Summary: 1. 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 CO₂ assimilation rates of the uppermost shoots (5th and 8th whorl) occurred in response to irradiance on sunny days, characterized by significant decreases in CO₂ 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.

Tropospheric concentrations of phytotoxic ozone (O₃) have undergone a great increase from preindustrial 10–15 ppbv to a present-day concentration of 35–40 ppbv in large parts of the industrialised world due to increased emissions of O₃ precursors including NO_x, CO, CH₄ and volatile organic compounds. The rate of increase in O₃ concentration ranges between 1 ppbv per decade in remote locations of the Southern hemisphere and 5 ppbv per decade in the Northern hemisphere, where largest sources of O₃ precursors are located. Molecules of O₃ penetrating into the leaves through the stomatal apertures trigger the formation of reactive oxygen species, leading thus to the damage of the photosynthetic apparatus. Accordingly, it is assumed, that O₃ increase reduces the terrestrial carbon uptake relative to the preindustrial era. Here we summarise the results of previous manipulative experiments in laboratory growth cabinets, field open-top chambers and free-air systems together with O₃ flux measurements under natural growth conditions. In particular, we focus on leaf-level physiological responses in trees, variability in stomatal O₃ flux and changes in carbon fluxes and biomass production in forest stands. As the results reported in the literature are highly variable, ranging from negligible to severe declines in photosynthetic carbon uptake, we also discuss the possible interactions of O₃ with other environmental factors including solar radiation, drought, temperature and nitrogen deposition. Those factors were found to have great potential to modulate stomata openness and O₃ fluxes.