Changing forest disturbance regimes and climate are driving accelerated tree mortality across temperate forests. However, it remains unknown if elevated mortality has induced decline of tree populations and the ecological, economic, and social benefits they provide. Here, we develop a novel demographic index and use it to conduct one of the first large-scale assessments of relative tree density shifts to be conducted in temperate forests. Drawing from over 24,000 remeasured plots in the western United States, we show over half of species examined have undergone significant population decline in the last two decades. The rate and pattern of change we observe across species, species size-distributions, and species ranges is alarming and often undesirable. These findings offer a stark warning of widespread change in forest composition and structure across the western US, and suggest that sustained anthropogenic and natural stress will likely result in broad-scale transformation of temperate forests globally.