Daily Photosynthetically Active Radiation Research Articles

Yield Performance of Intercropped Marantha arundinacea L. (Arrowroot) in Two Rubber Plantation Designs

Developing rubber agroforestry systems is crucial to ensure the sustainable development of natural rubber cultivation. This study focuses on the starch crop Maranta arundinacea (arrowroot) and assesses its productivity and influencing factors when intercropped in 6–7-year-old conventional single-row and double-row rubber plantations. We analyze various aspects, including light resources, root distribution, soil nutrients, arrowroot growth characteristics, and product quality. The results indicate that the daily average photosynthetically active radiation (PAR) in the double-row rubber agroforestry system intercropping area ranges from 896.4 to 940.2 μmol·m−2·s−1. Additionally, the rubber tree roots near the intercropping area are less dense (107.0 g cm−3). In contrast, the conventional single-row rubber agroforestry system has a significantly lower daily average PAR of only 145.7 μmol·m−2·s−1, and the nearby rubber tree roots are more abundant (616.2 g cm−3). Although soil nutrient levels were slightly lower in the intercropping area on the double-row treatment compared to the single-row treatment, there was no statistical difference (p < 0.05). Arrowroot’s photosynthetic capacity in the double-row rubber agroforestry system intercropping area is significantly greater than in the single-row rubber agroforestry system intercropping area. The yield per unit area in the former (23.46–27.47 t·ha−1) is also significantly higher than in the latter (2.87–4.75 t·ha−1, p < 0.05), with higher starch content. Therefore, arrowroot exhibits higher productivity when intercropped in double-row rubber agroforestry systems, making it suitable for establishing a “rubber–arrowroot” agroforestry model to enhance the yield per unit area of rubber plantations.

The Effects of Light Intensity and Flow Speed on Biogeochemical Variability Within a Fringing Coral Reef in Onna‐Son, Okinawa, Japan

AbstractGlobal warming and ocean acidification are driving declines in seawater dissolved oxygen (DO) concentrations and pH. Predicting how these changes will affect shallow, near‐shore environments such as coral reefs is challenging due to their high natural biogeochemical variability present over both spatial (m to km) and temporal (diel to seasonal) scales. To make predictions, we must understand the drivers of this variability. The impact of metabolic processes on coral reef biogeochemical variability has been the subject of significant research effort, however, physical factors, including flow speed and light intensity, have received less attention. Here, we measured seawater flow, photosynthetically active radiation (PAR), pH, and DO at three reef habitats (reef flat, lagoon, and outflow channel) in a fringing coral reef system in Okinawa, Japan for 3 weeks in October 2019. During the study, pH ranged from 7.86 to 8.37 units while DO varied from 127 to 369 μmol/kg. Circulation was primarily wave‐driven with mean flow speeds ranging from 14 to 26 cm/s. Flow direction became increasingly consistent at higher flow speeds and traced benthic striations visible in satellite imagery. Multiple linear regression models of daytime changes in pH and DO versus daily mean flow speed and PAR described 25%–74% of the observed variability across all sites while at night, flow speed alone accounted for 7%–75% of the observed variability. These results demonstrate PAR, water flow speed, and the path water takes play important roles in controlling biogeochemical variability within coral reefs and must be considered when assessing their vulnerability to both local and global environmental change.

Mapping long-term and high-resolution global gridded photosynthetically active radiation using the ISCCP H-series cloud product and reanalysis data

Abstract. Photosynthetically active radiation (PAR) is a fundamental physiological variable for research in the ecological, agricultural, and global change fields. In this study, we produced a 35-year (1984–2018) high-resolution (3 h, 10 km) global gridded PAR dataset using an effective physical-based model. The main inputs of the model were the latest International Satellite Cloud Climatology Project (ISCCP) H-series cloud products, MERRA-2 aerosol data, ERA5 surface routine variables, and MODIS and CLARRA-2 albedo products. Our gridded PAR product was evaluated against surface observations measured at 7 experimental stations of the SURFace RADiation budget network (SURFRAD), 42 experimental stations of the National Ecological Observatory Network (NEON), and 38 experimental stations of the Chinese Ecosystem Research Network (CERN). Instantaneous PAR was validated against SURFRAD and NEON data; mean bias errors (MBE) and root mean square errors (RMSE) were, on average 5.8 and 44.9 W m−2, respectively, and the correlation coefficient (R) was 0.94 at the 10 km scale. When upscaled to 30 km, the errors were markedly reduced. Daily PAR was validated against SURFRAD, NEON, and CERN data, and the RMSEs were 13.2, 13.1, and 19.6 W m−2, respectively, at the 10 km scale. The RMSEs were slightly reduced when upscaled to 30 km. Compared with the well-known global satellite-based PAR product of the Earth's Radiant Energy System (CERES), our PAR product was found to be a more accurate dataset with higher resolution. This new dataset is now available at https://doi.org/10.11888/RemoteSen.tpdc.271909 (Tang, 2021).

