•We assessed forest biomass dynamics in mountain forests over 25 years.•Spatiotemporal climate variability had a minor effect on tree biomass increment at the scale of individual plots.•We showed that climate, geomorphology, and stand characteristics modify stand dynamics.•Tree species proportion was a significant driver of stand dynamics.•The topographic wetness index indicated sites with higher recruitment and increment and lower mortality. Tree aboveground biomass and its increment are sensitive forest and landscape dynamics indicators. Although various methods can measure forest biomass dynamics, only in situ, direct monitoring can substantially reduce the uncertainty related to unknown or uncontrolled factors. Within the present study, we aimed to isolate drivers of tree biomass increment, mortality, and recruitment in the Gorce National Park (GNP), Outer Western Carpathians, Poland. We used a unique database consisting of information collected between 1992 and 2017 (in 5-year intervals) within 389 permanent monitoring plots regularly distributed in the GNP area. This allowed us to calculate tree biomass and its increments and model them using a set of explanatory variables: proportion of particular tree species, mean temperature of the coldest quarter (bio11), mean precipitation of the warmest quarter (bio18), elevation, topographic wetness index (TWI), stand basal area (BA), diameter heterogeneity expressed by the coefficient of variation (DBH CV), and conservation regime. We applied generalized linear mixed-effects models (GLMMs), assuming the beta distribution of response variables, i.e. biomass proportions of increment, recruitment, and mortality of three main tree species: Norway spruce Picea abies (L.) Karst, European beech Fagus sylvatica L., and silver fir Abies alba Mill. In a large part of the GNP area, tree biomass increased. In the central and southern parts of the park, tree mortality was higher than in other parts, especially between 2002 and 2007, due to bark beetle outbreaks and intense wind damage. Stand dynamics of all species depended mainly on species proportion in stand biomass. The increment of Abies alba increased with TWI but decreased with BA and DBH CV. Recruitment decreased with BA and slope but increased with TWI and DBH CV, while mortality decreased with DBH CV and TWI. For Fagus sylvatica, increment increased with TWI but decreased with BA, elevation, and bio18, while recruitment decreased with BA and increased with TWI. Mortality decreased with DBH CV, bio11, and TWI but increased with BA. Picea abies increment increased with elevation but decreased with BA, slope, and DBH CV and weakly increased with bio11. Recruitment increased with TWI but decreased with BA and DBH CV, while mortality decreased with DBH CV, BA, and bio18. Water retention at the plot level, approximated by TWI, decreased mortality and increased recruitment of the studied species. Therefore, it can serve as an indicator of suitable microsites for their persistence. In addition, the increment was lower in strictly protected forests, while mortality was higher. Our study provided quantitative evidence of how climate, geomorphology, and forest stand characteristics modify stand dynamics.