京都大学 / 新制・課程博士 / 博士(工学) / 甲第24221号 / 工博第5049号 / 新制||工||1788(附属図書館) / 京都大学大学院工学研究科都市環境工学専攻 / (主査)教授 清水 芳久, 教授 米田 稔, 准教授 松田 知成 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Forest degradation is a significant yet underestimated source of carbon emissions. Traditionally, monitoring forest degradation has been difficult due to a lack of sufficiently frequent satellite observations and reliable analysis methods. Recent advancements in satellite remote sensing provide new opportunities to monitor, attribute and analyze forest degradation. This dissertation develops methods to monitor and attribute forest degradation and analyzes the spatial-temporal patterns of forest degradation and associated carbon emissions. A new method, Continuous Change Detection and Classification - Spectral Mixture Analysis (CCDC-SMA), was developed on Google Earth Engine (GEE) to monitor abrupt and gradual forest degradation in temperate climate zones using Landsat time series. CCDC-SMA was applied to the Republic of Georgia from 1987-2019. Results show that forest degradation affected a much larger area than deforestation. In addition, CCDC-SMA was extended to monitor forest degradation in the tropics and applied in Laos. Attribution of the drivers of forest degradation was based on a combination of CCDC-SMA results, post-disturbance land cover classification and object-based image analysis. Shifting cultivation is the largest kind of forest disturbance in Laos, affecting 32.9% ± 1.9% of Laos during 1991-2020. The results show that shifting cultivation has been expanding and intensifying in Laos, especially in the last five years. Furthermore, the length of fallow periods has been continuously declining, which indicates that shifting cultivation is becoming increasingly unsustainable. Combining biomass estimates from the Global Ecosystem Dynamics Investigation (GEDI) and area estimates of shifting cultivation, the net carbon emissions from shifting cultivation during 1991-2020 in Laos are 1.28 ± 0.12 petagrams of CO2 equivalent (Pg CO2 eq). Tree canopy height and aboveground biomass density are strongly correlated with the years of regrowth since the latest year of slash-and-burn activities, which can be expressed using logarithmic models. It takes 131 years for the biomass to recover to pre-disturbed levels based on the logarithmic models. In addition to advancements in remote sensing of forest degradation, the results of this dissertation provide valuable information for policy related to forest management and reduction of carbon emissions.

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京都大学 / 新制・課程博士 / 博士(地球環境学) / 甲第24058号 / 地環博第221号 / 新制||地環||42(附属図書館) / 京都大学大学院地球環境学舎地球環境学専攻 / (主査)教授 柴田 昌三, 准教授 深町 加津枝, 教授 西前 出 / 学位規則第4条第1項該当 / Doctor of Global Environmental Studies / Kyoto University / DFAM

Lake ecosystems are sentinels of change in a landscape, integrating upstream terrestrial and aquatic effects of climate and land use drivers. Climate and land use change is mediated by socio-cultural and economic processes, resulting in complex responses in lake ecosystems as a part of coupled natural human (CNH) systems. I used multiple approaches within a CNH framework to better understand the effects of climate and land use on freshwater-human interactions. I first conducted a literature synthesis and found that slow processes (e.g., cultural change) are underrepresented in CNH-freshwater models relative to fast processes (e.g., daily decision-making), though both fast and slow processes are key to assessing decadal trajectories of change. I then examined the interaction of fast and slow variables in lakes through two ecosystem modeling assessments. I used a process-based model to assess drivers of annual chlorophyll-a concentration, a metric of phytoplankton biomass, over three decades in a low-nutrient lake and found that increases in summer median versus maximum chlorophyll-a are related to rising air temperatures and external phosphorus load, respectively. I also conducted a single-year study in the same lake to examine variability in site-specific gross primary production (GPP) and respiration (R), two fast-changing variables that serve as robust indicators of slowly-changing trophic state. I found that higher rates of near-shore GPP and R were partially due to stream-related variables, providing insight into how inflowing streams connect to in-lake processes. These two ecosystem assessment studies indicate fast-changing response variables can be indicative of specific slow-changing variables: annual maximum versus median chlorophyll-a can be used to assess differing impacts from climate and land use change, and estimation of GPP and R near inflow streams integrate sub-catchment drivers. Finally, I evaluated the effectiveness of an online model visualization relating current land use decisions, a fast process, to future water quality outcomes, a slow process, and found that the visualization was effective in altering property owner beliefs and intended behavior related to applying lawn fertilizer and installing waterfront buffers. Collectively, this work advances our understanding of how fast and slow variables interact to improve assessments of changes in CNH-lake systems. / Doctor of Philosophy / People depend on freshwater lakes for many reasons. However, lake water quality is threatened by climate change and increased land development. Here, I examined the complex interactions between people and freshwater lakes, using different types of computer simulation models to represent lake ecosystems. First, I found that the interaction of long-term and short-term processes are key in understanding trajectories of water quality change. For example, shorter term processes, such as people's decision-making about fertilizer use, may result in significant water quality changes over the long-term. Second, I used a lake ecosystem model to identify how long-term increases in air temperature due to climate change will cause average summer water quality to worsen. However, climate impacts may be offset if current land use change is properly managed to reduce runoff, thereby maintaining high water quality into the future. Third, I found that streams flowing into the lake directly influence short-term variations in water quality. Finally, I evaluated the effectiveness of an interactive online visualization in educating lakefront property owners about the importance of current human behavior for long-term water quality in the lake. After using the visualization, property owners were more likely to avoid applying lawn fertilizer and install waterfront buffers, in line with the educational goal. Management that incorporates models of short- and long-term processes in society and lakes will provide a better understanding of future trajectories of ecosystem change. Collectively, this work highlights connections between society and lake ecosystems that can be used to help manage water quality for generations to come.