The History Of Forest Fire In Yangzonghai Watershed And Its Relationship With Climate Change And Human Activities In Recent 1300 Years

High-resolution pollen and charcoal analyses of the lake sediments from Yangzonghai Lake in Yunnan Province reveal the forest fire history and its relationships with climate changes and human activities in the Yangzonghai Lake drainage area over the past 1300 years. Results show that forest fires around this valley has experienced four stages of changes during the last 1300 years, and the dominant factors influencing forest fires changed from climate changes in the early time to human activities in the late. During 780-1640 AD, forest fires of the study area were mainly controlled by climate changes. A small peak in charcoal influx appeared around 1180 AD, showing that several big forest fires occurred in the study area during this period and the main cause of big fires might be associated with the increase of population and thus the strengthening of human interference. In 1640-1870 AD, Poaceae pollen and charcoal influx started to rise, indicating that enhanced agricultural activities in this period, when people began to destroy forests for planting crops, led more frequent forest fires. Pollen and charcoal records also indicate that obvious influences of human activities on vegetation succession and forest fires began at this time, which is a transitional period of main factors influencing forest fires from climate factors to human activities. During 1870-1950 AD, Poaceae pollen reached its highest value over the past 1300 years, and the charcoal influx kept rising with fluctuations. The main causes of fluctuations in forest fires were probably associated with social background in this historical period. The social unrests in the late Qing Dynasty and the early Republic of China resulted in great fluctuations and movements of population in Yunnan Province, and thus strong disturbance intensity of human activities sometimes and weak at other times. After 1950 AD, Poaceae pollen decreased, but pollen of other cultivated plants began to increase. Charcoal influx reached its highest value over the past 1300 years. The long term cultivation of plants, especially the introduced corns, may lead to soil erosion easily, which compels people to plant other cultivated crops. The more frequent occurrence of forest fires in this period, partly may be related to the rapid growth of population and enhanced human activities, partly may be associated with the water project in which water of the Nanpan River was introduced into Yangzonghai Lake in 1965, causing an larger charcoal source area, therefore, the occurrence of forest fires at this stage was still affected by human activities.