I'm sorry to hear about your tree!

From the photos it's tough to narrow down the cause--a few different things could cause symptoms like this.  Can you tell me, please, in what conditions the tree is growing (a lawn, mulch, etc), how it is watered, and what is on the other side of the fence?

Cordially,

John

Thank you.

This could be caused by root damage or possibly by the honeylocust spider mite (or perhaps also the honeylocust right mite).  During heavy mite infestations, large portions of the crown may turn off-yellow and the leaves will drop prematurely.  Mites are usually problematic for trees in dry sites; if the tree is getting water from turfgrass they are less likely to be the culprit.  If you are able to look closely at the yellowing leaves you can check for the presence of the mites, which are light green/yellowish, and quite small.  

Root damage may have been caused by under- or over-watering, though if this is the first year for damage and your watering practices have not changed, this seems unlikely as well.

Another possible cause for root damage or leaf-drop like what you're seeing is damage from herbicides.  Do you know if any herbicides have been used in the rock areas on either side of the fence?

John

Hi Nick,

Watering is almost always helpful in Colorado--though you don't need to bother with a "deep-root" wand or anything--just set a low hose or low sprinkler to water the tree's rooting area, which includes several feet beyond the dripline.  

Fertilizer may make things worse--a stressed tree will at best not be able to use the resources; at worst it will respond by trying to push a lot of growth, ultimately deepening its stress.  If the tree recovers, you could consider a very light fertilizer application next spring.

Were you able to check for mites or other possible causes of root damage?

Cordially,

John

Hi Nick,

The ground swelling is likely not a sign of problems, though I do notice that you have weed fabric surrounding the tree, which can restrict water and airflow to the root zone.  Consider removing it from beneath the rocks from the area around the tree.  

Another thing to examine closely is where the trunk of the tree enters the ground--the side toward the photograph looks like it has good trunk flare, that is, the trunk begins to spread out just before transitioning to roots at the soil line.  On the side nearer the shrub, though, it looks like the trunk may enter the ground straight (like a telephone pole), which could indicate either a rot, or possibly a girdling root that could be damaging the canopy as well.  If you find that the trunk flare does not go all around the tree, gently excavate until you find the flare, and look for girdling roots or signs of decay.

More information about girdling roots can be found on page 4 of this document: https://cmg.extension.colostate.edu/Gardennotes/659.pdf

Cordially,

John