Dynamics of an ecosystem implies any changes in area borders location and replaces in the initial species composition. Generally, dynamics occurs under the impact of natural and anthropogenic factors. They always act together, diminishing or intensifying each other. The authors consider phenomenon woodlands case study, which are located on different mountain ranges both in semi damp and semiarid sectors of Altai-Sayan ecologic region. It should be noted that species composition of woodlands is various even within the same range. Thus, for example, the north part of West Sayan range is covered with dark coniferous forests with motley grasses, undershrubs and moss, while the southern one is characterized by the dominance of larch forests with steppe species in the underbrush at that. Natural factors of ecosystems dynamics imply long-term climate fluctuations, elemental forces of nature, invasions of pests at al. Since 1960s there’s an insignificant trend of temperature increasing at a practically constant level of precipitation in Altai-Sayan ecologic region. It’s well-known that the upper forest level is limited by cumulative growing degree-days, while the lower level is determined by the amount of precipitations. But the reaction to climate changes is feasible for at least one period of forest generation change. Therefore, the visible transformations may become remarkable at least in 100 years. Besides that, it should be pointed out that highland forest ecosystems are resistant regarding external actions by the microclimate function of biota. As to concerned region the changes in forest ecosystems are caused generally by the anthropogenic load. Nowadays the massive and rapid deforestation in Siberia is a serious problem which continues for many years. There are several main reasons of deforestation: agricultural development, logging, windbreaks and forest fire. Reforestation may take place in different ways depending on the initial biodiversity, geomorphological characteristics of area, slope orientation as well as intensity and duration of anthropogenic influence. For instance, on the hilly lands or steep slopes one of the features of forest succession is mentioned: the recovery of initial coniferous occurs except small-leaved forest intrusion. On the other hand, on overwetted plains fir and cedar forests are replaced by birch, alder and aspen communities. It’s important to note, that forest widening is observed in some areas at lower tree line, that leads to an extension of forest areas altitude diapason, but that isn’t an indicator of humidity increase. It just brings out the reduction in anthropogenic load (for example, abandoned plowed fields in Minusinsk or Tuva depressions are overgrown with cedar tracts). According to our investigation the anthropogenic impact on forest ecosystems dynamics in Altai-Sayan ecoregion is much more considerable than natural one.