Millions of years of evolution have selected the round shape of tree trunks. Maybe it's the way wind blows around a tree, helping it maintain its temperature or moisture content. Maybe it allows trees to sway in the wind, their seeds falling to the ground. Whatever the reason, tree trunks are round.
It would be nice, however, if they weren't--if they were, say, square. There would be less milling waste and they would stack nicely, making storage and transportation more efficient. Two Belgium-based designers, known collectively as DRAW ME A SHEEP, like the idea too. Here are some photos taken as part of their NATURE V2.01 project.