What causes this dark circle?

[3rock]

We just had a tulip poplar tree removed that was hollow in the trunk towards the top. The non-rotted cross-sections showed a dark circle in the center with lighter wood around it -- photo attached. Just out of curiosity, does anyone know what the difference is between the dark center and the lighter wood around it? Thanks!

 

[scottr]

dark circle

The dark wood in the center is the heartwood , the outer lighter wood is the sapwood . My Yellow Poplar usually has a greenish brown heartwood .

 

[NYCHA FORESTER]

Looks to be part of the "Decay Column". It's hard to tell from the picture but the tree "appears" have been injured approximately 22 years ago (Count the rings from the outside back to the beginning of the "Dark ring" ..... thats about how many years ago it was injured.