Control of atmospheric particles on diffuse radiation and terrestrial plant productivity

Abstract

Terrestrial plant productivity tends to increase under increasing but non-saturating photosynthetically active solar radiation when water, temperature and nutrients are not limiting. However, studies have shown that photosynthesis can also be higher under enhanced diffuse light despite a decrease in total irradiance. Clouds and atmospheric aerosols are two important variables that determine the total and proportion of diffuse light reaching the surface and thereby the rate of photosynthesis and carbon accumulation in plants. In addition to these factors, the response of plants to diffuse radiation is also dependant on plant characteristics such as functional types, leaf physiology, leaf area, leaf inclination, canopy structure and shape (i.e. clumping). Local environmental conditions (i.e. temperature, soil moisture, vapour pressure deficit, etc.) then modulate these plant responses. Changes in solar radiation as a consequence of clouds and aerosols thus can modify the carbon balance of terrestrial ecosystems. Therefore, understanding the role of solar radiation in terrestrial carbon processes has become one of the goals in terrestrial carbon cycle studies. It can help to identify the control and mechanisms of carbon processes and determines the geographical and temporal distribution of the major pools and fluxes in the global carbon cycle. Here we review the role of clouds and aerosols in partitioning solar radiation and their interactions with carbon processes of terrestrial plants. We also focus our review on vegetation characteristics that control the impact of radiation partitioning on vegetation carbon processes and the role of modelling approach to study this impact. We identify gaps in this field of research and further propose recommendations to bridge the gap.

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

“…9). These predictions align with previous observations of the diffuse light enhancement of primary productivity in agricultural and natural systems (Alton, 2008;Kanniah et al, 2012). However, it has been observed that, as LAI drops (e.g.…”

“…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.…”

“…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.…”

“…Given anticipated changes in fog and cloud cover in many places globally (Johnstone and Dawson, 2010;Brient and Bony, 2013), along with rising levels of aerosols (Carslaw et al, 2013), substantial research efforts have gone into improving predictions of how LUE is influenced by diffuse light (Kanniah et al, 2012). 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. Whether increases in LUE under diffuse light result in higher primary productivity, however, is unclear, as lower total surface irradiance tends to accompany diffuse light conditions (Kanniah et al, 2012). Moreover, a lack of consideration for plant phenology associated with seasonal changes in LAI and photosynthetic capacity has confounded most previous studies (for exceptions, see Li et al, 2014;Williams et al, 2014), potentially leading to a substantial overestimation of diffuse light photosynthetic enhancement at the canopy level (Williams et al, 2016).…”