Abstract: Climate warming has already made great impact on forest fires. Based on the daily meteorological data (temperature, precipitation, relative humidity, and wind speed), and by using the Canadian Fire Weather Index calculation formula, the daily forest fire weather indices (FWIs) of Yunnan Province in 1957-2007 were calculated, and through the statistical analysis of FWIs, the forest fire trends in this province over the past 50 years were studied. In the past 50 years, the forest fire season in Yunnan Province was from previous year November to current year June, lasting 8 months. Fire data (fire numbers, burned area, and burned forest area) had significant relationships with fire weather indices. The average daily duff moisture code (DMC) in whole fire season and the seasonal severity rating (SSR) were the good indices to evaluate the fire danger conditions among different fire seasons. The forest fire danger in Yunnan Province in 1957-2007 showed two change trends. One showed a clear cyclical change and a weak upward trend, i.e., the fire danger conditions in 1991-2007 was slightly severer than that in 1961-1990; and the another was that the fluctuation of forest fire danger conditions among different fire seasons decreased in 1991-2007, and the number of abnormal severe fire seasons was less than that in 1961-1990.

Key words: Transgenic Bt cotton, Nitrogen fertilization, Square loss, Cotton aphid, Cotton leafhopper, Cotton yield