Linkages between Atmospheric Circulation, Weather, Climate, Land Cover and Social Dynamics of the Tibetan Plateau

The Tibetan Plateau (TP) is an important landmass that plays a significant role in both regional and global climates. In recent decades, the TP has undergone significant changes due to climate and human activities. Since the 1980s anthropogenic activities, such as the stocking of livestock, land cover change, permafrost degradation, urbanization, highway construction, deforestation and desertification, and unsustainable land management practices, have greatly increased over the TP. As a result, grasslands have undergone rapid degradation and have altered the land surface which in turn has altered the exchange of heat and moisture properties between land and the atmosphere. But gaps still exist in our knowledge of land-atmosphere interactions in the TP and their impacts on weather and climate around the TP, making it difficult to understand the complete energy and water cycles over the region. Moreover, human, and ecological systems are interlinked, and the drivers of change include biophysical, economic, political, social, and cultural elements that operate at different temporal and spatial scales. Current studies do not holistically reflect the complex social-ecological dynamics of the Tibetan Plateau. To increase our understanding of this coupled human-natural system, there is a need for an integrated approach to rendering visible the deep interconnections between the biophysical and social systems of the TP. There is a need for an integrative framework to study the impacts of sedentary and individualized production systems on the health and livelihoods of local communities in the context of land degradation and climate change. To do so, there is a need to understand better the spatial variability and landscape patterns in grassland degradation across the TP. Therefore, the main goal of this dissertation is to contribute to our understanding of the changes over the land surface and how these changes impact the plateau\u27s weather, climate, and social dynamics. This dissertation is structured as three interrelated manuscripts, which each explore specific research questions relating to this larger goal. These manuscripts constitute the three primary papers of this dissertation. The first paper documents the significant association of surface energy flux with vegetation cover, as measured by satellite based AVHRR GIMMS3g normalized difference vegetation index (NDVI) data, during the early growing season of May in the western region of the Tibetan Plateau. In addition, a 1°K increase in the temperature at the 500 hPa level was observed. Based on the identified positive effects of vegetation on the temperature associated with decreased NDVI in the western region of the Tibetan Plateau, I propose a positive energy process for land-atmosphere associations. In the second paper, an increase in Landsat-derived NDVI, i.e., a greening, is identified within the TP, especially during 1990 to 2018 and 2000 to 2018 time periods. Larger median growing season NDVI change values were observed for the Southeast Tibet shrublands and meadows and Tibetan Plateau Alpine Shrublands and Meadows grassland regions, in comparison to the other three regions studied. Land degradation is prominent in the lower and intermediate hillslope positions in comparison to the higher relative topographic positions, and change is more pronounced in the eastern Southeast Tibet shrublands and meadows and Tibetan Plateau Alpine Shrublands and Meadows grasslands. Geomorphons were found to be an effective spatial unit for analysis of hillslope change patterns. Through the extensive literature review presented in third paper, this dissertation recommends using critical physical geography (CPG) to study environmental and social issues in the TP. The conceptual model proposed provides a framework for analysis of the dominant controls, feedback, and interactions between natural, human, socioeconomic, and governance activities, allowing researchers to untangle climate change, land degradation, and vulnerability in the Tibetan Plateau. CPG will further help improve our understanding of the exposure of local people to climate and socio-economic and political change and help policy makers devise appropriate strategies to combat future grassland degradation and to improve the lives and strengthen livelihoods of the inhabitants of the TP

Trends of land surface phenology derived from passive microwave and optical remote sensing systems and associated drivers across the dry tropics 1992–2012

Changes in vegetation phenology are among the most sensitive biological responses to global change. While land surface phenological changes in the Northern Hemisphere have been extensively studied from the widely used long-term AVHRR (Advanced Very High Resolution Radiometer) data, current knowledge on land surface phenological trends and the associated drivers remains uncertain for the tropics. This uncertainty is partly due to the well-known challenges of applying satellite-derived vegetation indices from the optical domain in areas prone to frequent cloud cover. The long-term vegetation optical depth (VOD) product from satellite passive microwaves features less sensitivity to atmospheric perturbations and measures different vegetation traits and functioning as compared to optical sensors. VOD thereby provides an independent and complementary data source for studying land surface phenology and here we performed a combined analysis of the VOD and AVHRR NDVI (Normalized Difference Vegetation Index) datasets for the dry tropics (25°N to 25°S) during 1992–2012. We find a general delay in the VOD derived start of season (SOS) and end of season (EOS) as compared to NDVI derived metrics, however with clear differences among land cover and continents. Pixels characterized by significant phenological trends (P < 0.05) account for up to 20% of the study area for each phenological metric of NDVI and VOD, with large spatial difference between the two sensor systems. About 50% of the pixels studied show significant phenological changes in either VOD or NDVI metrics. Drivers of phenological changes were assessed for pixels of high agreement between VOD and NDVI phenological metrics (serving as a means of reducing noise-related uncertainty). We find rainfall variability and woody vegetation change to be the main forcing variables of phenological trends for most of the dry tropical biomes, while fire events and land cover change are recognized as second-order drivers. Taken together, our study provides new insights on land surface phenological changes and the associated drivers in the dry tropics, as based on the complementary long-term data sources of VOD and NDVI, sensitive to changes in vegetation water content and greenness, respectively

