Direct Photosynthetically Active Radiation Research Articles

Impact of radiation variations on temporal upscaling of instantaneous Solar-Induced Chlorophyll Fluorescence

Solar-Induced Chlorophyll Fluorescence (SIF) has been increasingly used as a novel proxy for vegetation productivity. Several space-borne instruments can retrieve SIF at varying overpass time, which complicates the interpretation as SIF is driven by absorbed Photosynthetically Active Radiation (PAR) at the acquisition time. To facilitate comparisons across sensors, satellite-based SIF is upscaled to daily averages with a length-of-day correction factor (DC). In conventional DC calculations, the light intensity over a day is approximated geometrically by the cosine of the Solar Zenith Angle (SZA), neglecting changes in atmospheric extinction and topographic effects. Here, we use reanalysis PAR data for DC calculations to evaluate the impact of atmospheric extinction and diffuse radiation individually. We find that the simple SZA approach is a reliable approximation for flat surfaces, where the overall atmospheric impact on DC is less than 10% as large individual effects on direct and diffuse PAR partially compensate each other. At longer time-scales, a sampling (clear sky) bias might exist due to cloud-filtering of satellite data. We find that in the Amazon the true monthly mean PAR can be 25% lower than the one for cloud-filtered days, potentially inducing seasonal SIF biases on the same order. An additional factor impacting PAR during a day is topography. For complex terrain, direct light in the DC expression requires a correction for surface slopes. For example in the San Gabriel Mountains, California, USA, the modified DC is changed by as much as 500% for strongly tilted surfaces. This modification is especially important for satellite instruments with fine spatial resolutions, where surface slopes are not averaged out and can have a substantial impact on reflectance and SIF. Overall, our refined DC-corrections and averaging strategy can help satellite SIF interpretation as well as intercomparisons over a wide range of spatio-temporal scales and overpass times.

Photosynthetically active radiation decomposition models for agrivoltaic systems applications

Decomposition models of solar irradiance estimate the magnitude of diffuse horizontal irradiance from global horizontal irradiance. These two radiation components are well known to be essential for predicting the performance of solar photovoltaic systems. In open-field agrivoltaic systems (i.e., the dual use of land for both agricultural activities and solar power conversion), cultivated crops receive unequal amounts of direct, diffuse, and reflected photosynthetically active radiation (PAR). These uneven amounts depend on where the crops are growing due to the non-homogenous shadings caused by the presence of the installed solar panels (above the crops or vertically mounted). It is known that, per unit of total PAR, diffuse PAR is more efficient for canopy photosynthesis than is direct PAR. For this reason, it is essential to estimate the diffuse PAR component when agrivoltaic systems are being assessed, in order to properly predict the crop yield. Since PAR is the electromagnetic radiation in the 400–700 nm waveband that can be used for photosynthesis by the crops, several stand-alone decomposition models typically used to split global horizontal irradiance are selected in this study to decompose PAR into direct and diffuse. These models are applied and validated in three locations in Sweden (Lanna, Hyltemossa and Norunda) using the coefficients stated on the models’ original publications and locally fitted coefficients. The results showed weaker performances in all stand-alone models for non-locally fitted coefficients (nRMSE ranging from 27% to 43%). However, performances improve with re-parameterization, with a highest nRMSE of 35.24% in Lanna. The Yang2 decomposition model is the best-performing one, with the lowest nRMSE of 23.75% in Norunda when applying re-estimated coefficients. Country level sets of coefficients for the best-performing models (Yang2 and Starke) are given after parameterization using combined data for all three locations in Sweden. These Sweden-fitted models are tested and show an nRMSE of 25.08% (Yang2) and 28.60% (Starke). These results can be used to perform estimations of the PAR diffuse component in Sweden wherever ground measurements are not available. The overall methodology can be similarly applied to other countries.

