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

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 (P$_N$), water vapour stomatal conductance (g$_s$), and intrinsic water-use efficiency (WUE$_i$) of Norway spruce. More than 11 000 gas-exchange measurements during the summer of 2018 revealed that branch position significantly affects gas exchange and WUE$_i$ of juvenile Norway spruce trees. Northern branches showed on average 21% increased P$_N$, 35% higher g$_s$, and 8% lower WUE$_i$ compared to the southern branches (across T and VPD conditions). The P$_N$ and g$_s$ differences between the branches were temperature- and VPD-dependent. We observed the negative impact of raising temperature on gas exchange and WUE$_i$ for both treatments, with a 40°C threshold causing a rapid decline in WUE$_i$. Variability of the southern branches\u27 WUE$_i$ at 42°C was abruptly increased due to the decoupling of P$_N$ and g$_s$ (low P$_N$, high g$_s$). Surprisingly, raising VPD showed no significant impact on WUE$_i$ 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

Adaptive variation in physiological traits of beech provenances in Central Europe

Current climate changes can led to a decline of local beech populations fully adapted to previous climate conditions. In this context, the issue of variation in adaptive traits becomes important. A field experiment with 18-year-old trees of Fagus sylvatica L. was conducted on provenance plot located in Tále (Central Slovakia), where physiological responses of five beech provenances originating from contrasting sites along an altitudinal gradient from 55 to 1100 m a.s.l. across the range of the natural beech distribution were studied. Stomatal characteristics, parameters of chlorophyll a fluorescence and gas exchange parameters were determined. Overall, we observed a significant increase in physiological performance at the leaf level with increasing altitude of origin. Provenances from the higher altitudes showed higher CO2 assimilation rate, stomatal density, potential conductance indices and photochemical efficiency, and lower capability for dissipation of energy by heat. A similar pattern of response was recorded in relation to the precipitation regime of sites of origin. Moreover, in the context of the temperature regime, several negative trends were observed