Tree regeneration is key to forest resilience, and sparse postfire tree establishment has been observed recently in western North America. However, it remains unclear where and why postfire regeneration may succeed or fail in the future. We used a process-based model, iLand, to simulate subalpine forests of Greater Yellowstone (Wyoming, USA) in five landscapes. Fire and forest dynamics were simulated through 2100 under climate scenarios that varied in aridity and temperature. We asked (1) How do rates and composition of postfire tree regeneration vary with 21st-century warming and fire activity? (2) For large fires (> 400 ha), what factors (climate, fire size and severity, prefire basal area and proportion of young forest) explain variation in postfire tree regeneration during middle (2030-2055) and late century (2075-2100), and how do these vary among species? Within burned areas, we quantified the density of tree seedlings that established 5-years postfire by species and fit generalized linear models (GLMs) to estimate the rate of change in regeneration to 2100 for each species and scenario. To assess potential drivers of regeneration within large fire patches in both the middle and late century, we also fit GLMs to postfire regeneration. Rates of postfire regeneration were sustained for most species in wet scenarios and declined in dry scenarios. In some areas, Engelmann spruce (Picea engellmanii) declined by over 80% and dropped below 50 seedlings ha-1, indicating regeneration failure. Serotinous lodgepole pine (Pinus contorta var. latifolia) regeneration decreased by >50% in dry climate scenarios in landscapes where it now dominates. Regeneration rates of Douglas-fir (Pseudotsuga menziesii) always increased, with seedling densities increasing by >200% in dry scenarios. Across all species, prefire basal area was a strong driver of postfire regeneration, highlighting the importance of mature trees for postfire establishment. A negative effect of drier climate was significant for most species later in the century, except Douglas-fir and aspen (Populus tremuloides). The proportion of high-severity fire negatively affected regeneration of most species. Overall, regeneration was greater in the landscapes that included all dominant tree species, especially those at lower elevations where topographic complexity was higher. Postfire regeneration was sustained in most landscapes in all scenarios except the hot-dry scenario, however, in many cases we observed reassembly towards a forest community dominated by the lower montane fire-adapted species. These results may indicate that forest structure could be sustained during the 21st century, but shifts in tree species composition are likely and will favor species better adapted to a warmer climate and more frequent fire.