Some years ago I purchased a mountain ash which died after one year. I left the roots in the ground and this tree grew up. Can you identify this tree?
These appear to be the leaves of Sorbus x thuringiaca, also known as Oakleaf Mountain Ash.
This variety is a hybrid between S. aria and S. aucuparia – the latter being the more familiar European Mountain Ash, with its pinnate leaves and its white flowers in large flat heads. The leaves in your photograph are deeply lobed and veined, resembling oak leaves. Is it possible that your Mountain Ash was an Oakleaf? These are popular trees and readily available at large nurseries throughout the GTA.
By way of background, the genus Sorbus contains many species and they are known to hybridize easily, even with other genera such as Pyrus (pear) and Aronia (a native berry, known as the “chokeberry”), all members of the Rosaceae family. All this is to speculate that your Mountain Ash may have been one of the Sorbus hybrids, and so it is no surprise to see these leaves emerging from the stump of your tree.
Trees are capable of regenerating from stumps for many years after the tree has been cut down. When new growth emerges from the root, it does sometimes create another tree; more often, though, what emerges are known as “water sprouts”, weak, vertical, fast-growing branches that are not viable in the long term as a new tree or shrub. From your photograph it isn’t possible to tell if what you have is a tree that is, or will be, strong and well-shaped – in other words, a tree that you would like to continue to keep in your garden.