Exploring the link between clouds, radiation, and canopy productivity of tropical savannas

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

“…However, it has been observed that, as LAI drops (e.g. in savannahs and peat lands), the positive effect of diffuse light on primary productivity often becomes nonsignificant (Kanniah et al, 2012(Kanniah et al, , 2013. Assuming that a drop in LAI tends to correlate with a lower canopy-level extinction coefficient, our model suggests that the photosynthetic benefit of direct light may become more pronounced in low-LAI systems.…”

“…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.…”

Photosynthetic productivity and its efficiencies in ISIMIP2a biome models: benchmarking for impact assessment studies

“…Moreover, the forcing datasets did not contain a variable for the diffuse component of solar radiation. Several studies revealed the importance of diffuse radiation for the increased canopy radiation absorption and carbon assimilation (Roderick et al 2001, Gu et al 2002, Kanniah et al 2013, which would explain the low CO 2 growth rate after the huge volcanic eruption. By appropriately including solar radiation factors, several model studies have successfully simulated the change in terrestrial carbon budget after the volcanic eruption (Lucht et al 2002, Frölicher et al 2013.…”

The contribution of trees and grasses to productivity of an Australian tropical savanna

“…Yet diffuse radiation is known to penetrate the tree canopy more than direct radiation (Roderick et al, 2001), which should be beneficial to productivity. However, high variability in cloud cover and atmospheric aerosols in the wet and dry season changes the portion of direct and diffuse radiation reaching the land surface (Allen et al, 2008;Kanniah et al, 2013), which has a direct influence on available solar radiation to support productivity. A modelling study from Whitley et al (2011) suggests savanna productivity can be light limited due to a finite capacity of the vegetation to intercept light given limited canopy cover development.…”