Tropical cyclones (hurricanes, typhoons) are ecological disturbances that vary in severity. The immediate impacts of cyclones on forest trees range from partial defoliation to extensive uprooting and snapping of stems, which can lead to high tree mortality. More than 6 million square kilometers of land are impacted cyclones, with coastal areas experiencing the strongest winds. Because our warming oceans can potentially create more intense cyclones, there is increasing interest in their impact on, and subsequent recovery of, forests. Recent research suggests that forests frequently impacted by cyclones have developed some resilience, but also that frequent cyclones can change the structure and species composition of forests. Furthermore, interactions with other disturbances, such as fire, could be catastrophic.
The impacts of winds created by cyclones depend both on the characteristics of winds and the characteristics of trees. Recent work by Thomas Ibanez and colleagues (a global meta-analysis) and Tamara Ticktin and colleagues (a case study after category 5 cyclone ‘Pam’ in Vanuatu), indicates that, generally, trees that are taller and/or have lighter wood are more vulnerable to uprooting (Fig. 1) or stem snapping. Furthermore, these studies found that the maximum wind speed experienced by trees influences the amount of damage. For example, locations more sheltered from wind by the local topography tended to experience less damage.
While the immediate damage caused by tropical cyclones with strong winds can be severe, recovery may also be swift. Tamara Ticktin and colleagues found that both the canopy cover recovered within three years of Cyclone Pam and that regeneration (seedlings) included a wide range of species with different functional roles. Similarly, Gunnar Keppel and colleagues concluded that, in Samoa, 15 years after two successive cyclones had caused major damage to the forests, tree communities had recovered similar functional trait compositions. Both Samoa and Vanuatu are frequently impacted by cyclones and these findings suggest that the species in these communities have evolved adaptations to cope strong winds that occur regularly.
However, this does not mean that forests will not be impacted, if the intensity of cyclones changes. Firstly, in Samoa, the species composition of the tree community had changed 15 years after the cyclones. Secondly, previous studies have shown that frequent cyclones change the structure of forests, creating shorter canopies and denser vegetation. Furthermore, warmer oceans could bring cyclones to places that have hardly or never experienced them and hence support vegetation not adapted to this disturbance. For example, earlier this year the waters in Sydney were sufficiently warm to support the formation of cyclones in March, the Southern Hemisphere autumn, illustrating the increasing chances of cylones reaching this location that has historically rarely experienced cyclonic winds.
Multiple disturbances, where a second severe disturbance follows an earlier disturbance before the forest had time to recover, can have more severe impacts. A recent synthesis of interactions between fires and cyclones suggests that a successive cyclone and fire could produce biome shifts, changing forests into savannahs. Prolonged loss of forest cover was observed at Falealupo Rainforest Preserve in Samoa after two cyclones and a fire (Fig. 2). In a different Samoan protected area, a fire and two cyclones are believe to haven facilitate the invasion by the non-native African rubber tree (Funtumia elastica), which had become dominant in forest areas that burned after the first cyclone but not in others.
Recent research therefore provides a glimpse into what changed cyclone regimes could mean for forests. It seems that many forests currently experiencing frequent cyclones may be little impacted, as their species are adapted to frequently occurring severe winds. However, more severe cyclones could cause more damage that will take forests longer to recover from, making them more vulnerable to subsequent disturbances. Forest that will be affected by more frequent and/or severe cyclones in areas that are currently experiencing little cyclone activity will likely become reduced in stature and develop higher densities of trees, with the consequences for carbon stocks remaining unknown. Potentially disastrous consequences could arise where cyclones become more frequent or intense in fire-prone regions, as the likelihood of biome shifts would increase.
Further reading:
Biodiversity in Oceania 