Implications of spatial-temporal shading in agrivoltaics under fixed tilt & tracking bifacial photovoltaic panels

The spatial and temporal behavior of the incident sunlight can have important implications for agrivoltaic (AV) crop yield. Here we explore the short term (daily) and long term (monthly) variations of the photosynthetically active radiation (PAR) under various tracking and fixed-tilt agrivoltaic PV modules configurations and propose strategies to minimize the shade-induced crop yield loss. For a fixed row to row PV module spacing, vertically installed PV modules facing East/West provide the best spatial homogeneity and a higher intensity of net daily incident PAR, while the traditional North/South faced fixed tilt PV modules result in a high spatial contrast, i.e., a significantly lower PAR underneath the PV modules as compared to the open space between the adjacent rows of PV modules. The shading behavior for the horizontal single axis tracking PV modules shows a seasonal dependence, a higher shading vertically below the PV modules during winters and vice versa for summers. The spatial variation in the crop yield correlates well with that of the net daily PAR spatial pattern in the case of the shade sensitive crops (e.g., tomato) while the spatial yield for the shade tolerant crops (e.g., lettuce) is less affected by PAR variations. We propose an intercropping approach where the cultivated areas for crops having high and low shade sensitivity are identified based on the calculated spatial PAR pattern. A case study of intercropping lettuce with tomato under various PV module configurations shows that this approach can minimize the biomass loss for shade sensitive crops in the presence of spatial PAR heterogeneity.

The effects of temperature and light on ecosystem metabolism in a Japanese stream

Ecosystem metabolism is a fundamental process that governs biogeochemical C cycling through the fixation and mineralization of C by all the organisms in a system; however, the functional responses of ecosystem metabolism to light and temperature variability remain unknown in streams and rivers. In this study, we assessed the effects of temperature and light on ecosystem metabolism, namely gross primary production (GPP) and ecosystem respiration (ER), in a Japanese stream by monitoring the seasonal diel changes in dissolved oxygen. We found that the temperature dependence of metabolism, which is expressed as activation energy, was higher for ER (0.48 eV) than for GPP (0.39 eV), supporting the metabolic theory of ecology. Both daily GPP and ER increased with the daily integrated photosynthetically active radiation (PAR), peaking at moderate PAR values and decreasing at higher irradiance levels. This unimodal relationship between daily GPP and PAR was due to the rapid increase of instantaneous PAR during the early morning, suppressing the maximum rate of area-specific productivity, which was a main determinant of daily GPP. Moreover, daily GPP and ER were closely associated in this study stream, resulting in a unimodal pattern of daily ER with daily integrated PAR. Our results suggest that both temperature and light regulate stream ecosystem metabolism but that the sunlight regime (i.e., the rapid increase of solar irradiance in the early morning) rather than light intensity per se may be a critical factor for both GPP and ER in the study stream.

Upwelling Irradiance below Sea Ice—PAR Intensities and Spectral Distributions

Upwelling and downwelling spectral (320–920 nm) distributions and photosynthetic active radiation (PAR) intensities were measured below a first-year land-fast sea ice in a western Greenland fjord with and without a snow cover. Time-series of surface upwelling PAR, downwelling PAR, and under-ice PAR were also obtained. Spectral distributions of upwelling and downwelling irradiances were similar except for reduced intensities in the UV, the red, and NIR parts of the spectrum when the ice was snow-covered. Upwelling PAR amounted to about 10% of downwelling intensities, giving 5.1 µmol photons m−2 s−1 at the bottom of the ice with a snow cover and 8.2 µmol photons m−2 s−1 without. PAR partitioning analyses showed that the upwelling was related to scattering by suspended particles in the water column. A snow melt increased under-ice daily maximum downwelling PAR from 50 to 180 µmol photons m−2 s−1 and overall under-ice PAR of 55 and 198 µmol photons m−2 s−1 with 10% upwelling. It is concluded that upwelling PAR below sea ice might be an important factor regarding sea ice algae photophysiology and performance with a 10% higher PAR; specifically when PAR > Ek the light saturation point of the sea ice algae.

