I've been looking at three of the decks in my collection: Druid Plant Oracle, Green Man Tree Oracle, and Celtic Oracle. I noticed that three trees are grouped together in Druid Plant Oracle and called 'The Guardians', so I thought I'd see if they were in the other decks and then find out why they are called 'guardians'. They are Elder, Birch and Hawthorn.
According to Philip and Stephanie Carr-Gomm in the Druid Plant Oracle companion book, the Guardians act as 'preservers of our health, possibly even prolonging our lives, by strengthening the immune system and the key organs of the circulatory system, heart, liver and kidneys.' They go on to explain that the Elder is like the Mother, Hawthorn the Father, and Birch the child. This is because the Elder has the contradictory nature of the goddess, who can both create and destroy (its flowers and berries are fortifying, but its leaves and bark are poisonous). This does call to mind some of the more powerful Empress images in tarot, which show a female force that can give birth but also does not scruple to destroy (such as the rather fierce Empress in Anna K Tarot, who nurses a baby but has a rather malevolent bearing). Hawthorn is the Father because the tree is associated with sexuality and the circulatory system (a good circulatory system being key to male performance!). The Birch is associated with beginnings, birth and cleansing, so makes an appropriate symbol for a child. The sap can make wine and also a detoxing tonic.
In John Matthews' Green Man Tree Oracle, the association of the Elder tree and witchcraft is made. Lots of folk stories have witches turning themselves into Elder trees, and in the Scandinavian countries in particular, there is a folk tradition of honouring a figure called 'The Elder Woman'. The Elder is also seen as a protection against dark magic. Associated with the benefits of the Elder tree, though, is that some sort of sacrifice will be made on your part as part of the deal. And so the Elder can be interpreted as some sort of sacrifice or trade off.
The Birch, John Matthews points out in Green Man Tree Oracle, is knows as the Lady of the Woods. It does have a feminine, elegant look. It is one of the first trees to blossom in the spring, and its light colour links it to the moon, sun and stars. Birches were traditionally used to make switches for punishing children, and also for making cradles -- and broom handles, as in the witch's broom, which lends them flight.
The Hawthorn is linked to May Day, Matthews says, and Beltane is closely associated with fertility and rites of love and marriage.
May the Guardians protect me as I move through this week. :)