These Platanus × acerifolia trees (London Planetree) were pruned to have this short, stubby branch structure. What is this called, and how is is done?
That is called pollarding. It is a pruning system that promotes uniform yearly growth, in dense heads over the tree. You basically cut the branch ends at a certain diameter, and remove all smaller growth. Then on a yearly to almost a 20 year basis, remove the shoots that form, at the base. the result is the formation of these clublike heads, which grow in diameter, and a smaller mature tree.
When left alone, these trees can revert back to normal growth, but the branch structure will always look peculiar in the areas where the pollarding had been performed.
This pruning method is only suitable for certain species, and the most common of these are the Platanus (Sycamore) species.
Examples of trees that do well as pollards include the following: