Re: Moss growth in different hemisphers.

Let me preface my answer with the statement that what follows is simply my (well) educated take on the subject. I am aware of no study on this topic nor have I done a library search. Also, I have not made careful observations of this phenomenon although I have spent many hours in the field looking at plants in general.

First of all, the notion that moss grows on the north side of a tree in the Northern Hemisphere is nothing more than a piece of conventional wisdom used as a crude compass for those lost in the woods. In fact, moss in the Northern Hemisphere does not grow only on the north side of a tree. Furthermore, moss is not the only green plant or plant-like organism growing on trees (such organisms are called epiphytes). In addition to moss you will find lichens (combinations of a fungus and photosynthetic bacteria), algae (typically one-celled types), and a variety of vascular (not mosses) plants, including both flowering and non- flowering (ferns and similar plants) kinds.

For moss the primary consideration is the availability of water and light. Clearly, the south side of trees in the Northern Hemisphere would be, on average, best lit by the Sun. The presence of more light would also make them dryer. Similarly, trees tend to be better lit and dryer in the upper parts as opposed to the lower regions of the trunk. Given the tendency for many types of mosses to grow well in low light, it is likely that the north side of the lower part of the trunk of a typical, well exposed tree would provide the best habitat. For trees in less than ideal exposure, as in swamps, this would not be true and I would expect to find mosses growing evenly around the lower part of the tree trunk. If, in fact, a particular tree were more shaded (hence wetter) on the south side (perhaps by another, larger tree) I would expect the abundance of growth to be on that side.

As for the Southern Hemisphere, the direction of the Sun should be reversed. Your dad is right to the extent that the basic assumption is true.