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These ash trees are fast growing trees and as such have weaker wood overall. A healthy tree is often able to heal these wounds with a callus that grows together to
heal the split in the trunk.
In the first picture, it appears the crotch of these two branches is very narrow which is a weak connection between these branches. As the tree grows, the weight will make the split worse.
In the second picture, it appears this part of the trunk is younger than the one on the left. If the split is on the south side, it may have been caused by sun scald in the winter when the tree warms during the day but when temperatures drop at night, the sap in the tree freezes.
You have probably heard the Emerald Ash Borer has moved beyond the border of Boulder. This is a deadly pest that has decimated the ash in the Midwest states.
This link will tell you more about the emerald ash borer.
You should do nothing at this time to the splits but let the tree do it's own healing.
Keep careful watch to ensure the safety of people and property. Should the limbs continue to decline, an arborist should be contacted to discuss the possible options.