EFFECT OF COMPANION SPECIES ON THE GROWTH OF JACK PINE (PINUS-BANKSIANA)

The growth and yield of jack pine (Pinus banksiana Lamb.) was studied in even-aged stands of three types: pure jack pine, jack pine mixed wi th paper birch (Betula papyrifera Marsh.), and jack pine mixed with tr embling aspen (Populus tremuloides Michx.) growing on moderately well- drained glaciolacustrine clay soils in the southern tip of the Clay Be lt of northwestern Quebec. Site index, average DBH, and average height of jack pine, and the availability of nutrients in the forest floor w ere compared among stand types. No differences were found in the heigh t growth of jack pine among stand types. However, diameters of jack pi ne in mixtures with paper birch were significantly greater than in eit her pure stands or in mixtures with aspen despite the fact that both m ixed stand types showed higher forest floor pH and greater concentrati ons of exchangeable calcium and magnesium than the pure stands. These results, together with the pattern of height growth of the three speci es, suggest that the beneficial effect of paper birch on the diameter growth of jack pine is caused mainly by a reduction in the competition for light among individual jack pine trees. The silvicultural implica tions of the results are discussed.