Canopy Absorption Research Articles

Evaluating Numerical Methods to Investigate Spectral Solar Radiative Transfer in Plant Canopies

AbstractThe disposition of spectral solar irradiance in plant canopies is crucially important to understand processes such as photolysis of molecules amenable to absorbing actinic light. Thus, one objective of this study is to evaluate the most commonly applied radiative transfer approaches to estimate spectral irradiance as a function of plant canopy depth. Eight radiative transfer approaches are ascertained. Another objective is to determine the impacts of the spectral resolution assumed in radiative transfer calculations and model choice on key processes such as canopy absorption and reflection of irradiance. By comparing results from broadband‐only and spectrally‐resolved canopy radiative transfer, we aim to quantitatively determine the uncertainties associated with failing to resolve the sunlight spectra. We determine the optimal spectral resolution required to estimate canopy radiative transfer results such as air‐chemistry‐specific quantities related to photolysis of a select group of molecules. In addition, we evaluate techniques for binning leaf and soil optical properties. Results showed that high spectral resolution is ideally desired to compute photolysis of molecules such as ozone, nitrogen dioxide, nitrate radical, nitrous acid, and formaldehyde. For in‐canopy photolysis of molecules, a waveband resolution of at least 10 nm is sufficient to obtain accurate estimates for most photochemical reactions. Positive reaction‐dependent uncertainties in canopy‐mean relative photolysis values for individual molecules can be as high as 30% compared to estimates derived with broad‐band solar irradiance.

Incorporating canopy radiation enhances the explanation of maize yield change and increases model accuracy under film mulching

Plastic film mulching (PM) has been extensively used to increase crop yields in dryland regions in China. However, the impact of differently colored PM on crop radiation utilization due to the optical properties of the underlying PM surfaces remains unclear. We conducted a two-year field experiment in northwest China to evaluate the dynamics of photosynthetically active radiation interception and absorption (fIPAR and fAPAR), as well as the radiation use efficiency for intercepted and absorbed radiation (RUEi and RUEa) during spring maize (Zea mays L.) growth periods. Two fertilization levels (high and low) and 3 PM treatments (transparent film, black film, and no film) were considered in this study. We also developed empirical regression functions to calculate canopy fAPAR, accounting for both canopy and underlying surface characteristics. Furthermore, we used canopy absorption radiation to drive the Soil-Plant-Atmosphere Continuum System (SPACSYS) model to simulate maize biomass and yield. We found that transparent film mulching (TM) exhibited higher fIPAR and fAPAR than black film mulching and no mulching (CK). TM had an increase of RUEi and RUEa by 10.6% and 9.7%, respectively, relative to CK, resulting in a 9.3–19.2% increase in maize yield and a 25.8–30.2% increase in the partial factor productivity of nitrogen. These increases were mainly attributed to the increased relative chlorophyll content, net photosynthesis rate, and extended grain-filling period under TM. We found that the modified radiation input increased the accuracy of the SPACSYS model in simulating maize biomass and yield. We underscore the importance of using TM in dryland areas to increase crop yields and advocate for incorporating canopy radiation data into crop models, especially in agricultural regions where plastic film mulching is used, to enhance the accuracy of yield simulations.

Nitrogen budgets of a lower subtropical forest as affected by 6 years of over-canopy and understory nitrogen additions

Although tropical and subtropical regions have replaced temperate regions as the global-change hotspots for increased atmosphere nitrogen (N) deposition, whether the regional forests reach N saturation is still unclear. Understory or floor N addition has been commonly used in N-deposition studies, but the results of such studies have recently been challenged because they fail to account for canopy interception, assimilation, and leaching processes. Here, we conducted a field experiment to quantify the effects of over-canopy and understory N addition on N budgets in a lower subtropical monsoon evergreen broadleaved (LSMEB) forest. We found that the LSMEB forest was not N saturated after receiving additional N at 25 and 50 kg ha−1 yr−1 for 6 years. Plants were able to absorb the added N by increasing the N concentrations in their organs, with 120–412 % increasing trend of plant N pools under N-addition treatments. Canopy absorption of N resulting from over-canopy N addition led to increases in N concentrations in tree organs but not to increases in tree biomass. Understory N addition could underestimate the effects of N deposition in forests due to neglecting canopy N interception and canopy effects on N redistribution. Additional experiments using over-canopy N addition are needed to assess the true effects of N deposition on different forest ecosystems in different climate zones.

