That should hsve been Rhus aromatica of course!

I think you stated the problem -- not enough sun. In partial shade, sumacs will get lanky and thin searching for more sun.

I know. But I am hoping there is a way to avoid transplanting them!

Can you do some selective pruning on the trees to let a bit more sun reach them? I have R. aromatica ’Low Grow’ on a slope at the base of a large White Pine, and it has spread and filled in nicely.

Did you do anything to make it denser?

Not really, other than supplemental watering when it was getting started. It is growing on a very tough site, with the adjacent White Pine and a large Red Maple within 15'-20'. I've never measured the amount of sun it gets, but it can't be more than four hours of angled sunlight.

I should note that this was planted as a single container grown plant about eight years ago. It took several years to look like this.