Retrieving high-resolution surface photosynthetically active radiation from the MODIS and GOES-16 ABI data

Incident photosynthetically active radiation (PAR) is a key parameter in plant physiological, biological and physical process-based terrestrial ecosystem models. Deriving highly accurate and spatially continuous PAR from remote sensing data can fill in the gaps of insufficient ground-based measurements. In this study, we provide an efficient approach for estimating PAR by employing look-up table (LUT) method using observations from both MODIS and GOES-16. The GOES-16 is the first satellite of the new-generation US Geostationary Operational Environmental Satellite R-series (GOES-R), which can capture dynamic changes in the atmosphere every 5 min. First, the diurnal variation of surface bidirectional reflectance is characterized assuming that the surface characteristics and solar zenith angles in tropical and middle latitudes at a given time of day do not change dramatically within a month. Next, the aerosol optical depth (AOD) and cloud optical depth (COD) are retrieved from the GOES-16 visible observations under both clear-sky and cloudy conditions. Finally, PAR is estimated via the retrieved AOD and COD using the LUT method and a MODIS-based surface albedo product. The estimated PAR was compared against ground-based measurements collected from seven SURFace RADiation budget network (SURFRAD) sites and 23 National Ecological Observatory Network (NEON) sites at instantaneous, hourly and daily scales. The instantaneous PAR validation results achieved R2 values of 0.97 and 0.82, and root mean square errors (RMSEs) of 18.1 and 51.1 W m‐2, for clear and cloudy skies, respectively. The RMSEs of the hourly averaged PAR were reduced to 16.8 W m−2 and 40.4 W m−2, respectively, for clear-sky and cloudy conditions. The daily mean PAR evaluation for all sky conditions shows an overall mean bias error (MBE) of 0.97 W m−2 and an RMSE of 10.5 W m−2. Compared with other global PAR datasets, the estimated PAR values in this study have lower RMSEs and higher spatial-temporal resolution. Uncertainties in the retrieved PAR caused by topography and solar-cloud-geometry effects (SCGE) can be reduced by spatial and temporal averaging. Based on an evaluation of retrievals at different spatial-temporal scales, we recommend that a 20 × 20 km2 domain-averaged PAR be utilized for instantaneous PAR validation, whereas hourly and daily averaged PAR values are nearly independent of spatial scale.

Intercepted Photosynthetically Active Radiation (PAR) and Spatial and Temporal Distribution of Transmitted PAR under High-Density and Super High-Density Olive Orchards

We quantified the photosynthetically active radiation (PAR) interception in a high-density (HD) and a super high-density (SHD) or hedgerow olive system, by measuring the PAR transmitted under the canopy along transects at increasing distance from the tree rows. Transmitted PAR was measured every minute, then cumulated over the day and the season. The frequencies of the different PAR levels occurring during the day were calculated. SHD intercepted significantly but slightly less overall PAR than HD (0.57 ± 0.002 vs. 0.62 ± 0.03 of the PAR incident above the canopy) but had a much greater spatial variability of transmitted PAR (0.21 under the tree row, up to 0.59 in the alley center), compared to HD (range: 0.34–0.43). This corresponded to greater variability in the frequencies of daily PAR values, with the more shaded positions receiving greater frequencies of low PAR values. The much lower PAR level under the tree row in SHD, compared to any position in HD, implies greater self-shading in lower-canopy layers, despite similar overall interception. Therefore, knowing overall PAR interception does not allow an understanding of differences in PAR distribution on the ground and within the canopy and their possible effects on canopy radiation use efficiency (RUE) and performance, between different architectural systems.

