The role of land use change and land management in the global carbon cycle: simulation as a test of process understanding
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Humans have left their mark on Earth's ecosystems for centuries. Since 1900, the human population has grown more than 400%. Land conversion and land management have helped meet an ever-increasing demand for natural resources. Forests have been cleared for agriculture, grasslands have been used for grazing by farmed animals, and extensive logging activity has provided fuelwood for energy and raw materials for building. But a long history of land management has also led to a change in forest production, leaving century-old legacies of human activity on Earth's ecosystems. As land is deforested, wood can be used for building or other products. Unused biomass can be burned for fuel or naturally broken down by microbes into soils, ultimately being converted to carbon dioxide. This phase conversion of carbon, from solid to gas, is a natural process but humans have sped up this process, leading to more carbon dioxide in the atmosphere than would otherwise occur naturally. Increasing levels of carbon dioxide in the atmosphere is a direct cause of increasing global temperatures and changes to regional climates. For these reasons, the focus of research in this Dissertation has been to track each and every process during land use change and land management, to provide a better accounting of where and how much carbon gets transferred from solid to gas during land use activities, and to identify any alteration to the productivity of ecosystems long after timber harvest has removed wood for products or agricultural lands have been abandoned and the forest allowed to regrow. The research papers in Chapter Two and Three have been published in peer-reviewed scientific journals, and Chapter Four is prepared for submission for publication. Each chapter focuses on a very specific problem, but the thread connecting all these works is carbon -- How much carbon is transferred to a gas when natural lands are modified and resources extracted to meet human demand? Does deforestation leave a unique and long-lasting signal in the atmosphere? Land management creates more young, fast-growing forests, but can models represent forests of different ages at global scales?