Floristic diversity in two high - rainfall mediterranean - climate ecosystems: Lessons from the analysis of phylogenetic and compositional patterns

The world’s five mediterranean-type environments (MTEs) harbour exceptional levels of plant diversity. High-rainfall mediterranean-climate zones have been relatively under-researched in between-region comparative analyses of floristic diversity patterns. However, high-rainfall MTEs have been associated with both high species compositional turnover and high phylogenetic diversity, and may provide refugial habitat for rare and endemic plant species. Larger environmental contexts and evolutionary histories differ markedly between the world’s five mediterranean systems, which may influence relevant processes and mechanisms involved in plant community assembly. The thesis firstly explores the climatic characteristics of high-rainfall MTEs to derive an overview of patterns of climatic within - and between - region diversity. Climatic between - region differences are anticipated to result in distinct distributions of species and phylogenetic diversity within individual high-rainfall zones. A numerical analysis of the high-rainfall mediterranean climates established climatic similarities between the high-rainfall zones of the Klamath - Siskiyou Mountains, USA and the South-West Australian Floristic Region (SWAFR). Both regions have been attributed high conservation significance and are distinguished by exceptional floristic diversity. While the Klamath-Siskiyou Mountains are differentiated by a highly varied mosaic of topography, geology and climate, the SWAFR is characterized by a subdued topography, and diverse and ancient soils. The thesis evaluates whether pronounced between-region differences in larger landscape contexts, environmental attributes and evolutionary histories are reflected by the distribution of species and phylogenetic diversity within the two study areas. By assessing the relative importance of niche-differentiation and dispersal-limited processes for species compositional turnover, this thesis tests for fine-scale relationships between species and environment. Patterns of species composition were related to the distribution of plant phylogenetic diversity to estimate how closely plant diversity has evolved with the local environment. Plant diversity is expected to be especially vulnerable to future climatic changes if the distribution of plant phylogenetic diversity is spatially clustered within the landscape, and strongly associated with environmental factors. The importance of environmental and spatial variables for species compositional turnover and phylogenetic diversity patterns were assessed within canonical analyses and Mantel tests using large quadrat-based floristic data sets. Within both regions, species compositional turnover strongly corresponded to environmental within-region heterogeneity, while spatial factors were only weakly significant. Species compositional turnover at the landscape level and within floristic assemblage types was considerably higher within the SWAFR. Climatic factors were mainly important for species and phylogenetic turnover within the Klamath-Siskiyou Mountains where phylogenetic diversity was dispersed across a steep topographic and climatic gradient. Since phylogenetic diversity within the Klamath-Siskiyou Mountains is distributed across a mosaic of topographic and climatic micro-habitats, refugial habitats may continue to harbour floristic diversity during periods of moderate climate change. Significant relationships between species distributional patterns and environment were reflected within the distribution of phylogenetic diversity in the SWAFR. Because phylogenetic diversity and potential is clustered in edaphic and climatic micro-habitats, plant diversity appears highly susceptible to climate change in the SWAFR. The effects of varying levels of soil nutrient availability on species compositional turnover, phylogenetic diversity and dispersion were contrasted between three soil types associated with open forest vegetation within the SWAFR. Species richness, phylogenetic diversity and phylogenetic dispersion increased with a decrease in soil nutrients that is most likely related to soil development and leaching. The distributional patterns of phylogenetic diversity varied according to soil type and soil nutrient availability. It is concluded that high-rainfall mediterranean climate regions harbour floristic diversity within refugial habitats, independent of between-region differences in larger-scale environmental contexts and evolutionary histories. Region-specific environmental and geologic characteristics, however, significantly influence distributional patterns of phylogenetic diversity, and consequently affect climate change susceptibility. High-rainfall refugial habitats for phylogenetic plant diversity are dynamic and complexly linked with interacting edaphic and climatic determinants. For assessing climate change susceptibility, and the management and conservation of individual high-rainfall mediterranean plant refugia, these complex relationships have to be further evaluated using refined molecular phylogenies. The successful conservation of a large diversity of climatic and edaphic micro-habitats will likely increase the probability of maintenance of phylogenetic diversity within individual high-rainfall MTEs