Divergent Responses of Dominant Forest Tree Species to Manipulated Canopy and Understory N Additions in Terms of Foliage Stoichiometric, Photosynthetic, and Hydraulic Traits

AbstractNitrogen (N) deposition effects on the stoichiometric balance and photosynthetic and hydraulic couplings in subtropical forests has drawn wide attentions. The previously adopted understory application of N fertilization is criticized because it might ignore foliar N retention for different species. This paper reports a fertilizing application from the canopy (CAN) and under the canopy (UAN) in a phosphorus (P) limited ecosystem. Foliage stoichiometric, photosynthetic, and hydraulic traits of six dominant species were measured and analyzed. Both treatments equally enhanced foliage N and N/P, but not foliage P, who was highly species‐specific depending on tree height, which implied enhanced P limitation. Decreased isotope abundance of 15N (δ15N) that approaching to the level in the urea fertilizer under CAN suggested the existence of canopy retention of N. Besides, N response sensitivity of N, P and δ15N that positively related to tree height (H) under CAN indicated different exposure to the added N, which promoted stoichiometric imbalance among species. The photosynthetic traits represented by net photosynthesis (An) increased under both treatments. A divergent foliar photosynthetic and hydraulic traits variation was identified by significant decreased stomatal conductance (gs) and An/gs for CAN treatments, which induced the elevated isotope abundance of 13C (δ13C). Correspondingly, foliage hydraulic traits that shifted to water use efficiency axis were identified only under CAN in principal component analysis. Overall, our results proved that the canopy absorption and species heterogeneity should be considered regarding foliar safety versus efficiency trade‐off in response to nitrogen additions in the future.

Canopy uptake of volatilized 15NH3 derived from urea applications by tomato (Lycopersicum esculentum Mill.) in plastic-shed vegetable production systems

The absorption of NH3 by the tomato canopy and the dynamics of this process in the plastic-shed vegetable production systems have not been reported. To quantify the contribution of the leaf absorption of 15NH3, two field experiments were established to measure the amount of NH3 translocated and recovered cumulatively (Experiment Ⅰ) and instantaneously (Experiment Ⅱ) by tomato canopy. Four applications of 15N-labeled urea as side dressing using a plastic tray method to simulate field management conditions. This experimental procedure prevented the uptake of N from the labeled urea by the roots. The results showed that total NH3 loss accounted for 17.9 % of the N applied using the tray method, with average losses of 21.5 % and 14.3 % during the summer and winter growing seasons, respectively. The average canopy absorption of volatilized 15NH3 was 12.2 % of the total N applied and the average canopy absorption rate of 15NH3 reached to 2.81 % of the total 15N-urea applied. Additionally, an average of 44.13 % of the absorbed 15NH3-N was transferred into the tomato fruit. Across four growth stages, whole-plant 15N accumulation increased gradually over the first three stages, with peak absorption occurring during the topping and pruning stage. Then, absorption rates decreased significantly during harvesting. NH3 uptake by the tomato canopy was significantly more dynamic in summer than in winter. Our findings shed significant light on how NH3 is taken up by the greenhouse tomato canopy and can contribute to improving the sustainability of agricultural systems.

Translocation and recovery of 15N-labeled N derived from the foliar uptake of 15NH3 by the greenhouse tomato (Lycopersicon esculentum Mill.)

In order to completely evaluate ammonia emission from greenhouse vegetable fields, crop canopy absorption should not be neglected. The foliar uptake of NH3 applied at two growth stages and the subsequent 15N-labeled N translocation to other plant components were investigated under greenhouse conditions using chambers covered with the soil of a tomato field. Treatments comprised three NH3-N application rates (70, 140, and 210 mg/plot) using 15N-labeled ammonium sulfate. Plants were harvested immediately after exposure for 24 h, and the total N concentrations and 15N/14N ratios were determined. With increased NH3 concentration, total 15NH3-N absorption increased considerably, whereas the applied 15NH3-N uptake decreased gradually. The tomato plants absorbed 33–38% and 24–31% of the 15NH3-N generated at the anthesis and fruit growth stages, respectively. A total of 71–80% of the recovered NH3 was observed in the leaves and 20–30% of the recovered NH3 was remobilized to other components. Among them, an average of 10% of the absorbed 15NH3-N was transferred into the tomato fruits. All these results indicated the potential of the tested tomatoes for the foliar uptake of atmospheric 15NH3 and the distribution of 15N-labeled vegetative N among different plant components. The results are of great importance for the complete evaluation of nitrogen use efficiency in the greenhouse tomato fields.

