In order to decipher phenomenon of tree line changing with climate variety, the trend of tree line on the northern slope of Changbai Mountain was studied. Based on the meteorological data of Changbai Mountain, the January temperature (the limiting effect for tree line) and annual mean temperature were mainly investigated. In the ecotone between Betula ermanii and alpine tundra, the number and diameter at ground level of Betula ermanii in the plots were measured. According to the correlation between diameter at ground level (DGL) and age, the diameter at ground level can represent age directly. The results showed that the distribution age of Betula ermanii was in the trend of decreasing with elevation rising. In resent years, the annual mean temperature near Changbai Mountain is rising, which has led to the tree line ascending.
The objective of this study was to analyze the effects of climate change and doubled atmospheric CO2 concentrations, as well as the combined effects of climate change and doubling atmospheric CO2 concentrations on soil organic carbon （SOC） in the alpine steppe of the northern Tibetan Plateau using the CENTURY model. The results indicate that SOC loss in climate change scenarios varied from 49.77 52.36% in the top 20 cm. The simulation results obtained for a P1T0 scenario （increased precipitation and unchanged temperature）, POT1 scenario （unchanged precipitation and increased temperature）, and P1T1 scenario （increased precipitation and increased temperature） were similar. The alpine steppe in the P1T1 scenarios lost the greatest amount of SOC （844.40 g C m-2, representing the least amount of SOC） by the end of the simulation. The simulation for POT1 scenarios resulted in a 49.77% loss of SOC. However, SOC increased 12.87% under the COs doubling scenario, compared with the unchanged CO2 scenario. CO2 enhancement effects on SOC were greater than the climate change effects on SOC alone. The simulation of combined climate change and doubling atmospheric CO2 led to a decrease in SOC. This result indicated a decrease of 52.39% in SOC for the P1T1 ＋ 2 × CO2 scenario, 49.81% for the POT1 ＋ 2 ×CO2 scenario, and 52.30% for the P1T0 ＋ 2 ×CO2 scenario over the next 50 years. Therefore, SOC content in the alpine steppe will change because of changes in precipitation, temperature and atmospheric CO2 concentrations.
