I have seen this twice this winter, both on cold days. I though that it was a fungus. It turns out that it is actually ice. It gets called "ice beard" because of the hair-like quality of the ice. Apparently the sap cools down towards freezing point and expands as it does so. This cracks the bark. Water then comes out through something like capillary action and freezes into these fine filaments. Wikipedia says that Alfred Wegener, the man responsible for Continental Drift theory, was interested in this phenomenon too.
