The effect of humidity on photosynthesis and water relations of white spruce seedlings during the early establishment phase

Newly transplanted, 3-year-old bare-root white spruce (Piceaglauca (Moench) Voss) seedlings were grown in three levels of absolute humidity difference (AHD), under water-stressed and well-watered conditions in growth chambers. Stomatal conductance to CO2 as well as net assimilation were more than twice as high in the low AHD treatment (6.8 μg H2O•cm−3) as in the high AHD treatment (21.0 μg H2O•cm−3). Transpiration rates were uniform among humidity treatments, but in the low AHD treatment water use efficiency was more than double that of the high AHD treatment. Water use efficiency was greatest in the low AHD conditions, in the water-stressed seedlings, and during the time immediately after planting. There were no differences in the number of new roots produced among humidity treatments, despite the different levels of photosynthesis. In the stress treatment, bud flush of seedlings in the high AHD treatment occurred 3 days later than in the low AHD treatment. A field study on the effects of varying aspen (Populustremuloides Michx.) canopies on humidity levels showed that AHD under partial and full canopies was lower than that of the clearcut. Results suggest that silvicultural treatments that promote higher humidity levels on planting sites should improve white spruce seedling photosynthesis immediately after planting.