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>