| Summary: | Accumulated carbon uptake, apparent quantum yield (AQY) and light-saturated net CO 2 assimilation (A sat ) were used to assess the responses of photosynthesis to environmental conditions during spring for three consecutive years. Whole-tree chambers were used to expose 40-year-old field-grown Norway spruce trees in northern Sweden to an elevated atmospheric CO 2 concentration, [CO 2 ], of 700 µmol CO 2 mol -1 (C E ) and an air temperature ( T ) between 2.8 and 5.6°C above ambient T (T E ), during summer and winter. Net shoot CO 2 exchange (A net ) was measured continuously on 1-year-old shoots and was used to calculate the accumulated carbon uptake and daily A sat and AQY. The accumulated carbon uptake, from 1 March to 30 June, was stimulated by 33, 44 and 61% when trees were exposed to C E , T E , and C E and T E combined, respectively. Air temperature strongly influenced the timing and extent of photosynthetic recovery expressed as AQY and A sat during the spring. Under elevated T (T E ), the recovery of AQY and A sat commenced ㅂ days earlier and the activity of these parameters was significantly higher throughout the recovery period. In the absence of frost events, the photosynthetic recovery period was less than a week. However, frost events during spring slowed recovery so that full recovery could take up to 60 days to complete. Elevated [CO 2 ] stimulated AQY and A sat on average by ㅂ and ㅪ%, respectively, throughout the recovery period, but had minimal or no effect on the onset and length of the photosynthetic recovery period during the spring. However, AQY, A sat and A net all recovered at significantly higher T (average +2.2 °C) in T E than in T A , possibly caused by acclimation or by shorter days and lower light levels during the early part of the recovery in T E compared with T A . The results suggest that predicted future climate changes will cause prominent stimulation of photosynthetic CO 2 uptake in boreal Norway spruce forest during spring, mainly caused by elevated T, but also elevated [CO 2 ]. ...
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