If the tree view is data bound you could track for changes in the data source collection. Meaning that you can detect changes in your data and perform the operations you want.
However there is a DragEnded event fired by the destination tree when the drop operation is complete. So you can initiate you incremental save from handler of that event (this is the scenario when you are adding items).
Also when you want to track the removing of items from the tree view you could use the DragStarted event (rose by the source tree) and mark the source tree view for saving. Additionally you should attach handler to the RadDragAndDropManager's DropInfo event. When the event is fired and its status is DragComplete you should perform the saving of your source tree view.
Hope this helps. Let me know if you need more info?
the Telerik team
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