When drag and drop a node, the 'onDrop' event will be triggered, in which you can save the tree node state to server.
<ul id="tt2" class="easyui-tree" data-options="url:'tree_data.json',
		dnd:true,
		onDrop:function(target,source,point){
			var targetNodeId = $(this).tree('getNode',target).id;
			var sourceNodeId = source.id;
			$.post(url, {
				targetNodeId: targetNodeId,
				sourceNodeId: sourceNodeId,
				point: point
			}, function(){
				alert('save ok');
			});
		}"></ul>