The Effect of Diffuse Radiation on Ecosystem Carbon Fluxes Across China From FLUXNET Forest Observations

Aerosol loading and cloud cover can alter the composition of radiation reaching the Earth’s surface and affect the ecosystem’s carbon cycle. In this study, we established an empirical model of the diffuse radiation fraction (Kd) based on a clearness index (Kt) to obtain the Kd of four FLUXNET forest sites in China. We focused on the relationships among the Kd, photosynthetically active radiation (PAR), light-use efficiency (LUE) and gross primary productivity (GPP) through mechanistic analysis. The relationships between carbon fluxes [including GPP, ecosystem respiration (ER), and net ecosystem exchange (NEE)] and the Kd were explored. Furthermore, we investigated the influence of environmental factors on carbon fluxes. The results showed that the Kd models were accurate in estimating Kd (R2= 0.88–0.93). Overall, the GPP first increased and then decreased with increasing Kd. When Kd< Ko (Ko, the diffuse radiation fraction corresponding to the maximum value of GPP), the direct PAR decreased as Kd increased, while the diffuse PAR increased rapidly. At this stage, the diffuse fertilization effect led to an increase in GPP. When Ko<Kd<Kdiff-max (Kdiff-max, the diffuse radiation fraction corresponding to the maximum value of diffuse PAR), as Kd increased the direct PAR still decreased and the diffuse PAR still increased, but the GPP declined. When Kd>Kdiff-max, the diffuse PAR began to decrease, and the reduction in the superimposed direct PAR caused the GPP of the canopy to drop rapidly. The LUE of the vegetation canopy was higher under diffuse light conditions than under direct light. Furthermore, with an increase in the Kd, the negative value change of the NEE was consistent with the GPP, but the ER was less affected by the Kd. Finally, the impact of temperature (TA) and vapor pressure deficit (VPD) on the GPP was unimodal, and the impact on the NEE was U-shaped. In addition, latent heat (LE) had a significant positive effect on GPP and NEE. Our study emphasized the relationship between the change in PAR composition and the Kd, as well as its impact on the carbon fluxes change, which is highly important to the study of carbon neutralization.

The Response of the Amazon Ecosystem to the Photosynthetically Active Radiation Fields: Integrating Impacts of Biomass Burning Aerosol and Clouds in the NASA GEOS ESM

Abstract. The Amazon experiences fires every year, and the resulting biomass burning aerosols, together with cloud particles, influence the penetration of sunlight through the atmosphere, increasing the ratio of diffuse to direct photosynthetically active radiation (PAR) reaching the vegetation canopy and thereby potentially increasing ecosystem productivity. In this study, we use the NASA Goddard Earth Observing System (GEOS) model running with coupled aerosol, cloud, radiation, and ecosystem modules to investigate the impact of Amazon biomass burning aerosols on ecosystem productivity, as well as the role of the Amazon’s clouds in tempering the impact. The study focuses on a seven-year period (2010–2016) during which the Amazon experienced a variety of dynamic environments (e.g., La Nina, normal years, and El Nino). The radiative impacts of biomass burning aerosols on ecosystem productivity – call here the aerosol light fertilizer effect – are found to increase Amazonian Gross Primary Production (GPP) by 2.6 % via a 3.8 % increase in diffuse PAR (DFPAR) despite a 5.4 % decrease in direct PAR (DRPAR) on multiyear average. On a monthly basis, this increase in GPP can be as large as 9.9 % (occurring in August 2010). Consequently, the net primary production (NPP) in the Amazon is increased by 1.5 %, or ~92 TgCyr−1– equivalent to ~37 % of the carbon lost due to Amazon fires over the seven years considered. Clouds, however, strongly regulate the effectiveness of the aerosol light fertilizer effect. The efficiency of the fertilizer effect is highest for cloud-free conditions and linearly decreases with increasing cloud amount until the cloud fraction reaches ~0.8, at which point the aerosol-influenced light changes from being a stimulator to an inhibitor of plant growth. Nevertheless, interannual changes in the overall strength of the aerosol light fertilizer effect are primarily controlled by the large interannual changes in biomass burning aerosols rather than by changes in cloudiness during the studied period.

