Effects of frost on shaded and exposed spruce and pine seedlings planted in the field

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Abstract
Spruce (Piceaabies L.) and pine (Pinussylvestris L.) seedlings were planted at different positions relative to artificial tree shades. Two sites in northern Sweden known as difficult to regenerate owing to frosts during the growing season were used. No deficiency of soil water was recorded during the growing season and all seedlings experienced similar temperature conditions during night frosts. Exposed seedlings received on clear days 7 h more of direct solar radiation than did shaded seedlings. After the first severe night frost in August, discoloration of needles and a decrease in chlorophyll concentration as well as lowered photosynthetic rates were more pronounced for the exposed seedlings. This was more obvious for spruce than pine seedlings. The differences in radiant load seem to be of ultimate importance for the results observed. It is proposed that photoinhibition and subsequent photooxidation of planted seedlings occur in the field when severe night frosts are followed by clear days with high irradiance.