Witness trees were used in the early days of this country to mark property boundaries as the nation expanded westward.  Many of them still stand. 

Civil War witness trees are those that appeared in Matthew Brady's photographs of battle sites. 

There are several efforts across the country to preserve these old trees, which have stood as silent witnesses to our national history and the history of individual families who lived on the marked properties.

Other sites where you can read about witness trees:

http://www.witnesstrees.org/home.html

http://www.gettysburgdaily.com/?p=142





Robert Frost published a book of poems entitled Witness Tree, in which this poem appears:

BEECH
Where my imaginary line
Bends square in woods, an iron spine
And pile of real rocks have been founded.
And off this corner in the wild,
Where these are driven in and piled,
One tree, by being deeply wounded,
Has been impressed as Witness Tree
And made commit to memory
My proof of being not unbounded.
Thus truth's established and borne out,
Through circumstance with dark and doubt--
Though by a world of doubt surrounded.