Ecophysiological controls over the net ecosystem exchange of mountain spruce stand. Comparison of the response in direct vs. diffuse solar radiation

Abstract

Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25-year-old, leaf area index 11 m 2 m À2 ) during two successive 7-day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO 2 to PPFD. NEE and stomatal conductance at the canopy level (G canopy ) was estimated from halfhourly eddy-covariance measurements of CO 2 and H 2 O fluxes. In addition, daily courses of CO 2 assimilation rate (A N ) and stomatal conductance (G s ) at shoot level were measured using a gas-exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer.We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun-acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves.Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels.

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.…”

“…Deeper diffuse light penetration along canopy depth has been suggested as the primary mechanism underlying the corresponding increases of canopy-level LUE (Urban et al, 2007;Li et al, 2014). This mechanism is further supported by observations of increasing LUE with higher leaf area index (LAI; Greenwald et al, 2006;Alton et al, 2007), which varies with genotype/species differences in leaf morphology, orientation, and distribution.…”

“…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.…”

Photosynthetic response of European beech to atmospheric and soil drought

“…To test whether the changes in the CO 2 assimilation rate (P N ) were caused by the changes in stomatal conductance (g S ), assimilation rate was plotted against conductance (Urban et al 2007). The relationship of P N to g S was very tight in both groups (Fig.…”

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

“…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).…”