Try this:
<script>
$.extend($.fn.treegrid.methods, {
	myRefresh: function(jq){
		return jq.each(function(){
			var t = $(this);
			var opts = t.treegrid('options');
			var n = t.treegrid('getSelected');
			var d = t.treegrid('getData');
			t.treegrid('loadData', d);
			if (n){
				t.treegrid('select', n[opts.idField]);
			}
		})
	}
});
</script>
Now you can call 'myRefresh' method.
$('#tg').treegrid('myRefresh');