Summary1. Cloud cover affects carbon exchange between biota and the atmosphere. Recent studies have demonstrated that an increase in the diffuse radiation fraction enhances the photosynthetic efficiency of canopies. Although the exact mechanism behind this effect is not clear, a more even distribution of light among leaves across the vertical profile of the canopy is considered to be the most important cause of this difference. 2. To test this hypothesis, the net ecosystem production (NEP) of a Norway spruce forest (30-year-old) was measured under cloudy and sunny skies by the eddy covariance method. In parallel, measurements of the diurnal courses of gas exchange and chlorophyll fluorescence parameters were made in the upper sun (5th whorl; 1-year-old needles), middle (8th and 10th whorl; 1-and 2-year-old needles) and lower shade (15th whorl; >2-year-old needles) shoots. 3. The higher diffuse radiation fraction during cloudy days resulted in significantly higher ecosystem carbon uptake than at corresponding incident photosynthetic photon flux density on sunny days. Our shoot-level data show that shoots from deep within the canopy contribute substantially to the overall carbon balance during cloudy days. But, although shade-adapted shoots had a markedly positive carbon balance over a 24-h period on cloudy days, their performance was impaired on sunny days contributing only a marginal or even negative carbon balance from the middle and shaded parts of the canopy. The uppermost sun shoots contributed 78% of the total carbon assimilated during a sunny day, but only 43% during a cloudy day. 4. In addition, afternoon depression of canopy NEP and CO 2 assimilation rates of the uppermost shoots (5th and 8th whorl) occurred in response to irradiance on sunny days, characterized by significant decreases in CO 2 uptake and apparent quantum yield; however, this depression did not occur under cloudy conditions. Stomatal and non-stomatal regulations of carbon assimilation in the afternoon are discussed.

It has been suggested that atmospheric CO2 concentration and frequency of cloud cover will increase in future. It remains unclear, however, how elevated CO2 influences photosynthesis under complex clear versus cloudy sky conditions. Accordingly, diurnal changes in photosynthetic responses among beech trees grown at ambient (AC) and doubled (EC) CO2 concentrations were studied under contrasting sky conditions. EC stimulated the daily sum of fixed CO2 and light use efficiency under clear sky. Meanwhile, both these parameters were reduced under cloudy sky as compared with AC treatment. Reduction in photosynthesis rate under cloudy sky was particularly associated with EC-stimulated, xanthophyll-dependent thermal dissipation of absorbed light energy. Under clear sky, a pronounced afternoon depression of CO2 assimilation rate was found in sun-adapted leaves under EC compared with AC conditions. This was caused in particular by stomata closure mediated by vapour pressure deficit.

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Elevated CO2 instantaneously increases temperature optima of Amax due to reduced photorespiration and limitation of photosynthesis by RuBP regeneration. However, this increase disappears when plants are exposed to identical CO2 concentrations. In addition, increased heat-stress tolerance of primary photochemistry in plants grown at elevated CO2 is unlikely. The hypothesis that long-term cultivation at elevated CO2 leads to acclimation of photosynthesis to higher temperatures is therefore rejected. Nevertheless, incorporating acclimation mechanisms into models simulating carbon flux between the atmosphere and vegetation is necessary.

Natural fluctuations in light intensity may significantly affect the amount of CO assimilated by plants and ecosystems. Little is known, however, about the interactive effect of dynamic light conditions and atmospheric CO concentrations. The hypothesis that elevated CO concentration (EC; 700 μmol CO mol) increases photosynthetic efficiency in dynamic light environments as compared to ambient CO concentration (AC; 385 μmol CO mol) was tested. Sun leaves of European beech ( L.) and current-year shoots of Norway spruce [ (L). Karst.] were exposed to five dynamic light regimes (LRs) occurring within forest canopies due to variable cloud cover or self-shading of leaves and to a steady-state LR. The LRs differed in the time course of incident irradiance, whereas the overall duration (600 s) and total amount of radiation (35.88 mmol photons m) were the same in all LRs. The EC treatment enhanced the amount of CO assimilated under all LRs tested. While the stimulation was only 37 to 50% in beech, it was 52 to 85% in spruce. The hypothesis that photosynthetic efficiency is stimulated by EC was confirmed in LRs when the leaves were pre-exposed to low light intensity and photosynthetic induction was required. By contrast, only a minor effect of EC treatment was found on the rate of induction loss and postillumination CO fixation in both species studied.