[HRLand]

My neighbor has two zelkova trees in his front yard that are starting to split. Can anyone tell me what might be causing this and what can be done to save them?

This is the south facing side.

[HRLand]

North facing side.

[TrickyDick]

Hard to really tell but it looks like it's just a bad crotch (to small an angle between the stems) that formed years ago and is finally starting to split from the weight of the branches. They could have an arborist cable it.

[phototropic1]

Tricky Dick is probably right. Zelkovas and Elms are notoriously bad about having tight angled branch unions. As the branches grow in girth, the bark between the two branches gets included in the union. That is the rough ridge that you see coming straight down from the low part of the joint. An arborist might be able to cable it, but beyond that, start shopping for another tree.

[Dale Wiley]

Here in the wet Northwest, all the rain we get runs down the treeinto the crack and hastens the eventual failure of that tree along with other opportunistic types of vectors, bugs and problems.

[klaaar]

Pruning to thin the crown and remove deadwood, a rod or two through the split area, and cables in the crown. Follow this up with annual inspections.

The sort of action just described might be warranted for a valuable shade tree, etc. But it looks like this (these) trees are fairly young, and the cracks look substantial. Five or more leads connected to the trunk at one location looks pretty scary, too. Unless the trees' owner is desperate to save them, it may be advisable to do away with them completely.

[HRLand]

These trees have been there 20 years and the owner would really like to keep them. We just installed some lights under them this spring and he loves the way they look at night. I think he'll spend a few bucks to try and save them.

[klaaar]

Do as the others suggest and consult an experienced arborist to prune, brace, and possibly cable the trees, or learn to do it yourself. Good luck to you and the trees.