In the Mongolian forest steppe vegetation, woody trees play a crucial role in nature management. This study focuses on the dominant woodland type found in this region, characterized by stands primarily dominated by Betula platyphylla, restricted to north-facing slopes, with coexisting shrubby species of B. fusca. We investigate the architectural properties of the two Betula species, specifically analyzing tree size structure, dynamics, and allometric relationships at the scale of current-year shoots to quantify climax open woodlands. Special attention is given to the inter-specific differences between B. platyphylla and B. fusca. Field censuses were conducted in Hustai National Park (HNP) in central Mongolia.  We set census plot of 0.7 ha to demonstrate size structure of a B. platyphylla stand. Total basal area and estimated aboveground biomass for stems ≥ 3 cm in diameter at breast height were 5.5 m² ha^–1 and 11.8 Mg ha^–1 respectively. Leaf area index was 0.80 reflecting open canopy layer. Aboveground net primary production rate as the sum of coarse wood production and current-year shoot mass accounted for 1.84 Mg ha^–1 year^–1, which suggests quick turnover of biomass at 0.16 year^–1, about three-times those in typical closed forest ecosystems in East and Southeast Asia. Additionally, an analysis of current-year shoots at the upper crowns of both Betula species revealed distinct characteristics. B. fusca had smaller leaves compared to B. platyphylla, and consistent trends in shoot and leaf size, specific leaf area, and lamina-versus-stem allometries were observed, all decreasing with tree height. This study enhances our understanding of the ecological dynamics and structural attributes of climax open woodlands in the Mongolian forest steppe.