Are giant eucalypt forests rain forest?
Creators: name: Tng, DYP
Publisher: name: University Of Tasmania
Year: 2023

description: Forests dominated by giant eucalypts ( eucalypt species with the potential to attain heights exceeding 70m) occur along the Australian east coast in association with rain forest. These forests contribute significantly to the global carbon budget but their ecological classification suffers from ambiguities around the definition of rain forest. The belief that eucalypts are 'sclerophyllous' and therefore not rain forest is a subjective view that has led to problems with conservation policies and management strategies of giant eucalypt forests. Understanding these forests from a global and functional viewpoint is paramount for their effective management. Eucalyptus grandis-dominated giant eucalypt forests in the Wet Tropics of Australia serve as a case study. Observing that rain forest species continuously regenerate in the understories of these eucalypt forests and believing that rain forest incursion will lead to the local elimination of the giant eucalypts, land managers prescribe frequent, low intensity fires. This management strategy is contentious and not underpinned by robust ecological understanding. To resolve these classificatory problems around eucalypts occurring in rain forest, I take a multidisciplinary approach to address the specific question: Are giant eucalypt forests rain forests? To obtain an in depth understanding of the ecology of giant eucalypts and the forests they dominate, and to provide a global context for these systems, I synthesise over a century's worth of literature on these systems (Chapter 2). Based on these data I propose that giant eucalypts are ecologically akin to rain forest emergent pioneers with a unique dependence on fire for regeneration, and that their habitat should be considered a type of secondary rain forest. Using a GIS-based approach I investigate the landscape scale vegetation dynamics of rain forest and E. grandis forest in the Wet Tropics, where E. grandis forests are considered to be threatened (Chapter 3). Using a environmentally stratified sample of sites, I show that rain forest has expanded over the past 50 years, and that this expansion is most likely a response to a global driver such as increased atmospheric CO2 rather than with local environmental factors. Projective modelling of this rain forest expansion predicts that, even at the fastest estimated rate known for the region, it will be more than 2000 years before rain forest fully engulfs giant eucalypt forests. In Chapter 4, I present a seedling growth experiment to examine if the regeneration niche of E. grandis exhibits ecological convergence with that of well-studied temperate giant eucalypts. I show that E. grandis seedlings grow poorly in unburnt rain forest soils because of the unavailability of phosphorus. The addition of phosphorus lifts phosphorus-deficiency symptoms in seedlings in rain forest soils, and accords well with the idea of E. grandis being a rain forest pioneer with the unique requirement of fire as a disturbance mechanism to create suitable open habitats for regeneration. To contextualize the rain forest- giant eucalypt forest - savanna transitions in Australia from a functional and macroecological perspective, I present a plant functional trait analysis of representative plants across these vegetation transitions in both tropical and temperate Australia (Chapter 5). I show that both tropical and temperate giant eucalypt forest are functionally convergent with rain forest and not with savanna. These results suggest that a classification of giant eucalypt forest based on functional attributes of the whole forest will be more useful for management policy than the established classification based on canopy dominants. In conclusion (Chapter 6), the synthesis ofmy landscape ecology and functional biology data supports my overarching hypothesis that giant eucalypt forests are functionally and ecologically rain forests and should be managed as such. I discuss the implications of my research for the management of Wet Tropics giant eucalypt forest and recommend that E. grandis forest should be managed under a regime of total fire suppression. Given that rare natural fires can be expected to occur under this management, the resulting regime will mimic the inherently long fire return times of these systems.