You don't need a single tree across the entire timespan you want to look at. You can assemble a series of overlapping samples from a number of trees that lived at different times during the timespan of interest.

From Wikipedia [1]:

> A fully anchored and cross-matched chronology for oak and pine in central Europe extends back 12,460 years, and an oak chronology goes back 7,429 years in Ireland and 6,939 years in England. Comparison of radiocarbon and dendrochronological ages supports the consistency of these two independent dendrochronological sequences. Another fully anchored chronology that extends back 8500 years exists for the bristlecone pine in the Southwest US (White Mountains of California).

[1] https://en.wikipedia.org/wiki/Dendrochronology#Growth_rings (the final paragraph of this section)