Using the main menu, "Menu: Tree/Collapse" you can collapse a subtree.
There are two options available inside this menu item:
"Menu: Tree/Collapse/Subtree (one level)"
This will result in all children (of the currently selected node) becoming hidden.
"Menu: Tree/Collapse/Subtree (all levels)".
This will fully collapse a subtree. All descendant nodes will become collapsed as well.