(_f_) Frost splits are most numerous less than three feet above the ground.
[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse.

Forstwiss.
Centralb., XXXII, 2, 1910, pp.

74-81.] When a tree is swayed by the wind the roots are counteracting forces, and the wood fibres are tested in tension and compression by the opposing forces; where the roots exercise tension stresses most effectively the effect of compression stresses is at a minimum; only where the pressure is in excess of the tension, _i.e._, between the roots, can a separation of the fibre result.

Hence, when by frost a tension on the entire periphery is established, and the wind localizes additional strains, failure occurs.

The stronger the compression and tension, the severer the strains and the oftener failures occur.
The occurrence of reports of frost splits on wind-still days is believed by Busse to be due to the opening of old frost splits where the tension produced by the frost alone is sufficient.
Frost splits may heal over temporarily, but usually open up again during the following winter.