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 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.

Excess Diffuse Light Absorption in Upper Mesophyll Limits CO2 Drawdown and Depresses Photosynthesis

“…Furthermore, we demonstrated that sun-grown sunflower leaves excessively absorb diffuse light in the upper palisade cells at the expense of light availability in the lower palisade and spongy mesophyll. Thus, the leaf-level photosynthetic response to diffuse light in sunflower occurred opposite to the expected canopylevel increases in LUE (Gu et al, 2002;Alton et al, 2007;Urban et al, 2007Urban et al, , 2012Alton, 2008;Kanniah et al, 2013;Williams et al, 2014;Cheng et al, 2015). We combined our leaf-level model with canopy-level observations of diffuse and direct light penetration to investigate this tradeoff (Fig.…”

“…A higher fraction of diffuse light tends to occur due to light-scattering particles in the atmosphere, such as clouds, aerosols, and anthropogenic emissions or volcanic ejecta (Mercado et al, 2009). At the canopy level, in both agricultural and natural systems, diffuse light illuminates more total leaf area, which has been repeatedly associated with increased light use efficiency (LUE [net primary productivity divided by absorbed photosynthetically active radiation]; Gu et al, 2002;Alton et al, 2007;Urban et al, 2007Urban et al, , 2012; Alton, 2008;Kanniah et al, 2013;Williams et al, 2014;Cheng et al, 2015) and an insignificant to positive effect on primary productivity (Kanniah et al, 2012). Although studies at the individual leaf level are less common, several lines of evidence suggest that leaf developmental environment underlies internal light absorption and subsequent photosynthetic responses to diffuse versus direct light.…”

Forest Ecosystem as a Source of Co2 During Growing Season: Relation to Weather Conditions

“…Data were collected as half-hourly averages (from 30 s readings). For further details on the eddy covariance system and micrometeorological measurements (Urban et al, 2012).…”

“…The locality is situated within a cool and humid area having high precipitation in the Moravian-Silesian Beskydy Mountains (Czech Republic, 49°30´N, 18°32´E, 800-900 m a.s.l. ; for detailed description of the site, Urban et al, 2012). The investigated forest was planted in 1981 with four-year-old Picea abies (L.) Karst.…”

“…They often demonstrate the impact of anomalies, such as drought (Schwalm et al, 2012), flood (Dušek et al, 2009), and volcanic eruption (Gu et al, 2003). Other studies have analysed responses of CO 2 uptake to different types of radiation (Knohl and Baldocchi, 2008;Niyogi et al, 2004;Oliphant et al, 2011;Urban et al, 2007Urban et al, , 2012, vapour pressure deficit, and temperature (Dewar et al, 1999;Pingintha et al, 2010). There are a few studies based on year-round data sets obtained on a daily basis (Dore et al, 2012).…”