Hi Wendy and welcome to GoY, sorry for the loss of your dog. If you want a tree to grow and thrive in that soil your are going to need to get it dug over and some good humus rich compost dug in, in fact you'll need to do that whatever you plant aside from summer bedding or annuals. Assuming you do plant a small tree are you looking for something that is colourful or flowers or... also what height are you wanting the tree to end up at and how big is the space to be planted up?

Some good choices for heavy clay soil are: Magnolia, Hawthorn, Crab Apple, Holly, Indian Bean Tree and Hazel. Sorry about you dog.

What about a standard rose...they love clay. Sorry to hear about your dog..i like the green dog memorial. ?

Weeping standard rose..even better!