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Stellenbosch University (2018)

Understanding distributional responses of vegetation to changes in climate and fire regime in the Cape Floristic Region

Magadzire, Nyasha

Titre : Understanding distributional responses of vegetation to changes in climate and fire regime in the Cape Floristic Region

Auteur : Magadzire, Nyasha

Université de soutenance : Stellenbosch University

Grade : Doctor of Philosophy (PhD) 2018

Résumé
As evidence of climate change and its impact on biodiversity continues to grow, anticipating and understanding ecological responses to climate change is ever more critical. In fire-prone ecosystems, such as those found in the Cape Floristic Region (CFR), a major concern is that changes in climate will likely lead to dramatic shifts in fire activity (e.g. increased fire frequency) that will significantly affect the distribution, composition, and functioning of vegetation. Effectively mitigating and/or adapting to the potential loss of biodiversity and altered ecosystem function in this region hinges on an in-depth understanding of how vegetation in the CFR interacts with the environment and, more importantly, how vegetation will respond to changes in both climate and fire regime. Our understanding of how climate change may impact vegetation is largely derived from distribution models. Vegetation distribution models have been used for decades to investigate species-environment relationships, predict future distribution patterns, and test ecological theories. These models are founded on the premise that vegetation distributions are determined by the spatial distribution of environmental variables that are significantly correlated with, or limit, plant distributions. However, while much emphasis is placed on the role of climate and topography as key determinants of vegetation distributions, other critical ecosystem components (e.g. fire regime) that have significant effects on the composition and distribution of vegetation are rarely incorporated in vegetation distribution models. Given the importance of fire as a driver of vegetation formations and assemblages in the CFR, the exclusion of fire variables from vegetation distribution models potentially constrains the generation of accurate and appropriate information, critical for the management and conservation of biodiversity in the region. The exclusion of fire covariates from distribution studies is partly a result of a lack of fire data, coupled with the widely accepted, but limited, view that climate is the chief determinant of species distributions and also a key determinant of fire regime. To this end, a proxy for fire return interval data, derived from vegetation recovery rates estimated from satellite data, combined with climate and edaphic data, was used to model and analyse the distribution of fynbos vegetation in CFR. Firstly, the importance of fire as a determinant of fynbos species distributions, relative to climate, was evaluated. This was complimented by an assessment of the effect of life history traits on plant species sensitivity to changes in ecological regime. To achieve this first objective, the distributions of 52 closely related fynbos plant species pairs (104 species), classified across two growth forms Stellenbosch University https://scholar.sun.ac.za iii (graminoids and shrubs) and their respective fire response strategies (seeders and resprouters) were modelled using Maxent, and subsequently analyzed. Secondly, the potential impacts of changes in climate and fire regime on future fynbos distributions were assessed by modelling the future distributions of 22 fynbos vegetation types under 44 Phase 5 Coupled Model Intercomparison Project (CMIP5) general circulation models (GCMs) using multinomial linear regression. Lastly, an overlay analysis of projected distributions of fynbos species and vegetation types was used to assess whether fynbos species respond in unison or as individuals to changes in climate and fire regime

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