Very simply, evergreen trees are dominant in areas where it is
cold, and deciduous trees (trees that loose their leaves in autumn)
live in slightly warmer areas. We also know this was true in the
not-too-distant past. Therefore, by looking at fossil pollen trapped
in the bottom of lakes, we can tell how cold it was when this
pollen dropped from nearby trees. This is what scientists will
be doing with the sediments from Lozier Pond, where the Hyde Park
Mastodon was discovered in August of 1999.
Pollen images courtesy of Gretchen D. Jones
(USDA-ARS, APMRU) and Meredith H. Lieux (U. of Louisiana, retired).