Answer:
1.it means don't swing on birches >:D
2. the most imeadiate thought that comes into my head is based on the silver birch "betula pendula" which translated in Galic is "maidens skirt" referring to the long hanging branches of the tree that only get longer as the tree grows, the silver birch is a pioneer tree which means it was one of the discovered before civilisation in Britain, the silver birch is very common all across western Europe and even in some parts of north Asia. it is such a beautiful tree that "Burns" even made referencing in some of his poems.