Final answer:
To assess tree species migration in the U.S., scientists measure temperature changes, which directly impact the geographical distribution and phenology of trees. Changes in greenhouse gases are indicative of climate shifts that can drive tree migrations. Additional evidence comes from observing plant phenology and migration patterns in other species affected by climate change.
Step-by-step explanation:
To determine migration patterns of tree species in the United States over certain time periods, scientists typically measure temperature changes. These changes serve as a primary indicator of the shifts in climate that influence the geographical distribution of various tree species. For example, if the Earth's temperature rises due to an increase in greenhouse gases, tree species may move toward cooler areas, which often means migrating in latitude or altitude.
Surveys of plant phenology, which is the study of the timing of life cycle events, can also provide evidence of trees adapting to climate changes. This includes observing earlier flowering times for certain species that can indicate that they are responding to warmer conditions. Furthermore, range shifts noted in other species, such as European birds and plants, which have moved northward due to temperature increases, can be paralleled in tree migrations. These shifts in boreal forest ranges and similar plant communities are often a consequence of changing temperature and precipitation patterns, both of which are key factors determining the distribution of plants and animals.