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Atmospheric science: Pinatubo, diffuse light, and the carbon cycle
Abstract
Similar to clouds, volcanic eruptions increase the proportion of diffuse light reaching Earth's surface. As
Farquhar and Roderick
show in their Perspective, this change in the geometry (rather than intensity) of light can have a profound influence on photosynthesis
and the carbon cycle. They highlight the research article by
Gu
et al
., who have measured changes in net COexchange following the eruption of Mt. Pinatubo in 1991. Volcanoes, pollution, and greenhouse
gases may all increase diffuse light, with important consequences for Earth's carbon cycle and climate.
... LVEs modify the global climate by sending large amounts of sulfate aerosols into the stratosphere where they can persist for 1-3 years and spread over the globe (Robock, 2000). Ensuing increased light scattering leads to a short-term decrease in total solar radiation and a parallel large increase in the diffuse light fraction (DF) reaching the Earth surface (Farquhar, 2003). Reduced total radiation in turn results in longer-term cooling of the troposphere that typically lasts about a decade (Stoffel et al., 2015). ...
... These models nevertheless often have a simplistic representation of vegetation C allocation and differ in their representation of DF effect on photosynthesis, both processes being likely fundamental to vegetation response to LVEs. Higher DF following LVEs has long been hypothesized to improve forest gross primary productivity (GPP) worldwide, due to increased light distribution in the canopy and overall enhanced light use efficiency (Farquhar, 2003;Roderick et al., 2001). Eddy covariance estimates of GPP under different light conditions at numerous forest sites provide ample evidence that increased DF generally leads to a fertilization effect on stand-level photosynthesis (Kanniah et al., 2012;Knohl & Baldocchi, 2008;Rocha et al., 2004), and models suggest that this effect could yield substantial GPP gains at the global scale, potentially explaining as much as half of the effect of recent LVEs on the land C sequestration (Mercado et al., 2009;O'Sullivan et al., 2021;Rap et al., 2018). ...
... Namely, during the first 4 years following LVEs and compared with the following period, RW increased more at warmer sites, where a larger photosynthesis increase was found, but less at drier sites, where photosynthesis fertilization was likely limited. Together, these results provide empirical evidence of a positive effect of DF fertilization of photosynthesis on tree growth (Farquhar, 2003) and suggest additive interaction of C source and sink activities (see Figures S6 and S7 for other tree-ring proxies). Overall, our results nevertheless indicate temporally limited source control of tree growth response to LVEs and highlight the dominant and widespread role of sink limitations over extratropical Northern Hemisphere forests. ...
Forest productivity projections remain highly uncertain, notably because underpinning physiological controls are delicate to disentangle. Transient perturbation of global climate by large volcanic eruptions provides a unique opportunity to retrospectively isolate underlying processes. Here, we use a multi‐proxy dataset of tree‐ring records distributed over the Northern Hemisphere to investigate the effect of eruptions on tree growth and photosynthesis and evaluate CMIP6 models. Tree‐ring isotope records denoted a widespread 2–4 years increase of photosynthesis following eruptions, likely as a result of diffuse light fertilization. We found evidence that enhanced photosynthesis transiently drove ring width, but the latter further exhibited a decadal anomaly that evidenced independent growth and photosynthesis responses. CMIP6 simulations reproduced overall tree growth decline but did not capture observed photosynthesis anomaly, its decoupling from tree growth or the climate sensitivities of either processes, highlighting key disconnects that deserve further attention to improve forest productivity projections under climate change.
... In particular, the directional composition of sunlight plays a pivotal role in light distribution (Gourdriaan and Van Larr, 1994). Under diffuse light, light distribution on plants tends to be more homogeneous (Farquhar and Roderick, 2003). The combined effect of the homogeneity of light distribution and the non-linear light saturation response of C 3 plant photosynthesis generally increases canopy light use efficiency (LUE) (Hollinger et al., 1994;Choudhury, 2001;Gu et al., 2002) and subsequently plant growth (Li et al., 2014a(Li et al., , 2014b. ...
... Under diffuse covering materials, light profiles became more homogeneous (Li et al., 2014a), and crop growth and yield also increased (Hemming et al., 2007;Li et al., 2014aLi et al., , 2014b. To evaluate the effect of diffuse light on plants, estimations with mathematical models have been used (Gu et al., 2002;Farquhar and Roderick, 2003) and meaningful results on light interception and carbon fixation of terrestrial ecosystem (Cheng et al., 2015;Lee et al., 2018) and field crops (Choudhury, 2000(Choudhury, , 2001Williams et al., 2016) have been obtained. However, the mathematical approach is challenging to apply to greenhouses owing to the complexity of greenhouse structures and the complicated optical interactions between structural components and plants. ...
... The light profile on plants tends to be homogeneous under diffuse light (Farquhar and Roderick, 2003). Similarly, the light profile becomes homogeneous under highly diffusive films showing a lower standard deviation of absorbed light intensities under HILD, but not under LIHD (Fig. 10b). ...
Diffuse fraction, which can be increased by using diffuse films, has been considered to influence light interception and photosynthesis of crops in greenhouses. However, quantifying the influence of diffuse films is challenging owing to the complicated optical interactions between climatic factors inside and outside greenhouses. Thus, versatile methods for evaluating the effect of diffuse films are required. The objective of this study was to evaluate the effect of diffuse films on the improvement of the light profile and photosynthesis of tomatoes in greenhouses according to film diffuseness and regional solar radiation using ray-tracing simulation. The structural and optical properties of the greenhouse components were applied in a 3D-framework combined with a ray-tracing module. The light transmission patterns of diffuse films and solar radiation properties were incorporated. The reliability of the simulation was confirmed by comparing measured and estimated irradiances inside greenhouses covered with films having different haze factors. For scenarios, the diffuse film efficiency was assessed under typically different solar radiations, a low irradiance, high diffuse radiation fraction (LIHD) and a high irradiance, low diffuse radiation fraction (HILD). The light interception was estimated through the simulation and used to calculate the photosynthesis using the Farquhar-von Caemmerer-Berry model. The simulation was found to be reliable with R² of 0.95 and 0.94 for the two greenhouses covered with different diffuse films. The light distribution on the tomato plants were less affected by film diffuseness under LIHD than HILD. With increasing film diffuseness, carbon uptake and light use efficiency increased by 5.30% and 4.58% under HILD, but did not change under LIHD. The light distribution and photosynthesis in diffuse film-covered greenhouses under different light environments could be reasonably estimated by the simulation. Thus, this method can be used to evaluate the applicability of diffuse films to various regions with diverse meteorological characteristics.
... However, in reality, due to environmental stress factors, such as temperature and water availability, ε max deviates from its theoretical value (Monteith 1972;Pei et al. 2022;Yuan et al. 2014). As a key input variable in LUE models, the influence of radiation intensity must be considered from multiple perspectives (Farquhar and Roderick 2003;He et al. 2013). Previous LUE models, such as EC-LUE (Yuan et al. 2010), TCF (He et al. 2016), and CI-EF (de Almeida et al. 2018), overlook the impact of radiation intensity on LUE by simply assuming a linear relationship between radiation intensity and LUE, which results in inaccurate model prediction. ...
Accurately quantifying terrestrial gross primary productivity (GPP) can provide insights into the dynamics of global ecosystems. Light Use Efficiency (LUE) models with simple and effective structures have been widely used to estimate GPP. However, GPP estimation using LUE models was biased due to inaccurately representing the nonlinear effect of solar radiation on photosynthesis. To address this issue, we developed a hybrid LUE model, termed LUE-MLP, which integrates a multilayer perceptron (MLP) neural network simulating the relationship between GPP and solar radiation with a big-leaf LUE model. LUE-MLP was optimized and tested for GPP simulation on data from 166 flux sites from the FLUXNET2015 dataset, which covers 10 biome types. Results show that LUE-MLP exhibited a reliable performance (R2 = 0.76 and RMSE = 4.17 μmol m⁻² s⁻¹) in estimating (half-)hourly GPP on test data. Compared to the original big-leaf and two-leaf LUE models, LUE-MLP's R² (RMSE) for (half-)hourly GPP simulation across biomes rose (fell) by – 0.01-0.13 (−7.6%−16.8%) and 0.01-0.15 (0.39%−29.1%), respectively. Additionally, LUE-MLP well captured the seasonal variations in tower-based GPP. The proposed model can improve GPP estimation on a global scale, enhancing our understanding of global ecosystem dynamics.
... Although much work has been devoted to 61 understanding Mt. Pinatubo's impacts on plant productivity, the literature has been dominated by 62 studies focusing on impacts from changes to the quantity and quality of incoming solar radiation 63 (Farquhar and Roderick, 2003;Gu et al., 2002Gu et al., , 2003Jones and Cox, 2001;Robock, 2005). However, rainfall alone provides an incomplete understanding of the drought conditions relevant 79 for plant productivity; a rainfall deficit could in principle be overcome by moisture stored in soil. ...
Large volcanic eruptions can significantly affect the state of the climate, including stratospheric sulfate concentrations, surface and top-of-atmosphere radiative fluxes, stratospheric and surface temperature, and regional hydroclimate. The prevalence of higher natural variability in how the regional rainfall responds to the volcanic-induced climate perturbations creates a knowledge gap in understanding of how eruptions affect ecohydrological conditions and plant productivity. Here we will explore the understudied store (soil moisture) and flux (evapotranspiration) of water as the short-term ecohydrological control over plant productivity in response to the 1991 eruption of Mt. Pinatubo. We used the NASA’s Earth system model for modeling of the 1991’s Mt. Pinatubo eruption and detection of hydroclimate response. The model simulates a radiative perturbation of -5 Wm-2 and mean surface cooling of ~ 0.5 °C following the Mt. Pinatubo eruption in 1991. The rainfall response is spatially heterogenous, due to dominating variability, yet still shows suppressed rainfall in the northern hemisphere after the eruption. We find that up to 10–15 % of land regions show a statistically significant agricultural response. Results confirm that these higher-order impacts successfully present a more robust understanding of inferred plant productivity impacts. Our results also explain the geographical dependence of various contributing factors to the compound response and their implications for exploring the climate impacts of such episodic forcings.
... This change enhances the frequency of light saturation, making the canopy more responsive to variations in radiation levels, thereby increasing photosynthesis. [25,41]. The growing cloud cover up to a certain extent simultaneously decreases temperature, which in turn reduces the ecosystem respiration [68,85]. ...
A five-year study on net ecosystem exchange (NEE) to assess the impact of climatic drivers on inter-annual variability (IAV) was conducted using eddy covariance measurements in a lowland tropical rice–rice system. The IAV of carbon dioxide fluxes in two crop growing phases, i.e., dry and wet seasons along with fallow periods, was analyzed. The gross primary productivity (GPP) flux builds up during the crop growing season with corresponding increase in ecosystem respiration (RE) during the study period. Annual cumulative value of NEE was negative (sink) in both the crop growing seasons. The variability of climate drivers and changes in the ecosystem responses to drivers revealed a large intra-annual as well as inter-annual variability of net ecosystem fluxes. The NEE was correlated with different meteorological variables such as photosynthetically active radiation (PAR), precipitation, air temperature and soil temperature. The anomalies of NEE, GPP and RE were observed to be less in 2017 and 2018, which may be due to lower temperature anomalies recorded in these years. This work improves further understanding of biological mechanisms which are involved in the variation of climatological variables to improve our ability to predict future IAV of NEE.
