What happens when cyclones and fire meet?

Research published in Trends in Plant Science asks for the first time about the interactions between tropical cyclones and fires that affect biomes in many islands and coastal areas. One of their conclusions is that cyclone-induced tree damage can increase fuel loads on the ground and dryness, increasing the probability, intensity, and area of ​​fires.

Cyclones and fires meet

The researchers begin by defining both phenomena. Tropical cyclones originate over warm tropical oceans, generating high-intensity winds and heavy rainfall over large areas. Cyclone-prone land regions cover more than 6.2 million square kilometres or about 4% of the global land area. Meanwhile, fires are widespread disturbances in many terrestrial ecosystems. Lightning is the most common natural cause of fire globally, occurring with the highest density in the North Atlantic and Eastern Pacific basins.

Fires and cyclones are interacting in close succession and may be having a significant impact on the environment. This is stated by an international research group that includes, among others, scientists from the University of South Australia.

Although fires that follow tropical cyclones are known to increase tree mortality rates in forests, this interaction is still unclear. "Cyclones and fires are formidable meteorological phenomena in their own right, but when they occur in close succession, their effect can more than double," said UniSA researcher and ecologist, Associate Professor Gunnar Keppel.

Favours fire spread

Tropical cyclones open up forest canopies. This, the researchers explain, often results in a drier microclimate that reduces moisture in the fuel and favours fire spread. This process, the research says, is particularly critical in humid tropical forests, where fires are often rare and fuel moisture is the main limiting factor for fires.

For example, the winds generated by category 2 tropical cyclone Ofelia in 2017 caused massive fires in the temperate and Mediterranean forests of the Iberian Peninsula, an area historically not prone to cyclones. On the other hand, droughts related to climate change are increasing the frequency of fires and burned areas in many regions. Over the last 40 years, the proportion of major tropical cyclones (Categories 3 to 5) has increased and this trend is expected to continue.

“When a tropical cyclone or storm hits, it opens up the forest canopy, creating a lot of debris and drier, warmer conditions on the ground. In turn, this dried material increases the probability, intensity, and area of ​​subsequent fires,” Keppel added.

Cyclone-caused fires effects tropical moist forests

Cyclone-caused fires should have increasing effects on many temperate and tropical moist forests that historically rarely experienced fire and are therefore primarily composed of fire-intolerant species, the authors say in the research. Increased cyclone-fire interactions in these forests are further fueled by continued forest loss and fragmentation.

Fires, in turn, can directly increase the impacts of subsequent tropical cyclones by damaging trees and changing the composition and structure of tree communities, reducing their resistance to cyclones. In the humid tropical forests and scrublands of New Caledonia, for example, trees affected by previous fires appeared to be less resilient to cyclonic winds than unburned trees, perhaps due to damage to their wood structure, the researcher hint.

Fires could also increase the spacing between trees, which could make them more susceptible to wind damage, as suggested by the higher mortality of oak trees in burned savannahs compared to unburned ones in Panhandle savannahs of Florida during and after the category 2 and 4 cyclones that this area has suffered.

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