Dear Allison:

Thank you for your question, which has been routed to the CSU Extension Office Master Gardener Help Desk. And thank you for including photos. To help determine what is going on with your Mugo pine, I ask you to do the following:

1. Look carefully at pine's needles and bark, as well as the ground surrounding the tree. Do you notice any insects, fine sawdust, holes that look like they could have been made by insects, or black spots (that look like pepper) at the base of sick-looking needles? Have you (or your neighbors) used herbicides near the pine? When did you first notice discolored needles/dieback?
2. Dog urine can cause coniferous tree scorch, and you are wise to keep your dog (and others that may be visiting your yard) away from the pine. It is unlikely that dog urine is causing all of the issues, however.
3. You mentioned that the pine was planted in the fall of 2023, so it likely still has a small root system. According to the Purdue University Cooperative Extension Service, "On needled evergreens, the first symptom of water stress is an overall gray-green coloration to the foliage" that, if not alleviated, can progress to needle ends turning tan with possible branch dieback." In Colorado, it is imperative to water even during the winter (when temperatures climb over 40 degrees F). So it is possible that you are seeing some later effects of an underwatered tree that did not tolerate winter drought very well as a new transplant. Attached is a handout with more information on watering during fall and winter. You might consider increasing the mulch around the pine as well. 
4. You mentioned that the pine continues to put on new growth, which is a great sign. It is far too early to give up on the pine at this point.
5. Please remember that it is normal for pines to drop their oldest needles, which is easy to confuse with tree illness. I think that your pine has more than normal needle shedding going on; but (in future) please don't worry if you periodically see the tree shed some older needles.
6. If you wish, you may submit a specimen sample to CSU for analysis (please see second attachment). This is likely to get you the most definitive diagnosis. 

Thank you for contacting Ask Extension. If you wish to get in touch with the El Paso County Master Gardener Help Desk directly, please feel free to drop by our office at 17 N. Spruce Street, Colorado Springs, on Mondays or Wednesdays from 9-12 and 1-4 p. m. or email us at <personal data hidden>. Happy gardening, and please don't give up on your pine at this point!

Best wishes,

Heidi V.

Master Gardener Volunteer