The Siberian boreal forest is the largest continuous forest region on Earth and plays a crucial role in regulating global climate. However, the distribution and environmental processes behind this ecosystem are still not well understood. Here, we first develop Sentinel-2-based classified maps to show forest-type distribution in five regions along a southwest-northeast transect in eastern Siberia. Then, we constrain the environmental factors of the forest-type distribution based on a multivariate analysis of bioclimatic variables, topography, and ground-surface temperatur at the local and regional scales. Furthermore, we identify potential versus realized forest-type niches and their applicability to other sites. Our results show that mean annual temperature and mean summer and winter temperatures are the most influential predictors of forest-type distribution. Furthermore, we show that topography, specifically slope, provides an additional but smaller impact at the local scale. We find that the filling of climatic environmental niches by forest types decreases with geographic distance, but that the filling of topographic niches varies from one site to another. Our findings suggest that boreal forests in eastern Siberia are driven by current climate and topographical factors, but that there remains a portion of the variability that cannot be fully accounted for by these factors alone. While we hypothesize that this unexplained variance may be linked to legacies of the Late Glacial, further evidence is needed to substantiate this claim. Such results are crucial to understanding and predicting the response of boreal forests to ongoing climate change and rising temperatures.