>If you plant a tree that lasts for 10,000 years (is there such?), I wonder if you can use its height or the shadow it casts (trunk thickness) to measure time.
You need to make some sort of sign that indicates the age of the oldest trees in the forest is the clock. And make it impossible or unprofitable to log the forest for wood, maybe by building hundreds of annoying stone pillars at inconvenient spacings
https://en.wikipedia.org/wiki/Increment_borer