Coastal and island ecosystems may be threatened because we are modifying cyclone-fire interactions

While the impacts of single disturbances have been well studied, we still know little about what hapens when ecosystems experience two (or more) successive disturbances. The interactive effect of two disturbances should be larger than that of each individual disturbance combined, as the ecosystem may be more vulnerable while recovering from the initial disturbance. Cyclones and fires provide an excellent case study, as they co-occur over large area (4% of the Earth’s surface in total; Fig. 1), are formidable disturbances in their own right, and have disturbance regimes that are predicted to change as a result of climate change. Interactions between these disturbances occur mostly on islands and in coastal ranges, as cyclone quickly dissipate when travelling over land.

Fig 1 Mean annual percentage burned area between 1997-2014 from the GFED4 database with contour lines that show areas that experience at least 1 tropical storm or cyclone every 20 years based on the IBTrACS database. Creator: Thomas Ibanez.

Cyclones and fires can have major impacts on coastal and island ecosystems. In extreme cases, either disturbance can denude large tracks of the landcape with long recovery periods (Fig. 2). As a result of the loss of canopy, environmental conditions in the ecosystem change considerably. More solar radiation and rain directly reach the ground, producing warmer and drier conditions and more erosion. However, ecosystems that regularly experience such disturbances are often well adapted and often recover relatively quickly. So what happens when cyclones and fires interact?

Fig 2 Remnant stags of trees at the Falealupo Rainforests Preserve in Samoa in December 2002, 11 years after being impacted by two successive cyclones; cyclone Ofa (February 1990) and cyclone Val (December 1991). Photo: GK.

Theoretically, one disturbance should increase the impact of the other (Fig. 3). For example, a cyclone would create large amounts of debris and drier and warmer conditions near the ground – both of which should promote the occurrence and high intensities of fires. Fires, on the other hand, would be expected to weaken the strengths of stems, making them more susceptible to fires. Evidence for these effects has been reported.

However, a comprehensie overview of the combined effects of these disturbances on ecosystems has been lacking until now. Thomas Ibanez and colleagues show how the interactions between these disturbances can maintain savannas in landscapes that otherwise would be expected to support closed forests (Fig. 3).

Fig 3 Conceptual model of tropical cyclones-fires interactions and hypothetical impacts of changes in interactions on ecosystem states through time. (A) Feedback loop maintaining open canopy ecosystems. (B) Long fire interval (e.g., exceptional wet years or fire suppression) shifting an open ecoystem into a closed ecosystem. (C) Feedback loop maintaining closed canopy ecosystems. (D) Cyclone-fire interaction shifting a closed ecosystem into an open ecosystem (or maintaining an open ecosystem). Creator: Thomas Ibanez.

Climate change will alter the regimes of both disturbances. Both cyclones and fires are becoming more intense and are moving into areas where they rarely (or never) occurred before. For example, tropical cyclones are increasingly affecting higher latitudes and the 2019-20 bushfires in Australia were the most widespread and (in many locations) the most intense in history. Therefore, several coastal and island ecosystems are likely to be highly threatened by changing cyclone-fire interactions and we urgently need to understand how future changes will impact these ecosystems.

However, the future of ecosystems impacted by both disturbances is extremely difficult to predict. Not just because of the complex interactions among cyclones and fires, but also because of the strong effect humans have on fire regimes through deliberate fire suppression and ingnition. Attempts at forecasting the effects of cyclones and fires on ecosystems will therefore need to consider the effect of each disturbance and their interactions, as well as impacts of human actions – directly through fire moderation and indirectly through climate change.

Further reading: Ibanez T, WJ Platt, PJ Bellingham, G Vieilledent, J Franklin, PH Martin, C Menkes, DR Pérez Salicrup, J Russell-Smith & G Keppel (in press) Altered cyclone–fire interactions are changing ecosystems. Trends in Plant Sciences:

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