The principle of wood growth
Trees increase in volume each year. This happens when the treetop adds height and a new annual ring increases the width of the tree. Together the height and width increase create a wood sheathing that settles on top of previous years sheath.
The thickness of the annual rings varies. This is why the trees never grow to a pure cone or cylinder shape. The closer the tree grows to a cylinder shape, the better the narrowing class is.
The body shape of the wood is determined by genetics and also by the density of the forest. In cramped situations the width growth decreases and the stem becomes slender. The width growth can be increased by thinning down forests.
Source: Stig Nordman