Characterizing the three-dimensional spatiotemporal variation of forest photosynthetically active radiation using terrestrial laser scanning data

The three-dimensional (3-D) spatiotemporal distribution of forest canopy photosynthetically active radiation (PAR) is a major determinant of within-crown variations of leaf photosynthesis and transpiration. However, it remains challenging to map the 3-D spatiotemporal distribution of PAR within and under the forest canopy due to the limited ability to capture the detailed 3-D structures of forest canopies.This study proposes a simple approach to estimate the 3-D spatiotemporal distribution of forest canopy PAR at a fine spatial resolution based on terrestrial laser scanner (TLS) data by characterizing difference in transfer of incident solar direct PAR and diffuse PAR and by considering the effect of the orientation angles of foliage elements on the 3-D distribution of direct PAR. Estimated PAR was validated using 10-minute PAR measurements from pyranometers at different locations within four forest plots, at which effective leaf area index (LAIe) ranged from 0.83–4.25. It showed that the proposed method captured 85% (N=2831, p<0.001) of the variations in the field pyranometer measurements. Furthermore, the proposed method was also applicable for estimating the vertical distribution of PAR (R2=0.95, N=15, p<0.001). Direct, diffuse, and total PAR exhibited similar vertical distributions changing with LAIe. It was shown that without differentiation of direct and diffuse PAR or ignoring effects of orientation angles of foliage elements may cause errors of about 20% in the estimated 3-D PAR distribution. Furthermore, the results indicated that the proposed method can be used to estimate the vertical distribution of PAR when the average neighbor point distance (NPD) is ≤20 cm. This proposed method will be usable for accurate separation of sunlit and shaded leaves and estimation of vertical distribution of PAR, which will benefit to reduce uncertainties in the estimation of terrestrial carbon fluxes and retrieval of vegetation structural and biochemical parameters.

Direct Radiative Effect of Aerosols on Net Ecosystem Carbon Exchange in the Pearl River Delta Region

The environmental impact of aerosols is currently a hot issue that has received worldwide attention. Lacking simultaneous observations of aerosols and carbon flux, the understanding of the aerosol radiative effect of urban agglomeration on the net ecosystem carbon exchange (NEE) is restricted. In 2009-2010, an observation of the aerosol optical property and CO₂ flux was carried out at the Dongguan Meteorological Bureau Station (DMBS) using a sun photometer and eddy covariance systems. The different components of photosynthetically active radiation (PAR), including global PAR (GPAR), direct PAR (DPAR), and scattered PAR (FPAR), were calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The effects of PAR on the NEE between land-atmosphere systems were investigated. The results demonstrated that during the study period the aerosol optical depth (AOD) reduced the DPAR by 519.28±232.89 μmol photons · m⁻² s⁻¹, but increased the FPAR by 324.93±169.85μmol photons · m⁻² s⁻¹, ultimately leading to 194.34±92.62 μmol photons·m⁻² s⁻¹ decrease in the GPAR. All the PARs (including GPAR, DPAR, and FPAR) resulted in increases in the NEE (improved carbon absorption), but the FPAR has the strongest effect with the light use efficiency (LUE) being 1.12 times the values for the DPAR. The absorption of DPAR by the vegetation exhibited photo-inhibition in the radiation intensity &gt; 600 photons · m⁻² s⁻¹; in contrast, the absorptions of FPAR did not exhibit apparent photo-inhibition. Compared with the FPAR caused by aerosols, the DPAR was not the primary factor affecting the NEE. On the contrary, the increase in AOD significantly increased the FPAR, enhancing the LUE of vegetation ecosystems and finally promoting the photosynthetic CO₂ absorption.

Spatio-Temporal Variation in AOD and Correlation Analysis with PAR and NPP in China from 2001 to 2017

Atmospheric aerosols can elicit variations in how much solar radiation reaches the ground surface due to scattering and absorption, which may affect plant photosynthesis and carbon uptake in terrestrial ecosystems. In this study, the spatio-temporal variations in aerosol optical depth (AOD) are compared with that in photosynthetically active radiation (PAR) and net primary productivity (NPP) during 2001–2017 in China using multiple remote sensing data. The correlations between them are analyzed at different scales. Overall, the AOD exhibited a northeast-to-southwest decreasing pattern in space. A national increasing trend of 0.004 year−1 and a declining trend of −0.007 year−1 of AOD are observed during 2001–2008 and 2009–2017. The direct PAR (PARdir) and diffuse PAR (PARdif) present consistent and opposite tendency with AOD during two periods, respectively. The total PAR (PARtotal) shows a similar variation pattern with PARdir. In terms of annual variation, the peaks of AOD coincide with the peaks of PARdif and the troughs of PARdir, indicating that aerosols have a significant positive impact on PARdir and a negative impact on PARdif. Furthermore, the PARdir has a stronger negative association with AOD than the positive correlation between PARdif and AOD at national and regional scales, indicating that PARdir is more sensitive to aerosol changes. The NPP has higher values in the east than in the west and exhibits a significant increasing trend of 0.035 gCm−2day−1 after 2008. The NPP has a negative correlation (−0.4–0) with AOD and PARdif and a positive correlation (0–0.4) with PARdir in most areas of China. The area covered by forests has the highest NPP-PAR correlation, indicating that NPP in forests is more sensitive to the PAR than is the NPP in grasslands and croplands. This study is beneficial for interpreting the aerosol-induced PAR impact on plant growth and for predicting plant production on haze days.

