, 5-12% of regional forests were completely burned – this is compared with less than 1% in non-drought years. These forest fires were particularly intense. The weather had become so hot and dry that much of the small twigs, leaves, and branches had completely dried out, and provided an abundant source of fuel for the fires.
In addition, many tropical tree species actually have very thin barks, making them extremely vulnerable to fire damage. With higher-than-normal fuel loads and air temperatures, many trees were unable to withstand the intensity of the fires, resulting in a massive rise in tree death by the end of the drought periods.
These conditions are also exacerbated by ongoing changes in land-use in the Amazon forest. Logging, conversion to cattle ranches, and fragmentation all contribute to the thinning of canopies which ultimately causes the forest floor to dry out more rapidly during rainless periods. These practices also promote the growth of flammable herbs and increase other sources of fire ignition. It even results in increased surface air temperatures and lower local precipitation.
So it seems that even a few extreme drought years, in combination with human activities, can trigger dangerous wildfires across Amazon forests and lead to widespread tree death. To prevent further deterioration of these forests, it is imperative that serious efforts are made to stop deforestation and that measures are put in place to control forest fires once they start.
Reference: Brando PM, Balch JK, Nepstad DC, Morton DC, Putz FE, Coe MT, Silvério D, Macedo MN, Davidson EA, Nóbrega CC, Alencar A, & Soares-Filho BS (2014). Abrupt increases in Amazonian tree mortality due to drought-fire interactions. PNAS PMID: 24733937
Image: Nigel Dickinson, WWF