Hi, 

Did anything hit the tree that you know of and it could have caused a large piece of bark to fall off? The bark can minimally split in Redbuds as they mature but these seems extreme. 

Do you notice any borer holes in the bark or any sawdust looking material coming out from the trunk anywhere or around the base of the tree on the mulch? 

If there are borers there isn't anything you can do unfortunately to help the tree at this point. If the canopy still looks good and healthy, we are not sure why this is happening then, maybe the issue hasn't worked up to the canopy yet. 

You can monitor for signs of dying or additional stress in the canopy. We can't see a root flare at the bottom so that may be the issue with the bark. There could be a girdling root under the mulch. 

Emily

There is unfortunately little that can be done to prevent tree decline if an issue is systemic enough to be affecting the trunk or canopy (perhaps later), but you can keep monitoring the tree for watering needs in the meantime to make sure its root zone doesn't get too dry in this ongoing drought. We do not see any distinct borer holes either, though some could be as small as the diameter of a toothpick, so may be hard to detect in the photos. As Emily mentioned, one main concern is the lack of a visible root flare at the trunk base, if the tree was planted too deeply (a common occurrence given how trees are situated in their pots or burlap at garden centers). A girdling root worsening over time and slowly "strangling" one side of the trunk could explain sapwood tissue damage that leads to bark loss, and direct evidence of girdling would not be visible if the root is covered with soil and/or mulch.

You could have a certified arborist assess the tree in person, since they can do a more detailed inspection for issues like borers, girdling roots, and other conditions. They won't be able to treat or cure every condition, but can at least help to narrow-down a possible diagnosis and find any other sources of tree stress that might be able to be alleviated. As you noted, the canopy still looks quite good, which is an encouraging sign that this might be caught early enough to not progress to canopy dieback. If a girdling root is found, an arborist will probably be able to cut it to stop it from worsening if the root is not too thick or fused with the trunk already.

Miri