The only 'votive' tree I've seen in my travels was near West Kennet Long Barrow in Wiltshire. The old practice still survives - it was common in post-Reformation Scotland to tie bits of fabric from the garments of the sick or ailing to trees near holy wells in the hope that the saints would intercede on behalf of the supplicant. The practice still remains in places.

And I'm so glad that the Clava Cairns have already accumulated a devoted band of followers. They deserve it!!