Abstract Background Wildfires impact forests including understory composition and modify environmental conditions such as canopy cover and soil fertility. The recovery of understory plant community composition over time after the fire event is also affected by differences in heat and moisture induced by climate and topography. However, the relative importance of these drivers in shaping understory plant communities in subalpine forests remains unclear. We studied the impact of fire on plant communities at 9 burned patches (24 plots) and its adjacent unburned forests (18 plots) across a 79-year chronosequence in subalpine forests located on the southeastern Tibetan Plateau. Using a space-for-time approach, we examined changes in plant α and β diversity at 14–20, 52–53, and 68–79 years following fire, revealing the main drivers influencing these fire-induced shifts. Results We found that fires enhanced plant α diversity but diminished β diversity over time. Such an opposite pattern was reflected in a notable increase in plants adapting to high radiation levels, xeric conditions, and lower elevations after fire. Elevation and climate variables were weak drivers of those changes. Direct effects of time since fire drove α diversity, while indirect effects, mediated by changes in canopy cover and soil fertility, drove plant β diversity. The decrease in plant β diversity was primarily attributed to a reduction in canopy cover and an increase in soil fertility. When canopy cover was below 0.5, increasing canopy cover did not lead to an increase in vegetation heterogeneity. Conclusions High-severity fire in subalpine forests of the southeastern Tibetan Plateau has lasting effects on understory plant communities, showing signs of community thermophilization. We propose afforestation strategies that aim to increase canopy cover above 0.5 and prioritize the restoration of forests with high soil fertility. Within a climate change context, these measures are intended to counteract the increase in community homogenization over time since fire in subalpine forests.