Well, you are thinking correctly!! The species of
moss that grow on trees do indeed tend to grow on
the north side of the tree, if the tree is at
least somewhat exposed and not heavily sheltered
by other trees. There are a couple of reasons for
this as you stated, "because it is shady and
cool". And due to those facts more water moisture
is retained by the moss growing in this location
which is a major requirement for it to survive.
If the moss is on a tree that is heavily sheltered
by other trees in can grow on any side because the
moss is protected from the wind and sun which
would remove the water moisture from the moss.
Your hypothesis that moss grows best in shady,
cool places is right. Mosses usually need a lot
of water because they don't transport water very
well. Mosses also need a layer of water to
reproduce. Your hypothesis that the north side of
trees tends to be shadier and cooler than the
other sides is also right. However, moss
*doesn't* always grow (only) on the north side of
trees. In some forests, trees are cool and moist
enough on all sides to allow the moss to grow.
Consider a tree that is crowded with other trees.
Is much light filtering in to any side of the
trunk? Sometimes one side of a tree will be
moister or cooler than the north side due to local
shading or moisture.
To find directions in
the wild, a compass is best. The position of the
sun and the north star will give you a rough idea
of the direction you're facing. If you're ever
lost in the woods, though, your best bet is to
stay where you are so that you're easier to
Your ideas about this question are very good. It
is true that moss likes to grow in places where
it's moist, shady, and cool. The question then
becomes, what would make the north side of a tree
moist, shady, and/or cool and thus good for the
growth of mosses?
Fortunately for us,
moist, shady, and cool conditions all tend to
occur together where the sun doesn't shine as
much. Shady places are, by definition, places
where the sun isn't directly shining. These
places are also cool because there is no direct
sunlight to warm them up and they are more moist
because there is no direct sunlight to evaporate
So we now want to find a place where
the sun shines less. Because we live in the
northern hemisphere (latitude about 34.5 north),
the path of the sun is always to the south of us
(another discussion that you can ask about if
you're curious). With the sun always to the
south, the south side of trees get more light
throughout the day, leaving the north side in
shade. As I mentioned above, with shade you also
get coolness and
moisture, providing good
conditions for moss!
This doesn't mean that
moss only grows on the north side of trees. Moss
will grow other places that you get coolness,
shade, and moisture and in some forests (more
likely in northern California where it's more
wet), you can get moss covering the entire tree.
Still, the moss will prefer the north side as a
place to grow, because of the characteristics you
mentioned. Good ideas!
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