Browsing by Author "de Beurs, Kirsten M."

Now showing 1 - 4 of 4
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    Analysis of Dryland Forest Phenology using Fused Landsat and MODIS Satellite Imagery
    Walker, Jessica (Virginia Tech, 2012-09-07)
    This dissertation investigated the practicality and expediency of applying remote sensing data fusion products to the analysis of dryland vegetation phenology. The objective of the first study was to verify the quality of the output products of the spatial and temporal adaptive reflectance fusion method (STARFM) over the dryland Arizona study site. Synthetic 30 m resolution images were generated from Landsat-5 Thematic Mapper (TM) data and a range of 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance datasets and assessed via correlation analysis with temporally coincident Landsat-5 imagery. The accuracy of the results (0.61 < R2 < 0.94) justified subsequent use of STARFM data in this environment, particularly when the imagery were generated from Nadir Bi-directional Reflectance Factor (BRDF)-Adjusted Reflectance (NBAR) MODIS datasets. The primary objective of the second study was to assess whether synthetic Landsat data could contribute meaningful information to the phenological analyses of a range of dryland vegetation classes. Start-of-season (SOS) and date of peak greenness phenology metrics were calculated for each STARFM and MODIS pixel on the basis of enhanced vegetation index (EVI) and normalized difference vegetation index (NDVI) time series over a single growing season. The variability of each metric was calculated for all STARFM pixels within 500 m MODIS extents. Colorado Plateau Pinyon Juniper displayed high amounts of temporal and spatial variability that justified the use of STARFM data, while the benefit to the remaining classes depended on the specific vegetation index and phenology metric. The third study expanded the STARFM time series to five years (2005-2009) to examine the influence of site characteristics and climatic conditions on dryland ponderosa pine (Pinus ponderosa) forest phenological patterns. The results showed that elevation and slope controlled the variability of peak timing across years, with lower elevations and shallower slopes linked to higher levels of variability. During drought conditions, the number of site variables that controlled the timing and variability of vegetation peak increased.
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    Deforestation and the Transformation of the Landscape of North China: prehistory - present
    Moore, Alan H. (Virginia Tech, 2010-09-24)
    Environmental evidence shows that 10,000 years ago North China was primarily a lush deciduous forest. Like many other regions of the planet, this landscape has been dramatically transformed by human activity, yet unusually this mostly occurred long ago under pre-industrial conditions. Fortunately China has a long recorded history of human activity. Complementary environmental evidence helps to extend this record into prehistory, for even prehistoric Chinese substantially altered their environment. The first half of this study examines historical and physical evidence in order to better explain how North China's forests disappeared. Only recently have there been regional scale activities focused on reversing this tragic trend. Despite many claims of successes in afforestation, there are serious shortcomings in the collection of government statistics and known limitations to area-based forest assessments, so it is difficult to say with much confidence what is happening with North China's forests today. Phenological measurements from space-based instruments have been effectively used to characterize vegetation trends. In the second half of this study, MODIS sensor observations for 2000-2009 are collected for five study sites and are used to characterize vegetation change over the past decade, independent of government statistics and area-based estimates. Forests provide tangible benefits to environmental and human well-being. Forest health and growth are critical to addressing global climate change. Much attention has been focused on China's efforts to combat deforestation. A better understanding of North China's forest trends — both past and present — may offer valuable lessons for our environmental future.
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    Land Surface Phenology of North American Mountain Environments Using the Terra Moderate Resolution Imaging Spectroradiometer
    Hudson Dunn, Allisyn (Virginia Tech, 2009-07-07)
    Monitoring and understanding plant phenology is becoming an increasingly important way to identify and model global changes in vegetation life cycle events. Although numerous studies have used synoptically sensed data to study phenological patterns at the continental and global scale, relatively few have focused on characterizing the land surface phenology of specific ecosystems. Mountain environments provide excellent examples of how variations in topography, elevation, solar radiation, temperature, and spatial location affect vegetation phenology. High elevation biomes cover twenty percent of the Earth's land surface and provide essential resources to both the human and non-human population. These areas experience limited resource availability for plant growth, development, and reproduction, and are one of the first ecosystems to reflect the harmful impact of climate change. Despite this, the phenology of mountain ecosystems has historically been understudied due to the rough and variable terrain and inaccessibility of the area. Here, we use two MODIS/Terra satellite 16-day products, Vegetation Index and Nadir BRDF Adjusted Reflectance, to assess start of season (SOS) for the 2007 calendar year. Independent data for elevation, slope, aspect, solar radiation, and temperature as well as longitude and latitude were then related to the SOS output. Based on the results of these analyses, we found that SOS can be predicted with a significant R² (0.55-0.64) for each individual zone as well as the entire western mountain range. While both elevation and latitude have significant influences on the timing of SOS for all six study areas. When examined at the regional scale and accounting for aspect, SOS follows closely with Hopkins' findings in regard to both elevation and latitude.
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    A Phenological Comparison of NDVI Products within Contiguous United States
    Chai, Jiaxun (Virginia Tech, 2011-05-16)
    This study computed the Normalized Difference Vegetation Index (NDVI) products derived from NOAA AVHRR, MODIS, and SPOT VGT sensors. NDVI products from different instruments vary in spatial resolution, temporal coverage and spectral range. As a result, multi-sensor NDVI products are rarely used in a single phenological study. In order to evaluate the difference and similarity of NDVI records from the three sensors, I used EPA Eco-region frameworks to determine the average annual Start of Season (SOS) and End of Season (EOS) of Contiguous United States, and analyzed dates among datasets. In addition, I created 1127 sample points within the study area, and compared relationship between SOS/EOS based on land cover. The objectives of this thesis are to: 1) compare multi-sensor NDVI data using phenological models, 2) define a strategy to merge multi-sensor NDVI products to a single phenological product without direct NDVI conversion. The spatial and statistical analysis revealed that the Land Surface Phenology (LSP) measurements retrieved from NDVI time series from different sensors follow linear and positive relationships where compared by either eco-region or sample point. The historical record of AVHRR combined with the modern MODIS and SPOT data provides a critical and reliable perspective on phenological patterns in Contiguous United States area. The success of this study will help LSP by providing understanding of how different instruments can be combined to generate multi-sensor NDVI data for phenology.