Interactions between climatic and sociopolitical factors in the expansion of the Chiefdom of Lijiang during the Ming Dynasty
Although many studies have linked complex social processes with climate change, few have examined the linkages between changes in environmental factors, resources, or energy and the evolution of civilizations on the Tibetan Plateau. The Chiefdom of Lijiang was a powerful chiefdom located on the eastern Tibetan Plateau during the Ming Dynasty that expanded following the 1460s. Although many studies have analyzed the political and economic motivations responsible for this expansion, no high-resolution climate records representing the expansion period of the Chiefdom of Lijiang were available until now. Here, we obtain a 621-year reconstruction of the April-July normalized difference vegetation index (NDVI) values derived from moisture-sensitive tree rings in northwestern Yunnan Province. Our NDVI reconstruction accounts for 40.5% of the variability in instrumentally measured NDVI values and can effectively represent the historical changes in regional vegetation productivity that occurred on the eastern Tibetan Plateau. In combination with a reconstruction of summer temperatures on the eastern Tibetan Plateau, the results reveal that the regional climate was relatively warm and persistently wet during the expansion of the Chiefdom of Lijiang. This period was characterized by long periods of above-mean vegetation productivity on the eastern Tibetan Plateau that coincided with the expansion of the Chiefdom of Lijiang; these phases were unprecedented in the past six centuries. We propose that the wet-warm climate promoted high regional vegetation productivity and favored the expansion of the sphere of influence of the Chiefdom of Lijiang. Instrumental climate data and tree rings also reveal that the early 21st-century drought on the eastern Tibetan Plateau was the hottest drought recorded over the past six centuries, in accordance with projections of warming over the Tibetan Plateau. Future climate warming may lead to the occurrence of similar droughts, with potentially severe consequences for modern Asia.