| The first objective of the thesis was to measure the growth response of eastern white pine (Pinus strobus L.) seedlings to silvicultural treatments in the context of the shelterwood system. A second objective was to determine the acclimation capacity, both morphological and physiological, of white pine seedlings to environmental conditions created by such a system, particularly after the regeneration cut. Two distinct experimental designs were used to address these objectives. The first one was a complete block factorial design that included three regeneration cut intensities in a mature white pine forest (Petawawa, Ontario), two levels of vegetation control using a chemical herbicide, and two levels of soil scarification. Results of this study demonstrate that maximum seedling growth was attained under the moderate regeneration cut intensity (33% residual basal area relative to that in the uncut control), with limited additional growth occurring under a more intense regeneration cut (17% residual basal area). Regression analyses reveal that growth increment also levelled off at 50% of full light. The highest level of regeneration cut promoted the invasion of competing vegetation, while the moderate regeneration cut intensity proved to be an efficient means of limiting the development of this competing vegetation. Measures of leaf-level photosynthetic capacity in seedlings reveal a weak physiological plasticity of white pine in response to increasing light availability, particularly under conditions of vegetation control. Biomass allocation patterns in seedlings indicate that white pine can compensate for this lack of plasticity by increased allocation to foliage. White pine seedlings were subjected to three levels of soil temperature and three levels of light availability in a second factorial experimental design under greenhouse conditions. These two factors interacted to affect both growth and physiological variables, indicating that soil temperature should be considered explicitly as a driving environmental factor under natural conditions when relating growth of white pine to available photosynthetic radiation. In conclusion, a moderate regeneration cut intensity combined with soil scarification is recommended as an optimum treatment combination to promote both growth and protection of white pine seedlings under a shelterwood system. |
