Best to consult with a tree expert (Arborist) sourced in the link from the Arborist Society:

https://www.treesaregood.org/findanarborist

Hi Ron,

Reviewing your photos further, there is a notable issue for this tree which is likely to be one of the primary conditions leading to its decline.

The tree's trunk has been buried too deeply in the soil. That can happen at the time it is planted but also through generous applications of compost or mulch over the years. Many trees, lindens included, are likely to grow girdling roots in these situations and you can see that happening in your photo. Roots grow up into the extra soil above the root system where they circle the trunk and gradually choke the plant.

You will need to have a certified arborist examine the tree on site for other issues and dig into the excess soil to look at the extent of the girdling roots. In some situations, these strangling roots can be removed and the tree restored to good health if it is still generally vigorous, but not always.

It is possible that your Cornus kousa suffered the same conditions. Soil built up against the trunk at the level of the root flare and above can lead to other problems as well. The soil holds dampness against the bark which is not designed to withstand the extended exposure. It can lead to bark weakness and support disease or insects entering the plant.

With best regards,

Leslie