Modelling Three-Dimensional Spatiotemporal Distributions of Forest Photosynthetically Active Radiation Using UAV-Based Lidar Data

The three dimensional (3-D) spatiotemporal variations of forest photosynthetically active radiation (PAR) dictate the exchange rates of matter and energy in the carbon and water cycle processes between the plant-soil system and the atmosphere. It is still challenging to explicitly simulate spatial PAR values at any specific position within or under a discontinuous forest canopy. In this study, we propose a novel lidar-based approach to estimate both direct and diffuse forest PAR components from a 3-D perspective. An improved path length-based direct PAR estimation method was developed by incorporating the point density along a light transmission path, and we also obtained the diffuse PAR components using a point-based sky view analysis by assuming the anisotropic sky diffuse distribution. We compared the total PAR modelled using three light path length-based parameters with reference data measured by radiometers on a five-minute time scale during a daily solar course. Our results show that, in a discontinuous forest canopy, the effective path length is a feasible and powerful (R2 = 0.92, p &lt; 0.01) parameter to capture the spatiotemporal variations of total PAR along a light transmission path with a mean bias of −53.04 μmol·m−2·s−1(−6.8%). Furthermore, incorporating point density and spatial distribution factors will further improve the final estimation accuracy (R2 = 0.97, p &lt; 0.01). In the meantime, diffuse PAR tends to be overestimated by 17% at noon and underestimated by about 10% at sunrise and sunset periods by assuming the isotropic sky diffuse distribution. The proposed lidar-based 3-D PAR model will provide a solid foundation to various process-based eco-hydrological models for simulating plant physiological processes such as photosynthesis and evapotranspiration, intra-species competition and succession, and snowmelt dynamics purposes.

Border row effects on light interception in wheat/maize strip intercropping systems

In strip intercropping systems, different crop rows in a complete strip usually behave differently in growth dynamics and have distinct contribution to the final yield of the system, so resources capture and utilization processes of different crop rows should be treated separately in the performance evaluation of the whole system. This study was conducted to investigate the differences in light capture among different crop rows in wheat/maize strip intercropping systems. The field experiment comprised four planting patterns: monoculture wheat, monoculture maize, six rows of wheat alternated with two rows of maize (I62), and twelve rows of wheat alternated with four rows of maize (I124). A geometrical radiation transmission model was modified to estimate instantaneous light capture by different intercropped rows, which was tested with photosynthetically active radiation (PAR) measured above and beneath the intercrop canopy. The results demonstrated that the model accurately predicted PAR transmitted through the canopy with mean absolute error, root mean square error (RMSE) and normalized RMSE of 60.2μmolm−2s−1, 82.6μmolm−2s−1 and 5.2% respectively for I62 and of 65.9μmolm−2s−1, 95.3μmolm−2s−1 and 5.7% respectively for I124. With assistance of the model, we found that border rows in wheat strip intercepted far greater amount of both direct and diffuse PAR than the inner rows, whereas maize border rows showed no advantage in PAR absorption due to its canopy development was constrained by the competition from wheat strip. Canopy structure character showed positive effects on radiation interception in wheat border rows and all maize rows, and plant phenotypic plasticity showed positive effects on wheat border rows but negative effects on maize border rows. The radiation transmission model presented in this work was proved to be a useful tool for calculating spatial and temporal variability of radiation in strip intercropping systems and it can be further served as a base for modeling the plant growth dynamics in strip intercrops.

Leaf color is fine-tuned on the solar spectra to avoid strand direct solar radiation.

