On the island of the cyclone

Leaves torn off, branches broken, trunks snapped, trees uprooted... When the wind approaches the speed of a high-speed train, how can trees resist? This is a crucial question for the forests of the South-West Pacific islands, which face between five and eight cyclones every austral summer, sculpting these island ecosystems.

How do tropical forests survive cyclones? To find out more about these ecosystems, Thomas Ibanez, researcher at the Amap¹laboratory, took part in a study to analyze the resilience of forests to cyclones on 76 islands in New Caledonia, Vanuatu, Wallis and Futuna, Fiji, Samoa and Tonga. Between 2000 and 2020, 74 cyclones passed through the region, with varying effects on vegetation. " We wanted to characterize, in relation to the strength of the cyclones, the degree of change that vegetation has undergone and the time it takes to recover its previous state", explains the tropical forest conservation specialist.

Forest resilience

This study mobilized several disciplines, from remote sensing to modeling, to reconstruct cyclone paths, wind speeds and visualize the state of forests before and after storms. " For the most violent winds, reaching 280 km/h, vegetation losses reach 40% and regeneration time exceeds 5 months", explains Thomas Ibanez. And in 20% of the areas affected by these devastating winds, the forest had still not regenerated after one year.

"The resilience of forests has limits that may be exceeded in the future, because with global warming, the cyclones that affect this area could intensify, not in frequency but in strength," explains Thomas Ibanez. The warming of surface waters, which are the cradle of cyclones, will have another important consequence: the modification of their trajectory. " Some forests that have been spared until now are likely to be swept away by these violent winds", stresses the researcher.

Adaptation

How will trees that have been caressed by gentle breezes react in the storm? " Some are better adapted, the small, stocky ones, those with less foliage and therefore less wind," explains Thomas Ibanez, who also mentions possible adaptation strategies in which trees lose their leaves and branches more easily, in order to increase the chances of sparing the trunk.

"We also want to determine the characteristics of trees that make them more resistant: height, diameter, wood density, but it's difficult to carry out experiments designed to simulate wind stress. To get as close as possible to these island forests, Thomas Ibanez will be spending three years in New Caledonia, studying the impact of cyclones on forest structure and function. "Islands are at the heart of the biodiversity challenge, representing just 6.7% of the earth's surface, but home to 20% of the world's biodiversity, 50% of species recognized as threatened, and 75% of known extinctions.

1. Amap (UM, Cirad, CNRS, Inrae, IRD)
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