Aspen tree failing health
We have several Aspen trees on our property that are either near death or yellowing with serious health issues. Looking closely at the trees they appear not to have an insect infestation. There is damage to the bark but no bore holes. In some cases the bark has deteriorated. The leaves that are yellowing have black specks on them that look to be internal to the leaf structure. I have attached photos. Can you identify the problem and provide a possible solution?
Larimer County Colorado
Your aspen have iron chlorosis and possibly one of the many other leaf spots these plants can get. Unfortunately, aspen at lower elevation are very susceptible to a host of diseases and insects.
It also looks like these trees are quite tall and older. Aspen are considered to be short-lived trees in urban areas. It would be important for you to remove the dead limbs from the tree and have the trees assessed for structural health.
We have a good fact sheet on aspen leaf spots: http://extension.colostate.edu/topic-areas/yard-garden/aspen-and-poplar-leaf-spots-2-920/
Essentially, the recommendation (at this point in the season) is to make sure to clean up the leaves in the fall and throw them in the trash--do not compost. Also keep the trees as healthy as possible with regular watering.
The bark splitting on the trunk could be from winter damage or other storm damage. The wounds have sealed over and formed callous tissue, so there is nothing further you can do.
At this point, preventing additional stress is important and increase good cultural practices as much as possible.