The consequences of changing cyclone activity for forests

Tropical cyclones are large, rotating low-pressure systems that have high wind speeds and carry large amounts of precipitation. In recent years, cyclones have become increasingly intense, causing many deaths and extreme damage to human livelihoods and infrastructure. The recent Cyclone Idai killed more than 1,000 people in southern Africa and has created a humanitarian crisis. In addition, cyclones can have severe impacts on ecosystems, killing and injuring plants and animals. Climate change predictions suggest that cyclones will increase in intensity and occur at progressively higher latitudes over the next decades, as our oceans continue to get warmer.

What do the predicted changes in cyclone activity mean for forest ecosystems? The effect of immediate effects of cyclones on forests have been well documented: cyclonic winds rip off leaves, break branches and may cause entire trees to snap, ultimately increasing the mortality rates of trees. However, this also creates more opportunities for trees. The additional light due to canopy openings and the additional space due to trees dying, gives other plants the opportunity to utilise the newly available resources.

Defoliated canopies and broken branches in the Mount Koroyanitu Range in Fiji 2 months after Cyclone Winston.

Therefore, cyclones should have important long-term consequences for forest ecosystems. Frequent cyclones would be expected to regularly “prune” the upper canopy of the tallest trees, reducing the canopy height of forests. The more frequent openings in the canopy should also encourage more trees to grow (as more light becomes available), resulting in denser forests (forests that have more stems per unit area).  Our recent, global study comparing the structure of forests experiencing vastly different cyclone activity strongly supported this. It found that forests experiencing more frequent and more intense cyclones did indeed have lower canopies and higher stem densities.

This means that forests around the world will change over the next decades. More intense cyclones will cause shorter canopies and greater stem densities in forests. The occurrence of cyclones at increasingly higher latitudes implies that some forest ecosystems will experience cyclones for the first time, possibly meaning they will reduce in height and become denser. However, other changes are harder to predict. For example, species differ in their susceptibility and responses to cyclonic winds, meaning that changes cyclone activity will likely alter the species composition of forests. Furthermore, characteristics, such as wood density, make species more susceptible to cyclonic winds, meaning that changes in cyclone patterns could alter carbon stocks and nutrient cycling. Therefore, more is needed to better understand what changed cyclone activity means for forest ecosystem.

Further Reading: Ibanez T., Keppel G., Menkes C., Gillespie T. W., Lengaigne M., Mangeas M., Rivas-Torres G. & Birnbaum P. (2019) Globally consistent impact of tropical cyclones on the structure of tropical and subtropical forests. Journal of Ecology 107, 279-92.

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