n. An extremely old tree, particularly one that was present at one or more important historical events.
2017
Today, only four trees survive from Washington’s time—he died at Mount Vernon in 1799….No one is more aware of the mortality of the witness trees near the mansion than Joel King, a Mount Vernon gardener who is on a mission to propagate them.
2016
Many tourists come to the Laurel Hill Cemetery, and they loved the tree because it was one of those that touched history; it was alive when famous figures from the past were alive," said Civil War author and historian Andy Waskie…" Many battlefields, such as Gettysburg, have 'witness trees,' which survived the fighting and are honored.
2015
The white oak on top of the Devil’s Den is one of the “witness trees” still on the field from the time of the battle.
1996 (earliest)
Up close the "witness tree" is larger than it seems from the road—a good five feet in diameter at chest height. It's roughly 300 years old, so it would have been around 100 when the quake hit in 1811-'12.
This is a relatively new sense of the phrase. An older sense—a specially-marked tree used by surveryors to designate the corners of lots (that is, to "witness" the accuracy of their survey)—dates to the late 18th century.