Remote sensing analysis of croplands, woody plant encroachment and carbon fluxes of woody savanna

Since 1990s, much attention has been paid to Land use/land cover change (LULCC) studies because it is an important component of global change. The vegetation change is a critical factor of land cover changes, which interacts with climate, ecosystem processes, biogeochemical cycles and biodiversity. Remote sensing is a good tool to detect the changes of land use and land cover. To date, most of studies on vegetation changes have been conducted at biome scales, but have not examined changes at the species level. This lack of studies on species inhibits analysis of ecosystem functions caused by the shifts of vegetation types. This dissertation aims to explore the potential of remote sensing images to produce long-term products on specific vegetation type and study the interactions between vegetation type, climate and gross primary production. In Chapter 2, a simple algorithms was developed to identify paddy rice by selecting a unique temporal window (flooding/transplanting period) at regional scale using time series Landsat-8 images. A wheat-rice double-cropped area from China was selected as the study area. The resultant paddy rice map had overall accuracy and Kappa coefficient of 89.8% and 0.79, respectively. In comparison with the National Land Cover Data (China) from 2010, the resultant map had a better detection of the changes in the paddy rice fields. These results demonstrate the efficacy of using images from multiple sources to generate paddy rice maps for two-crop rotation systems. Chapter 3 developed a pixel and phenology-based mapping algorithm, and used it to analyze PALSAR mosaic data in 2010 and all the available Landsat 5/7 data during 1984-2010. This study analyzed 4,233 images covering more than 10 counties in the central region of Oklahoma, and generated eastern redcedar forest maps for 2010 and five historical time periods: the late 1980s (1984-1989), early 1990s (1990-1994), late 1990s (1995-1999), early 2000s (2000-2004), and late 2000s (2005-2010). The resultant maps clearly illustrated an increase in red cedar encroachment within the study area at an annual rate of ~8% during 1984-2010. Chapter 4 investigates the dynamics of juniper encroachment on the grasslands of Oklahoma by generating multi-period maps of juniper encroachment from 1984 to 2010 at a 30-m spatial resolution. The juniper forest maps in 1984 to 2010 were produced by the algorithms developed in Chapter 3. The resultant maps revealed the spatio-temporal dynamics of juniper forest encroachment at state and county scales. This study also characterized the juniper forest encroachment by geographical pattern and soil conditions. The resultant maps can be used to support studies on ecosystem processes, sustainability, and ecosystem services. Chapter 5 compared dynamics of major climatic variables, eddy covariance tower-based GPP, and vegetation indices (VIs) over the last decade in a deciduous savanna and an evergreen savanna under a Mediterranean climate. The relationships were also examined among VIs, GPP, and major climatic variables in dry, normal, and wet hydrological years. GPP of these two savanna sites were also simulated using a light-use efficiency based Vegetation Photosynthesis Model (VPM). The results of this study help better understanding the eco-physiological response of evergreen and deciduous savannas, and also suggest the potential of VPM to simulate interannual variations of GPP in different types of Mediterranean-climate savannas

Climate-driven change in Himalayan Rhododendron phenology

Phenology – the seasonal timing of life-history events – is a critical dimension of natural history. It is also one of the earliest and most noticeable traits by which organisms respond to climate change. However, these responses are complex, and only beginning to be understood, especially in the montane and alpine environments that are among the ecosystems most vulnerable to climate change. Drawing from diverse data sets and employing multiple methodologies, I examined how climate affects phenology in Himalayan Rhododendron spp. Comprehensively monitoring flowering phenology over gradients of season and elevation on Mt. Yulong, China – home to a diverse assemblage of Rhododendron spp. that is ecologically and culturally salient – I showed that the species have an elaborate sequenced progression of flowering over season and elevation. Both phenology and the responses to annual temperatures directly impact reproductive success, making this progression vulnerable to the effects of future climate change. A community phylogeny suggests this flowering progression experiences constraints from shared evolutionary history and morphology: the last flowering day of each species shows phylogenetic signal, and is associated with flower size and fruit size. This pattern may be driven by the limited time for fruit development before the onset of cold temperatures in autumn, underscoring the asymmetric pressures on phenology at seasonal extremes. To gain a longer-term perspective, I contextualized Rhododendron phenological progressions within a 125 year proxy record of phenology based on herbarium collections, demonstrating that although increased annual temperatures are associated with earlier flowering, increased fall temperatures are associated with delayed flowering. These contrasting effects have resulted in opposing changes in flowering time, even during rapid recent anthropogenic warming. Finally, interviews with Mt. Yulong villagers revealed a rich knowledge base, with local understandings of changes in and drivers of Rhododendron phenology paralleling the results from ecological studies. In combination, these studies reveal the complexity of phenological responses to climate change. The multiple methodologies allowed a deeper exploration than simple ‘earlier spring’ models of phenological response. Each illuminated distinct details, while agreement among diverse data sources increased confidence in both the methodologies and the conclusions

