understory management of river birch tree
My daughter has a very small back yard with a large river birch tree. See attached. We removed the grass and planted some PA sedge and Itea Virginica. See 2nd photo. 2 questions - there is a neighborhood cat using the area near the base of the tree as a litter box and killed the PA sedge and heuchera. Actually saw the cat in that exact area. What do you recommend to repel this cat (chemical, motion detector water sprayers)? Fortunately the shrubs and pa sedge further from the base of the tree are still doing well. After solving the cat problem. We would like to vegetate (foam flowers, pachysandra procumbens are some ideas). Of course the area around the tree is most delicate. I was thinking for the winter to add pine bark wood chips to give time to get rid of the cat. I don't know about adding any soil. The roots are very close to surface. Also, we had an arborist look at the tree. It's in very good condition. It will need some pruning in near future. Thank you!
You can discourage cat activity under the tree by applying a commercial repellent (your vet may recommend a good brand) or spreading chicken wire in the area temporarily. If you put down mulch now, put the chicken wire or repellent over the mulch. https://extension.umd.edu/hgic/domestic-and-feral-cats
The closer your plantings get to the river birch, the more they will struggle with root competition from the birch. Surface roots make this extra problematic. They will require care to supply extra water in order to get established. That may have contributed to your plant loss.