It is well known that atmospheric CO_２ concentration and temperature are increasing as a consequence of human activities. In past decades, considerable efforts have been put into investigating the effects of climate change on processes of forest ecological system. In general, studies have been mainly focused on the effects of elevated atmospheric CO_２ on plant physiology and development, litter quality, and soil microorganisms. Studies showed that there was variation in the responses of root development and below-ground processes to climate between different plant communities. Since the concentration of CO_２ in soil is much higher(10～50 times) than in the atmosphere, increasing levels of atmospheric CO_２ may not directly influence below-ground processes. Rhizosphere was defined as the small district in soil that was influnced by living roots. In the small district, microbial population was great, the study and utilization of the rich microbial resource pool has been paid more and more attention to. However, there was lack of information and studies about the effect of elevated CO_２ concentration and increased temperature on rhizospheric microbe as so far. 
Betula albo-sinensis, as a dominate tree species of sub-alpine dark coniferous in the west Sichuan province, which plays an important role in the structure and function of this kind of forest ecosystem. In our study, effects of elevated atmospheric CO_２ concentration (350±25)μmol•mol^－１, increased temperature (2.0±0.5)℃ and interaction between elevated CO_２ concentration and increased temperature on the number of rhizospheric microbe were studied by the independent and enclosed-top chamber’ system under high-frigid conditions. Responses of rhizospheric bacteria, actinomycetes and fungi number of Betula albo-sinensis under different densities(high density with 86 stems•m^－２, low density with 28 stems•m^－2 ) to elevated CO_２ concentration and increased temperature were analyzed and discussed. Results of the present study indicate that in comparing with the control, treatment EC increased the number of rhizospheric bacteria under high density significantly, in one growing season, the greatest increment of rhizospheric bacteria in July. However EC had no effect on the number of rhizospheric bacteria under low density. Except May and June, treatment ET increased the number of rhizospheric signifcantly. The effect of treatment ECT on the number of rhizospheric bacteria under different density was not significant. Under treatment EC, the number of rhizospheric actinomycetes with low density increased significantly, however, treatment EC did not increase the number of rhizospheric actinomycetes with high density. Simultaneously, treatment ET and ECT did not affect the number of rhizospheric actinomycetes. Finally, treatment EC and ET did not increase the number of rhizospheric fungi under different density, but in treatment ECT, the number of rhizospheric fungi under high density increased significantly.