... In addition, diffuse radiation also plays an important role to terrestrial carbon uptake due to radiative disturbances of aerosols and cloud , Jarvis et al 1985, Hollinger et al 1994, Freedman et al 2001, Gu et al 2002, Alton et al 2007. Observations have shown that diffuse radiation can help increase the light use efficiency of canopy and therefore promote ecosystem GPP (Farquhar and Roderick 2003, Gu et al 2003, Cirino et al 2014, Yang et al 2019, Zhou et al 2022. Nevertheless, the impact of diffuse radiation on GPP variability is not fully explored due to the lack of diffuse radiation datasets with wide spatial coverage and high temporal frequency (Zhou et al 2021c). ...
Climate exerts both short-term and long-term impacts on the ecosystem carbon assimilation. However, the main climatic drivers for the variability of gross primary productivity (GPP) remain unclear across various timescales and vegetation types. Here, we combine the state-of-the-art machine learning algorithms with a well-established explanatory method to explore the impacts of climatic factors on long-term GPP variability at global FLUXNET sites across four timescales and six plant functional types. Results showed that diffuse shortwave radiation (SWdif) dominates GPP variability at the sub-daily (half-hourly to 3-hourly) timescales especially for the tree species, and acts as the secondary contributor after air temperature at the daily or longer timescales. Attribution analyses further showed that the main effects of SWdif are much higher than their interactive effects with other climatic factors in regulating the GPP variability. By identifying the main climatic drivers, this study improves the understanding of the climate-driven GPP variability and provides important implications for the future projection of ecosystem carbon assimilation under global climate change.
... Clouds organize on scales ranging from few meters to hundreds of kilometers, affecting plant photosynthesis by increasing the fraction of diffuse solar radiation that arrives at the top of the canopy.64 The isotropic property of diffuse solar radiation enables the radiation to spread more equally over all the leaves and thereby increases the light-use efficiency of a canopy.65,66 Following pioneering work by Freedman et al.,67 and as shown in the observations inFigure 10from Vilà-Guerau de Arellano et al.,23 cloud radiation perturbations have opposite effects on the net primary production (enhancing) and latent heat (diminishing), which is still not well understood nor well represented in our weather and carbon-climate models. ...
Vegetation and atmosphere processes are coupled through a myriad of interactions linking plant transpiration, carbon dioxide assimilation, turbulent transport of moisture, heat and atmospheric constituents, aerosol formation, moist convection, and precipitation. Advances in our understanding are hampered by discipline barriers and challenges in understanding the role of small spatiotemporal scales. In this perspective, we propose to study the atmosphere–ecosystem interaction as a continuum by integrating leaf to regional scales (multiscale) and integrating biochemical and physical processes (multiprocesses). The challenges ahead are (1) How do clouds and canopies affect the transferring and in‐canopy penetration of radiation, thereby impacting photosynthesis and biogenic chemical transformations? (2) How is the radiative energy spatially distributed and converted into turbulent fluxes of heat, moisture, carbon, and reactive compounds? (3) How do local (leaf‐canopy‐clouds, 1 m to kilometers) biochemical and physical processes interact with regional meteorology and atmospheric composition (kilometers to 100 km)? (4) How can we integrate the feedbacks between cloud radiative effects and plant physiology to reduce uncertainties in our climate projections driven by regional warming and enhanced carbon dioxide levels? Our methodology integrates fine‐scale explicit simulations with new observational techniques to determine the role of unresolved small‐scale spatiotemporal processes in weather and climate models.
... a is ecosystem apparent quantum yield, P max is the maximum ecosystem photosynthetic capacity, P 600 is GPP at low PAR (600 μmol m −2 s −1 ) and P 1800 is GPP at high PAR (1800 μmol m −2 s −1 ). respect to I g (Farquhar and Roderick, 2003;Alton et al., 2007). In recent years, the increasing aerosols due to human activities and climate change have enlarged the photosynthetic rate . ...
Diffuse radiation (I f) is one of important variables determining photosynthetic rate and carbon uptake of forest ecosystems. However, the responses of gross primary productivity (GPP) and light use efficiency (LUE) to diffuse fraction (DF) are still poorly understood. We used a 6-year dataset of carbon flux at a warm-temperate mixed plantation site in North China to explore the impacts of DF on GPP and LUE. During 2011-2017, ecosystem apparent quantum yield (α) and photosynthesis at photosynthetically active radiation (PAR) of 1800 µmol m⁻² s⁻¹ (P 1800) on cloudy days were 63% and 17% higher than on clear days, respectively. Under lower vapor pressure deficit (VPD) and air temperature (T a) conditions, canopy photosynthesis was significantly higher on cloudy skies than on clear skies. On half-hourly scale, increased DF enhanced α and P 1800. Daily GPP peaked at a median DF (=0.5), while daily LUE significantly increased with DF (p<0.01). Both GPP and LUE were mainly controlled directly by DF and PAR. DF had an indirect effect on LUE and GPP mainly through PAR. At high DF levels (>0.5), the increase in LUE did not make GPP enhancement. The direct effect of DF on GPP and LUE under lower T a and VPD was more sensitive than under higher T a and VPD. When DF was incorporated into the Michaelis-Menten model, it performed well in the GPP estimation, and the determination coefficient increased by 32.61% and the root mean square error decreased by 25.74%. These findings highlight the importance of incorporating DF into carbon sequestration estimation in North China.
... This makes anthropogenically forced trends challenging to identify in both observations ( Sanchez-Lorenzo et al., 2008;Wild, 2016 ) and models ( Moseid et al., 2020;Storelvmo et al., 2018 ). The multi-decadal changes of SSR, termed dimming and brightening ( Wild et al., 2005 ), are known to affect the climate system through different mechanisms, concerning the hydrological cycle ( e.g., Ramanathan, 2001;Wild et al., 2008 ), global warming ( Wild et al., 2007 ), the cryosphere Wild, 2009;Wild et al., 2008 ), and the terrestrial biosphere and carbon cycle ( e.g., Farquhar, 2003;Jones & Cox, 2001;Mercado et al., 2009 ). Global dimming and brightening trends are observed, for example, in China from 1958 to 1999, the sustained trend magnitudes for the 41-year period being −7.4 W m −2 /decade We take the internal variability analysis as a starting point for understanding the behavior of a climate variable and apply it more specifically to SSR, a key component of the global energy balance ( Wild et al., 2014 ). ...
Internal variability comprises all processes that occur within the climate system without any natural or anthropogenic forcing. Climate‐driving variables like the surface solar radiation (SSR) are shown to exhibit unforced trends (i.e., trends due to internal variability) of magnitudes comparable to the magnitude of the forced signal even on decadal timescales. We use annual mean data from 50 models participating in the preindustrial control experiment (piControl) of the Coupled Model Intercomparison Project‐Phase 6 (CMIP6) to give quantitative grid‐box specific estimates of the magnitudes of unforced trends. To characterize a trend distribution, symmetrical around 0, we use the 75th percentile of all possible values, which corresponds to a positive trend with 25% chance of occurrence. For 30‐year periods and depending on geographical location, this trend has a magnitude between 0.15 and 2.1 W m⁻²/decade for all‐sky and between 0.04 and 0.38 W m⁻²/decade for clear‐sky SSR. The corresponding area‐weighted medians are 0.69 W m⁻²/decade for all‐sky trends and 0.17 W m⁻²/decade for clear‐sky trends. The influence of internal variability is on average six times smaller in clear‐sky, compared to all‐sky SSR. The relative uncertainties in the physical representation, derived from the CMIP6 inter‐model spread, are ±32% for all‐sky and ±43% for clear‐sky SSR trends. Reasons for differences between models like horizontal resolution, aerosol handling, and the representation of atmospheric and oceanic phenomena are investigated. The results can be used in the analysis of observational time series by attributing a probability for a trend to be caused by internal variability, given its magnitude, length, and location.
... For instance, the photosynthetic behaviour of cucumbers was enhanced when light penetration throughout the canopy was increased [42]. A similar boost in productivity was observed during cloudy skies, or under forests [43][44][45][46]. Therefore, diffuse light can penetrate deeper into the lower leaves of the canopy [47][48]. ...
To safeguard future renewable energy and food supply the use of agrophotovoltaic (APV) systems was investigated, which enable simultaneous production under the same piece of land. As conventional photovoltaic (PV) array topologies lead to unfavourable conditions for crop growth, the application of APV is limited to areas with high solar insolation. By optimizing the APV array’s design, compatibility with various climates and crop species can be attained. Therefore, the aim of this research was to establish a multi-scale modelling approach and determine the optimal topology for a medium-to-large-scale fixed bifacial APV array. Three main topologies were analyzed under the climate of Boston, USA: S-N facing, E-W wings, and E-W vertical. For each topology, respectively, specific yield was amplified by 39%, 18%, and 13% in comparison to a conventional monofacial ground mounted PV array. E-W vertical is more appropriate for permanent crop species, while S-N facing necessitates the cultivation of shade tolerant crops during summer as electricity generation is prioritized. The E-W wings APV topology combines the best of both; light is distributed homogeneously, and crops are effectively shaded at noon. To promote the growth rate of blueberries under shade, customized bifacial modules were integrated (arranged as the E-W wings). Land productivity enhanced by 50%, whereas electrical AC yield reduced by 33% relative to the conventional and separate production. Through this holistic approach, it is possible to achieve a comprehensive understanding of the limitations and potential synergies associated with the dual use of land; ultimately, encouraging the transition of the agricultural sector into sustainability.
... Diffuse radiation has a great impact on the IAV of NEE (Cox et al. 2013) in different ways. Increase in net radiation (NR), cloud cover largely during WS decrease direct radiation, and the frequency of light saturation and making the canopy photosynthesis more responsive to radiation changes (Farquhar and Roderick, 2003;Knohl and Baldocchi, 2008). The growing cloud cover upto a certain extent simultaneously decrease temperature, which in turn reduce the ecosystem respiration (Zhang et al. 2013). ...
Consecutive five-year long eddy covariance measurements in a lowland tropical rice-rice system were used to investigate the impacts of gross primary productivity (GPP), climate drivers and ecosystem responses (i.e. ecosystem respiration, RE) on the inter-annual variability (IAV) of the net ecosystem exchange (NEE), which is directly related to the agricultural productivity and climate change. The IAV of carbon dioxide fluxes in two crop growing phases i.e. dry and wet season along with fallow period were analysed. The respiratory fluxes build up during the non-growing season were lower by net uptake in growing season. Annual cumulative value of NEE was negative (sink) in both the crop growing season. The variability of climate drivers and changes in the ecosystem responses to drivers revealed a large intra-annual as well as inter-annual variability of net ecosystem fluxes. NEE was found to be strongly correlated with GPP and RE and also with other metrological variables such as photosynthetically active radiation (PAR), precipitation, air temperature and soil temperature. The anomalies of NEE, GPP and RE were observed to be less in 2017 and 2018 which may be due to lower temperature anomalies recorded in these years. Further understanding of biological mechanisms is needed which is involved in the variation of climatological variables to improve our ability to predict future IAV of NEE.
