Two systems or one? Vegetation dynamics in Australia's Wet Tropics

Around the world tropical rainforests intergrade with open, fire-dependent (pyrophytic) vegetation forming landscape-scale mosaics. However, rainforests and open vegetation are mostly studied independently from each other. This has been the case in the Australian Wet Tropics. This thesis begins by arguing that the vegetation of north-eastern Queensland can be considered as a complex system where rainforests and pyrophytic vegetation represent alternative stable states. This is important because it creates a new context for understanding the region’s biological dynamics. It allows us to move forward from an understanding where only abiotic factors determine the distribution of vegetation, to one in which the vegetation itself plays an important and active role at several scales. Globally, studies of abrupt boundaries between closed forests and pyrophytic vegetation focus on the contrast between abiotic factors on either side of the boundaries. I used a novel approach considering controls in non-boundary vegetation and comparing them to parameters across boundaries at a regional scale. My results show that contrasts in soil chemistry across boundaries are not greater than the variance found within either vegetation type, and that the vegetation itself can change soil chemistry remarkably quickly. This highlights the active, rather than passive, role of vegetation in the region. In contrast to other systems presenting alternative stable states (such as coral reefs, savannas or kelp forests), I found herbivory in the Wet Tropics was similar across vegetation types. I also show for the first time that the advantages of compound vs. simple leaves do not lie in leaf construction or herbivore avoidance, addressing a long debated topic to which there is still no satisfying answer. Lastly, a computer model compares the effects on landscape-scale vegetation patterns of changing species interactions under varying environmental conditions. The model evaluates the outcome of obligate rainforest and pyrophytic “species” competing with pioneers and ‘traitors’ and proposes long-term hypotheses that may be tested in the field. In conclusion, using ideas about alternative stable states as a starting point has allowed me to make contributions towards understanding the interaction between different factors involved in large-scale vegetation dynamics in the Australian Wet Tropics