Stuart Williamson, of Dunedin, asks :-

Why do trees, even on the side of a hill, always grow vertically?

John Walker, of the School of Forestry, University of Canterbury, responded.

Think `leaning Tower of Piza'; think of large cranes. Leaning structures are always fighting to stay upright.

Soil movement and snow creep on steep hillsides can gradually topple trees, even large trees. Young trees growing in the shade of taller trees need to weave sideways to capture more light and so survive. They need a mechanism to straighten and to reorient.

A leaning tree in a Park can be stabilized by guying with steel wire (pulling the tree) or by propping it up (a strut under the tree to carry a compressive load).

A straight leaning tree does not become upright by rotating back to the vertical (like lifting a flag pole). In fact leaning stems that 'straighten' are actually shaped like hockey sticks. At the cambium millions and millions of wood cells are laid down that can either expand (pushing) or contract (pulling). Softwoods can bend a straight stem by expanding its cambial cells on the underside of the lean (pushing); hardwood straighten by shortening its cambial cells on the upper side of the leaning stem (pulling). Straightening can occur over years, over decades. These additions cells, infinitesimally as individuals, in their millions gradually bend the existing straight stem into a hockey stick.

Correcting the lean corrects any tendency of the tree to topple. Why hardwoods pull and softwoods push is more of a mystery than scientists would care to admit!