In mountainous regions, the timberline is the highest altitude at which trees grow  In the Rocky Mountains, the current timberline is at the altitude above which growing season temperatures remain cooler than 10 degrees centigrade  Fossilized remains of trees that grew 10, 000 years ago have been found 100 meters above the current Rocky Mountain timberline
Clearly, ttherefore, the climate of the Rocky Mountains is cooler now than it was 10, 000 years ago
The fossils are not of species of trees that were able to tolerate cooler growing temperatures than are the species that currently grow near the timberline.