It's a natural time capsule. The ground under the tree is stabilized for the duration of the tree's life, effectively snapshotting its contents.
The first time someone explained this to me while on a hike, as they excitedly dug through a mess of exposed tree roots, was one of those rare, surreal perspective-shifting moments. Suddenly I saw the forest surrounding me as a versioned filesystem of sorts, with every tree holding a snapshot of ground for its respective age beneath.
‘Yes,’ said the home’s owner, ‘colonial times...’”
-Weekend Update, Saturday Night Live, December 1, 2012
http://decoy.tvpassport.com/talk/late-laughs-week-november-2...
They can tell skeletons significantly older than 1000 years were from someone, for example, killed by a tiger, due to teeth marks on bones.