The impacts of tropical cyclones on rain forest structure and functioning.

HSF19105 | Amount: $84,000 | Project Leader: G Keppel | Project Period: Jul ’19 – Jul ‘22

A project undertaken at The University of South Australia, and supervised by A/Prof Gunnar Keppel.

Forests are of extraordinary importance for global processes, sustaining biodiversity, and supporting our livelihoods. Tropical cyclones, also known as hurricanes or typhoons, are large-scale weather events with destructive winds that can cause severe damage to forests and infrastructure. While short-term impacts of cyclones, such as broken branches and fallen trees are clearly visible, their lasting impacts on the structure and functioning of ecosystems remain poorly understood. This is a problem, given that cyclones are becoming more intense and are occurring in new regions due to climate change.

Our project investigated the impacts of cyclones on the species composition, structure and functioning of tropical rainforests. Due to COVID-related travel restrictions, it used existing data available in the literature and with researchers to create databases of 1) forest plots that have been impacted by cyclones, 2) studies investigating the immediate impact of cyclones (such as defoliation or broken stems), and 3) characteristics of tree species in Oceania from species descriptions. The project was led by Gunnar Keppel (University of South Australia) and Thomas Ibanez (IRD, Montpellier), and also involved Christophe Menkes (IRD, Nouméa), Dan Metcalfe (CSIRO), John Dwyer (University of Queensland), Edward L. Webb (University of Helsinki), and Peter Vesk (University of Melbourne).

The project had several key outputs. We developed an application (or package), called StormR, for the statistical environment R that will allow users to calculate vital cyclone parameters (maximum wind speed, power dissipated by the cyclone, period of exposure to various wind speeds) at any place for any time over the last 4 decades. For example, a user could determine the maximum wind speed of all cyclones that have impacted the city of Cairns and relate that to the damages in infrastructure recorded. Similarly, maps of the total wind energy dissipated over large areas, such as the Australian Wet Tropics, can be created and related to characteristics, such as structure or function, of rainforests.

Another important output was a database that includes key characteristics of tree species, such as the sizes of leaves, fruits, and flowers, from the Australian Wet Tropics and several islands in the South-west Pacific. The data were extracted from detailed descriptions of the species and will now be available digitally. Such data are particularly important for regions that are understudied and often difficult to access, like the rainforests on Pacific Islands. These data on the characteristics of tree species will allow researchers to understand how climate and disturbances may filter for trees with certain characteristics. Such knowledge will be important as climate change progresses.

While our analyses are ongoing, the project has already produced important new insights into the short- and long-term impacts of tropical cyclones on ecosystems. For example, forests exposed to frequent and stronger cyclones tend to have shorter heights and a greater number of trees per unit area. The former is likely due to the pruning-effect of frequent winds at high speeds and the latter due to the canopy being regularly defoliated, allowing more light to reach the forest floor, which would favour regeneration. Furthermore, frequent cyclones appear to exclude some species that would occur at low frequencies in less affected forests, reducing species diversity.

Another key finding was that the impacts of cyclones can be exacerbated by subsequent fires and vice versa, Therefore, the combined impacts of fire and cyclones may prevent forests formation in regions where they would be expected to occur based on climatic conditions alone. Indeed, the interaction of fires and cyclones can help explain the distribution of savannah and forest biomes worldwide.

Besides improving our knowledge about the biological impacts of cyclones, our project raised awareness among the wider scientific community about cyclones being an important, potentially destructive, disturbance that will affect experienced by many ecosystems in new ways due to climate change.
Therefore, our project has laid the foundation for better understanding the impacts of tropical cyclones on terrestrial ecosystems. This will be important to allow predicting the likely impacts on our forests and timber plantations as cyclones continue to become stronger and to affect areas further north and south than they have in recorded history. We will continue to work on providing the toolkit that will allow us to manage these changes as effectively.