Morning, noon, or afternoon:: does timing of direct radiation influence the growth of Picea abies seedlings in mountain forests?

We studied the influence of the timing of direct sunlight on the growth of Picea abies seedlings in a field experiment on a north-facing slope in the subalpine zone of the Swiss Alps. Vertical walls were established to expose three- year- old P. abies seedlings to direct sunlight at different times of day (morning, noon, afternoon), and to diffuse light only, during two growing seasons. The light treatments were chosen in order to simulate microsites in forest gaps with different orientations. While the light treatments resulted in different daily soil temperature curves, they affected neither average growing season soil temperature nor the frequency of soil temperatures above 10 degrees C, which are assumed to be particularly beneficial for seedling growth. Final seedling biomass was unaffected by the timing of direct sunlight, but lower for seedlings receiving diffuse light only. These findings suggest that the orientation of slit- shaped gaps in subalpine forests to promote P. abies seedling growth is unimportant.