3D city models for urban farming site identification in buildings

Studies have suggested that there is farming potential in urban residential buildings. However, these studies are limited in scope, require field visits and time-consuming measurements. Furthermore, they have not suggested ways to identify suitable sites on a larger scale let alone means of surveying numerous micro-locations across the same building. Using a case study area focused on high-rise buildings in Singapore, this paper examines a novel application of three-dimensional (3D) city models to identify suitable farming micro-locations (level and orientation) in residential buildings. We specifically investigate whether the vertical spaces of these buildings comprising outdoor corridors, façades and windows receive sufficient photosynthetically active radiation (PAR) for growing food crops and do so at a high resolution. We also analyze the spatio-temporal characteristics of PAR, and the impact of shadows and different weather conditions on PAR in the building. Environmental simulations on the 3D model of the study area indicated that the cumulative daily PAR or Daily Light Integral (DLI) at a location in the building was dependent on its orientation and shape, sun's diurnal and annual motion, weather conditions, and shadowing effects of the building's own façades and surrounding buildings. The DLI in the study area generally increased with building's levels and, depending on the particular micro-location, was found suitable for growing moderately light-demanding crops such as lettuce and sweet pepper. These variations in DLI at different locations of the same building affirmed the need for such simulations. The simulations were validated with field measurements of PAR, and correlation coefficients between them exceeded 0.5 in most cases thus, making a case that 3D city models offer a promising practical solution to identifying suitable farming locations in residential buildings, and have the potential for urban-scale applications.

Spatial-Temporal Shading Under Mobile & Fixed Tilt Bifacial Agrivoltaic Panels & Implications for the Cropping Practices

Spatial and temporal behavior of the incident sunlight can have important implications for agrivoltaic (AV) crop yield. We explore the daily and monthly variations of the photosynthetically active radiation (PAR) under various tracking and fixed-tilt agrivoltaic solar panels and propose strategies to minimize the shade-induced crop yield loss. Vertically installed solar panels facing East/West provide the best spatial homogeneity and a higher intensity of net daily incident PAR, while North/South faced fixed tilt panels result in a high spatial contrast, i.e., a significantly lower PAR underneath the panels as compared to the spacing between the adjacent panel rows. The shading behavior for the horizontal single axis tracking panels shows a seasonal dependence, a higher shading below the panels during winters and vice versa for summers. The spatial variation in the crop yield correlates well with that of the net daily PAR pattern in the case of the shade sensitive crops (e.g., tomato) while the spatial yield for the shade tolerant crops (e.g., lettuce) is less affected by PAR variations. We propose an intercropping approach using crops with high and low shade sensitivity to minimize the biomass loss related to PAR heterogeneity and present lettuce/tomato intercropping schemes under various agrivoltaic configurations

Radiación fotosintéticamente activa evaluada en la ciudad de Zacatecas

The present investigation is related to one of the most important processes for the development of life on Earth; photosynthesis, an essential process in the cycle and development of living beings, centered on solar radiation that is useful for plants to carry out this process, Photosynthetically Active Radiation (PAR). The objective of this work is to generate information on the PAR through a database to collaborate in the decision-making of farmers in the area. For this purpose, a quantum sensor installed in building 6 of the UAZ Siglo XXI Campus was used. According to Abal (2013), in agricultural and production planning, it is especially important to have a detailed knowledge of incident solar radiation on the earth's surface (Abal and Durañona, 2013). When collecting, treating and analyzing the data, it was found that the daily average PAR is 819.52 μmol of photons m-2 s-1 (179.47 W m-2), if only the sunny hours are taken into account. It can be concluded that according to the PAR received in the evaluation region and the type of nutrients in the soil, other crop alternatives to those traditionally used can be sought.

A New Set of MODIS Land Products (MCD18): Downward Shortwave Radiation and Photosynthetically Active Radiation

Surface downward shortwave radiation (DSR) and photosynthetically active radiation (PAR), its visible component, are key parameters needed for many land process models and terrestrial applications. Most existing DSR and PAR products were developed for climate studies and therefore have coarse spatial resolutions, which cannot satisfy the requirements of many applications. This paper introduces a new global high-resolution product of DSR (MCD18A1) and PAR (MCD18A2) over land surfaces using the MODIS data. The current version is Collection 6.0 at the spatial resolution of 5 km and two temporal resolutions (instantaneous and three-hour). A look-up table (LUT) based retrieval approach was chosen as the main operational algorithm so as to generate the products from the MODIS top-of-atmosphere (TOA) reflectance and other ancillary data sets. The new MCD18 products are archived and distributed via NASA’s Land Processes Distributed Active Archive Center (LP DAAC). The products have been validated based on one year of ground radiation measurements at 33 Baseline Surface Radiation Network (BSRN) and 25 AmeriFlux stations. The instantaneous DSR has a bias of −15.4 W/m2 and root mean square error (RMSE) of 101.0 W/m2, while the instantaneous PAR has a bias of −0.6 W/m2 and RMSE of 45.7 W/m2. RMSE of daily DSR is 32.3 W/m2, and that of the daily PAR is 13.1 W/m2. The accuracy of the new MODIS daily DSR data is higher than the GLASS product and lower than the CERES product, while the latter incorporates additional geostationary data with better capturing DSR diurnal variability. MCD18 products are currently under reprocessing and the new version (Collection 6.1) will provide improved spatial resolution (1 km) and accuracy.

