Dave Cheetham (February)
I've seen these clusters of twigs in trees many times and always dismissed them from a distance as disused birds nests. Recently I looked a little closer only to find they are actually intense clusters of shoot growth.
After seeking advice from a local environmental expert, it turns out that this is the explanation:
These are actually dense bunches of short shoots with small leaves caused by the fungus Taphrina Betulina. It is commonly known as Witches' Broom because if all the extra shoots grow in the same direction it can take the shape of a traditional broomstick.
The fungus enters the tree and stimulates extra growth in the place of one shoot, and then feeds on this extra growth. It does not seriously harm the tree, and because of this the Witches' Broom is classed as a gall. Plant galls are part of a plant that have been caused by a parasitic attack within the plant cells, causing abnormal growth within the tissues.
It used to be believed that witches had flown over the tree to make them appear. More often, the shoots grow as a cluster of growth, with no fixed direction and it resembles a bird's nest instead. These can be found wherever birch trees are found.
Witches' Broom generally appears only on birches (Betula species) in Britain, but in different parts of the world can appear on elms, pines or other kinds of trees; the growths can be safely removed, although they might reappear elsewhere on the tree. Mature birches can support up to around 100 growths. Traditional (and witches') broomsticks are made from birch twigs.