... Sulfate aerosols scatter sunlight and increase diffuse sunlight, which is available to be used by plant canopy that remains shaded (Farquhar & Roderick, 2003;Kanniah et al., 2012;Mercado et al., 2009). Thus, aerosol-induced Figure S12 in Supporting Information S1. ...
Stratospheric aerosol injection is one of the most widely studied solar radiation modification methods to reduce some effects of anthropogenic warming. We perform idealized simulations to analyze climate response to different latitudinal and altitudinal distributions of additional stratospheric sulphate aerosols. We analyze climate response to both large volcanic‐size aerosols that are representative of sulphate aerosols which form when SO2 is injected into the stratosphere by major volcanic eruptions or by proposed stratospheric sulphate aerosol geoengineering option, and small background‐size aerosols which are present in the stratosphere during quiescent periods with no major volcanic eruptions. Our simulations show that under the climate state with doubled atmospheric CO2, for the same total mass, independent of size‐distributions, aerosols concentrated at higher latitudes produce less negative effective radiative forcing but larger surface cooling as a result of larger forcing efficacy. For the same latitudinal distribution, small background‐size aerosols added at lower altitudes cause larger surface cooling as a result of hygroscopic growth that increases scattering of sunlight. In contrast, large volcanic‐size aerosols added at lower altitudes cause smaller surface cooling than at higher altitudes as a result of enhanced stratospheric heating and associated water vapor feedback that enhance absorption of solar and terrestrial radiation. For both background‐size and volcanic‐size aerosols, asymmetric distributions about the equator induce substantial cross‐equatorial energy transport, causing the Intertropical Convergence Zone to move to the warmer hemisphere with less aerosol loading. Our study helps to understand the climate consequences of different spatial patterns and size distributions of stratospheric aerosol perturbation.
... Diffuse radiation has been found to increase total canopy ε since more light can be diffused into deeper canopy layers (Cheng et al., 2015;Farquhar and Roderick, 2003;Gu et al., 1999;Huang et al., 2014;Knohl and Baldocchi, 2008;Mercado et al., 2009). Therefore, the effect of the diffuse radiation ratio, represented by cloudiness index (CI), has been incorporated into some LUE models (He et al., 2013;Turner et al., 2006a;Wang et al., 2015;Wang et al., 2018;Yan et al., 2017), and is backed by the classical theory of radiative transfer (Ross, 1981) and adopted by mechanistic canopy models (Ibrom et al., 2006). ...
The sensitivity of photosynthesis to environmental changes is essential for understanding carbon cycle responses to global climate change and for the development of modeling approaches that explains its spatial and temporal variability. We collected a large variety of published sensitivity functions of gross primary productivity (GPP) to different forcing variables to assess the response of GPP to environmental factors. These include the responses of GPP to temperature; vapor pressure deficit, some of which include the response to atmospheric CO2 concentrations; soil water availability (W); light intensity; and cloudiness. These functions were combined in a full factorial light use efficiency (LUE) model structure, leading to a collection of 5600 distinct LUE models. Each model was optimized against daily GPP and evapotranspiration fluxes from 196 FLUXNET sites and ranked across sites based on a bootstrap approach. The GPP sensitivity to each environmental factor, including CO2 fertilization, was shown to be significant, and that none of the previously published model structures performed as well as the best model selected. From daily and weekly to monthly scales, the best model's median Nash-Sutcliffe model efficiency across sites was 0.73, 0.79 and 0.82, respectively, but poorer at annual scales (0.23), emphasizing the common limitation of current models in describing the interannual variability of GPP. Although the best global model did not match the local best model at each site, the selection was robust across ecosystem types. The contribution of light saturation and cloudiness to GPP was observed across all biomes (from 23% to 43%). Temperature and W dominates GPP and LUE but responses of GPP to temperature and W are lagged in cold and arid ecosystems, respectively. The findings of this study provide a foundation towards more robust LUE-based estimates of global GPP and may provide a benchmark for other empirical GPP products.
... In addition, as atmospheric levels of carbon dioxide increase, photosynthesis becomes more efficient as plants can fix more carbon dioxide using the same amount of water (Farquhar 1997). Increased cloud cover (associated with increased rainfall) is not necessarily an impediment as photosynthesis utilizes diffuse as well as direct solar energy (Farquhar and Roderick 2003), and it could even enhance photosynthesis in multi-layered vegetation canopies (Hollinger et al. 1998). ...
... Increases in diffuse light are known to reduce canopy shading of interior leaves, yielding higher light use efficiencies and high levels of canopy GPP following volcanic eruptions and downwind of cities (Farquhar and Roderick 2003;FIG. 7. The annual mean cycle of fire-induced changes in aerosol optical depth (AOD; unitless), downwelling shortwave radiation (S in ; W m 22 ), surface air temperature (TAS; 8C), precipitation (PPT; mm day 21 ), relative humidity (RH; %), surface wind speed (U; m s 21 ), evapotranspiration (ET; mm day 21 ), and gross primary production (GPP; g C m 22 month 21 ) for selected regions in the southern Amazon, central Africa, and the Maritime Continent region of tropical Asia. ...
Fire-emitted aerosols play an important role in influencing Earth’s climate, directly by scattering and absorbing radiation and indirectly by influencing cloud microphysics. The quantification of fire-aerosol interactions, however, remains challenging and subject to uncertainties in emissions, plume parameterizations, and aerosol properties. Here we optimized fire-associated aerosol emissions in the Energy Exascale Earth System Model (E3SM) using the Global Fire Emissions Database (GFED) and AERONET aerosol optical depth (AOD) observations during 1997-2016. We distributed fire emissions vertically using smoke plume heights from Multi-angle Imaging SpectroRadiometer (MISR) satellite observations. From the optimization, we estimate that global fires emit 45.5 Tg y ⁻¹ of primary particulate organic matter and 3.9 Tg y ⁻¹ of black carbon. We then performed two climate simulations with and without the optimized fire emissions. We find that fire aerosols significantly increase global AOD by 14 ± 7% and contribute to a reduction in net shortwave radiation at the surface (-2.3 ± 0.5 W m ⁻² ). Together, fire-induced direct and indirect aerosol effects cause annual mean global land surface air temperature to decrease by 0.17 ± 0.15°C, relative humidity to increase by 0.4 ± 0.3%, and diffuse light fraction to increase by 0.5 ± 0.3%. In response, GPP declines by 2.8 Pg C y ⁻¹ , as a result of large positive drivers (decreases in temperature and increases in humidity and diffuse light) nearly cancelling out large negative drivers (decreases shortwave radiation and soil moisture). Our analysis highlights the importance of fire aerosols in modifying surface climate and photosynthesis across the tropics.
... We hypothesized that a primary mechanism underlying enhanced LUE of strawberry crops during fog events is the change in geometry of the incident light, such that diffuse light irradiates more leaves within the canopy than direct-beam radiation. This could effectively elevate whole-plant carbon uptake (Farquhar & Roderick, 2003). Emmel et al. (2020) demonstrate that GPP was significantly greater under diffuse rather than direct radiation conditions for six different arable crops, which varied in their canopy structures. ...
In coastal California, the peak growing season of economically important crops is concurrent with fog events, which buffer drought stress during the dry season. Coastal fog patterns are changing, so we quantified its effects on the energy, water, and carbon fluxes of a strawberry farm located in the fog‐belt of the Salinas Valley, California. We used Geostationary Operational Environmental Satellite (GOES) total albedo to detect and quantify large scale patterns of coastal fog. We used eddy covariance (EC) to quantify actual evapotranspiration and gross primary productivity (GPP) at the field scale (approximately 0.5–3 hectares) from June to September 2016. We measured canopy‐scale (approximately 0.6 m²) strawberry physiology on foggy and non‐foggy days within the measurement footprint of the EC tower. Downwelling longwave radiation (L↓), observed by a surface‐mounted pyrgeometer, was consistently higher on foggy compared to clear‐sky days (regardless of fog‐drip), indicating that emission of longwave radiation was derived almost entirely from the cloud base. L↓ and total GOES albedo were positively and strongly correlated (R² = 0.68, P < 0.01). For both field‐ and canopy‐scales, water‐use and light‐use efficiency increased by as much as 50% and 70%, respectively, during foggy compared to non‐foggy conditions. The initial slope of the curvilinear relationship fit between GPP and photosynthetically active radiation was twice as steep during foggy (α = 0.0395) than non‐foggy (α = 0.0210) conditions, suggesting that the scattering of light during fog events enhances photosynthetic output of whole‐plants. Our results suggest that irrigation for these fields could be rescheduled during foggy periods without sacrificing plant productivity.
... Inside greenhouses, the distribution of light on the different leaves of a plant shows great variation depending on the solar angle, shadow-producing points, and areas of direct sunlight. Damage caused by light can occur particularly at these points of direct sunlight [8], whereas diffuse light is more uniformly distributed over crops than direct light [9][10][11][12]. ...
The efficient use of light is one of the most important factors for the development of greenhouse crops. It is increasingly necessary to use film covers that enhance transmittance and the proportion of diffuse light to generate a more homogeneous light distribution. The objective of this study was to evaluate the effect that an experimental film cover with high transmittance and high light diffusivity produces on the microclimate and the growth and yield of tomato crops (Solanum lycopersicum L.), compared with a commercial thermal film cover. The trial was developed during a spring–summer growing cycle in a multispan greenhouse divided into two compartments (sectors) separated by a vertical polyethylene sheet. In the East sector, a commercial film was installed (transmittance of 85% and diffusivity of 60%) and in the West sector, an experimental film was used (transmittance of 90% and diffusivity of 55%). The results show an increase in the marketable yield of 0.25 kg·m−2 in the sector with the experimental film, which represents 3.2% growth with respect to the commercial film. The photosynthetic activity measured in tomato leaves was 21.5% higher in plants growing in the sector with the experimental film, with had the highest transmittance. The increase in radiation transmittance of 14% produced greater photosynthetic activity without generating a higher inside air temperature at the crop level (at the height of 2 m above the floor). However, the mean temperature of the soil surface was statistically higher on the side with the diffuse experimental cover film, as a logical consequence of the higher level of intercepted solar radiation.
