Impact of clear and cloudy sky conditions on the vertical distribution of photosynthetic CO2 uptake within a spruce canopy

Abstract

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 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.