Evaluation of leaf-level optical properties employed in land surface models

Abstract. Vegetation optical properties have a direct impact on canopy absorption and scattering and are thus needed for modeling surface fluxes. Although plant functional type (PFT) classification varies between different land surface models (LSMs), their optical properties must be specified. The aim of this study is to revisit the “time-invariant optical properties table” of the Simple Biosphere (SiB) model (later referred to as the “SiB table”) presented 30 years ago by Dorman and Sellers (1989), which has since been adopted by many LSMs. This revisit was needed as many of the data underlying the SiB table were not formally reviewed or published or were based on older papers or on personal communications (i.e., the validity of the optical property source data cannot be inspected due to missing data sources, outdated citation practices, and varied estimation methods). As many of today's LSMs (e.g., the Community Land Model (CLM), the Jena Scheme of Atmosphere Biosphere Coupling in Hamburg (JSBACH), and the Joint UK Land Environment Simulator (JULES)) either rely on the optical properties of the SiB table or lack references altogether for those they do employ, there is a clear need to assess (and confirm or correct) the appropriateness of those being used in today's LSMs. Here, we use various spectral databases to synthesize and harmonize the key optical property information of PFT classification shared by many leading LSMs. For forests, such classifications typically differentiate PFTs by broad geo-climatic zones (i.e., tropical, boreal, temperate) and phenology (i.e., deciduous vs. evergreen). For short-statured vegetation, such classifications typically differentiate between crops, grasses, and photosynthetic pathway. Using the PFT classification of the CLM (version 5) as an example, we found the optical properties of the visible band (VIS; 400–700 nm) to fall within the range of measured values. However, in the near-infrared and shortwave infrared bands (NIR and SWIR; e.g., 701–2500 nm, referred to as “NIR”) notable differences between CLM default and measured values were observed, thus suggesting that NIR optical properties are in need of an update. For example, for conifer PFTs, the measured mean needle single scattering albedo (SSA, i.e., the sum of reflectance and transmittance) estimates in NIR were 62 % and 78 % larger than the CLM default parameters, and for PFTs with flat leaves, the measured mean leaf SSA values in NIR were 20 %, 14 %, and 19 % larger than the CLM defaults. We also found that while the CLM5 PFT-dependent leaf angle values were sufficient for forested PFTs and grasses, for crop PFTs the default parameterization appeared too vertically oriented, thus warranting an update. In addition, we propose using separate bark reflectance values for conifer and deciduous PFTs and demonstrate how shoot-level clumping correction can be incorporated into LSMs to mitigate violations of turbid media assumption and Beer's law caused by the nonrandomness of finite-sized foliage elements.

Estimation of relative canopy absorption and scattering at L-, C- and X-bands

Quantification of microwave vegetation properties (i.e. vegetation optical depth (VOD) and single scattering albedo (ω)) is of interest not only to reliably estimate soil moisture from the Earth's microwave emission, but also for broad applications across plant physiology and hydrology. Estimated VOD is often used as a biomass proxy or to investigate plant water dynamics on multiple scales, as it is dependent on the vegetation water content, dry woody biomass, and canopy structure. For most but not all microwave satellite remote sensing applications, VOD and ω are assigned a priori from auxiliary vegetation information and used as inputs to retrieve soil moisture. Alternatively, polarized brightness temperature has been used to estimate VOD and ω. Retrieval algorithms typically use the zeroth-order solution to the radiative transfer equation (tau-omega model) to simultaneously characterize surface and vegetation emission. In this study we investigate one year of estimated VOD and ω at L- (1.4 GHz), C– (6.9 GHz) and X-band (10.7 GHz), from SMAP and AMSR2 satellites respectively. Since VOD and ω describe absorption and scattering integrated over a path length, we derive estimates of relative absorption and scattering through normalization by a vegetation height derived from GLAS light detection and ranging (lidar) measurements.We assess whether relative attenuation is independent of vegetation height and land cover class. We also test how the relative absorption and scattering estimates follow a spectrum across microwave frequencies. We apply this analysis globally and find relative canopy absorption and scattering to follow distinct patterns, relatively independent of the spatial distributions of vegetation height. We find that relative canopy absorption generally peaks for shorter vegetation (5–10 m) and high density (shrub lands) while high forest canopies have relatively lower values. The VOD values that include the path length (taller vegetation) however are higher for forests relative to shorter vegetation. Seasonal amplitudes of VOD are higher for C- and X-band than for L-band and have maxima in regions with strong rainfall seasonality. This study presents first results for global relative canopy attenuation, based on one consistent retrieval for three frequencies, and discusses temporal dynamics as well as the role of multiple frequencies in assessing attenuation in canopies.

