Dégagement chimique ou manuel de plantations d'épinette noire? Étude de cas dans le domaine de la sapinière à bouleau blanc au Québec

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This study aimed at comparing manual and chemical release treatments in black spruce plantations within the balsam fir – white birch ecosystems located in the Lower-Saint-Lawrence and inland Gaspé région of Québec. Three experimental sites characterized by competition dominated by red raspberry (Rubus idaeus) were selected for the study. On each site, a completely randomized block design with seven repetitions was used. Each block was composed of four plots, each representing a vegetation treatment: a manual release, a manual release to be repeated two years later, a chemical release using Vision® (Monsanto Canada), and no treatment on a control plot. The effects of treatment on spruce growth were assessed 1, 2, and 5 years after treatment. The spruce seedlings benefited from the release treatments. After five years, spruce diameter was 25.2 to 42.9% superior, height was 12.5 to 24.8% superior and the mean volume index was 89.3 to 168.6% superior to seedlings on control plots. Moreover, growth profiles over time for the released and control seedlings were not parallel; the seedlings in the released plots showed a growth rate which was superior to that of the control plots, and differences in growth trajectories increased over time. For the three experimental sites, chemical release showed no significant advantage over manual release when assessed after five years of spruce growth variables. This result is attributed to a relatively poor vegetation regrowth after treatment. A manual release treatment was applied a second time on only one of the three sites. In this case, spruce demonstrated a significantly higher growth than on plots which were only manually released once. The survival rate was not significantly affected by either of the treatments except when seedlings suffered mechanical damage. This study supports the hypothesis whereby sites located within the balsam fir – white birch ecosystems for which vigor of competing vegetation is moderate, a manual release treatment produces similar results to a chemical release treatment in terms of black spruce growth in the ensuing years. Key words: black spruce, chemical release, competition, manual release, red raspberry, spruce plantations.