A still more recent theory is that of Busse[36] who considers the mechanical action of the wind a very important factor.  He observed:  (a) Frost splits sometimes occur at higher temperatures than 14 deg.F. (b) Most splits take place shortly before sunrise, i.e., at the time of lowest air and soil temperature; they are never heard to take place at noon, afternoon, or evening. (c) They always occur between two roots or between the collars of two roots, (d) They are most frequent in old, stout-rooted, broad-crowned trees; in younger stands it is always the stoutest members that are found with frost splits, while in quite young stands they are altogether absent, (e) Trees on wet sites are most liable to splits, due to difference in wood structure, just as difference in wood structure makes different species vary in this regard. (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.  The presence of old splits is often indicated by a ridge of callous, the result of the cambium’s effort to occlude the wound.  Frost splits not only affect the value of lumber, but also afford an entrance into the living tree for disease and decay.

SHAKES, GALLS, PITCH POCKETS

Heart shake occurs in nearly all overmature timber, being more frequent in hardwoods (especially oak) than in conifers.  In typical heart shake the centre of the hole shows indications of becoming hollow and radial clefts of varying size extend outward from the pith, being widest inward.  It frequently affects only the butt log, but may extend to the entire hole and even the larger branches.  It usually results from a shrinkage of the heartwood due probably to chemical changes in the wood.