Influence of Leaf Specular Reflection on Canopy Radiative Regime Using an Improved Version of the Stochastic Radiative Transfer Model

Interpreting remotely-sensed data requires realistic, but simple, models of radiative transfer that occurs within a vegetation canopy. In this paper, an improved version of the stochastic radiative transfer model (SRTM) is proposed by assuming that all photons that have not been specularly reflected enter the leaf interior. The contribution of leaf specular reflection is considered by modifying leaf scattering phase function using Fresnel reflectance. The canopy bidirectional reflectance factor (BRF) estimated from this model is evaluated through comparisons with field-measured maize BRF. The result shows that accounting for leaf specular reflection can provide better performance than that when leaf specular reflection is neglected over a wide range of view zenith angles. The improved version of the SRTM is further adopted to investigate the influence of leaf specular reflection on the canopy radiative regime, with emphases on vertical profiles of mean radiation flux density, canopy absorptance, BRF, and normalized difference vegetation index (NDVI). It is demonstrated that accounting for leaf specular reflection can increase leaf albedo, which consequently increases canopy mean upward/downward mean radiation flux density and canopy nadir BRF and decreases canopy absorptance and canopy nadir NDVI when leaf angles are spherically distributed. The influence is greater for downward/upward radiation flux densities and canopy nadir BRF than that for canopy absorptance and NDVI. The results provide knowledge of leaf specular reflection and canopy radiative regime, and are helpful for forward reflectance simulations and backward inversions. Moreover, polarization measurements are suggested for studies of leaf specular reflection, as leaf specular reflection is closely related to the canopy polarization.

Fast matrix treatment of 3-D radiative transfer in vegetation canopies: SPARTACUS-Vegetation 1.1

Abstract. A fast scheme is described to compute the 3-D interaction of solar radiation with vegetation canopies. The canopy is split in the horizontal plane into one clear region and one or more vegetated regions, and the two-stream equations are used for each, but with additional terms representing lateral exchange of radiation between regions that are proportional to the area of the interface between them. The resulting coupled set of ordinary differential equations is solved using the matrix-exponential method. The scheme is compared to solar Monte Carlo calculations for idealized scenes from the RAMI4PILPS intercomparison project, for open forest canopies and shrublands both with and without snow on the ground. Agreement is good in both the visible and infrared: for the cases compared, the root-mean-squared difference in reflectance, transmittance and canopy absorptance is 0.020, 0.038 and 0.033, respectively. The technique has potential application to weather and climate modelling.

Photon recollision probability in modelling the radiation regime of canopies — A review

Abstract Nearly two decades ago, the idea of the ‘spectral invariants theory’ was put forth as a new tool to model the shortwave radiation absorbed or scattered by vegetation. The theory states that the amount of radiation absorbed by a canopy should to a great accuracy depend only on the wavelength and a wavelength-independent parameter describing canopy structure. The revolutionary idea behind this theory was that it would be possible to approximate vegetation canopy absorptance, transmittance and reflectance based on only the optical properties of foliage elements and the spectrally invariant parameter(s). This paper explains how this so-called spectral invariant is related to photon recollision probability and to canopy structural variables. Other spectral invariants were later introduced to quantify the directionality of canopy scattering. Moreover, the paper reviews the advances in the theoretical development of the photon recollision probability (p) concept and demonstrates some of its applications in global and local monitoring of vegetation using remote sensing data.

