Well, we have a sad reason to plant a new tree now. Our dog Douglas went downhill very quickly (he was diagnosed with lymphatic sarcoma about two months ago), and we asked the vet to put him down last Wednesday. He is buried next to the pond he loved and has left a big dog sized hole in our lives.
The soil there is pretty wet most of the time, so a willow or alder look the best contenders, although it is pretty sheltered, so there may be some other options. Dyson seems to be taking it well and is now able to enjoy his soft toys without Douglas taking them off him and ripping the stuffing out.