Assessment of phytoplankton photosynthetic efficiency based on measurement of fluorescence parameters and radiocarbon uptake in the Kara Sea

Spatial distribution of relative electron transport rate (rETR) values, the maximum quantum efficiency of PSII (Fv/Fm), and biomass-specific primary production (PB, mg C (mg Chl a)−1 h−1) were described for the eastern part of the Kara Sea in the autumn (September). A characteristic feature of this period was a noticeable decrease in the length of the day and the elevation angle of the sun, leading to a significant decrease in surface photosynthetically active radiation (PAR). Despite low light in the euphotic zone, the phytoplankton maximum quantum efficiency of PSII (Fv/Fm) was high (0.5–0.7) in the upper 30-m layer of the water column, which indicates a potentially active state of phytoplankton. At the same time, the main productive activity of phytoplankton was linked to the surface 0–3 m and was closely related to the daily incident PAR and the share of diatoms in the total phytoplankton biomass. Despite a decrease in light level and following a reduction in the values for productive characteristics of the phytoplankton community, diatoms continued to play a major role in primary productivity in the eastern Kara Sea at the end of the vegetative season. Comparative analysis of data obtained by two different techniques – fluorescence measurements and experimental carbon fixation estimations – demonstrated a close relationship between rETR and PB. However, surface values of PB changed more strongly than rETR, which may, apparently, reflect different efficiency in the use of absorbed light energy in the synthesis processes. Photosynthetic efficiency, reflecting the extent of use of the light energy caught in processes of organic matter synthesis, can be expressed through the ratio between PB and rETR values. The PB/rETR ratio increased in areas characterized by drastic gradients of hydrophysical conditions: over the external coastal shelf, central shelf and lower continental slope of the St Anna Trough.

Spatiotemporal variations of photosynthetically active radiation and the influencing factors in China from 1961 to 2016

Daily measurements of photosynthetically active radiation (PAR) and other meteorological parameters at 16 stations in China during 2005–2015 were used to develop the all-weather Adaptive-Network-based Fuzzy Inference System (ANFIS) PAR models. The ANFIS models were compared with other semi-empirical and semi-physical methods, and the results showed that ANFIS models using global solar radiation (G) as input parameter provided more accurate estimates than other models. Daily PAR values during 1961–2016 were reconstructed using meteorological parameters at 908 meteorological stations, and the spatiotemporal characteristics of PAR in China were analyzed. The daily PAR ranged from 20.03 to 36.29 mol m−2 day−1 with annual mean value being about 29.03 mol m−2 day−1 in China during 1961–2016. The PAR values were generally higher in Qinghai-Tibet Plateau (QTP) and Inner Mongolian Plateau (IMP) and lower in Northeast and Southeast China, especially in the Sichuan Basin. PAR decreased in most part of China, especially in Northeast China, with the minimum value being about − 0.11 mol m−2 day−1 year−1 during 1961–2016. The increasing trend only appeared in Southwest China with the maximum value being about 0.02 mol m−2 day−1 year−1. Annual mean daily G, PAR fraction (Fp, the ratio of PAR and G), sunshine duration (SSD), relative humidity (RH), and water pressure (MP) were further analyzed to find the influencing parameters for PAR. The results showed that G, Fp, and SSD were the major factors for influencing PAR in most part of China, but in some areas, RH and MP influenced the PAR more effectively. Finally, the monthly variations of aerosol optical depth (AOD) in the Yangtze River Basin gathered from the Moderate Resolution Imaging Spectroradiometer (MODIS) Level 3 Atmosphere Product during 2002–2016 were analyzed, which was one important factors influencing the PAR values in summer and winter months.

