Effect of branch position and light availability on shoot growth of understory sugar maple and yellow birch saplings

Phenotypic plasticity enables tree saplings to change their morphology acco rding to their environment to grow toward a better light micro-habitat. The refore, processes of crown development could be expected to vary as a funct ion of light. The objectives of this study were to (i) evaluate the effects of position and light availability on shoot growth within the crowns of un derstory saplings of sugar maple (Acer saccharum Marsh.) and yellow birch ( Betula alleghaniensis Britton.); (ii) develop a new vigour index for shoots ; and (iii) evaluate the possible factors relating to branch mortality in t he crown of sugar maple saplings. The results revealed that there is a clea r branch position effect on shoot growth in the crown for yellow birch sapl ings and that it is partly related to the presence of two types of shoots. Dead branches were located at the bottom of the crown of sugar maple saplin gs; they were smaller in size, had wider angles and had lower indexes of vi gour than live branches found nearby. Preliminary results obtained on the v igour index indicate that it is a potentially useful tool for predicting th e growth and vigour status of a branch.