Pollen viewer   Lee E. Frelich
  Dec 21, 2006 17:16 PST 


If you haven't used the pollen viewer, it is at this website:


It shows the distribution of numerous tree species over North America
during the last 21,000 years. You just click on a species from the list and
click play, and you get a movie showing how the tree migrated in response
to retreat of the last glaciation.

Also very interesting is how the glaciers changed over time, as well as the
shape of the land itself as glacial Lake Agassiz came and went, the oceans
went up 300 feet, and glacial rebound also changed the area around Hudson's

Most species of trees reached their maximum northward extent 6,000-8,000
ybp, and have retreated southward in recent years as the interglacial we
are in began to end (now that trend will reverse with global warming).

Hemlock is very interesting. You see how widespread and abundant it was 500
years ago, and how little of it there is in the 'modern' picture which
represents the late 20th Century.