... Chameides et al. (1999) and Xiong et al. (2012) have suggested that because of the reduction in total radiation, rising aerosol loading has resulted in a 5%-30% reduction in crop yields in China, which is similar to the assessed reduction caused by climate change and ozone pollution (Tian et al., 2016). However, other studies (Cohan et al., 2002;Liu et al., 2016) also suggested that the negative impacts of aerosols on crop yields may be overestimated due to the ignorance of the presence of aerosols that could promote the penetration of solar radiation into the canopy and enhance the photosynthesis of shaded leaves (Farquhar & Roderick, 2003;Kanniah et al., 2012;Mercado et al., 2009;Roderick et al., 2001). Process-based model studies that considered the diffuse fertilization effect did find that aerosols promoted crop yields (Greenwald et al., 2006;Schiferl & Heald, 2018). ...
Aerosols can affect crop photosynthesis by altering radiation and meteorological conditions. By combining field observations, mechanistic modeling, and satellite‐retrieved solar‐induced chlorophyll fluorescence (SIF), we assessed aerosols' impacts on crop photosynthesis from leaf to regional scale. We found that the initial increase in aerosol optical depth (AOD) enhanced photosynthesis of sun leaves, shade leaves, and canopy, which reached their maximum at AOD=0.76, 1.13, and 0.93, respectively, and then decreased. Aerosol‐induced changes in radiation regime and the concurrent high relative humidity led to such nonlinear responses. Similarly, the SIF of croplands in the North China Plain also showed a bell‐shaped response to aerosols. The optimal AOD level at which SIF reached the maximum value varied from 0.56 to 1.04, depending on the background meteorological conditions. Approximately 76‐90% of the North China Plain exceeded the optimal AOD level, suggesting that stringent aerosol pollution control could promote cropland productivity in this region.
... The summer LCF regime has a consistent and strong impact on the surface radiation budget for ecosystems and agriculture located along the narrow coastal strip of central and northern California, and these are likely to have significant implications for plant functioning. Despite a lower availability of light for photosynthesis, the high proportion of diffuse beam PAR means the incident light is more isotropic in direction and therefore will better penetrate canopies enhancing light availability for leaves at lower levels in the canopy (Farquhar and Roderick 2003;Urban et al. 2007). Most ecosystem light use efficiency studies show that gross primary production saturates from moderate to high levels of PAR, meaning the reduction in magnitude of PAR G is unlikely to have significant negative effect on photosynthesis. ...
Advection fog processes produce frequent low cloud and fog (LCF) during summer along the coast of central and northern California. Four radiometer datasets from sites in San Francisco and Monterey Bay as well as fog-drip and meteorological observations were used to characterize the role of LCF on surface radiation budget components and scattering of solar radiation. LCF produced distinct and consistent impacts on surface radiation relative to clear skies including large reductions (approximately half) in solar irradiance but large increases (approximately 180%) in the diffuse beam component, consistent increases of approximately 70 W m−2 in down-welling longwave radiation (L↓), as well as reductions in net radiation and leaf surface temperature by day and increases at night. L↓ can be a useful and accurate tool for monitoring the presence of LCF, using simple thresholds: LCF = L↓ > 360 W m−2 and clear-sky = L↓ < 340 W m−2. Based on this, LCF conditions prevailed 60% of hours during summer, peaking consistently at 08:00 PDT, with clear skies occurring 33% and mixed conditions 7% of hours. The ratio of the incident surface to extraterrestrial solar radiation (τ) is also useful to determine LCF presence during daylight hours, using LCF = τ < 0.0059β + 0.3 and clear-sky = τ > 0.0059β + 0.3, where β is the solar elevation angle. We also present a model to partition the diffuse and direct beam fractions of solar irradiance specifically for coastal LCF, which improved accuracy by more than 10% relative to existing universal models. These radiation characteristics are important for understanding energy balance and sunlight conditions experienced by coastal ecosystems.
... Under high solar radiation level, vegetation photosynthesis may be light saturated and may even be suppressed by excessive light (Garbulsky et al., 2014;Hilker et al., 2008), resulting in low LUE (Figures S3e and S3f;Tong et al., 2009;Hilker et al., 2012). Due to data limitation, our study, however, could not well explore the impacts of diffuse radiation on LUE, which was found to mostly affect vegetation canopy with complex structure (Farquhar & Roderick, 2003;Min, 2005) or under cloudy days (Gao et al., 2018;Urban et al., 2007). Despite previous studies indicating significant potential influence of diffusive radiation on global photosynthesis (Mercado et al., 2009;Rap et al., 2018), due to lack of consistent global data set on diffuse radiation, few process-based models and data-driven models have accounted this process, which became a major source of uncertainties to global LUE variations. ...
Light use efficiency (LUE) is a critical parameter, on which we rely to quantify the vegetation's capability to harvest solar energy and thus the photosynthesis carbon uptake. However, our understanding on its magnitude, global pattern, and climatic drivers remains uncertain. In this study, we investigated global mean annual LUE during 1980–2013 based on state‐of‐the‐art estimates from scaled‐up eddy covariance measurements (data‐driven models) and 10 process‐based models used in Global Carbon Project. We found comparable global LUE estimates from data‐driven models (0.90 ± 0.25 g C MJ⁻¹) and process‐based models (0.73 ± 0.22 g C MJ⁻¹), with individual model estimates ranging from 0.49 to 0.98 g C MJ⁻¹. While both types of models agreed on highest LUE over the tropical evergreen forests, discrepancies remained over middle and high latitudes of the Northern Hemisphere, which probably resulted from different response of LUE to climatic factors. Spatial analyses revealed that LUE variations were more driven by temperature and precipitation than by solar radiation. While the response of LUE to precipitation was almost always positive globally, the response of LUE to temperature was only positive in part of the nontropical regions. Process‐based models showed that precipitation strongly regulated the response of LUE to temperature, which shifted from negative to positive when mean annual precipitation was larger than 1,000 mm yr⁻¹. Our results provide converging global LUE estimates and highlight the need to take fully account of interactive effects of climatic factors in regulating LUE and thus the response of photosynthesis to climate change.
... Remote Sens. 2020, 12, 3355 2 of 21 photosynthetic saturation. Therefore, higher Rd increases the canopy light use efficiency (LUE) and thus leads to the enhancement of GPP [4][5][6][7]. Consequently, under cloudy or severe haze conditions, even if Rg decreased, vegetation GPP can increase as a result of the higher Rd component. ...
Solar radiation significantly affects terrestrial gross primary productivity (GPP). However, the relationship between GPP and solar radiation is nonlinear because it is affected by diffuse radiation. Solar radiation has undergone a shift from darker to brighter values over the past 30 years in China. However, the effects on GPP of variation in solar radiation because of changes in diffuse radiation are unclear. In this study, national global radiation in conjunction with other meteorological data and remotely sensed data were used as input into a two-leaf light use efficiency model (TL-LUE) that simulated GPP separately for sunlit and shaded leaves for the period from 1981 to 2012. The results showed that the nationwide annual global radiation experienced a significant reduction (2.18 MJ m −2 y −1 ; p < 0.05) from 1981 to 2012, decreasing by 1.3% over this 32-year interval. However, the nationwide annual diffuse radiation increased significantly (p < 0.05). The reduction in global radiation from 1981 to 2012 decreased the average annual GPP of terrestrial ecosystems in China by 0.09 Pg C y −1 , whereas the gain in diffuse radiation from 1981 to 2012 increased the average annual GPP in China by about 50%. Therefore, the increase in canopy light use efficiency under higher diffuse radiation only partially offsets the loss of GPP caused by lower global radiation.
... Clouds affect plant photosynthesis by increasing the fraction of diffuse solar radiation that arrives at the top of the canopy (Kanniah et al., 2012). With a larger contribution of diffuse solar radiation and within the canopy, the radiation spreads more equally over all leaves and thereby increases the light-use efficiency of a canopy (Farquhar and Roderick, 2003). At a constant level of radiation at the top of the canopy, the increased light-use efficiency results in enhanced canopy photosynthesis, which is known as the diffuse fertilization effect (Roderick et al., 2001). ...
The CloudRoots field experiment was designed to obtain a comprehensive observational dataset that includes soil, plant, and atmospheric variables to investigate the interaction between a heterogeneous land surface and its overlying atmospheric boundary layer at the sub-hourly and sub-kilometre scale. Our findings demonstrate the need to include measurements at leaf level to better understand the relations between stomatal aperture and evapotranspiration (ET) during the growing season at the diurnal scale. Based on these observations, we obtain accurate parameters for the mechanistic representation of photosynthesis and stomatal aperture. Once the new parameters are implemented, the model reproduces the stomatal leaf conductance and the leaf-level photosynthesis satisfactorily. At the canopy scale, we find a consistent diurnal pattern on the contributions of plant transpiration and soil evaporation using different measurement techniques. From highly resolved vertical profile measurements of carbon dioxide (CO2) and other state variables, we infer a profile of the CO2 assimilation in the canopy with non-linear variations with height. Observations taken with a laser scintillometer allow us to quantify the non-steadiness of the surface turbulent fluxes during the rapid changes driven by perturbation of photosynthetically active radiation by cloud flecks. More specifically, we find 2 min delays between the cloud radiation perturbation and ET. To study the relevance of advection and surface heterogeneity for the land–atmosphere interaction, we employ a coupled surface–atmospheric conceptual model that integrates the surface and upper-air observations made at different scales from leaf to the landscape. At the landscape scale, we calculate a composite sensible heat flux by weighting measured fluxes with two different land use categories, which is consistent with the diurnal evolution of the boundary layer depth. Using sun-induced fluorescence measurements, we also quantify the spatial variability of ET and find large variations at the sub-kilometre scale around the CloudRoots site. Our study shows that throughout the entire growing season, the wide variations in stomatal opening and photosynthesis lead to large diurnal variations of plant transpiration at the leaf, plant, canopy, and landscape scales. Integrating different advanced instrumental techniques with modelling also enables us to determine variations of ET that depend on the scale where the measurement were taken and on the plant growing stage.
... In the analysis of plant shape at the community level, two main functional concepts involving plant interactions are discussed: the occupation of the space and the shading of neighbors. As there are important world-scale modifications in overall light availability and in the various components of solar radiation, in particular in the ratio of direct to diffuse irradiance (Roderick et al. 2001, Gu et al. 2002, Farquhar and Roderick 2003, understanding the fundamental relationships between plant architecture and efficiency of harvesting light is the precondition in grasping the global change effects on vegetation productivity. ...
... Comparisons of mean light interception (I, μmol m −2 s −1 ) and photosynthetic rate (A, μmol CO 2 m −2 s −1 ) of the scanned parametric model (SPM) and lowcurvature parametric model (LPM) according to light intensity at the upper canopy of the whole-plant. photosynthetic efficiency could be changed according to the overall light intensity distribution in a plant (Farquhar and Roderick, 2003;Mercado et al., 2009). At the leaf scale, the mean photosynthetic rate showed a difference similar to the change in light intensity. ...
