Apical control of branch movements in white pine: biological aspects.

Two-year-old branches on control trees (Pinus strobus L.) were compared through a season with branches on trees stem-girdled just above, or below, the branch whorl. All branches first sagged down for 20 days and then moved up for 40 days. Then, control branches reversed and moved back down while branches in both girdle treatments continued to move up. Movement reversal correlated with cessation of both elongation and diameter growth in control branches. Diameter growth continued in branches of girdled trees. Control branches continued to stiffen even after diameter growth stopped. Differences in movements due to girdling are from compression wood formed after cessation of branch elongation. Apical control stops cambial activity and compression wood formation in branches after branch elongation ceases, allowing photosynthate produced in the branch to move to the stem. Control branches bend down from increasing self-weight after cambial activity ceases.