[thejoneser]

Not necessarily. You can compare dead trees in the same vicinity that were alive at overlapping time intervals. Patterns in the width of the rings will reveal the period over which their lives overlapped. With enough dead trees, if they are preserved well enough, you could go back significantly further and age trees that are long dead.

[schrectacular]

It's called "Dendrochronology" and this is exactly how it is done. In the Northern Hemisphere they can date back at least 13,910 years.