BRANCH ABSCISSION IN BALSAM POPLAR (POPULUS-BALSAMIFERA) - CHARACTERIZATION OF THE PHENOMENON AND THE INFLUENCE OF WIND

We studied the annual shedding of small branches in balsam poplar (Populus balsamifera) and found that in Calgary the majority of these branches fall in October, prior to leaf abscission in November. The proportions of abscised branches with attached leaves, secondary branches, and viable buds were approximately 0.5, 0.3, and 0.4. The mean age of abscised branches was 6.7 yr, and the mean length was 20.4 cm. The degree of branch abscission in a tree may depend on its age and vigor, since abscission appears to be the consequence of self-shading in older branches by the new growth that forms at the periphery of the canopy each year. The nitrogen and potassium content of buds in abscised branches was shown to be the same as that in buds of still-attached branches, but the leaf area of abscised branches was significantly less than that of attached branches of similar size and age. The relationship of self-pruning to tree strength and wood quality is discussed. Though most branches abscise mainly during October, superimposed on that seasonal pattern are other times of increased branch abscission that correspond to days when strong winds occurred. During the early period of branch life, there may often be a partial separation layer. This constitutes a weak link, and if strong winds occur the branch may be shed. If conditions are more moderate, the partial separation layer will heal over, leading to the formation of swollen bases on the younger branches. Branch breakage also occurs (without separation layer formation), but to a lesser extent than branch abscission, and breakage is more common during May and June. Breakage may serve to prune out poorly located and unproductive new growth.