Woody plant diversity in tropical savannas of Africa and Australia

Tropical savannas are a dominant biome on Earth characterised by co-dominance of woody plants (trees and shrubs) and grasses that is fundamental to understanding of the global carbon cycle. Tropical savannas display a high level of biogeographic contingency, where both the constituent flora and ecosystem dynamics vary widely among regions, and woody plant composition is dissimilar between continents. To date, there remains a limited understanding of patterns of woody diversity related to climate, fire and drought. Although a new understanding of patterns of diversity and composition will help inform ecosystem models of these regions alongside informing land use policies and highlighting vulnerabilities to climate change. My thesis aims to: 1) quantify patterns of woody diversity in tropical savannas of Africa and Australia in relation to climate and fire; 2) explore use of woody diversity to map the limits of savanna distributions in Africa and Australia; and 3) examine the impacts of drought and fire on community composition in juvenile and adult woody species in a semi-arid savanna in South Africa. To address these aims, I used a combination of data synthesis, experimental field data and statistical modelling. In Chapter one, I introduce the topic and field of research to highlight data gaps related to understanding history, composition and patterns of woody diversity across African and Australian savannas. In Chapter two, using a compilation of 314 plot inventories from Australian and African savannas I quantified the relationship between site level taxonomic richness (alpha diversity) with climate and fire. I found that alpha diversity is higher in African than Australian savannas at any given location. In Africa, patterns of woody diversity have a non-linear relationship with water availability, while in Australia diversity is linearly related to water availability. This chapter is written as a research manuscript to be submitted to the Journal of Biogeography. In Chapter three, I use the same inventory plot data as in Chapter two to quantify woody taxonomic turnover (beta diversity) across gradients of climate and fire. Species turnover is higher over shorter distances in Australia, but decreases for genera and disappears for families, likely due to the dominance of the Eucalyptus and Corymbia genera, and the Myrtaceae family, respectively. This chapter was compiled out of curiosity about complementing the exploration of savanna woody diversity across environmental gradients, but will likely not be published. In Chapter four, using species distribution models and occurrence data for 366 African and 189 Australian savanna woody species, I asked whether savanna distribution can be defined by ensembles of species distributions, and related these ensembles with definitions of savanna based on tree cover and presence of an herbaceous layer. In Australia, mapping savannas using an ensemble of a minimum of 50 species per 0.05 x 0.05° grid cell coincided well with current understanding of savanna limits. However, in Africa where savanna - forest alternative states are more prevalent, the potential to use ensembles of species to map the potential limits of savanna was less clear. This chapter explores the use of the taxonomic data in Chapter two and three to map savanna ecosystems, and is written as a research manuscript to be published in the journal Diversity and Distributions. In Chapter five, using a time series of experimental field data, I quantified the tree demographic responses of juvenile and adult woody species to the 2014-2016 El Nino drought and manipulations of fire frequency in a semi-arid savanna in Kruger National Park, South Africa. Analysing multivariate woody abundance data from 160 sample points over a four year period, it was apparent that juvenile woody diversity increased post-drought on frequently burned and unburned points. However, different fire frequencies favoured different species. Finally, adults suffered substantial drought induced mortality alongside the top-kill from fire. This chapter is written as a research manuscript, which has been accepted in April 2022 as an article in the journal Biotropica. In Chapter six, and in conclusion, I integrate my research findings to suggest that dynamics of woody diversity in savannas of Africa and Australia do not have a universal relationship with environment, likely due to the geography and deep time differences in the development of the biome on each continent. Although, diversity does offer a useful tool to help understand savanna limits. Ecosystem analyses and studies should not treat savannas globally as a single entity given the differences in the woody taxa, unique relationships with environment among continents and the disparity even within species at different life stage to environment.