Microbiological analyses, establishment of biomass and biological activity of microbial communities in the monocultures of poplar stands were carried out. Their age moved from 10 to 40 years (T1–T4). The results show that microbial communities are less stable in variants T1–T2 than in variants T3–T4 (in the year 1994). The comparison of the years 2001–2005 showed that microbial communities are more stable and the differences between stands T1–T2 and T3–T4 are minimum.