Hi Adam,

I'm including a link that you probably already saw;

https://extension.unl.edu/statewide/buffalo/Heart%20Rot%20in%20Trees%20November%2015%202014.pdf

The tree still looks relatively healthy but, I think you are probably correct about it having Heart Rot. 

I think it will be a few years before it dies ( weather being a stress factor). If it was my yard and I wanted a tree there I would be planting another small tree close to it (different variety) so you have one well established when this one goes down.

An arborist would be able to tell you for sure .

I can see that all your trees need to have the grass pulled away from their trunk area. No grass should be growing over the flare roots of trees, no soil piled on tope either. You should be able to see the tops of the root flare, they need oxogen. Take away at least 3-4 feet of grass around the base of all trees and lightly mulch with wood mulch (2 inches). Leaving soil and grass over the root flare stresses the trees and makes them susceptible to disease and pests.

I hope this helps,

Sally Granath

St. Louis County MG