Relationships between ecosystem factors and contents of megastigmatrienones in flue-cured tobacco leaves in Liangshan,Sichuan Province Quantitative Study on the Drive Factors of Wetland Change in Three Rivers‘ Source Area Influences of row spacing on fertile tillers and characters of seed yield of Elymus sibiricus cv. Chuancao No.2 Remote sensing estimation of gross primary productivity and its response to climate change in the upstream of Heihe River Basin Gradient analysis on the influence of terrain on the forest landscape pattern in the burned blanks of the north slope of Mt. Daxing‘anling Summer and Autumn Meteorological Effects on the Emergence of Turf Weeds in the Nanjing Area