Plant structure is a significant factor for influencing the light interception and photosynthesis of plants. The light interception on the plant surface can be analyzed by three-dimensional (3D) plant model and optical simulation, but its accuracy is directly affected by the structural accuracy of the 3D model. This study aims to analyze and compare the effect of the accuracy of 3D structural models on light interception and photosynthesis. 3D-scanned plant models with different structural accuracies were constructed, and the light interception and photosynthetic rate were analyzed at single leaf and whole-plant scales. When using a low accuracy model that lacked the fine structural details of the plant, it was overestimated in light interception and photosynthetic rate compared to the 3D-scanned model that has high structural accuracy. At the single leaf scale, the light interception was higher in the low-accuracy model than that in the 3D-scanned model due to self-shadings from higher curvature in the leaf surface. At the whole-plant scale, the light interception and the subsequent photosynthetic rate in the low-accuracy model were 18% and 45 to 58% higher than those in the 3D-scanned model at light intensities of 700–2000 μmol m⁻² s⁻¹ at the upper canopy. The 3D-scanned plant model could accurately estimate the light interception and photosynthetic rate of the plants through optical simulation. The presented methodology can contribute to accurate analyses of plant light environment, plant physiological response, and plant growth modelling.
... shade levels (Farquhar & Roderick, 2003). Moreover, D'Odorico et al. (2019) showed that crop plants at this site reached light saturation in the lower canopy at lower PAR than in the higher canopy, which indicates a high plasticity of the plants to adapt to their local environment. ...
Diffuse radiation generally increases photosynthetic rates if total radiation is kept constant. Different hypotheses have been proposed to explain this enhancement of photosynthesis, but conclusive results over a wide range of diffuse conditions or about the effect of canopy architecture are lacking. Here, we show the response of canopy photosynthesis to different fractions of diffuse light conditions for five major arable crops (pea, potato, wheat, barley, rapeseed) and cover crops characterized by different canopy architecture. We used 13 years of flux and microclimate measurements over a field with a typical four‐year crop rotation scheme in Switzerland. We investigated the effect of diffuse light on photosynthesis over a gradient of diffuse light fractions ranging from 100% diffuse (overcast sky) to 11% diffuse light (clear‐sky conditions).
Gross primary productivity increased with diffuse fraction and thus was greater under diffuse than direct light conditions if the absolute photon flux density per unit surface area was kept constant. Mean leaf tilt angle (MTA) and canopy height were found to be the best predictors of the diffuse vs. direct radiation effect on photosynthesis. Climatic factors, such as the drought index and growing degree days (GDD), had a significant influence on initial quantum yield under direct but not diffuse light conditions, which depended primarily on MTA. The maximum photosynthetic rate at 2000 µmol m–2 s–1 PAR under direct conditions strongly depended on GDD, MTA, leaf area index (LAI), and the interaction between MTA and LAI, while under diffuse conditions this parameter depended mostly on MTA and only to a minor extent on canopy height and their interaction. The strongest photosynthesis enhancement under diffuse light was found for wheat, barley and rapeseed, whereas the lowest was for pea. Thus, we suggest that measuring canopy architecture and diffuse radiation will greatly improve gross primary productivity estimates of global cropping systems.
... I d increases under cloudy sky conditions, which enables the shade leaves to receive more light for improving photosynthesis Farquhar and Roderick, 2003;Alton et al., 2007). Reduced direct radiation also slows down "light saturation". ...
Dynamic changes in solar radiation have an important influence on ecosystem carbon sequestration, but the effects of changes caused by sky conditions on net ecosystem CO 2 exchange (NEE) are unclear. This study analyzed the effects of sunny, cloudy, and overcast sky conditions on NEE using carbon flux and meteorological data for a subtropical coniferous plantation in 2012. Based on one-year data, we found no seasonal variation in the light response curve under various sky conditions. Compared with sunny sky conditions, the apparent quantum yield (α) and potential photosynthetic rate at a light intensity of 150 and 750 W m −2 (P 150 and P 750) under cloudy sky conditions increased by an average of 82.3%, 217.7%, and 22.5%; α and P 150 under overcast sky conditions increased by 118.5% and 301% on average. Moderate radiation conditions were more favorable for maximum NEE, while low radiation conditions inhibited NEE. In most cases, when the sunny NEE was used as a baseline for comparison, the relative change in NEE (%NEE) was positive under cloudy sky conditions and negative under overcast sky conditions. The average maximal %NEE under cloudy sky conditions was 42.4% in spring, 34.1% in summer, 1.6% in autumn and-87.3% in winter. This study indicates that cloudy sky conditions promote photosyn-thetic rates and NEE in subtropical coniferous plantations.
... Clouds affect plant photosynthesis by increasing the fraction of diffuse solar radiation that arrives at the top of the canopy (Kanniah, et al., 2012). With a larger contribution of diffuse solar radiation, and within the canopy, the radiation spreads 10 more equally over all leaves and thereby increasing the light-use efficiency of a canopy (Farquhar & Roderick, 2003). At a constant level of radiation at the top of the canopy, the increased light-use efficiency results in enhanced canopy photosynthesis which is known as the diffuse fertilization effect (Roderick, et al., 2001). ...
Abstract. The CloudRoots field experiment was designed to obtain a comprehensive observational data set that includes soil, plant and atmospheric variables to investigate the interaction between a heterogeneous land surface and its overlying atmospheric boundary layer at the sub-hourly and sub–kilometre scale. Our findings demonstrate the need to include measurements at leaf level in order to obtain accurate parameters for the mechanistic representation of photosynthesis and stomatal aperture. Once the new parameters are implemented, the mechanistic model reproduces satisfactorily the stomatal leaf conductance and the leaf-level photosynthesis. At the canopy scale, we find a consistent diurnal pattern on the contributions of plant transpiration and soil evaporation using different measurement techniques. From the high frequency and vertical resolution state variables and CO2 measurements, we infer a profile of the plant assimilation that shows a strong non-linear behaviour. Observations taken by a laser scintillometer allow us to quantify the non-steadiness of the surface turbulent fluxes during the rapid changes driven by perturbation of the photosynthetically active radiation (PAR) by clouds, the so-called cloud flecks. More specifically, we find two-minute delays between the cloud radiation perturbation and ET. The impact of surface heterogeneity was further studied using ET estimates infer from the sun-induced fluorescence data and show small variation of ET in spite of the plant functional type differences. To study the relevance of advection and surface heterogeneity on the land-atmosphere interaction, we employ a coupled surface-atmospheric conceptual model that integrates the surface and upper-air observations taken at different scales: from the leaf-level to the landscape. At the landscape scale, we obtain the representative sensible heat flux that is consistent with the evolution of the boundary-layer depth evolution. Finally, throughout the entire growing season, the wide variations in stomatal opening and photosynthesis lead to large variations of plant transpiration at the leaf and canopy scales. The use of different instrumental techniques enables us to compare the total ET at various growing stages, from booting to senescence. There is satisfactory agreement between evapotranspiration of total ET, but the values remain sensitive to the scale at which ET is measured or modelled.
... Alterations in sky conditions influence the fractions of direct and diffuse incident light, with direct light occurring under clear sky conditions and diffuse light occurring under cloudy sky conditions (Brodersen et al. 2008;Urban et al. 2012;Williams et al. 2014). The fraction of the diffuse component in total radiation (diffuse index, DI) is increasing because of anthropogenic emissions and volcanic eruptions (Ezhova et al. 2018;Farquhar and Roderick 2003;Kanniah et al. 2012;Mercado et al. 2009;Rap et al. 2018). Diffuse light is more efficient for CO 2 assimilation than direct light at the same level of intensity (Brodersen et al. 2008;Mercado et al. 2009;Urban et al. 2012;Williams et al. 2014). ...
Plants can change leaf forms, adjusting light conditions on their adaxial and abaxial surfaces, to adapt to light environments and enhance their light use efficiencies. The difference between photosynthesis on the two leaf sides (dorsoventral asymmetry) is an important factor that affects light use efficiency. However, photosynthetic dorsoventral asymmetry is rarely compared under direct and diffuse light conditions. To estimate the impacts of recently reported alterations in direct and diffuse light in the sky radiation on plant carbon assimilation, variations in morphology between the two leaf sides in tobacco (Nicotiana tabacum L.) were investigated, and the dorsoventral responses of photosynthesis to illuminating directions were compared in direct and diffuse light. Dorsoventral asymmetry was reflected in stomatal densities, anatomic structures, and photochemical traits, which caused markedly different photosynthetic rates as well as stomatal conductances both in direct and diffuse light. However, the degree of photosynthetic asymmetry was weakened in diffuse light. The diffuse light caused a greater stomatal conductance on the abaxial side than direct light, which resulted in reduced photosynthetic asymmetry. In addition, the photosynthetic dorsoventral asymmetry could be affected by the photosynthetic photon flux density. These results contribute to understanding the dorsoventral regulation of photosynthesis in bifacial leaves, and provide a reference for breeding to cope with the increase in the proportion of diffuse light in the future.
... Solar dimming/brightening also significantly influences plant photosynthesis in terrestrial ecosystems. However, in addition to the total amount of SSR, diffuse radiation is also key in this context , as it reduces the increased atmospheric CO2 concentration (Farquhar and Roderick, 2003) and affects the global sink of terrestrial carbon (Mercado et al., 2009). Gu et al. (2003) estimated that diffuse radiation from the volcanic eruption of Mount Pinatubo in June 1991 enhanced photosynthetic production of a deciduous forest by 23% in 1992 and 8% in 1993 in North America. ...
We projected incident surface solar radiation (SSR) over China in the middle (2040–2059) and end (2080–2099) of the 21st century in the Representative Concentration Pathway (RCP) 8.5 scenario using a multi-model ensemble derived from the weighted average of seven global climate models (GCMs). The multi-model ensemble captured the contemporary (1979–2005) spatial and temporal characteristics of SSR and reproduced the long-term temporal evolution of the mean annual SSR in China. However, it tended to overestimate values compared to observations due to the absence of aerosol effects in the simulations. The future changes in SSR showed increases over eastern and southern China, and decreases over the Tibetan Plateau (TP) and northwest China relative to the present day. At the end of the 21st century, there were SSR increases of 9–21 W m−2 over northwest, central, and south China, and decreases of 18–30 W m−2 over the TP in June–July–August (JJA). In northeast China, SSR showed seasonal variation with increases in JJA and decreases in December–January–February. The time series of annual SSR had a decreased linear trend for the TP, and a slightly increased trend for China during 2006–2099. The results of our study suggest that solar energy resources will likely decrease in the TP under future climate change scenarios.
... A common belief is that compared with direct radiation, diffuse radiation is more uniformly distributed over all the leaves in a canopy, thereby, resulting in an improved whole-canopy light distribution and interception (Kanniah, Beringer, North, & Hutley, 2013;Li & Yang, 2015;Wang et al., 2018). Such a spatial distribution in a canopy allows the incoming radiation being more efficiently utilized by plants (Farquhar & Roderick, 2003;Williams et al., 2014). ...