Seasonal variations in a polysaccharide composition of Far Eastern red seaweed Ahnfeltiopsis flabelliformis (Phyllophoraceae)

The quantitative and qualitative composition of polysaccharides from red alga Ahnfeltiopsis flabelliformis collected in the Amur Bay (Sea of Japan) from February to December 2009, depending on the water temperature and daily dose of photosynthetically active radiation (PAR), was analyzed. The highest content of polysaccharides was observed in February and November–December at low water temperature and daily PAR dose. The polysaccharide amount produced by the seaweed significantly decreased from March to September and increased from September to November. The main components of the polysaccharides were galactose and 3,6-anhydrogalactose containing sulfate groups. The galactose content was high throughout the studied period (28–42%), 3,6-anhydrogalactose ones in the summer (17.4%) and did not depend on environmental factors. The sulfate group amount was high in April–May and July–August. A statistically significant positive correlation was found between the sulfate group amount in the polysaccharide and the average daily PAR dose in the habitat of seaweed. The dynamics of dry weight accumulation during the year was similar to the polysaccharides. According to IR spectroscopy, the polysaccharides mainly contained disaccharide units of kappa-carrageenans and iota-carrageenans. The polysaccharide from A. flabelliformis collected in June had more regular structure than that in other months.

An efficient algorithm for calculating photosynthetically active radiation with MODIS products

Photosynthetically active radiation (PAR) is the critical forcing data in ecological and agricultural fields. Remote sensing can be utilized to derive spatiotemporally continuous PAR. Empirical algorithms can be used to quickly retrieve surface PAR data sets, but their accuracy cannot be guaranteed in regions without local calibration. Physical algorithms generally incorporate all relevant physical processes and can be used globally, but their computational efficiency is often low. In this paper, an efficient algorithm is developed to calculate surface PAR by combining a clear-sky PAR model and the parameterizations for cloud transmittances. In the algorithm, the transmittances for water vapor, ozone, Rayleigh, aerosol, and cloud are each handled across the whole PAR band (400–700nm). In addition, the contribution of the multiple reflections between surface ground and the atmosphere are also expressly considered. The new algorithm is applied to estimate instantaneous PAR with inputs from Moderate Resolution Imaging Spectroradiometer (MODIS) products onboard both Terra and Aqua platforms. The daily PAR is estimated from these two instantaneous values by an upscaling method. The instantaneous and daily PAR estimates were validated with in situ data collected in the USA and China. The results indicate that the new algorithm, based on MODIS products, can effectively retrieve PAR with root mean square errors (RMSE) of about 40Wm−2 and 15Wm−2 at instantaneous and daily-mean time scales, respectively. These performances are generally better than those of previous studies.

Photosynthetically active radiation and carbon gain drives the southern orientation of Myrtillocactus geometrizans fruits.

The equatorial orientation of reproductive structures is known in some columnar cacti from extratropical deserts. It has been hypothesised that photosynthetically active radiation (PAR) interception is the main reason for this orientation, because of its key effect on nocturnal CO2 uptake. However, there are no studies addressing both the effect of PAR and its consequence, carbon gain, on fruit orientation. Accordingly, we tested whether PAR and carbon gain could explain the southern fruit orientation of Myrtillocactus geometrizans, an inter-tropical columnar cactus. We studied three populations of M.geometrizans in Mexico. For each population, azimuth of fruits, total daily PAR, nocturnal acid accumulation (NAA) and fruit production were measured. The relationships between rib orientation and number of fruits, as well as total daily PAR, were evaluated using periodic regressions. The effect of total daily PAR and NAA on number of fruits was assessed using generalised linear models. During spring, mean fruit orientation had a south azimuth for three populations. Likewise, rib orientation had a significant effect on fruit production, with the south-facing ribs having the maximum number of fruits. Total daily PAR was highest in the south-facing ribs, at least for those in the northern and central populations. Furthermore, during spring, there was a significant positive effect of total daily PAR and NAA on fruit production. Our results provide strong evidence that the higher carbon gain in equatorial ribs, through a highest interception of PAR, would be the responsible factor for equatorial orientation of fruits in an inter-tropical columnar cactus.

Estimation of daily photosynthetically active radiation (PAR) in presence of low to high aerosol loads: application to OLCI-like satellite data.

