WHITE PINE SHOOTS: ROLES OF GRAVITY AND EPINASTY IN MOVEMENTS AND COMPRESSION WOOD LOCATION

Pinus strobus L. trees were grown on a clinostat. Although elongating shoots moved in almost the reverse of the normal pattern, they reached a normal original angle. Compression wood (CW) formed only on the upper side of branches opposite where CW had previously formed. Neither treatments to remove epinasty (girdling branches, removing leaders and branches), nor treatments to bend branches out of position changed CW distribution. On trees grown off the clinostat, branches bending upwards after removal of the terminal formed CW on the upper side before reaching vertical. Main stems could form CW on the upper side after having been tipped to induce CW formation on the under side. It is suggested that epinasty is not involved in either early shoot movements or in compression wood location and that when gravity stimulates CW formation on the under side of a shoot it preconditions the upper side for CW formation, thus establishing an equilibrium position for the shoot.