Global dimming, a decadal decrease in incident global radiation, is often accompanied with an increase in the diffuse‐radiation fraction, and therefore the impact of global dimming on crop production is hard to predict. A popular approach to quantify this impact is statistical analysis of historical climate and crop data, or use of dynamic crop simulation modelling approach. Here we show that statistical analysis of historical data did not provide plausible values for the effect of diffuse radiation versus direct radiation on rice or wheat yield. In contrast, our field experimental study of three years demonstrated a fertilization effect of increased diffuse‐radiation fraction, which partly offset yield losses caused by decreased global radiation, in both crops. The fertilization effect was not attributed to any improved canopy light interception but mainly to the increased radiation use efficiency (RUE). The increased RUE was explained not only by the saturating shape of photosynthetic light‐response curves but also by plant acclimation to dimming that gradually increased leaf nitrogen concentration. Crop harvest index slightly decreased under dimming, thereby discounting the fertilization effect on crop yields. These results challenge existing modelling paradigms, which assume that the fertilization effect on crop yields is mainly attributed to an improved light interception. Further studies on the physiological mechanism of plant acclimation are required to better quantify the global‐dimming impact on agroecosystem productivity under future climate change. This article is protected by copyright. All rights reserved.
... 1) Under diffuse light conditions, sunlit leaves receive less direct beam radiation, but shaded leaves receive more diffuse radiation, which comes from all directions and penetrates the canopy to a fuller extent. Leaves in deep shade generally benefit from an increase in radiation more than leaves in full sun suffer from an equivalent decrease; therefore, a more even vertical distribution of radiation should enhance the photosynthetic light use efficiency of the canopy as a whole (Farquhar and Roderick, 2003;Rocha et al., 2004). 2) Diffuse fraction also covaries with other environmental factors that influence the carbon exchange process. ...
... In (a), (c), (d), and (e), the anomaly is shown as percentage relative to mean prior to eruption. radiation in the calculation of plant canopy photosynthesis, that is, our model does not account for the effect that the volcanic aerosol-induced increase in diffuse radiation allows plant canopies to photosynthesize more efficiently (e.g., Farquhar & Roderick, 2003;Gu et al., 2003;Mercado et al., 2009;Robock, 2005). Therefore, for example, the decrease in global land NPP of 3 GtC/year with the most reduced insolation at year 1 in PT and ELD (Figures 4a and 4d) may be corrected to 2 GtC/year or less following the diffuse radiation-induced NPP increase of about 1 GtC/year estimated for the 1982 El Chichón and 1991 Pinatubo eruptions by Mercado et al. (2009; Figure 2b). ...
We used the Meteorological Research Institute Earth System Model to simulate the climate response to massive sulfur dioxide (SO2) emission from the volcanic eruptions of Paektusan (China/North Korea) and Eldgjá (Iceland) into the stratosphere in the tenth century Common Era (CE). Assuming 3 times the SO2 emission of the 1883 Krakatau eruption, as recorded by Greenland ice core sulfate concentrations, simulations of Paektusan and Eldgjá had roughly similar global mean impacts within 2 years of erupting: decreases in surface insolation (−10 W/m², −5%), surface air temperature (−1 °C), land net primary production (NPP; −3 GtC/year, −5%), and soil respiration (−5 GtC/year, −10%). While both simulations had severe impacts on the extratropical Northern Hemisphere, the simulated response to Paektusan is twice as strong as Eldgjá in the tropics (cooling [1 °C], precipitation decrease [10%], and NPP increase [7%]). Simulation‐Eldgjá had almost no impact on the extratropical Southern Hemisphere because of its initial latitude. These regional differences combine so that Simulation‐Paektusan has slightly larger global mean impacts, including an atmospheric CO2 decrease of ~2 ppm. Tropical NPP primarily increases due to the fact that photosynthesis maximizes at temperatures below the tropical mean temperature and secondarily to the cooling‐induced decrease in respiration, which indicates relatively rich tropical harvests despite large volcanic eruptions. In contrast, the severe cooling (−2.5 °C) and decreased NPP (−20%) in the northern extratropics could mean poor harvests and famines, which can lead to social turmoil such as a rebellion in northeast Japan around the same time as the eruptions.
... This is expected to be effective for outdoor cultivation, as the intensity of the sun varies in time, season, region, and weather, where Chlb expressing mutants will have better chance for photosynthesis during any time with low intensity light. Indeed, there have been numerous researches on the importance of diffuse radiation compared to direct radiation on photosynthesis in plants [53,54]. Even though we did not achieve increased lipid contents, this opens the opportunity of exploiting heterologous expression of CAO in industrial production of biofuels and biomaterials in Nannochloropsis. ...
Background
Chlorophylls play important roles in photosynthesis, and thus are critical for growth and related metabolic pathways in photosynthetic organisms. They are particularly important in microalgae, emerging as the next generation feedstock for biomass and biofuels. Nannochloropsis are industrial microalgae for these purposes, but are peculiar in that they lack accessory chlorophylls. In addition, the localization of heterologous proteins to the chloroplast of Nannochloropsis has not been fully studied, due to the secondary plastid surrounded by four membranes. This study addressed questions of correct localization and functional benefits of heterologous expression of chlorophyllide a oxygenase from Chlamydomonas (CrCAO) in Nannochloropsis.
Results
We cloned CrCAO from Chlamydomonas, which catalyzes oxidation of Chla producing Chlb, and overexpressed it in N. salina to reveal effects of the heterologous Chlb for photosynthesis, growth, and lipid production. For correct localization of CrCAO into the secondary plastid in N. salina, we added the signal-recognition sequence and the transit peptide (cloned from an endogenous chloroplast-localized protein) to the N terminus of CrCAO. We obtained two transformants that expressed CrCAO and produced Chlb. They showed improved growth under medium light (90 μmol/m²/s) conditions, and their photosynthetic efficiency was increased compared to WT. They also showed increased expression of certain photosynthetic proteins, accompanied by an increased maximum electron-transfer rate up to 15.8% and quantum yields up to 17%, likely supporting the faster growth. This improved growth resulted in increased biomass production, and more importantly lipid productivity particularly with medium light.
Conclusions
We demonstrated beneficial effects of heterologous expression of CrCAO in Chlb-less organism N. salina, where the newly produced Chlb enhanced photosynthesis and growth. Accordingly, transformants showed improved production of biomass and lipids, important traits of microalgae from the industrial perspectives. Our transformants are the first Nannochloropsis cells that produced Chlb in the whole evolutionary path. We also succeeded in delivering a heterologous protein into the secondary plastid for the first time in Nannochloropsis. Taken together, our data showed that manipulation of photosynthetic pigments, including Chlb, can be employed in genetic improvements of microalgae for production of biofuels and other biomaterials.
Electronic supplementary material
The online version of this article (10.1186/s13068-019-1462-3) contains supplementary material, which is available to authorized users.
Fruit vegetables are a component in a healthy diet. The dry weight gain and fruit production in fruit vegetables, being dependent on their photosynthetic activities, have been studied, and strategies that enhance fruit development have been discussed based on their source-sink associations. The efficiencies of the photosynthetic leaves and green colour herbaceous stems as the source capacity are critical in influencing the fruit load as fruits and seeds within them are plant parts that demand the photosynthetic product. This review paper discusses light intensity and quality, mineral metabolism, notably nitrogen balance, and leaf-to-fruit rationing that affect the leaf area index and net assimilation rate in crops. Manual leaf removal is among the husbandry practices that reduce the shaded leaves enabling the crops to utilize the resources in their environment more effectively. However, the studies on the source-sink relationship have been focused on a limited number of fruit vegetables, namely tomato and capsicum. This paper aims to gather some useful insights on source-sink manipulation on crops for future studies on many other fruit vegetables like cucurbits, okra etc. This paper also mentions the application of source-sink interaction models as a potential tool in predicting the economic yield of the fruit crops.
This two-year experiment with different planting seasons (April 2018 and August 2021) aimed to determine the effect of deficit irrigation (DI) levels (DI0: 100%, DI30: 70%, and DI60: 40% field capacity) on light interception, light extinction coefficient (k), dry biomass and radiation use efficiency (RUE) of two basil cultivars (Green and Purple). The greenhouse experiment was arranged in a split-plot design with three replications. The results showed that the k values ranged from 0.57 to 0.68 and 0.31 to 0.43 in April and August planting, respectively. The cumulative solar radiation interception, dry biomass, and RUE depended on DI levels, harvest numbers, and seasons. In two study years, dry biomass decreased and RUE increased with the reduction in irrigation water at all harvests and their total. Compared to DI0, basil dry biomass did not show a considerable decrease under DI30 in three harvests in April planting and in the first and second harvests in August planting. April planting had the higher cumulative solar radiation interception in the first and third harvest than the August planting. The basil dry biomass and RUE in the April planting were higher than those in the August planting at all three harvests and their total.
Protected cultivation systems, such as high tunnels, have gained popularity for their ability to enhance growth conditions and extend the growing season of horticultural crops. The choice of the covering film in these systems can significantly impact crop productivity and quality while influencing pest and pathogen incidence. This study aimed to evaluate the yield, physiological responses, and nutritional quality of green and purple basil grown in high tunnels using different plastic film covers. The study was conducted in Central Pennsylvania using twelve research high tunnels covered with four alternative polyethylene films with varying light diffusion and ultraviolet (UV) radiation transparency levels: TuffLite IV (TIV), KoolLite Plus (KLP), UV-transparent (UVT), and UV-opaque (UVO). Green ‘Genovese Compact Improved’ and purple ‘Amethyst Improved’ basil (Ocimum basilicum L.) cultivars were grown as test crops. The plastic covering film had a significant effect on the growth of both basil cultivars, with a significant interaction between the film and basil genotype. Green basil generally exhibited higher fresh and dry biomass compared to purple basil, and the film effect varied with the basil genotype. Leaf area, stem, leaf, and total plant biomass were influenced by the plastic cover, with UVO and UVT films resulting in higher biomass production. The plastic covering films showed varying effects on the mineral content, total chlorophyll, carotenoids, total phenolic compounds, and antioxidant activity, with UVO and UVT films often resulting in improved nutritional quality compared to traditional films. Furthermore, covering films influenced the downy mildew severity on both cultivars and the UVT film consistently limited the severity of the foliar disease in both genotypes. Overall, this study highlights the importance of selecting appropriate plastic covering films with varying levels of UV transparency in high tunnel production systems.
Anthropogenic emissions affect vegetation photosynthesis and carbon flux through meteorological variations induced by aerosols and clouds. However, the insufficient consideration of meteorological conditions limits the understanding of relevant mechanisms, and further inhibits the projection of future terrestrial carbon balance. Based on multiple sets of model simulations, we characterized changes in gross primary production (GPP) due to three typical individual pollutants emissions (black carbon, organic carbon, and sulfate), quantified the relative contributions of co-varied environmental factors, and explored the regulatory roles of background meteorological conditions across China. Our results showed that the heterogeneous GPP enhancement induced by emissions was dominated by cloud cover (CC) change. During its short-term effect, air temperature (Tair), vapor pressure deficit (VPD), and radiation (both quality and quantity) played a collectively non-negligible role in GPP variation, among which the universal diffuse radiation fertilization effect was generally far less than the benefits of brighter, cooler, and wetter environmental conditions. However, the sensitivity of GPP to an individual environmental variable was also altered by background meteorological gradients, whose changing pattern differed substantially among factors, indicating that the meteorological-regulated vegetation optimal photosynthetic range was a trade-off among heat, water, and light instead of being controlled by the univariable. This study implies that a deeper understanding of concurrent environmental variables is an effective way to reduce uncertainties in assessing the terrestrial carbon cycle perturbation exerted by human-induced emissions, especially under future scenarios with ongoing climate change.
