The reason the fallen birch was "rotten" is that as a species they are notorious for decaying fast if left out in the weather, within a few weeks or monthes logs left on the floor turn to a sort of crumbly crunchie bar texture, and it is then next to impossible to split, it breaks sideways....in other words its not weather resistant in the same way that eg oak or chestnut are. Ash is another species that has poor weather resistance, great for tool handles though...