Forest Service scientists compared the distribution of seedlings and mature trees for most tree species in California, Oregon, and Washington. Across all forestlands in the region, the mean annual temperature of the range of seedlings is 0.120 degrees CelsiusC (32.216 degrees Fahrenheit) colder than that of the range of mature trees. Individual species followed the same pattern, except for some species common in California mixed conifer forest. For most species, the estimated shift is less than the increase in temperature during the last decades, suggesting that tree migration may be lagging temperature increases. The broad scale, variety of disturbance regimes, exhaustive sampling of tree species, and direct relationship between temperature response and a warming climate, provide strong evidence to attribute the observed shifts to climate change.