Large spruce trees will not grow when 'y' coordinate is negative

I like digging and building large underground bases.  My current one goes to bedrock (-58 'y' coordinate) and I have found the large spruce trees will not grow at a negative 'y' coordinate.  The small ones will, but the large ones will not.

 

Tested with 16 blocks clear above saplings with no blocks being within 2 blocks of saplings at a negative 'y' coordinate.

Tested with 32+ blocks under same conditions

Tested with 32+ blocks under same conditions with the sky showing as well.

 

In the end, I could not get a large spruce tree to grow until I went to a positive 'y' value.