Over the past 40 years, the proportion of major tropical cyclones (categories 3–5) has increased and this trend is predicted to continue under the influence of climate change. The incidence of wildfires has also increased and it is possible that these two forms of ecological disturbance may be interacting with each other, causing alterations to ecosystems and having long-lasting impacts on biodiversity and the provision of ecosystem services.
The authors of a new review, published in the journal Trends in Plant Science, state that while the effects of tropical cyclones and fires on ecosystems have been considered in isolation from each other, it is likely that these two types of disturbance interact, causing ecosystem impacts that may have been missed in past investigations. They say that cyclones and fires fuel one another, with each type of disturbance worsening the effect of the other.
“Rainforests, for instance, are wet by definition, which makes them resistant to fire. But when they are disturbed by cyclones, it can allow fire to spread into those ecosystems. To have a fire, we need three things: fuel to burn, dry enough micro-climate, and a source of ignition,” says lead author Thomas Ibanez, a plant ecologist at the French National Research Institute for Sustainable Development working with the University of Montpellier. “Cyclones can affect all three elements.”
Cyclones are storms that originate in the South Pacific or Indian Oceans and, like hurricanes in the Atlantic or typhoons in the Nothwestern Pacific, they deliver heavy rains, storm surges, and gale-force winds that can rip off the leaves and branches of trees, consigning more potential fuel to the ground. In addition, microclimates are altered as there is more air circulation and less canopy cover where a cyclone has passed through.
“When the wind from a cyclone blows, it damages trees, bringing down a lot of leaves, twigs, branches, and logs to the ground, which make great fuel for future fire,” says Ibanez. “Wind also opens the canopy, bringing more light in the undergrowth, which can promote the growth of grasses or bushes that also make good fuel. Also, when the canopy is opened, it makes the undergrowth drier because the canopy usually shades it from the sun and locks in moisture.”
The authors reviewed the disturbance regimes, particularly in woody ecosystems, and found that humans have altered natural disturbance regimes directly by introducing novel disturbances or suppressing historical disturbances, and indirectly by changing land use, local environments, and global climate. They first identify those regions where tropical cyclones and fires co-occur and then summarize the evidence for interactive effects on woody ecosystems.
Cyclones may also indirectly increase the instances of human-generated ignition by rendering forests promising places to conduct agriculture. “This phenomenon is common where people depend on slash-and-burn agriculture or forest resources for livelihoods,” says Ibanez. “After cyclones, damaged forests may be burned to provide ash beds for planting new crops and for easier access to forest resources.”
Not only do cyclones make the likelihood of fire greater, but the fires may also change the ways that cyclones affect forests. “Of course, fire cannot directly affect the likelihood of cyclones, because the cyclones originate from oceans, but they can affect the response of the ecosystem to cyclones,” says Ibanez. “If you have a fire, it can make the trees weaker to resist cyclone winds or it can kill trees and promote the regrowth of trees that are less resistant to cyclones.”
In these ways, altered cyclone-fire interactions can change the composition of ecosystems and biomes, especially on islands and in coastal regions where these disturbances occur frequently. Alternatively, a cyclone or a fire could render any subsequent disturbance more severe because damage caused by the first disturbance lowers the resistance of established trees or favors the establishment of less resistant trees (compound interaction).
Where strong cyclones and fires have co-occurred historically, the events are a natural part of maintaining the land. “In cyclone-prone regions, there are also ecosystems that are adapted to frequent cyclones and fires, and interactions between these disturbances actually maintain original and species-rich ecosystems,” says Ibanez. “In these ecosystems, human activities that reduce fire, such as land use change and fire suppression, can threaten the stability of the ecosystem and cause biodiversity loss.”
However, what with increased numbers of tropical cyclones predicted in future, it is possible that some ecosystems that have already been damaged by the action of a cyclone will be more vulnerable to the effects of a subsequent fire. For example, winds generated by a tropical cyclone (Ophelia, category 2, in 2017) fueled massive fires in temperate and Mediterranean forests in the Iberian Peninsula, an area not historically prone to cyclones. Studies from the North Atlantic basin suggest that biomes influenced by cyclone-fire interactions are at conservation risk due to climate change and human manipulation of fire regimes.
“We would like to better understand how this phenomenon varies by location. We could then predict with climate change which places would be more likely to be affected by these changes,” says Ibanez. “An important component of global change is that ecosystems do not face just one disturbance, but a mix of several disturbances, and the interaction between new disturbances can result in unexpected effects.”
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