From 1999 to 2011 the Onon river basin experienced an enhancement in aridity of climate, which led to substantial changes in the water-thermal regime of geosystems, depletion of soil water storage, and to a decrease in groundwater table and in streamflow. Drying of birch forests on a mass scale showed their vulnerable position in steppe and forest-steppe areas of the basin. For revealing the territorial regularities of climatogenic dynamics of tree vegetation, geoinformation methods were used in analyzing the relief and constructing the map representing a combined influence of three parameters of the relief — absolute altitude, dissection, and the position relative to the direction of moisture transport by air masses. It is established that the status of birch forests is impaired from the tops of the mountain ranges to the hollows and intermontane depressions as well as from the surrounding mountains to the middle of the basin. The most unfavorable conditions have developed in flat, weakly dissected, inland areas which are protected the most from westerly moisture-carrying air masses. This is confirmed by results of field investigations assessing the vital status of the birch forests on the study territory.Keywords: drying of tree vegetation, birch forests, aridity of climate, digital elevation model, geosystem.