Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The present study examines the impact of intensive silvicultural treatments on environmental conditions, leaf level morphology and physiology, and growth of planted eastern white pine (Pinus strobus L.) saplings and evaluates how silvicultural treatments and the presence of competing vegetation influence the relationships between leaf nitrogen, leaf morphology, and leaf level photosynthetic capacity of saplings. The six silvicultural treatments evaluated consisted of combinations of scarification (removal of entire humus layer), vegetation control (herbicide), and fertilization (slow release fertilizer). Competing vegetation (mainly Populus tremuloides Michx.) had negative impacts on shoot water potential (Ψx), leaf nitrogen, leaf mass per unit leaf area (LMA), height, and basal area of 7-year-old saplings. Net CO2 assimilation rate at light saturation (A; both on a mass and area basis) and stomatal conductance for water vapor (gwv) were not significantly influenced by the presence of competing vegetation. The only significant impact of competition on gas exchange variables was to decrease water-use efficiency (both instantaneous and long-term WUE as expressed by carbon isotope discrimination or Δ). Scarification significantly increased predawn Ψx, height, and basal area of suppressed saplings, and Aarea, gwv, and LMA of saplings subject to herbicide application. These positive impacts of scarification were attributed to enhanced root growth due to higher soil temperatures. No positive impact of fertilization was observed in either suppressed or open conditions. Leaf nitrogen and LMA were both driving variables for photosynthetic capacity of saplings across all conditions created by silvicultural treatments, but also within both suppressed and open conditions. This suggests that the dependency of the photosynthetic apparatus on leaf nitrogen and LMA occurs whether there is competing vegetation (and/or shade) or not. However, the gradient of light availability under suppressed conditions was accompanied by a gradient of soil temperature due to scarification. It would be pertinent to investigate more fully the interactions between light availability and soil temperature, both influencing root growth and leaf level morphology and physiology of young eastern white pine.