Determining the Canopy Water Stress for Spring Wheat Using Canopy Hyperspectral Reflectance Data in Loess Plateau Semiarid Regions

Crop water stress significantly reduces crop yield. Several studies have employed optical and remote-sensing methods to obtain nondamage monitoring crop water content to understand the agriculture drought process. In this paper, the spectral information (i.e., the canopy absorption feature at the 350–2500 nm band range) from the field experiments was used to estimate and identify the canopy water stress. Five different levels of water treatments exist in the spring wheat field in the semiarid regions of Loess Plateau, Northwest China. The hyperspectral reflectance, soil moisture content, soil water potential, canopy water content, amount of chlorophyll, leaf area index, and environmental parameters were measured. The relationship between canopy reflectance and canopy water content was analyzed at different water stress levels. In addition, various spectral indices were tested by measurements. Results showed that a high correlation exists in semiarid water index-1, semiarid water index-2, and red-edge normalized difference vegetation index, thus denoting that these indices can indicate water stress effectively. We can conclude that canopy reflectance can identify crop water stress and can be used to develop a certain index for monitoring agriculture drought.

Arctic shrub effects on NDVI, summer albedo and soil shading

The influence of Arctic vegetation on albedo, latent and sensible heat fluxes, and active layer thickness is a crucial link between boundary layer climate and permafrost in the context of climate change. Shrubs have been observed to lower the albedo as compared to lichen or graminoid-tundra. Despite its importance, the quantification of the effect of shrubification on summer albedo has not been addressed in much detail. We manipulated shrub density and height in an Arctic dwarf birch (Betula nana) shrub canopy to test the effect on shortwave radiative fluxes and on the normalized difference vegetation index (NDVI), a proxy for vegetation productivity used in satellite-based studies. Additionally, we parametrised and validated the 3D radiative transfer model DART to simulate the amount of solar radiation reflected and transmitted by an Arctic shrub canopy. We compared results of model runs of different complexities to measured data from North-East Siberia. We achieved comparably good results with simple turbid medium approaches, including both leaf and branch optical property media, and detailed object based model parameterisations. It was important to explicitly parameterise branches as they accounted for up to 71% of the total canopy absorption and thus contributed significantly to soil shading. Increasing leaf biomass resulted in a significant increase of the NDVI, decrease of transmitted photosynthetically active radiation, and repartitioning of the absorption of shortwave radiation by the canopy components. However, experimental and modelling results show that canopy broadband nadir reflectance and albedo are not significantly decreasing with increasing shrub biomass. We conclude that the leaf to branch ratio, canopy background, and vegetation type replaced by shrubs need to be considered when predicting feedbacks of shrubification to summer albedo, permafrost thaw, and climate warming.

Do we (need to) care about canopy radiation schemes in DGVMs? Caveats and potential impacts

Abstract. Dynamic global vegetation models (DGVMs) are an essential part of current state-of-the-art Earth system models. In recent years, the complexity of DGVMs has increased by incorporating new important processes like, e.g., nutrient cycling and land cover dynamics, while biogeophysical processes like surface radiation have not been developed much further. Canopy radiation models are however very important for the estimation of absorption and reflected fluxes and are essential for a proper estimation of surface carbon, energy and water fluxes. The present study provides an overview of current implementations of canopy radiation schemes in a couple of state-of-the-art DGVMs and assesses their accuracy in simulating canopy absorption and reflection for a variety of different surface conditions. Systematic deviations in surface albedo and fractions of absorbed photosynthetic active radiation (faPAR) are identified and potential impacts are assessed. The results show clear deviations for both, absorbed and reflected, surface solar radiation fluxes. FaPAR is typically underestimated, which results in an underestimation of gross primary productivity (GPP) for the investigated cases. The deviation can be as large as 25% in extreme cases. Deviations in surface albedo range between −0.15 ≤ Δα ≤ 0.36, with a slight positive bias on the order of Δα ≈ 0.04. Potential radiative forcing caused by albedo deviations is estimated at −1.25 ≤ RF ≤ −0.8 (W m−2), caused by neglect of the diurnal cycle of surface albedo. The present study is the first one that provides an assessment of canopy RT schemes in different currently used DGVMs together with an assessment of the potential impact of the identified deviations. The paper illustrates that there is a general need to improve the canopy radiation schemes in DGVMs and provides different perspectives for their improvement.

