Hi there -

Bark splitting is common on trees in Denver when sunscald and frost cracks occur and can be exacerbated by root issues such as girdling roots.  When a tree is stressed, it is then susceptible to numerous insect pests, including the lilac-ash borer, a common pest of ash trees.  Another new pest of ash trees is the emerald ash borer which was found in Boulder (but not Denver yet) for the first time last September.  You can read more info. about this highly destructive insect here: www.eabcolorado.com

Thanks for your response. Are you saying that you think my tree has girdling roots? And do you think I have the lilac ash borer? 

What actions should I take to help my tree?

It is possible it has both.  Here is more information and see attached:
http://www.wci.colostate.edu/shtml/ash.borer.shtml

I would suggest seeing how the tree leafs-out in the spring and assessing the extent of dieback in the canopy (if any) before you proceed further.