The effect of elevated CO2 on photosynthesis is modulated by nitrogen supply and reduced water availability in Picea abies.

It is assumed that the stimulatory effects of elevated CO2 concentration ([CO2]) on photosynthesis and growth may be substantially reduced by co-occurring environmental factors and the length of CO2 treatment. Here, we present the study exploring the interactive effects of three manipulated factors ([CO2], nitrogen supply and water availability) on physiological (gas-exchange and chlorophyll fluorescence), morphological and stoichiometric traits of Norway spruce (Picea abies) saplings after 2 and 3 years of the treatment under natural field conditions. Such multifactorial studies, going beyond two-way interactions, have received only limited attention until now. Our findings imply a significant reduction of [CO2]-enhanced rate of CO2 assimilation under reduced water availability which deepens with the severity of water depletion. Similarly, insufficient nitrogen availability leads to a down-regulation of photosynthesis under elevated [CO2] being particularly associated with reduced carboxylation efficiency of the Rubisco enzyme. Such adjustments in the photosynthesis machinery result in the stimulation of water-use efficiency under elevated [CO2] only when it is combined with a high nitrogen supply and reduced water availability. These findings indicate limited effects of elevated [CO2] on carbon uptake in temperate coniferous forests when combined with naturally low nitrogen availability and intensifying droughts during the summer periods. Such interactions have to be incorporated into the mechanistic models predicting changes in terrestrial carbon sequestration and forest growth in the future.

Ozone flux and ozone deposition in a mountain spruce forest are modulated by sky conditions.

In order to understand the main driving factors of ozone (O3) deposition we tested the hypothesis that sky conditions (cloudy, partly cloudy, and clear sky) modulate O3 flux in forest ecosystems via stomatal regulation. The hypothesis is based on the fact that complex microclimate conditions under cloudy sky usually stimulate stomatal conductance. O3 fluxes were inferred from a concentration gradient in a mountainous Norway spruce forest in the Czech Republic (Central Europe) for years 2012-2016 and measured directly by eddy-covariance during the summer of 2017. Daily and seasonal O3 depositions were calculated separately for days with cloudy, partly cloudy, and clear sky conditions. The data show unequivocally that more O3 is taken up under cloudy and partially cloudy skies. Moreover, we found significant interactive effects of sky conditions and season on O3 flux. Though there are other mechanisms and pathways involved in the transport of O3 to the plant-soil system, the highest O3 deposition was associated to the highest stomatal conductance during partly cloudy and cloudy sky conditions in all seasons, while lower O3 ecosystem fluxes were observed under clear sky conditions despite the highest O3 concentrations at this time. These findings suggest that forests growing at sites where conditions are predominantly cloudy are expected to deposit higher extent of O3 than less-cloudy forests being thus more threatened by phytotoxic O3.