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Reconstructing 600 years of landscape change in the grassy woodlands of the Tasmanian Midlands

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posted on 2023-05-28, 09:44 authored by Romanin, LM
Temperate grassy woodlands are often foci for human activity. Globally these landscapes exist in a continuum from natural to synthetic. Woodlands in the Americas and Australia were occupied by humans for millennia but the transformations they effected were subtle. Woodlands in Europe have been occupied and used by humans for many thousands of years. Some landscapes have been severely degraded by human activity (e.g. the Fertile Crescent), while others exist as analogues of naturally occurring ecosystems containing a high proportion of species that are useful to humans (e.g. Dehesa of Spain). When Europeans expanded their empires they drastically altered North American and Australian woodlands to make them suitable for agriculture, creating 'Neo-European' landscapes. The Midlands of Australia is one such woodland landscape. After Europeans settled the region in the early 1800s Aboriginal land management ceased, and a suite of exotic plants and animals were introduced. In this thesis I examine landscape scale changes to the Midlands beginning in the period shortly before European arrival. Using multiple methods, I examined changes to fire regime, vegetation patterns and tree cover. I also examined the effectiveness of conservation methods in this largely privately-owned landscape, and predicted changes to tree cover under various climate change projections. Chapter 1. Using pollen and charcoal analysis I examined how vegetation and fire regimes have changed over the last 600 years in the Midlands of Tasmania. Sediment cores from seven lagoons were sampled, with a chronology developed at one site (Diprose Lagoon) using \\(^{210}\\)Pb and \\(^{14}\\)C dating. Statistical contrasts of six cores, where Pinus served as a marker of European settlement in the early 19th Century, showed significant changes in pollen composition following settlement with (a) an influx of ruderal exotic taxa including Plantago lanceolata L., Brassicaceae, Asteraceae (Liguliflorae) and Rumex, (b) an increase in pollen of the aquatics Myriophyllum spp. and Cyperaceae, (c) a decline in native herbaceous pollen taxa, including Chenopodiaceae and Asteraceae (Tubuliflorae) and (d) a decline in Allocasuarina and an initial decline and then increase of Poaceae. The presence of Asteraceae (Liguliflorae) in the pre-European period suggests that an important root vegetable Microseris lanceolata (Walp.) Sch.Bip. may have been abundant. Charcoal deposition was low in the pre-European period and significantly increased immediately after European arrival. Collectively, these changes suggest substantial ecological impacts following European settlement including cessation of Aboriginal traditions of fire management, a shift in hydrological conditions from open water lagoons to more ephemeral herb covered lagoons, and increased diversity of alien herbaceous species following pasture establishment. Chapter 2. The Midlands of Tasmania is a valuable model system for studying changes to tree cover and above ground biomass in neo-European landscapes. Aerial imagery, historical reconstructions, field surveys and future climate projections provided tools to chart changes in tree canopy cover and carbon stores in the Northern Midlands for the period 1788‚Äö-2070. In the 160 years between 1788 and 1940s, large areas of open woodland were cleared but carbon loss was modest (-14 %). In the 60 years between 1940s and 2010, carbon loss accelerated (a further -21%) as clearing shifted from woodlands to forests. An estimated 28% of the study area would need to be replanted with eucalypts to capture the carbon lost between 1788 and 2010. Three general circulation models (GCMs) representing climate predictions for 2070 suggest that carbon storage in the landscape would change by +13% to -13.2% of 2010 levels, without any restoration intervention. Chapter 3. The Tasmanian Midlands is now primarily privately owned, with very little area devoted to conservation of biodiversity. In this landscape, conservation covenants have been enacted on many private properties with the intention of encouraging tree recruitment and conservation of threatened plant communities and rare species. I compared the demographic structures of overstorey Eucalyptus species and midstorey tree genera on public and private properties with contrasting land use histories. Reserves on private lands had little tree recruitment, probably because exotic pasture species were common, whereas tree recruitment was abundant in public reserves, where pasture improvement has not occurred. Active measures are needed to restore ecological structure and function in grassy woodland conservation reserves on private land by encouraging regeneration of Eucalyptus and Acacia as well as returning the understorey to a functionally native state. This will entail reinstating fire disturbance, reducing exotic pasture species cover and managing domesticated (sheep), feral (deer) and native (macropod) herbivores. Synthesis. The landscape of the Midlands has been subject to phases of alteration according to the needs and values of its human inhabitants throughout time. The ways that humans have manipulated the landscape have interacted with available technologies and climatic conditions. Early Aboriginal inhabitants arrived in Tasmania during a time of glaciation and used fire to manipulate the landscape. Modern Tasmanians now live in a rapidly warming climate and possess a broad suite of technologies to cultivate, fertilise and irrigate the landscape. The drive to increase productivity conflicts with a desire for healthy landscapes. However, inhabitants of the Midlands see value in biodiversity and ecosystem function, and landowners and conservation agencies are working together to create a healthy landscape that will function under future climate conditions

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Copyright 2018 the author Chapter 1 is the following published article: Romanin, L. M., Hopf, F., Haberle, S. G., Bowman, D. M. J. S., 2016. Fire regime and vegetation change in the transition from Aboriginal to European land management in a Tasmanian eucalypt savanna, Australian journal of botany 64(5), 427-440 Chapter 2 is the following published article: Romanin, L. M., Prior, L. D., Williamson, G. J., Bowman, D. M. S. J., 2015. Trajectory of change in land cover and carbon stocks following European settlement in Tasmania, Australia, Anthropocene, 9, 33-40 Chapter 3 appears to be the equivalent of a post-print version of an article published as: Romanin, L. M., Prior, L. D., Bowman, D. M. S. J., 2019. The legacy of pasture improvement causes recruitment failure in grassy eucalypt woodland conservation reserves in the Midlands of Tasmania, Australian journal of botany 67(7), 558-570

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