A 3D Canopy Radiative Transfer Model for Global Climate Modeling: Description, Validation, and Application

Abstract The process of solar radiative transfer at the land surface is important to energy, water, and carbon balance, especially for vegetated areas. Currently the most commonly used two-stream model considers the plant functional types (PFTs) within a grid to be independent of each other and their leaves to be horizontally homogeneous. This assumption is unrealistic in most cases. To consider canopy three-dimensional (3D) structural effects, a new framework of 3D canopy radiative transfer model was developed and validated by numerical simulations and shows a good agreement. A comparison with the two-stream model in the offline Community Land Model (CLM4.0) shows that an increase of canopy absorption mainly happens with sparse vegetation or with multilayer canopies with a large sun zenith angle θsun and is due to increases of the ground and sky shadows and of the optical pathlength because of the shadow overlapping between bushes and canopy layers. A decrease of canopy absorption occurs in densely vegetated areas with small θsun. For a one-layer canopy, these decreases are due to crown shape effects that enhance the transmission through the canopy edge. For a multilayer canopy, aside from these shape effects, transmission is also increased by the decreased ground shadow due to the shadow overlapping between layers. Ground absorption usually changes with opposite sign as that of the canopy absorption. Somewhat lower albedos are found over most vegetated areas throughout the year. The 3D model also affects the calculation of the fraction of sunlit leaves and their corresponding absorption.

Evaluation of the JRC-TIP 0.01° products over a mid-latitude deciduous forest site

The Joint Research Centre Two-stream Inversion Package (JRC-TIP) makes use of white sky albedo products—derived from MODIS and MISR observations in the visible and near-infrared domain—to deliver consistent sets of information about the terrestrial environments that gave rise to these data. The baseline version of the JRC-TIP operates at a spatial resolution of 0.01° and yields estimates of the Probability Distribution Functions (PDFs) of the effective canopy Leaf Area Index (LAI), the canopy background albedo, the vegetation scattering properties, as well as, the absorbed, reflected and transmitted fluxes of the vegetation canopy. In this contribution the evaluation efforts of the JRC-TIP products are extended to the deciduous forest site of Hainich (Germany) where multiannual datasets of in-situ estimates of canopy transmission—derived from LAI-2000 observations—are available. As a Fluxnet site, Hainich offers access to camera acquisitions from fixed locations in and above the canopy that are being used in phenological studies. These images qualitatively confirm the seasonal patterns of the effective LAI, canopy transmission and canopy absorption products (in the visible range of the solar spectrum) derived with the JRC-TIP. Making use of the LAI-2000 observations it is found that 3/4 of the JRC-TIP products lie within a ± 0.15 interval around the in-situ estimates of canopy transmission and absorption. The largest discrepancies occur at the end of the senescence phase when the scattering properties of the vegetation (evidenced by the pictures) and the images qualitatively confirm the seasonal patterns of the effective LAI, canopy transmission and canopy absorption products (in the visible range of the solar spectrum) derived with the JRC-TIP. Making use of the LAI-2000 observations it is found that 3/4 of the JRC-TIP products lie within a ± 0.15 interval around the in-situ estimates of canopy transmission and absorption. The largest discrepancies occur at the end of the senescence phase when the scattering properties of the vegetation (evidenced by the pictures) and the effective LAI (also derived from LAI-2000 measurements) are experiencing large simultaneous changes. It was also found that the seasonal pattern of vegetation scattering properties derived from MISR observations in the near-infrared varies together with the Excess Green index computed from the various channels of the camera data acquired at the top of the canopy.

Spatial and Temporal Variability of Net Primary Productivity (NPP) over Terrestrial Biosphere of India Using NOAA-AVHRR Based GloPEM Model

The monitoring of terrestrial carbon dynamics is important in studies related with global climate change. This paper presents results of the inter-annual variability of Net Primary Productivity (NPP) from 1981 to 2000 derived using observations from NOAA-AVHRR data using Global Production Efficiency Model (GloPEM). The GloPEM model is based on physiological principles and uses the production efficiency concept, in which the canopy absorption of photosynthetically active radiation (APAR) is used with a conversion “efficiency” to estimate Gross Primary Production (GPP). NPP derived from GloPEM model over India showed maximum NPP about 3,000 gCm−2year−1 in west Bengal and lowest up to 500 gCm−2year−1 in Rajasthan. The India averaged NPP varied from 1,084.7 gCm−2year−1 to 1,390.8 gCm−2year−1 in the corresponding years of 1983 and 1998 respectively. The regression analysis of the 20 year NPP variability showed significant increase in NPP over India (r = 0.7, F = 17.53, p < 0.001). The mean rate of increase was observed as 10.43 gCm−2year−1. Carbon fixation ability of terrestrial ecosystem of India is increasing with rate of 34.3 TgC annually (t = 4.18, p < 0.001). The estimated net carbon fixation over Indian landmass ranged from 3.56 PgC (in 1983) to 4.57 PgC (in 1998). Grid level temporal correlation analysis showed that agricultural regions are the source of increase in terrestrial NPP of India. Parts of forest regions (Himalayan in Nepal, north east India) are relatively less influenced over the study period and showed lower or negative correlation (trend). Finding of the study would provide valuable input in understanding the global change associated with vegetation activities as a sink for atmospheric carbon dioxide.

