Change of Sulfur Content in the Bark and Its Application in Monitoring the Air SO2 Pollution in Winter Poplar Trees (Populus canadensis Moench) Initiate Vegetative Storage Protein Accumulation During New Shoot Development in Spring Potential Effect of Extracts of Roots of Malus pumila and Populus canadensis on Wheat Growth Comprehensive Factor Analysis about the Impact of Soil Pb Content on Seedling Growth of Populus canadensis