how might one keep a tree diary at each tree?

There are waterproof plastic 'paper's and pens that are made for scuba diving and rescue work. If you put one in a metal box chained to a stake by each tree, people could easily make notes and I think that the materials would stand up to the years.

How about a block of granite say six feet square with a hammer and chisel attached to it by a log chain and everyone could just carve in any infro they wanted to leave about the tree and when it dies because of improper care it can be a memorial. Just kidding.
I really am not into the woo woo thing about trees plants, my dogs I care about and take care of but I also remember they are just dogs, if they are serously ill and the vet says it will cost thousands, they are put to sleep. Rather than spend thousands on a animal there are humans that need medical help that the money would be better spent on.
As for trees we weed out the sick, dying deaseased, leave the best to grow and make better lumber to build shelters for people.
Today its on the list to go out and have a period of meditation around a bout 10 or 15 hickory trees thank them for keeping me and the wife warm during the winter of 07 / 08 and then chain saw time.

I think the poly pipe thing would work, and just place it in the crook of the tree where the branches meet the trunk.

The only draw back would be vandals taking them or someone not taking the care to replace them where they go, but there is almost nothing you can do to protect yourself from people like that.

Paul,
I'm not sure of the best way to go about the task of recording the "history" of each tree, but it sure is a fabulous idea. I don't think you're "woo, woo" at all

__________________
"Perhaps I'll have them string a clothesline from the hearse I am in, with my underwear waving in the breeze, as we drive to the cemetary. People worry about the dumbest things!"
by Wendy