Climatic variability and changes in greenness and primary productivity in the Southeastern United States
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The Southeastern United States is home to more than 80 million people, to key agricultural areas and to numerous unique ecosystems, ranging from diverse Appalachian Mountain forests to the swamps and prairies of the Everglades. The region has been experiencing high precipitation and temperature variability, including climate extremes. Following 2006, the Southeast witnessed an exceptionally severe drought, with below-normal precipitation being recorded for multiple years. As a result, water levels fell to record lows and local terrestrial ecosystems were affected by water stress and wildfires. Droughts in the region have been frequently exacerbated by increased temperatures. Wet periods and associated floods have also affected the Southeast in the recent past (e.g., in 2003 and in 2009). Under a climate-change scenario, with increased probability of extreme events affecting unique ecosystems and society, it is critical to expand our capacity to identify, understand and act upon the biosphere’s responses to future climates. In particular, the capacity to characterize vegetation responses to severe and extreme droughts and wet periods, including impacts to photosynthetic potential and primary productivity, as well as tipping points when systems collapse, are fundamental in order to develop adaptation strategies in face of a new climatic reality. This dissertation used a multi-scale, multi-temporal approach involving in-situ, remotely sensed and modeled data to characterize severe and extreme climate events (1896-2012) and vegetation status (2000-2013) over twelve states in the Southeast. Data processing included the generation of multiple descriptors (i.e., drought indices, vegetation and water content indices, primary productivity metrics). Increased variability in the frequency of dry and wet periods were observed for several time periods analyzed (5, 10, 20 and 30 years), and multiple severe and extreme dry (1925, 1954, 1986, 2007) and wet (1973, 1983, 2003, 2009) events were identified in the region. Negative anomalies of greenness were observed following the onset of major droughts, including the 2007 drought. Primary productivity during the 21st century stayed below the region’s potential for carbon uptake, and results suggest connections between reductions in productivity and water scarcity in the Southeast.