Spring photosynthetic recovery of boreal Norway spruce under conditions of elevated [CO2] and air temperature

Accumulated carbon uptake, apparent quantum yield (AQY) and light-saturated net CO₂ 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 CO₂ concentration, [CO₂], of 700 µmol CO₂ 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 CO₂ 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_satcommenced 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 [CO₂] 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 CO₂ uptake in boreal Norway spruce forest during spring, mainly caused by elevated T, but also elevated [CO₂]. However, the effects of elevated T may not be linearly extrapolated to future warmer climates.