Tree Runes more commonly known as Ogham come from the Celts who use them as a secret way of communication as well as divination purposes.  Examples of Ogam can still be seen today, carved on stones scattered throughout the British Isles and the European mainland.  These are mainly name places or sign posts or recording to great heros.  Some of them are clearly for more mystical purposes.

Ogam itself is a very complex language capable of many different subtle layers, some of which are lost to us, through its associations with trees different elements and colours one word could mean several things and in the hands of a true bard many political undercurrents could be construed that could crush the mightiest conquerer.

The celts themselves were a fiery creative people, quick tempered and renowned as fearless warriors. This stemmed from their religious beliefs which the afterlife was an integral part of. This belief also had a proficient effect on their culture and most notably their art. The Celtic mythology and art made a profound impact and influence through out Europe. Celts still survive to this day their descendants can now be found in Scotland, Ireland (which has always been the centre of their world), Wales and parts of Brittany and Spain.
As I expand the site this section will grow to include a detailed history, mythology, as well as their culture and art.

(extract from web site:  http://www.uponreflection.co.uk/ogham/history.htm)  This web sit is very good goes in to details of all the 20 letters of the Ogham plus the new five additional ogham created later on.
There are other web sites to look at on this subject to:
http://ogham.lyberty.com/ogmean.html
http://www.druidfellowship.org/Ogham.htm (this one goes into Norse Runes as well)

cud u plz let me know the celtic zodiac signs and their characteristics?

That's a tough one will have to do some research for you as my specialty is the Eldar Futhark.  I have been meaning to look in to Ogham and I know that there is a tree rune for every month like in my post about Runic Half months.  Might be a while though hope you don't mind as I am working on another project at the moment.

http://www.novareinna.com/constellation/celtic.html  I just found this after reading your query it might help.  If you get anything from it let me know please.

I have been fascinated by the Ogham for some time and was wondering if I create my own runes, would it be okay if I added in the Ogham for the tree I make the runes out of as the blank rune, rather than the blank rune itself standing for mystery or the unknowable? What should I use as the symbol for the tree in Ogham if the tree is not listed in Ogham? Would it be a bad idea to use the tree's wood to make the runes then if it is not listed?