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• Tree mortality increased in Yungas forests of Argentina since 1991–92. • Tree mortality increased only in old-growth forests. • Variability of tree mortality rates is explained by climate and forest structure. Tree mortality and its relationships with forest structure and environmental factors have been studied for many decades using permanent plots. In the last decades, plot monitoring showed that tree mortality rates were increasing in tropical and temperate regions. We analyzed tree mortality in subtropical montane forests of Northwestern Argentina to: (i) compare tree mortality rates between secondary and old-growth forests, (ii) describe relationships between tree mortality rates with elevation, climatic variables (mean annual temperature, total annual precipitation, temperature and precipitation seasonality), productivity, plot-mean wood, density and plot-mean square diameter, and (iii) analyze long-term trends of tree mortality rates at plot and species level. We used 54 permanent plots monitored for 25 years (1991–92 to 2016–17) to analyze individual-based mortality and basal area mortality rates. We found that tree mortality rates decreased significantly with elevation, precipitation seasonality, plot-mean wood density and plot-mean square diameter, and increased significantly with total annual precipitation, mean annual temperature and productivity. Individual-based mortality increased between 1991–92 and 2016–17 in old-growth forests. Although the main drivers of increasing tree mortality over time remain unknown, we suggest that it may be related to the increasing precipitation and temperature that occurred in the last decades in this region. Further research with forest plot monitoring is necessary to understand causes of increasing mortality and the consequences for forest structure, composition and dynamics in the long-term. [ABSTRACT FROM AUTHOR]