Thank you for your photographs; some additional information would be helpful.
1. What kind of tree? Cherry? Or?
2. Where in your landscape is the tree? (e.g. irrigated turf; south exposure, west/east/north exposure, etc.)
3. Where in relation to the affected branch are the other areas in photograph #3? (e.g. same side of tree; opposite side; 2 ft. below or above, etc.) 
4. Do you see any fruiting bodies, black or orange dots/spots in these cankers or nearby?

Thank you for sending this information.

The tree name was supposed to make it into the question, but I see it got omitted somehow. This is a Japanese Cherry Kwanzan tree. Yes, irrigated turf. The tree is in the backyard which faces west. The lawn is slightly sloped, making for perfect drainage.

The cankers in pic #3 are on the trunk, below and to the right of the diseased branch, about 8-10" away. Actually, I don't know whether to call them cankers, or splits, because the latter word better describes what they look like. I don't know if these are typical of this type of tree (from the tree growing?), but I count about 5 or 6 splits like these, of varying sizes, along the trunk but not along any of the branches that I can see. I do not see any fruiting bodies, black or orange dots/spots in any of the cankers, nearby, or in the crotch of the diseased branch.

The diseased branch is the lowest branch on the tree. The tree is 3-4 yrs old and was planted in spring 2011.

It seems that attaching pictures (all of mine are < 5MB) causes a problem because I kept getting an error message until I finally left the picture off. Boom, the message sent right away. The same thing happened with my original message. I tried three days in a row to send. Finally, I got lucky and the message sent with 3 pics attached. Not so lucky today. I was going to attach a different photo for perspective, which shows the tree from about 6 ft away.

Thank you for your reply. I am sorry you have had a difficult time attaching photos and with delays in transmission.

This has given me a bit of time to do some research on your tree. Kwanzan ornamental cherries seem to be prone to develop cracks in their trunks and branches. The cracks are usually not life-threatening for the trees, however the cracks can provide a way for insects and fungi to enter the tree, so it is important to keep an eye on these cracked areas to make sure they do not show any signs of disease.

Environmental conditions at different times of year contribute to the development of cracks. In late winter and early spring, rapid thawing of tissue after a bitter cold night can lead to frost cracks. If you recall we did have a series of bitterly cold nights and warm days and this temperature fluctuation can lead to cracks especially with a south or southwestern exposure. The tree may have had some slight damage that was not noticeable however, the tissue reacted to the cold/warm temperatures and the crack developed. Hopefully, the tree will form a callus where the crack occurs and thus seals off any entry into the inner tissue of the tree. Sunscald can also be a cause of cracks especially in younger trees. Wrapping the trunk with tree wrap during the winter months can help protect this tender, thin tissue from getting a "sunburn". You will remove the tree wrap in the spring.

Be sure to keep the tree as healthy as possible with adequate watering during hot weather and during our dry, windy winters. Supplemental watering during winter is a great idea if we have not had sufficient moisture from snow.

The branch that is cracked in the crotch area between branch and trunk should be removed. The following link should help you to make the correct pruning cuts. Be sure to use clean tools and clean tools again when finished. You can use rubbing alcohol or disinfectant to spray your pruning tools. This will ensure that you are not passing on any contamination in your landscape.

 http://www.ext.colostate.edu/mg/Gardennotes/612.html 

Good luck with your tree.

Thank you for researching our specific tree. We didn't know cherry Kwanzan trees could be prone to cracks. Also, the bit about environmental conditions contributing to cracking was insightful. Yes, we definitely commented to each other more than once what a roller coaster of a spring this was. I don't know when the deep wound in the diseased branch occurred, but it was sometime last winter or this spring.

We'll keep an eye on the cracks. I guess no point in worrying now about what we'd do should one of them get infected.

Thank you again for your researched and thoughtful responses. You've been very helpful.

You are very welcome.