Photosynthetic Physiological Response of Two Ecotypes of Leymus chinensis to the Double Concentration of CO2 Effect of CO2 Concentration Doubling on Photosynthesis and Dry Matter Production in Different Growth Stages of Soybean Plant Photosynthetic Response of Six Woody Species to Elevated CO2 in Quercus liaotungensis Forest in the Beijing Mountainous Areas Influence of CO2 Doubling on Water Transport Process at Root/Soil Interface of Pinus sylvestriformis Seedlings