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University of Cape Town (2018)

Firestorms in a mesic savanna-forest mosaic

Beckett, Heath

Titre : Firestorms in a mesic savanna-forest mosaic

Auteur : Beckett, Heath

Université de soutenance : University of Cape Town.

Grade : Doctor of Philosophy 2018

Résumé partiel
Forests and savannas in a savanna-forest mosaic are maintained by positive feedback loops with fires, promoting fires in savannas and excluding fires in forests. Occasional extreme fires do however burn into forest edges and cause extensive mortality. So, while our understanding of bistability in these systems has advanced, our understanding of biome-level change has remained rather static. Very few studies address the issue of forest to savanna transitions following extreme fire events. This study is posed to address the criticisms surrounding the bistability of savanna and forest vegetation and present evidence for catastrophic regime shifts in these mosaic systems. What sets it apart from previous research is the boundary between vegetation types is naturally occurring, not the result of logging within forests and the subsequent damage by fire. In HiP, forest patches naturally abut savannas, the system has its native herbivores, and human impacts are low. In my thesis I explored the circumstances causing a catastrophic regime shift or recovery of a resilient forest boundary. I initially set out to determine the drivers of vegetation distributions and dynamics in Hluhluwe-iMfolozi Park (HiP). Next I documented the aftermath of an extreme fire in thicket, forest and savanna vegetation focusing on the recovery of vegetation and evidence for catastrophic regime shifts. And lastly, I explored the association between bark thickness and commonly measured traits in order to understand trait evolution in response to fire regime and other selective pressures such as herbivory and drought. Using aerial photographs from six time periods between 1937 and 2013, I mapped vegetation changes in Hluhluwe iMfolozi Park. Using a Generalized Additive Model (GAM) I built a Habitat Suitability Index (HSI) map based on vegetation distribution maps and topographic variables related to fire behaviour. I investigated transitions between time periods based on the HSI map, as well as the effects of neighbourhood on transition probabilities. Forest distributions in HiP have not remained static over time and have expanded into areas that were once savannas. The habitat suitability index, using topographic predictors associated with fire behaviour, relates to the expansion and contraction of forest vegetation.

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