The interannual to decadal variability in natural carbon sinks limits the explanation of recent changes in atmospheric CO2 concentration. Here we account for interannual and decadal variability using a simple quasi-mechanistic model of the net land carbon exchange with terms scaling with atmospheric CO2 and a weighted spatial average of temperature anomalies. This approach reduces the unexplained residual in Earth’s carbon cycle budget from ±0.76 GtC per year obtained using process models to ±0.50 GtC per year, with the largest improvements on decadal timescales despite assuming constant dynamics. Our findings reveal remarkable stability of the carbon cycle and allow verification of reported global emissions to within 4.4% (95% confidence level) over the five-year stocktake cycle of the Paris Agreement—half the uncertainty reported previously.
Urban forests are recognized as eco-friendly filters to reduce PM2.5 concentrations. However, there remains a lack of consensus on how PM2.5 affects forests’ productivity in highly polluted urban areas by altering solar radiation components. Here, we tested the hypothesis that the effects of PM2.5 on the solar radiation components and forest productivity differ by season because of seasonal variations in solar elevation angle (β) and PM2.5 concentrations. The PM2.5 concentrations and CO2 fluxes were measured using optical particle counters and an eddy covariance system, respectively, from April 2018 to December 2019 at a natural deciduous oak stand and an evergreen pine plantation in the peri‑urban region of the Seoul Metropolitan Area, Republic of Korea. Complex relationships among PM2.5 concentration, direct radiation (Sdir), diffuse radiation (Sdf), air temperature (Ta), and net ecosystem production (NEP) were disentangled by season and β using structural equation modeling. We found that the Sdf was the controlling factor of NEP in both the stands, showing a more substantial effect than that of Sdir, Ta, and PM2.5. In the oak stand, when oaks hold their leaves (May–October), the increase in PM2.5 concentrations enhanced Sdf and NEP, and it was more significant when β increased from 10 to 20° to > 60° However, in the pine plantation, the increase in PM2.5 concentrations enhanced Sdf and NEP when β > 40° and PM2.5 < 10,100 cm–3. In contrast, when β < 40° and PM2.5 > 10,100 cm–3, PM2.5 increase resulted in a greater Sdir reduction than Sdf enhancement, which consequently led to a NEP reduction. Our study results demonstrated the contrasting seasonal effects of PM2.5 on solar radiation components and forest productivity and highlighted the importance of integrating them into ecosystem modeling and environment management plans in urban or peri‑urban regions.
The sunlight received by plants is affected by cloudiness and pollution. Future changes in cloud cover will differ among regions, while aerosol concentrations are expected to continue increasing globally as a result of wildfires, fossil fuel combustion, and industrial pollution. Clouds and aerosols increase the diffuse fraction and modify the spectral composition of incident solar radiation, and both will affect photosynthesis and terrestrial ecosystem productivity. Thus, an assessment of how canopy and leaf-level processes respond to these changes is needed as part of accurately forecasting future global carbon assimilation. To review these processes and their implications: first, we discuss the physical basis of the effect of clouds and aerosols on solar radiation as it penetrates the atmosphere; second, we consider how direct and diffuse radiation are absorbed and transmitted by plant canopies and their leaves; and finally, we assess the consequences for photosynthesis at the canopy and ecosystem levels. Photobiology will be affected at the atmospheric level by a shift in spectral composition toward shorter or longer wavelengths under clouds or aerosols, respectively, due to different scattering. Changes in the microclimate and spectral composition of radiation due to an enhanced diffuse fraction also depend on the acclimation of canopy architectural and physiological traits, such as leaf area index, orientation, and clumping. Together with an enhancement of light-use efficiency, this makes the effect of diffuse solar radiation on canopy photosynthesis a multilayered phenomenon, requiring experimental testing to capture those complex interactions that will determine whether it produces the persistent enhancement in carbon assimilation that land-surface models currently predict.
It has been long established that the terrestrial vegetation in spring has stronger photosynthetic capability than in autumn. However, this study challenges this consensus by comparing photosynthetic capability of terrestrial vegetation between the spring and autumn seasons based on measurements of 100 in situ eddy covariance towers over global extratropical ecosystems. At majority of the sites, photosynthetic capability, indicated by light use efficiency (LUE) and apparent quantum efficiency, is significantly higher in autumn than in spring, due to lower atmosphere vapor pressure deficit (VPD) at the same air temperature. Seasonal VPD differences also substantially explain the interannual variability of the differences in photosynthetic capability between spring and autumn. We further reveal that VPD in autumn is significantly lower than in spring over 74.14% extratropical areas based on a global climate dataset. In contrast, LUE derived from data-driven vegetation production dataset, is significantly higher in autumn over 61.02% extratropical vegetated areas. Six Earth System Models consistently projected continuous larger VPD values in spring as compared to autumn, which implies that the impacts on vegetation growth will long exist and should be adequately considered when assessing the seasonal responses of terrestrial ecosystems to future climate conditions.
Surface Solar Irradiance (SSI) is required for solar energy planning and adoption, and is a fundamental parameter in modelling weather, climate, ecosystem and agricultural activities. Herein a time series based radiative transfer model was developed to simultaneously retrieve properties of clouds, aerosols and surface albedo, which were in turn used to calculate the components of SSI: i.e., global, direct and diffuse irradiance. The calculated results were calibrated across the Australian continent against in-situ measurements to account for minor factors (e.g., water vapor) not considered by the physics-based method. Detailed validation of the SSI components was performed against three years of in-situ measurements at 11 sites across Australia, at a range of time scales (i.e., instantaneous, hourly, daily and monthly) and under both all-sky and cloudy-sky conditions. The main advantage of the present method is the reliable separation of the direct and diffuse components with consistently low biases (~4 W/m²) at all four time scales, while still maintaining relatively low RMSE (root-mean-square-error) and MAE (mean-absolute-error). Once calibration has been performed the model does not require any ancillary data when implemented operationally: that is the model only requires geostationary satellite data. This model may be implemented across the globe using widely available next generation geostationary satellite data with a handful of ground-data for calibration.
Downward shortwave radiation (SW) and photosynthetically active radiation (PAR) play crucial roles in Earth system dynamics. Spaceborne remote sensing techniques provide a unique means for mapping accurate spatiotemporally continuous SW–PAR, globally. However, any individual polar-orbiting or geostationary satellite cannot satisfy the desired high temporal resolution (sub-daily) and global coverage simultaneously, while integrating and fusing multisource data from complementary satellites/sensors is challenging because of co-registration, intercalibration, near real-time data delivery and the effects of discrepancies in orbital geometry. The Earth Polychromatic Imaging Camera (EPIC) on board the Deep Space Climate Observatory (DSCOVR), launched in February 2015, offers an unprecedented possibility to bridge the gap between high temporal resolution and global coverage and characterize the diurnal cycles of SW–PAR globally. In this study, we adopted a suite of well-validated data-driven machine-learning models to generate the first global land products of SW–PAR, from June 2015 to June 2019, based on DSCOVR/EPIC data. The derived products have high temporal resolution (hourly) and medium spatial resolution (0.1∘×0.1∘), and they include estimates of the direct and diffuse components of SW–PAR. We used independently widely distributed ground station data from the Baseline Surface Radiation Network (BSRN), the Surface Radiation Budget Network (SURFRAD), NOAA's Global Monitoring Division and the U.S. Department of Energy's Atmospheric System Research (ASR) program to evaluate the performance of our products, and we further analyzed and compared the spatiotemporal characteristics of the derived products with the benchmarking Clouds and the Earth's Radiant Energy System Synoptic (CERES) data. We found both the hourly and daily products to be consistent with ground-based observations (e.g., hourly and daily total SWs have low biases of -3.96 and -0.71 W m-2 and root-mean-square errors (RMSEs) of 103.50 and 35.40 W m-2, respectively). The developed products capture the complex spatiotemporal patterns well and accurately track substantial diurnal, monthly, and seasonal variations in SW–PAR when compared to CERES data. They provide a reliable and valuable alternative for solar photovoltaic applications worldwide and can be used to improve our understanding of the diurnal and seasonal variabilities of the terrestrial water, carbon and energy fluxes at various spatial scales. The products are freely available at 10.25584/1595069 (Hao et al., 2020).
Restrictions to reduce human interaction have helped to avoid greater suffering and death from the COVID-19 pandemic, but have also created socioeconomic hardship. This disruption is unprecedented in the modern era of global observing networks, pervasive sensing and large-scale tracking of human mobility and behaviour, creating a unique test bed for understanding the Earth System. In this Perspective, we hypothesize the immediate and long-term Earth System responses to COVID-19 along two multidisciplinary cascades: energy, emissions, climate and air quality; and poverty, globalization, food and biodiversity. While short-term impacts are dominated by direct effects arising from reduced human activity, longer-lasting impacts are likely to result from cascading effects of the economic recession on global poverty, green investment and human behaviour. These impacts offer the opportunity for novel insight, particularly with the careful deployment of targeted data collection, coordinated model experiments and solution-oriented randomized controlled trials, during and after the pandemic.
Radiation components have distinct effects on photosynthesis. In the desert steppe ecosystem, the influence of diffuse radiation on carbon fixation has not been thoroughly explored. We examined this diffusion and its effect on ecosystem productivity was examined during the growing season from 2014 to 2015 on the basis of eddy covariance measurements of CO 2 exchange in a desert steppe ecosystem in northwest China. Our results indicated that the gross ecosystem production (GEP) and diffuse photosynthetically active radiation (PAR dif ) peaked when the clearness index (CI) was around 0.5. The maximum canopy photosynthesis (P max ) under cloudy skies (CI < 0.7) was 23.7% greater than under clear skies (CI ≥ 0.7). When the skies became cloudy in the desert steppe ecosystem, PAR dif had a greater effect on GEP. Additionally, lower vapor pressure deficits (VPD ≤ 1 kPa), lower air temperatures (T a ≤ 20 °C), and non-stressed water conditions (REW ≥ 0.4) were more conducive for enhanced ecosystem photosynthesis under cloudy skies than under clear skies. This may be due to the comprehensive effects of VPD and T a on stomatal conductance. We concluded that cloudiness can influence diffuse radiation components and that diffuse radiation can increase the ecosystem production of desert steppe ecosystems in northwest China.
