The combined effect of branch position, temperature, and VPD on gas exchange and water-use efficiency of Norway spruce

Abstract

The impact of climate change on the physiological processes of Norway spruce in Central Europe is a significant concern. The increased temperature and evaporative demand associated with climate change may negatively affect its photosynthesis and carbon-water balance. This study tests the combined effect of branch position, temperature (T) and water vapour pressure deficit (VPD) on net photosynthetic rate (PN), water vapour stomatal conductance (gs), and intrinsic water-use efficiency (WUEi) of Norway spruce. More than 11 000 gas-exchange measurements during the summer of 2018 revealed that branch position significantly affects gas exchange and WUEi of juvenile Norway spruce trees. Northern branches showed on average 21% increased PN, 35% higher gs, and 8% lower WUEi compared to the southern branches (across T and VPD conditions). The PN and gs differences between the branches were temperature- and VPD-dependent. We observed the negative impact of raising temperature on gas exchange and WUEi for both treatments, with a 40°C threshold causing a rapid decline in WUEi. Variability of the southern branches' WUEi at 42°C was abruptly increased due to the decoupling of PN and gs (low PN, high gs). Surprisingly, raising VPD showed no significant impact on WUEi of Norway spruce. The results of this study provide necessary information for upscaling and process-based modelling of whole-crown gas exchange. Moreover, experimental studies of gas exchange should take into consideration the branch position effect to prevent possible bias errors.

Keywords

• intrinsic water-use efficiency
• photosynthesis
• picea abies
• stomatal conductance