Effects of phloem girdling in conifers on apical control of branches, growth allocation and air in wood

We investigated effects of stem phloem girdles on apical control of branch angle, stem and branch growth and stem air content in six conifer species. A stem girdle 2 cm above a branch caused the branch to bend upward in all six species. Upward bending was associated with increased formation and action of compression wood (CW) in the lower portion of the branch. Compression wood also formed in the main stem below the branch, suggesting increased auxin production in the branch. A stem girdle 2 cm below a branch (the branch remained directly connected to the apex and distal branches) released the branch from apical control in Tsuga canadensis (L.) Carr., Pinus contorta Dougl. ex Loud. and Pseudotsuga menziesii (Mirb.) Franco. The branch bent up, but there was no CW formation in the stem. In Pinus rigida Mill., the branch exhibited increased cambial activity but did not bend up. A stem girdle > 20 cm below a branch did not release the branch from apical control in any of the species. These results support the hypothesis that branches compete with the subjacent stem for branch-produced photosynthate and that when the branch lacks this competitive sink it is released from apical control. A stem girdle 2 cm below a branch did not cause release of apical control in either Juniperus virginiana L. or Picea abies (L.) Karst. In these species, decreased shoot elongation and cambial activity above the girdle probably prevented release. A stem girdle 2 cm below a branch increased air content in the stem below the girdle in four of five species, whereas the other girdle treatments had no significant effect on stem air content. Although growth was inhibited above the girdle in the two species with the largest increase in air content, growth was not inhibited in the other species. High air content in stem segments isolated from distal auxin and carbohydrate sources is consistent with the hypothesis that a carbohydrate supply is required to refill embolized cells.