An analysis of long-term effects of climate change and socioeconomic activities on grassland productivity of inner Mongolia

In recent years, researchers have recognized the complexity of the interactions between the ecological system and the economic development of human society. However, the complicated relationships overwhelm traditional statistical procedures and require an innovative approach to investigate their dynamics. We proposed this study to provide a unique perspective in analyzing the long-term causal relationships between the grassland productivity, climate change, and socioeconomic development of Inner Mongolia Autonomous Region (IMAR) of China. Our attempt began with acquiring remotely sensed satellite imagery, climatic variations, and aggregated annual reports of the socio-economy of the IMAR in vegetation growing seasons for 15 years. The spatial and temporal dissimilarities of the raw observations prevented us from exploiting the potential of this valuable dataset; thus, we interpolated and extrapolated the data to generate a panel dataset with consistent spatial and temporal resolutions. Then, we took another step to preprocess the panel data by applying a signal filter to isolate the long-term trend of change from the inter- and intra-annual cyclic patterns and used the trends as the input for a panel data model. The results from our statistical analysis indicated that the independent variables explained the variations in the dependent variable extremely well, while the polynomial terms of climatic variables were significant with limited marginal effect and most of the climatic variables showed negative linear impact on the grassland productivity. In the meantime, we found not all socioeconomic variables we attempted to include into the model significantly affected grassland productivity, especially the variables describing the financial status of the IMAR residents

Impacts of grazing intensity, precipitation and temperature on productivity, forage quality, species composition and diversity in typical steppe of Inner Mongolia

The overall goal of this study is to analyze and evaluate the long-term effects of sheep grazing intensity on yield performance, herbage quality, species diversity and specie composition in grassland ecosystems in Inner Mongolia, and then to provide efficient practices preventing grassland degradation

ASSESSING FOREST RESPONSES TO CLIMATE CHANGE AND RESOLVING PRODUCTIVITY MEASUREMENTS ACROSS SPATIAL SCALES

Informed decisions regarding forest and carbon resources require knowledge of the impacts of environmental changes on forest productivity. We also need to reconcile the diverging productivity estimates that are presently available. This dissertation assembles two publications addressing the impacts of climate change on forest productivity and one exploring the relationship between three estimates of forest productivity. In the first chapter, I evaluated whether forests have responded to recent changes in climatic conditions. Through combining published evidence I show that forests have responded to changes in the patterns of light, water, and temperatures over the last half of the 20th century. Most published studies showed a positive growth trend. Negative growth trends were found for drier study areas. Conclusions on the greening of the world\u27s forests, are difficult due to poor geographical coverage and measurement method disparity. In the second chapter, I compared three productivity estimation methods (two ground-based and one satellite-based) using 166 sites in Austria. Results of disturbance-free projections show the relevance of each method to actual site productivity and their combined usefulness in identifying the most appropriate scale for monitoring climate forcings. Each estimation method provides information on a portion of the underlying actual NPP. In the last chapter, I explore the effect of three IPCC climate change scenarios on forests of the US Northern Rockies. Results show an increase in growing season length and in water stress, and a decrease in snow quantities and in number of days with ground snow for all forests by 2089. Under the driest and warmest scenario, the majority of the sites became carbon sources, and I identify a water/temperature tipping point, past which system stored carbon drastically declines. For these disturbance-free projections, water availability drove the system. In this dissertation, I resolve a otential source of conflict among forest productivity estimates; combined, these estimates lead to a broader understanding of productivity. I also present evidence that forests are already responding to climate change, and that more drastic changes are likely in the future

China\u27s terrestrial carbon balance: Contributions from multiple global change factors

The magnitude, spatial, and temporal patterns of the terrestrial carbon sink and the underlying mechanisms remain uncertain and need to be investigated. China is important in determining the global carbon balance in terms of both carbon emission and carbon uptake. Of particular importance to climate-change policy and carbon management is the ability to evaluate the relative contributions of multiple environmental factors to net carbon source and sink in China\u27s terrestrial ecosystems. Here the effects of multiple environmental factors (climate, atmospheric CO2, ozone pollution, nitrogen deposition, nitrogen fertilizer application, and land cover/land use change) on net carbon balance in terrestrial ecosystems of China for the period 1961–2005 were modeled with newly developed, detailed historical information of these changes. For this period, results from two models indicated a mean land sink of 0.21 Pg C per year, with a multimodel range from 0.18 to 0.24 Pg C per year. The models\u27 results are consistent with field observations and national inventory data and provide insights into the biogeochemical mechanisms responsible for the carbon sink in China\u27s land ecosystems. In the simulations, nitrogen deposition and fertilizer applications together accounted for 61 percent of the net carbon storage in China\u27s land ecosystems in recent decades, with atmospheric CO2 increases and land use also functioning to stimulate carbon storage. The size of the modeled carbon sink over the period 1961–2005 was reduced by both ozone pollution and climate change. The modeled carbon sink in response to per unit nitrogen deposition shows a leveling off or a decline in some areas in recent years, although the nitrogen input levels have continued to increase