Summary — Containerized black spruce (Picea mariana [Mill] BSP) seedlings of 3 sizes (heights of 18, 35 and 45 cm) were placed in growth chambers and subjected to conditions of low evaporative (20°C, 60% RH) or high evaporative (30°C, 40% RH) demand, with 3 levels of soil water availability in each environment. The large seedlings had the highest rate of net photosynthesis in the cooler environment, but showed the greatest reduction in net photosynthetic rate in the warmer and drier environment, under conditions of limited water supply. The small seedlings were least affected by the warmer and drier environment in which they maintained photosynthetic rates higher than those of the larger seedlings. The decrease in net photosynthesis experienced by the large seedlings in the warmer and drier environment under conditions of limited water availability was associated with a large decrease in stomatal conductance. However, the maintenance of a high level of intercellular CO 2 concentration suggests that most of the limitations to net photosynthesis were of non-stomatal origin. Water content of the root plug was also reduced by increased seedling size, but the differences were least evident under conditions that produced the largest differences in net photosynthetic rates. These results, obtained under controlled conditions, suggest that after outplanting, large seedlings would experience greater reduction in growth than smaller ones only under conditions of high evaporative demand and low water availability.

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