A number of radiation modification approaches have been proposed to counteract anthropogenic warming by intentionally altering Earth's shortwave or longwave fluxes. While several previous studies have examined the climate effect of different radiation modification approaches, only a few have investigated the carbon cycle response. Our study examines the response of plant carbon uptake to four radiation modification approaches that are used to offset the global mean warming caused by a doubling of atmospheric CO2. Using the National Center for Atmospheric Research Community Earth System Model, we performed simulations that represent four idealized radiation modification options: solar constant reduction, sulfate aerosol increase (SAI), marine cloud brightening, and cirrus cloud thinning (CCT). Relative to the high CO2 state, all these approaches reduce gross primary production (GPP) and net primary production (NPP). In high latitudes, decrease in GPP is mainly due to the reduced plant growing season length, and in low latitudes, decrease in GPP is mainly caused by the enhanced nitrogen limitation due to surface cooling. The simulated GPP for sunlit leaves decreases for all approaches. Decrease in sunlit GPP is the largest for SAI which substantially decreases direct sunlight, and the smallest for CCT, which increases direct sunlight that reaches the land surface. GPP for shaded leaves increases in SAI associated with a substantial increase in surface diffuse sunlight, and decreases in all other cases. The combined effects of CO2 increase and radiation modification result in increases in primary production, indicating the dominant role of the CO2 fertilization effect on plant carbon uptake.
Paul Gordon Jarvis was a widely known and well-respected plant ecologist and physiologist, who pioneered the scientific analysis of the exchange of water and carbon dioxide between forests and the atmosphere, and laid the foundations for decades of study on the interplay between forests and the climate system. He was one of the first to measure directly the photosynthesis and transpiration of forests, and leading from this, his analysis of the relationships between the physiology of plants and the weather has informed and inspired a generation of young scientists. In particular, he was one of the first to address the linkage between knowledge gained at the microscopic scale of stomata and the landscape scale of forests, and the implications of that linkage for the climate system.
Cambridge Core - Plant Sciences - Leaf Optical Properties - by Stéphane Jacquemoud
Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.
CO2 samples were collected on board container ships between Japan and Australia and between Japan and the United States from April 1984 to December 1991. They were then analyzed for their delta13C values with a precision of 0.020/00. The seasonal cycle of delta13C was clearly observable at latitudes from 55°N to the equator. Peak-to-peak amplitude of the average seasonal cycle of delta13C was about 0.80/00 at latitudes north of 40°N and decreased going southward to 0.20/00 at the equator. The comparison of the observed seasonal cycles of delta13C with those of the CO2 concentration suggests that the seasonal CO2 cycles observed at latitudes north of the equator are due to the seasonal change in terrestrial biospheric activities. However, air transport from the southern hemisphere is also thought to be partly responsible for the seasonal CO2 cycles at northern low latitudes. Values of delta13C decreased secularly at a rate of about -0.030/00/year, owing mainly to increased amounts of isotopically light CO2 produced in fossil fuel combustion and deforestation. Interannual variations of the long-term trend of delta13C in association with the El Niño-Southern Oscillation (ENSO) event and other factors were clearly observed, which were almost opposite in phase with those of the CO2 concentration. From the comparison of the observed long-term trend of delta13C with those calculated using a box diffusion model, it is suggested that an imbalance of the CO2 exchange between the atmosphere and the terrestrial biosphere could be responsible for such interannual variations.
We used the eddy-correlation techn~que to investigate the exchange of COZ between an und~sturbed old-growth forest and the atmosphere at a remote Southern Hemi- sphere slte on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO, exchange. and to test whether present knowledge of leaf- level processes was sufficient to understand ecosystem-level exchange. On clear summer days the maxlmum rate of net ecosystem COY uptake exceeded 15 pmo1.m '.s I, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between - -2 and -7 pmol.m-'.s-l. Nighttime CO, efflux rate roughly doubled with a 10°C increase in temperature. Daytime variation in net ecosystem C02 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature. saturation deficit, and the diffuse PPFD were of lesser. but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However. at night. the short-term exchange of CO, between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between noc- turnal CO, exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO, exchange. The daytime light-use efficiency of the ecosystem (CO, uptakehncident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/ mol after frosts on clear winter days. Ecosystem COZ uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.
The volume of shade within vegetation canopies is reduced by more than an order of magnitude on cloudy and/or very hazy days compared to clear sunny days because of an increase in the diffuse fraction of the solar radiance. Here we show that vegetation is directly sensitive to changes in the diffuse fraction and we conclude that the productivity and structure of vegetation is strongly influenced by clouds and other atmospheric particles. We also propose that the unexpected decline in atmospheric [CO2] which was observed following the Mt. Pinatubo eruption was in part caused by increased vegetation uptake following an anomalous enhancement of the diffuse fraction by volcanic aerosols that would have reduced the volume of shade within vegetation canopies. These results have important implications for both understanding and modelling the productivity and structure of terrestrial vegetation as well as the global carbon cycle and the climate system.
A biogeochemical model of vegetation using observed climate data predicts the high northern latitude greening trend over the
past two decades observed by satellites and a marked setback in this trend after the Mount Pinatubo volcano eruption in 1991.
The observed trend toward earlier spring budburst and increased maximum leaf area is produced by the model as a consequence
of biogeochemical vegetation responses mainly to changes in temperature. The post-Pinatubo decline in vegetation in 1992–1993
is apparent as the effect of temporary cooling caused by the eruption. High-latitude CO2 uptake during these years is predicted as a consequence of the differential response of heterotrophic respiration and net
primary production.
Volcanic aerosols from the 1991 Mount Pinatubo eruption greatly increased diffuse radiation worldwide for the following 2 years. We estimated that this increase in diffuse radiation alone enhanced noontime photosynthesis of a deciduous forest by 23% in 1992 and 8% in 1993 under cloudless conditions. This finding indicates that the aerosol-induced increase in diffuse radiation by the volcano enhanced the terrestrial carbon sink and contributed to the temporary decline in the growth rate of atmospheric carbon dioxide after the eruption.
There is significant interannual variability in the atmospheric concentration of carbon dioxide even when the effect of anthropogenic sources has been accounted for. It has been shown that this variability is correlated with the El Nino Southern Oscillation (ENSO) cycle [Bacastow, 1976; Keeling et al., 1995]. However, there are periods during the atmospheric CO2 record when this correlation does not hold and CO2 levels are much lower than can be explained by the correlation with ENSO. These periods coincide with major volcanic eruptions. It has been well documented that a major eruption has a cooling effect on the surface and lower troposphere [McCormick, 1992; Hansen, et al., 1996]. Here we show that it is likely that this cooling has a significant and measurable effect on the carbon cycle. We use a coupled general circulation climate-carbon cycle model to study the mechanisms involved. The model simulates the observed temperature and CO2 response of the climate to the 1991 eruption of Mount Pinatubo. The surface cooling due to the eruption leads to reduced soil and plant respiration globally and increased gross primary productivity in the tropics. The result is significant uptake of carbon (1-2 GtC yr−1) by the terrestrial biosphere for several years after the eruption. There is no significant variation in uptake or release of carbon by the oceans.
Theories describing radiative transfer in plant canopies are very well developed for canopies approximating full cover. However, for canopies of partial cover such as orchards, wide-row crops or widely spaced individual plants, no general theory exists. This paper contains a description of a general radiative transfer model that is capable of accommodating nearly any canopy structure. The plant canopy is approximated by an array of ellipsoidal subcanopies that may be equally spaced, randomly spaced, or spaced in any manner desired. Ellipsoids were chosen because of the infinite variety of shapes that they can approximate. All of the foliage of any individual subcanopy is contained within the confines of its ellipsoidal subcanopy, and assumed to be statistically distributed within the subcanopy volume with respect to location, foliage angle, and foliage area per unit of subcanopy volume. The radiative transfer model includes attenuation of direct beam and sky diffuse radiation, multiple scattering for visible and near-infrared wavelength, and emission for thermal wavelengths. When the subcanopies overlap extensively, the predictions from the general ellipsoidal model approach the predictions from one-dimensional models applicable to canopies of full cover. Predictions from this three-dimensional model are compared with cross-row measurements in a corn canopy. In addition, sunfleck area and sunlit-leaf area comparisons are made between this three dimensional model and a one dimensional model for a developing corn canopy.
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A theory predicting the gap-size distribution of a canopy of horizontal, azimuthally symmetric, randomly located, flat leaves of any shape is compared to measurements in sumac (Rhus typhina L.), sunflower (Helianthus annuus L.), and an artificial canopy. Light intensity distributions of the solar beam component on a horizontal surface below these canopies are predicted by combining the gap-size distribution theory with penumbral effects of the finite solar disc. These predictions of the beamcomponent distribution are then combined with average diffuse penetration theory and a new scattering theory to produce a final light intensity distribution for comparison with measurements from a miniature light sensor in visible and near-infrared wavelength bands. The agreement is good and indicates that sizable penumbra effects occur in canopies only 2 m in height.
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A number of studies show that significant reductions in solar radiation reaching the Earth’s surface have occurred during the past 50 years. This review analyzes the most accurate measurements, those made with thermopile pyranometers, and concludes that the reduction has globally averaged 0.51±0.05Wm−2 per year, equivalent to a reduction of 2.7% per decade, and now totals 20Wm−2, seven times the errors of measurement. Possible causes of the reductions are considered. Based on current knowledge, the most probable is that increases in man made aerosols and other air pollutants have changed the optical properties of the atmosphere, in particular those of clouds. The effects of the observed solar radiation reductions on plant processes and agricultural productivity are reviewed. While model studies indicate that reductions in productivity and transpiration will be proportional to those in radiation this conclusion is not supported by some of the experimental evidence. This suggests a lesser sensitivity, especially in high-radiation, arid climates, due to the shade tolerance of many crops and anticipated reductions in water stress. Finally the steps needed to strengthen the evidence for global dimming, elucidate its causes and determine its agricultural consequences are outlined.
A versatile model for mechanistic simulation of the radiative transfer in discontinuous canopies is presented. Canopy structure is described with arrays of asymmetric crown envelopes, adaptable to various tree geometries and based on the following parameters: total tree height, height at crown insertion and at the greatest width of the crown, crown radii in four orthogonal directions, and shape coefficients of vertical crown profiles. Unlike previous models, this canopy model can simulate the high level of asymmetry in crown shape and displacement typical of natural and semi-natural forests. Within an individual crown, the vertical distribution of leaf area density (LAD) is modelled using the Beta or Weibull equation. The effect of the spatial pattern of leaf area is taken into account by simulating random, regular or clumped distributions. Parameters related to the canopy architecture (vertical and spatial distribution of the leaf area, angular distribution of the leaf normal) are treated as species-specific, so that mixed forests can be represented. Light penetration is modelled by the ‘turbid medium hypothesis’; that is, by computing the beam path length and the LAD within single crowns with a high angular resolution (0.05–5 deg). Diffuse fluxes generated by reflection and transmission of intercepted radiation are simulated by the Adding method, on the basis of the leaf scattering coefficients of each plant species. Using this model, the distribution of radiation intensity and spectra beneath heterogeneous canopies may be analysed in time and space at the required resolution, and could complement research activities involving remote sensing, plant physiology and ecology. The software has been written in C + + with an object oriented approach, and it does not impose any limit to the number of trees or species in an experimental plot.
We thankfor discussion and help with preparation of the manuscript
- D O'brien
- J Powles