The spectral distributions of light absorption rates by intact leaves are notably different from the incident solar radiation spectra, for reasons that remain elusive. Incident global radiation comprises two main components; direct radiation from the direction of the sun, and diffuse radiation, which is sunlight scattered by molecules, aerosols and clouds. Both irradiance and photon flux density spectra differ between direct and diffuse radiation in their magnitude and profile. However, most research has assumed that the spectra of photosynthetically active radiation (PAR) can be averaged, without considering the radiation classes. We used paired spectroradiometers to sample direct and diffuse solar radiation, and obtained relationships between the PAR spectra and the absorption spectra of photosynthetic pigments and organs. As monomers in solvent, the spectral absorbance of Chl a decreased with the increased spectral irradiance (Wm(-2)nm(-1)) of global PAR at noon (R(2)=0.76), and was suitable to avoid strong spectral irradiance (λmax=480nm) rather than absorb photon flux density (μmolm(-2)s(-1)nm(-1)) efficiently. The spectral absorption of photosystems and the intact thallus and leaves decreased linearly with the increased spectral irradiance of direct PAR at noon (I dir-max), where the wavelength was within the 450-650nm range (R(2)=0.81). The higher-order structure of photosystems systematically avoided the strong spectral irradiance of I dir-max. However, when whole leaves were considered, leaf anatomical structure and light scattering in leaf tissues made the leaves grey bodies for PAR and enabled high PAR use efficiency. Terrestrial green plants are fine-tuned to spectral dynamics of incident solar radiation and PAR absorption is increased in various structural hierarchies.

Seasonal Variation in Understory Light Near a Gap Edge and Its Association with Conifer Seedling Survival in a Southern New England Forest

Forest edge-generated gradients in understory light are known to influence the survival of tree seedlings in North American deciduous forests, but the seasonality of that influence has not been investigated for the forests of southern New England. We examined the winter versus summer difference in photosynthetically active radiation (PAR) transmitted through a southern New England forest canopy in the vicinity of a large experimental canopy gap and investigated if this difference influenced the survival of Pinus strobus L. (White Pine) and Tsuga canadensis (L.) Carr. (Eastern Hemlock) seedlings along a gap—forest gradient using Kaplan-Meier survival curves and Cox proportional hazards models. The mean and the spatial distribution of winter understory PAR transmittance (%T) across the gap—forest gradient were significantly different from those of summer %T. Distance from gap edge into the forest and variability in %T during the winter months were associated with the survival of White Pine seedlings, but not with that of Eastern Hemlock seedlings, which had a constant survival probability throughout the gap—forest gradient. Summer %T was not associated with the survival of seedlings of either species. Experimentally, we found that digital fisheye canopy photos did not accurately capture the range in understory light level along a gap—forest gradient. Thus they are a poor proxy for direct PAR transmittance measurements and should be used with caution in southern New England forests, which are highly fragmented and contain many edges. Forest management actions for the conservation of White Pine should take into account winter understory light regimes rather than summer regimes.

On the stability of radiometric ratios of photosynthetically active radiation to global solar radiation in Tsukuba, Japan

The precise measurement of incident photosynthetically active radiation (PAR) is crucial for the estimation of ecosystem vegetation productivity. However, reliable values of PAR are seldom measured routinely. Instead, it is sometimes estimated based on solar radiation (RS). One popular estimation method is by use of the conversion ratio PAR/RS. Depending on whether PAR is expressed in energy units (PE, W m−2) or photon units (PP, μmol m−2 s−1), there are two types of conversion ratios: PE/RS (unitless), or PP/RS (μmol J−1). Moreover, to translate PAR expressed in one unit to another, the ratio PP/PE (μmol J−1) is also important. However, past studies have not led to a general understanding of these ratios, mainly due to instrumental errors such as cosine errors. To reduce such errors, we developed a new PAR measurement system using grating spectroradiometers mounted on sun trackers to measure direct and diffuse PAR separately. The present study aims to clarify the characteristics of these three ratios using this new, more precise measurement system and radiative transfer simulation. We carried out measurements for one year in Tsukuba, Japan (36.05°N, 140.13°E). PE/RS increased with water vapor pressure (e) from 0.40 to 0.47 and increased with decreasing clearness index (kT), but did not strongly depend on solar zenith angle (θ). PP/RS also increased with e from 1.9μmol J−1 to 2.2μmol J−1. Its dependence on climatic factors was similar to that of PE/RS. PP/PE varied by about 3% around the value of 4.57μmol J−1.

