No single piece of natural navigation is better known than ‘moss grows on the north side of trees’. It therefore sometimes surprises people when I tell them that this technique wouldn’t get into my top 20.
Have a look at the top picture of moss growing on the north side of a tree and you’ll see why.
Ask yourself this: Why is there lots of moss at the bottom of the tree and none near the top? They are both the north side of the tree.
The answer is that moss is very sensitive to moisture and not at all sensitive to direction. There are occasions when the south side dries more quickly than the north side, but there are many more occasions when other factors throw this out. In the top picture, there is most moss at the bottom, because moisture constantly evaporates from the ground. Natural navigators can ignore any mosses growing below knee height; they are no use for navigation at all.
Next look how the mosses grow thinner as the tree curves towards the vertical. The steeper any surface is, the faster water runs off it and the drier it is on average.
The moss is growing on the north side of this tree not because it is north, but because it is fairly coarse bark, that is not vertical and is above a damp forest floor. If the bark was smooth and vertical, there would be a ring of moss all around the base of the tree and none above knee height.
This is why you won’t find mosses thriving on smooth vertical trees like most beeches.
Mosses on rooftops are a little more dependable, as you can compare two equal gradients and textures, that are well away from the ground moisture.
If you still need convincing that moisture, not aspect, is the deciding factor for mosses, then take a look at the second picture, which I took in the wild, dank Wistman’s Wood in Dartmoor. Which way is the moss telling you is north there? 🙂
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