Climate models predict that plants will migrate to higher elevations in response to a warmer environment. This is because plants are very sensitive to temperature and moisture changes. Studies over the past several decades have focused on the Catalina Mountains right here in Southern Arizona. As a hiker climbs into the mountain, they notice the plants changing as they increase their height above the surface. For example, saguaro cactus is only found at a specific elevation band, if you were to climb higher than this band, you will no longer see many saguaros.
Very precise data has been collected about the plants in the Catalina Mountains, which records the elevations in which they are found. Over the past several decades, researchers have found these plants are indeed being found higher in the mountains year after year. A full study of this data was conducted by the University of Arizona and was published in the "Ecology and Evolution" journal.