The alder tree is associated with death and resurrection, with courage and with balancing the male and female principles within a human being. Its wood is used for such elevated purposes as the making of magical flutes and whistles, the foundation of medieval cathedrals and the fabrication of cigar boxes.

The alder is the tree of Bran, the Celtic raven god. Bran the Blessed set out from Wales to Ireland to avenge the mistreatment of his sister. Mortally wounded in his foot, he ordered his men to execute a rather drastic amputation, leaving only his head to take back with them. That journey took no less than 87 years and many adventures, and all that time Brans head wouldn't shut up. At last, they buried it at the site where now is the Tower of London, where the head and its accompagnying ravens still gard Britain against invaders.

The animal associated with the alder-month is the bear. Although now extinct in the British isles, once it was a totem animal. The word "Arth", meaning bear, is of course the root of the name Arthur.
By the way, it is said that Arthur exhumed the head of Bran, which was buried facing France, and reburied it facing the other way. He couldn't stand another man defending his country... The Ogham sign attributed to this month is Fearn.