Hi,
Maybe, you can not use the UpdatePanel to prevent the whole page reload when you click on the treenode, because the treevoew and menu controls are not Compatible with UpdatePanel Controls.
The more you can check Controls that Are Not Compatible with UpdatePanel Controls section of this link: http://www.asp.net/ajax/documentation/live/overview/UpdatePanelOverview.aspx
If you want to prevent the whole page check, I would suggest you use the iframe instead the master page.
In fact, the MasterPage class derives from the UserControl class. When the application is executing,the master page just like another child control. so you also can say, the master page is not a page, it is a usercontrol that is on the content page.
The more about how the master page works, you can read this article: http://www.odetocode.com/Articles/450.aspx
Hope it helps.
Please remember to mark the replies as answers if they help and unmark them if they provide no help.
Yours sincerely,
Amanda Wang
Microsoft Online Community Support