Estimating the photosynthetically active radiation under clear skies by means of a new approach

Abstract. The k-distribution method and the correlated-k approximation of Kato et al. (1999) is a computationally efficient approach originally designed for calculations of the broadband solar radiation by dividing the solar spectrum in 32 specific spectral bands from 240 to 4606 nm. This paper describes a technique for an accurate assessment of the photosynthetically active radiation (PAR) from 400 to 700 nm at ground level, under clear-sky conditions using twelve of these spectral bands. It is validated against detailed spectral calculations of the PAR made by the radiative transfer model libRadtran. For the direct and global PAR irradiance, the bias is −0.4 W m−2 (−0.2%) and −4 W m−2 (−1.3%) and the root mean square error is 1.8 W m−2 (0.7%) and 4.5 W m−2 (1.5%). For the direct and global Photosynthetic Photon Flux Density, the biases are of about +10.3 μmol m−2 s−1 (+0.8%) and 1.9 μmol m−2 s−1 (−0.1%) respectively, and the root mean square error is 11.4 μmol m−2 s−1 (0.9%) and 4.0 μmol m−2 s−1 (0.3%). The correlation coefficient is greater than 0.99. This technique provides much better results than two state-of-the-art empirical methods computing the daily mean of PAR from the daily mean of broadband irradiance.

Using a Simple Apparatus to Measure Direct and Diffuse Photosynthetically Active Radiation at Remote Locations

Plant canopy interception of photosynthetically active radiation (PAR) drives carbon dioxide (CO2), water and energy cycling in the soil-plant-atmosphere system. Quantifying intercepted PAR requires accurate measurements of total incident PAR above canopies and direct beam and diffuse PAR components. While some regional data sets include these data, e.g. from Atmospheric Radiation Measurement (ARM) Program sites, they are not often applicable to local research sites because of the variable nature (spatial and temporal) of environmental variables that influence incoming PAR. Currently available instrumentation that measures diffuse and direct beam radiation separately can be cost prohibitive and require frequent adjustments. Alternatively, generalized empirical relationships that relate atmospheric variables and radiation components can be used but require assumptions that increase the potential for error. Our goal here was to construct and test a cheaper, highly portable instrument alternative that could be used at remote field sites to measure total, diffuse and direct beam PAR for extended time periods without supervision. The apparatus tested here uses a fabricated, solar powered rotating shadowband and other commercially available parts to collect continuous hourly PAR data. Measurements of total incident PAR had nearly a one-to-one relationship with total incident radiation measurements taken at the same research site by an unobstructed point quantum sensor. Additionally, measurements of diffuse PAR compared favorably with modeled estimates from previously published data, but displayed significant differences that were attributed to the important influence of rapidly changing local environmental conditions. The cost of the system is about 50% less than comparable commercially available systems that require periodic, but not continual adjustments. Overall, the data produced using this apparatus indicates that this instrumentation has the potential to support ecological research via a relatively inexpensive method to collect continuous measurements of total, direct beam and diffuse PAR in remote locations.

Estimation of direct, diffuse, and total FPARs from Landsat surface reflectance data and ground‐based estimates over six FLUXNET sites

AbstractThe fraction of photosynthetically active radiation (PAR) absorbed by green elements (FPAR) is an essential climate variable in quantifying canopy absorbed PAR (APAR) and gross and net primary production. Current satellite FPAR products typically correspond to black‐sky FPAR under direct illumination only, but the radiation transfer and vegetation absorption processes differ for direct and diffuse PARs. To address this, the present study developed a new approach to estimate direct, diffuse, and total FPARs, separately, from Landsat surface reflectance data. Field‐measured direct and diffuse FPARs were first derived for crops, deciduous broadleaf forests, and evergreen needleleaf forests at six FLUXNET sites. Then, a coupled soil‐leaf‐canopy radiative transfer model (SLC) was used to simulate surface reflectance under direct and diffuse illumination conditions. Direct, diffuse, and total FPARs were estimated by comparing Landsat‐5 Thematic Mapper (TM) data and simulated surface reflectances using a lookup table approach. The differences between the Landsat‐estimated and the field‐measured FPARs are less than 0.05 (10%). The diffuse FPAR is higher than the direct FPAR by up to 19.38%, whereas the total FPAR is larger than the direct FPAR by up to 16.07%. The direct APAR is higher than the diffuse APAR under clear‐sky conditions, but underestimates the total APAR by −277.72 µmol s−1 m−2 on average. The approach described here can be extended to estimate direct, diffuse, and total FPARs from other satellite data and the obtained FPAR variables could be helpful to improve modeling of vegetation processes.

