Aspen trees are subject to many diseases. We'd have to see the trees or sharp photos of the affected leaves, trunks and branches to determine what's causing problems.

Consider asking a certified arborist or forester to assess the trees' health onsite and recommend a course of action. The following information may be useful if you decide to hire an arborist:

How to hire a professional arborist to help care for your landscape trees
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/how-to-hire-a-professional-arborist/

I am going to send you pictures for you to assist in identifying the problem Bob.

Bob here are a few more pics from <personal data hidden>

Several things may be affecting the health of the trees. This is not uncommon with aspen because they are short-lived and susceptible to a number of diseases.

The first photo shows leaves affected by marssonina blight. Go here to learn more about the disease and what to do about it:

http://articles.extension.org/pages/41456/what-could-be-causing-spots-on-the-leaves-of-aspen-trees-a...

https://ask.extension.org/questions/422731

The second photo shows an aspen trunk infected with hypoxylon canker.   Go here to learn about this disease:

https://www.extension.umn.edu/garden/yard-garden/trees-shrubs/hypoxylon-canker-on-aspens-and-willows...

The other photos show trunks that also appear to be affected by hypoxylon. The branches with many dead leaves may have died due to the canker.

For a certain diagnosis and care recommendations, ask a certified arborist or forester to assess the trees' health onsite. Online diagnosis by photos is not definitive.

Hypoxylon can not be cured, so it's likely that the affected trees will continue to decline.

https://www.na.fs.fed.us/spfo/pubs/fidls/hypoxylon/hypoxylon.htm