Seasonal variations of photosynthetic capacities of white spruce (Picea glauca) and jack pine (Pinus banksiana) saplings

Abstract

Seasonal photosynthetic capacity (maximum rate of net photosynthesis at saturating light) was assessed in 30-year-old open-grown Pinus banksiana Lamb, and 20-year-old open-grown and understory Picea glauca (Moench) Voss in central Alberta. Photosynthesis commenced in early April despite cold soils (0 °C) and night frosts. It fluctuated greatly in the summertime during the periods of summer droughts and stopped abruptly in late October when night air temperature dropped below −10 °C. In comparing seasonal maximums, there was proportionally lower photosynthetic capacity in Pinus banksiana than in Picea glauca in the spring and autumn; however, in the summer, photosynthetic capacity in Pinus banksiana was less variable than in Picea glauca. The fluctuation of photosynthetic capacity in understory Picea glauca saplings was greatly reduced in the summertime compared with open-grown saplings. The data suggest that Picea glauca is able to use the periods of high light in the understory when the aspen is leafless by quickly regaining photosynthetic capacity in the spring and maintaining photosynthesis well into the late autumn. Key words: Picea glauca, Pinus banksiana, photosynthesis, season.