A comparison of two canopy radiative models in land surface processes

This paper compares the predictions by two radiative transfer models—the two-stream approximation model and the generalized layered model (developed by the authors) in land surface processes—for different canopies under direct or diffuse radiation conditions. The comparison indicates that there are significant differences between the two models, especially in the near infrared (NIR) band. Results of canopy reflectance from the two-stream model are larger than those from the generalized model. However, results of canopy absorptance from the two-stream model are larger in some cases and smaller in others compared to those from the generalized model, depending on the cases involved. In the visible (VIS) band, canopy reflectance is smaller and canopy absorptance larger from the two-stream model compared to the generalized model when the Leaf Area Index (LAI) is low and soil reflectance is high. In cases of canopies with vertical leaf angles, the differences of reflectance and absorptance in the VIS and NIR bands between the two models are especially large.

A simplified scheme of the generalized layered radiative transfer model

In this paper, firstly, a simplified version (SGRTM) of the generalized layered radiative transfer model (GRTM) within the canopy, developed by us, is presented. It reduces the information requirement of inputted sky diffuse radiation, as well as of canopy morphology, and in turn saves computer resources. Results from the SGRTM agree perfectly with those of the GRTM. Secondly, by applying the linear superposition principle of the optics and by using the basic solutions of the GRTM for radiative transfer within the canopy under the condition of assumed zero soil reflectance, two sets of explicit analytical solutions of radiative transfer within the canopy with any soil reflectance magnitude are derived: one for incident diffuse, and the other for direct beam radiation. The explicit analytical solutions need two sets of basic solutions of canopy reflectance and transmittance under zero soil reflectance, run by the model for both diffuse and direct beam radiation. One set of basic solutions is the canopy reflectance αf (written as α1 for direct beam radiation) and transmittance βf (written as β1 for direction beam radiation) with zero soil reflectance for the downward radiation from above the canopy (i.e. sky), and the other set is the canopy reflectance (αb) and transmittance βb for the upward radiation from below the canopy (i.e., ground). Under the condition of the same plant architecture in the vertical layers, and the same leaf adaxial and abaxial optical properties in the canopies for the uniform diffuse radiation, the explicit solutions need only one set of basic solutions, because under this condition the two basic solutions are equal, i.e., αf = αb and βf = βb. Using the explicit analytical solutions, the fractions of any kind of incident solar radiation reflected from (defined as surface albedo, or canopy reflectance), transmitted through (defined as canopy transmittance), and absorbed by (defined as canopy absorptance) the canopy and other properties pertinent to the radiative transfer within the canopy can be estimated easily on the ground surface below the canopy (soil or snow surface) with any reflectance magnitudes. The simplified transfer model is proven to have a similar accuracy compared to the detailed model, as well as very efficient computing.

Highly predictable photosynthetic production in natural macroalgal communities from incoming and absorbed light

Photosynthesis-irradiance relationships of macroalgal communities and thalli of dominant species in shallow coastal Danish waters were measured over a full year to test how well community production can be predicted from environmental (incident irradiance and temperature) and community variables (canopy absorptance, species number and thallus metabolism). Detached thalli of dominant species performed optimally at different times of the year, but showed no general seasonal changes in photosynthetic features. Production capacity of communities at high light varied only 1.8-fold over the year and was unrelated to incident irradiance, temperature and mean thallus photosynthesis, while community absorptance was a highly significant predictor. Actual rates of community photosynthesis were closely related to incident and absorbed irradiance alone. Community absorptance in turn was correlated to canopy height and species richness. The close relationship of community photosynthesis to irradiance is due to the fact that (1) large differences in thallus photosynthesis of individual species are averaged out in communities composed of several species, (2) seasonal replacement of species keeps communities metabolically active, and (3) maximum possible absorptance at 100% constrains the total photosynthesis of all species. Our results imply that the photosynthetic production of macroalgal communities is more predictable than their complex and dynamic nature suggest and that predictions are possible over wide spatial scales in coastal waters by measurements of vegetation cover, incoming irradiance and canopy absorptance.