Interactive Effect of Elevated CO2 and Reduced Summer Precipitation on Photosynthesis is Species-Specific: The Case Study with Soil-Planted Norway Spruce and Sessile Oak in a Mountainous Forest Plot

John Grace,Thomas Agyei,Karel Klem,Petr Holub,Otmar Urban,Kojo Kwakye Ofori-Amanfo,Michal V. Marek,Barbora Veselá

doi:10.3390/f12010042

Abstract

We investigated how reduced summer precipitation modifies photosynthetic responses of two model tree species—coniferous Norway spruce and broadleaved sessile oak—to changes in atmospheric CO2 concentration. Saplings were grown under mountainous conditions for two growing seasons at ambient (400 μmol CO2 mol–1) and elevated (700 μmol CO2 mol–1) CO2 concentration. Half were not exposed to precipitation during the summer (June–August). After two seasons of cultivation under modified conditions, basic photosynthetic characteristics including light-saturated rate of CO2 assimilation (Amax), stomatal conductance (GSmax), and water use efficiency (WUE) were measured under their growth CO2 concentrations together with in vivo carboxylation rate (VC) and electron transport rate (J) derived from CO2-response curves at saturating light. An increase in Amax under elevated CO2 was observed in oak saplings, whereas it remained unchanged or slightly declined in Norway spruce, indicating a down-regulation of photosynthesis. Such acclimation was associated with an acclimation of both J and VC. Both species had increased WUE under elevated CO2 although, in well-watered oaks, WUE remained unchanged. Significant interactive effects of tree species, CO2 concentration, and water availability on gas-exchange parameters (Amax, GSmax, WUE) were observed, while there was no effect on biochemical (VC, J) and chlorophyll fluorescence parameters. The assimilation capacity (Asat; CO2 assimilation rate at saturating light intensity and CO2 concentration) was substantially reduced in spruce under the combined conditions of water deficiency and elevated CO2, but not in oak. In addition, the stimulatory effect of elevated CO2 on Amax persisted in oak, but completely diminished in water-limited spruce saplings. Our results suggest a strong species-specific response of trees to reduced summer precipitation under future conditions of elevated CO2 and a limited compensatory effect of elevated CO2 on CO2 uptake under water-limited conditions in coniferous spruce.

Highlights

Forest ecosystems play dominant roles in the ecology and evolution of life on the Earth.Forests have a critical function in regulating the global climate through their effects on the biogeochemical cycles, especially the cycles of carbon and water [1]
Pre-dawn and noontime leaf water potentials were significantly influenced by tree species (TS) and water availability (WA), but CO2 treatment had significant effect only on Ψpre-dawn (Table 1)
Our experiment showed that photosynthesis of Q. petraea was stimulated by elevated [CO2], but the stimulatory effect of elevated [CO2] diminished in P. abies under the WL conditions

Summary

Introduction

Forests have a critical function in regulating the global climate through their effects on the biogeochemical cycles, especially the cycles of carbon and water [1]. Since CO2 serves as a substrate for photosynthesis, atmospheric [CO2] strongly influences plant physiology and growth. The effects of elevated [CO2] on trees have received great attention [3,4,5,6], investigations of combined effects with other environmental factors are still insufficient to predict future responses, as several other factors are likely to co-vary with the increase in [CO2]. Global climate models suggest an increasing probability of changes in the seasonal pattern of precipitation and severe drought periods in future decades [1,2]. It is important to evaluate the sensitivity of trees grown under elevated [CO2] to limited water availability and to understand the physiological mechanisms contributing to their adaptation capacity

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