Testing arbitrary classes of light in a physiognomically heterogeneous area of "campo rupestre" vegetation

The Brazilian "campo rupestre" vegetation shows high light heterogeneity, characteristic which is often disregarded in ecophysiological studies due the difficult of obtaining accurate measurements in a complex environment. The aim of the present study it was to describe how much heterogeneous is the light distribution in this environment, as well as tried to define classes in relation to the photosynthetically active radiation (PAR) for 30 rupicolous individuals of Bulbophyllum involutum. The individuals were marked and previously classified in light classes visually predetermined ("full sun", "partially shaded" and "shaded"). Once defined these classes, principal component analysis, cluster and dispersal analyses using direct and indirect PAR measures were performed to characterize the environment as well as to validate the inclusion of the individuals in each class. The use of multivariate analysis increased the accuracy of predetermined classes of light, and some individuals from "shaded" and "partially shaded" classes needed to be regrouped. After individuals' reclassification, amplitude of PAR variance was reduced in each class, enhancing homogeneity. Ours results show the possibility to evaluate light environment conditions more precisely in "campo rupestre" vegetation using multivariate analysis, allowing the increase in accuracy of this parameter for ecophysiological researches.

Simulated effects of changes in direct and diffuse radiation on canopy scale isoprene emissions from vegetation following volcanic eruptions

Abstract. Volcanic eruptions can alter the quality of incoming solar irradiance reaching the Earth's surface thereby influencing the interactions between vegetation and the Earth system. Isoprene (C5H8) is a biogenic volatile organic compound emitted from leaves at a rate that is strongly dependent on the received flux of photosynthetically active radiation (PAR). We used a theoretical approach to investigate the potential for volcanic eruptions to change the isoprene flux from terrestrial forests using canopy-scale isoprene emission simulations that vary either the relative or absolute amount of diffuse (Idiff) and direct (Idir) PAR. According to our simulations for a northern hardwood deciduous forest, if the total amount of PAR during summer remains constant while the proportion of Idiff increases, canopy-scale isoprene emissions increase. This effect increases as leaf area index (LAI) increases. Simulating a decrease in the total amount of PAR, and a corresponding increase in Idiff fraction, as measured during the 1992 Pinatubo eruption, changes daily total canopy-scale isoprene emissions from terrestrial vegetation in summertime by +2.8% and −1.4% for LAI of 6 and 2, respectively. These effects have not previously been realized or quantified. Better capturing the effects of volcanic eruptions (and other major perturbations to the atmospheric aerosol content) on isoprene emissions from the terrestrial biosphere, and hence on the chemistry of the atmosphere, therefore may require inclusion of the effects of aerosols they produce on climate and the quality of PAR.

Model parameterization to simulate and compare the PAR absorption potential of two competing plant species.