Estimation of daily photosynthetically active radiation (PAR) is of primary importance for monitoring the ocean primary production and the subsequent production of carbon by phytoplankton at global scale from remote sensing ocean color sensors. On the other hand, aerosol abundance and composition play a critical role in the modulation of PAR. In this study, an original algorithm, so-called OLCIPAR, is proposed for routinely determining the daily PAR from optical satellite sensors such as the OLCI sensor aboard Sentinel-3 (ESA). The OLCIPAR algorithm has been developed to overcome some of the limitations of the current existing methods. In particular, multiple scattering effects induced by the atmospheric layer are taken into account based on exact radiative transfer calculations. Another advantage of OLCIPAR method is to consider a great variety of aerosol models to better account for their optical variability as observed in real world conditions. The OLCIPAR algorithm was applied to the archive of MERIS data, whose sensor is similar to OLCI. The validation of the retrieved daily PAR was carried out based on comparison with the time series acquired by the BOUSSOLE oceanographic buoy moored in the Mediterranean Sea. Results show a regression slope of 1% and an accuracy within 10% which confirms the robustness of the algorithm. The comparison of OLCIPAR retrievals with the products routinely distributed by NASA shows that estimates of PAR differ by up to 20% in the subtropical Atlantic Ocean where important amounts of dust aerosols are present. The improvements brought by OLCIPAR method for deriving the daily PAR could thus permit to better assess the impact of aerosols on reduction of PAR with implications on the estimation of oceanic primary production.

Effects of the partitioning of diffuse and direct solar radiation on satellite-based modeling of crop gross primary production

Modeling crop gross primary production (GPP) is critical to understanding the carbon dynamics of agro-ecosystems. Satellite-based studies have widely used production efficiency models (PEM) to estimate cropland GPP, wherein light use efficiency (LUE) is a key model parameter. One factor that has not been well considered in many PEMs is that canopy LUE could vary with illumination conditions. This study investigates how the partitioning of diffuse and direct solar radiation influences cropland GPP using both flux tower and satellite data. The field-measured hourly LUE under cloudy conditions was 1.50 and 1.70 times higher than that under near clear-sky conditions for irrigated corn and soybean, respectively. We applied a two-leaf model to simulate the canopy radiative transfer process, where modeled photosynthetically active radiation (PAR) absorbed by canopy agreed with tower measurements (R2=0.959 and 0.914 for corn and soybean, respectively). Derived canopy LUE became similar after accounting for the impact of light saturation on leaf photosynthetic capacity under varied illumination conditions. The impacts of solar radiation partitioning on satellite-based modeling of crop GPP was examined using vegetation indices (VI) derived from MODIS data. Consistent with the field modeling results, the relationship between daily GPP and PAR×VI under varied illumination conditions showed different patterns in terms of regression slope and intercept. We proposed a function to correct the influences of direct and diffuse radiation partitioning and the explained variance of flux tower GPP increased in all experiments. Our results suggest that the non-linear response of leaf photosynthesis to light absorption contributes to higher canopy LUE on cloudy days than on clear days. We conclude that accounting for the impacts of solar radiation partitioning is necessary for modeling crop GPP on a daily or shorter basis.

Explaining the variability of the photochemical reflectance index (PRI) at the canopy-scale: Disentangling the effects of phenological and physiological changes

Assessing photosynthesis rates at the ecosystem scale and over large regions is important for tracking the global carbon cycle and remote sensing has provided new and useful approaches for performing this assessment. The photochemical reflectance index (PRI) is a good estimator of short-term light-use efficiency (LUE) at the leaf scale; however, confounding factors appear at larger temporal and spatial scales. In this study, canopy-scale PRI variability was investigated for three species (Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L.) growing under contrasting soil moisture conditions. Throughout the growing season, no significant differences in chlorophyll content and in violaxanthin, antheraxanthin and zeaxanthin were found between species or treatments. The daily PRI vs PAR (photosynthetically active radiation) relationships were determined using continuous measurements obtained at high frequency throughout the entire growing season, from early spring budburst to later autumn senescence, and were used to deconvolute the physiological PRI variability related to LUE variations due to phenological variability and related to temporal changes in the biochemical and structural canopy attributes. The PRI vs PAR relationship is used to show that the canopy-scale PRI measured at low radiation depends on the chlorophyll content of the canopy. The range of PRI variations at an intra-daily scale and the dynamics of the xanthophyll pool do not vary between days, which suggests that the PRI responds to a xanthophyll ratio. The PAR values at PRI saturation are mainly related to the canopy chlorophyll content during budburst and senescence and to the soil moisture content when the chlorophyll content is no longer a limiting factor. This parameter is significantly lower in the oak species that experience less stress from variations in soil moisture and is species dependant. These results provide new insights regarding the analysis and the meaning of PRI variability as a proxy for LUE at the canopy scale.