Investigating Changes In Forest Growth And Atmospheric Circulation In The Himalayan Region During The Past Four Centuries
2020-07
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Investigating Changes In Forest Growth And Atmospheric Circulation In The Himalayan Region During The Past Four Centuries
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2020-07
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The Himalayan regions have experienced rapid changes in climate and hydrology in the recent decades. However, because weather and hydrological stations as well as radiosonde balloons across the Himalayas are sparsely located and relatively newly launched (1970s, on average), our understanding of the region’s surface and upper atmospheric processes and their effects on climate as well as natural resources including forests are limited to only past few decades. My dissertation applies the field and laboratory methods of dendrochronology and combines observed and gridded climatic and atmospheric variables research to examine the climatic and human effects on Nepal’s forest growth and to investigate the dynamics of the subtropical jet (STJ) over central Asia during the past four hundred years. I combined tree-ring chronologies across Nepal to produce a nation-wide index of forest growth spanning the past four hundred years, which showed that both short-term disturbances related to climate extremes such as volcanic eruptions as well as recent atmospheric warming can exert a lasting influence on the vigor of Nepal’s forests. At a smaller scale, my study also showed that human use practices have overall negative impacts on the growth of pine forests in eastern Nepal’s Koshi River watershed. Winter moisture is the primary limiting factor to the growth of Koshi pines, which suggests modeling future distribution of pine forests in eastern Nepal should consider winter precipitation even though summer is the wet season. These results indicate that it is important to include the competing effects of climate and humans on tree growth to achieve sustainable forest management in Nepal. My final dissertation chapter finds that, during the past four centuries, the ‘Himalayan jet’ (the latitudinal position of the STJ over the Himalayas during spring) has become more variable and experienced the highest number of poleward excursions since the 1950s. During these poleward excursions, most parts of central Asia experienced heat waves and droughts, which suggests that incorporating the latitudinal position of the jet into forecasting models may improve the accuracy of dry season weather predictions in the Himalayan regions. This four-century long tree-ring record of the Himalayan jet provides crucial datasets to validate model simulations required to detect the role of internal variability and climate change in the unique behavior of the Himalayan jet.
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University of Minnesota Ph.D. dissertation. July 2020. Major: Geography. Advisor: Scott St. George. 1 computer file (PDF); 134 pages.
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Thapa, Udya. (2020). Investigating Changes In Forest Growth And Atmospheric Circulation In The Himalayan Region During The Past Four Centuries. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/216328.
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