Mountain pastures dominated by the pasture grass Setaria sphacelata in the Andes of southern Ecuador are heavily infested by southern bracken (Pteridium arachnoideum), a major problem for pasture management. Field observations suggest that bracken might outcompete the grass due to its competitive strength with regard to the absorption of photosynthetically active radiation (PAR). To understand the PAR absorption potential of both species, the aims of the current paper are to (1) parameterize a radiation scheme of a two-big-leaf model by deriving structural (LAI, leaf angle parameter) and optical (leaf albedo, transmittance) plant traits for average individuals from field surveys, (2) to initialize the properly parameterized radiation scheme with realistic global irradiation conditions of the Rio San Francisco Valley in the Andes of southern Ecuador, and (3) to compare the PAR absorption capabilities of both species under typical local weather conditions. Field data show that bracken reveals a slightly higher average leaf area index (LAI) and more horizontally oriented leaves in comparison to Setaria. Spectrometer measurements reveal that bracken and Setaria are characterized by a similar average leaf absorptance. Simulations with the average diurnal course of incoming solar radiation (1998–2005) and the mean leaf–sun geometry reveal that PAR absorption is fairly equal for both species. However, the comparison of typical clear and overcast days show that two parameters, (1) the relation of incoming diffuse and direct irradiance, and (2) the leaf–sun geometry play a major role for PAR absorption in the two-big-leaf approach: Under cloudy sky conditions (mainly diffuse irradiance), PAR absorption is slightly higher for Setaria while under clear sky conditions (mainly direct irradiance), the average bracken individual is characterized by a higher PAR absorption potential. (∼74 MJ m−2 year−1). The latter situation which occurs if the maximum daily irradiance exceeds 615 W m−2 is mainly due to the nearly orthogonal incidence of the direct solar beam onto the horizontally oriented frond area which implies a high amount of direct PAR absorption during the noon maximum of direct irradiance. Such situations of solar irradiance favoring a higher PAR absorptance of bracken occur in ∼36% of the observation period (1998–2005). By considering the annual course of PAR irradiance in the San Francisco Valley, the clear advantage of bracken on clear days (36% of all days) is completely compensated by the slight but more frequent advantage of Setaria under overcast conditions (64% of all days). This means that neither bracken nor Setaria show a distinct advantage in PAR absorption capability under the current climatic conditions of the study area.

Scaling up of CO 2 fluxes from leaf to canopy in maize-based agroecosystems

Carbon dioxide fluxes are being measured in three maize-based agroecosystems in eastern Nebraska in an effort to better understand the potential for these systems to sequester carbon in the soil. Landscape-level fluxes of carbon, water and energy were measured using tower eddy covariance systems. In order to better understand the landscape-level results, measurements at smaller scales, using techniques promoted by John Norman, were made and scaled up to the landscape-level. Single leaf gas exchange properties (CO 2 assimilation rate and stomatal conductance) and optical properties, direct and diffuse radiation incident on the canopy, and photosynthetically active radiation (PAR) reflected and transmitted by the canopy were measured at regular intervals throughout the growing season. In addition, soil surface CO 2 fluxes were measured using chamber techniques. From leaf measurements, the responses of net CO 2 assimilation rate to relevant biophysical controlling factors were quantified. Single leaf gas exchange data were scaled up to the canopy level using a simple radiative model that considers direct beam and diffuse PAR penetration into the canopy. Canopy level photosynthesis was estimated, coupled with the soil surface CO 2 fluxes, and compared to measured net ecosystem CO 2 exchange (NEE) values from the eddy covariance approach. Estimated values of canopy level absorbed PAR was also compared to measured values. The agreement between estimated and observed values increases our confidence in the measured carbon pools and fluxes in these agroecosystems and enhances our understanding of biophysical controls on carbon sequestration.

Estimating photosynthetically active radiation distribution in maize canopies by a three-dimensional incident radiation model

The three-dimensional (3-D) radiation distribution model in plant canopy is pivotal for understanding and modelling plant eco-physiological processes. Diffuse and direct radiations penetrate into plant canopies in different ways and may present different intensity and wavelength composition. Sunfleck (the canopy surfaces where the direct radiation reaches) distribution in the plant canopy is usually regarded as an important index for crop development, especially under dense canopy conditions. Distributions of direct and diffuse components of photosynthetically active radiation (PAR) in maize (Zea mays L.) canopies were estimated respectively using a 3-D incident radiation model (3DIRM). The 3DIRM model was set up for computing incident radiation in crop canopies by applying a parallel-projection based submodel for direct solar radiation and a central-projection based submodel for incident diffuse radiation simulation in crop canopy. It was well assessed with a field experiment with multi-point PAR measurement in maize canopies with relative errors of 2.6, 4.5 and 2.6%, respectively, for sunfleck area ratio, diffuse PAR and total PAR. The results suggest that the 3DIRM model could be used to estimate the direct, diffuse and total PAR at any specific surface part in the 3-D canopy space. The exponential distinction model for direct, diffuse and total PAR along with leaf area index in different heights in maize canopies was also evaluated based on the 3DIRM simulation results.