tree允许显示根节点，但不允许选择根节点（即根节点不允许点击成选中状态）...

var _root = new Ext.tree.AsyncTreeNode({
text: '根节点',
draggable:false,
id:'1',
listeners:{//监听根节点的beforeclick事件
beforeclick:function(n){
return false;
}
}