Presented are the results from studying the productive function of the geosystems of Cisbaikalia by considering the example of three key areas located on the spurs of the Primorskii mountain range and on the northern macroslope of the Khamar-Daban mountain range. Data of field investigations and from analyzing additional evidence (remotely sensed data, and data from published sources) were used for a classification of the geosystems for each key area. We determined the intervals of variability in the productive function of geosystems: the formation of the reserves of woody and ground grass vegetation. For each area we generated the landscape GIS-based database. Quantitative indicators of the productive function were compared with some environmental conditions and geosystem formation factors. The widest ranges of variability, and also the highest arithmetic mean values and maxima of productivity indicators correspond to the vegetation in the key area on the northern macroslope of the Khamar-Daban mountain range. The geosystems in the Priol’khonie region are characterized by minimum variability intervals for the productivity indicators. It was found that the distribution of the indicators of the reserves of grass communities in the key area of Priol’khonie approaches an exponential one, which is due to a predominance of low values in the sample. This is explained by a high degree of degradation of the local geosystems. For Northern Cisbaikalia the phytomass reserves are relatively high, whereas the accumulative active temperatures are minimal. It is shown that the differences in the phytomass reserves are caused by the different influences of the natural factors (largely the precipitation amount, and the depth of snow cover) as well as of the anthropogenic factors.productive function of geosystems, landscape GIS, woody phytomass, grass phytomass, landscape differentiation factors.